FIELD

The present disclosure relates to an interferometric optical sensor system and method for use in optical sensing of a target substance and, in particular though not exclusively, for use in optical sensing of a target substance contained in a fluid, such as a protein contained in a fluid, or for use in sensing a target substance comprising a fluid.

BACKGROUND

It is known to use photonic sensors to sense or detect a target substance contained in a fluid, such as a protein contained in a fluid, or for sensing or detecting a target substance comprising a fluid. For example, it is known to use an optical sensor comprising a grating structure which is configured to support one or more guided-mode resonances (GM Rs) to sense or detect a concentration of a target substance to which the grating structure is exposed. In this regard, it should be understood that the term “guided-mode resonance” is a term of the art which is used to describe an optical phenomenon which may be exhibited by some grating structures as exemplified in “Theory and applications of guided-mode resonance filters”, S. S. Wang and R. Magnusson, Applied Optics, Vol. 32, No. 14, 10 May 1993, pp. 2606-2613. Depending on the configuration of the grating structure and the properties of the incident light, a guided mode resonance may be excited in the grating structure which results in at least a portion of the light which is incident on the grating structure being reflected from the grating structure and/or which results in at least a portion of the light which is incident on the grating structure being transmitted through the grating structure, wherein the quantity of light which is reflected and/or transmitted depends on the nature of the guided mode resonance excited in the grating structure.

Interferometric optical sensor systems are also known which exploit a phase response of a GMR sensor. For example, a known interferometric optical sensor system includes a GMR sensor, a laser for exciting different GM Rs in a plurality of different sensor regions of a 1 D grating structure of the GMR sensor and a plurality of pinholes aligned with the plurality of sensor regions of the 1 D grating structure of the GMR sensor so as to transmit and interfere light output from the plurality of sensor regions of the 1 D grating structure of the GMR sensor to thereby generate an interferogram. The interferogram depends on the phase shifts imposed on light incident on the plurality of sensor regions of the 1 D grating structure of the GMR sensor as a result of excitation of the different GMRs in the different sensor regions, wherein the phase shifts are themselves dependent on a concentration of a target substance to which the sensor surface of the GMR sensor is exposed. However, such interferometric optical sensor systems may require precise alignment of the plurality of apertures with the plurality of sensor regions of the GMR sensor. Moreover, use of a plurality of apertures may reduce optical power resulting in an interferogram which has limited contrast. Consequently, such known interferometric optical sensor systems may be susceptible to noise. Moreover, such interferometric optical sensor systems rely upon the use of a laser to excite different GMRs in the different sensor regions of the 1 D grating structure of the GMR sensor, which may add to the cost and complexity of the optical sensor system, which may lead to speckle noise, and which may not be compatible with eye safety regulations. Furthermore, although it may be possible to use a low cost diode laser to excite the different GMRs in the different sensor regions of the 1 D grating structure of the GMR sensor, such a low cost diode laser may suffer from wavelength hopping, polarization instability and intensity noise.

Interferometric optical sensor systems are also known which include a GMR sensor, a laser for exciting orthogonally polarised GMRs having different resonant spectral responses in two different sensor regions of the same 1 D grating structure of the GMR sensor, and a polarisation shearing arrangement comprising a Wollaston prism and a polarisation analyser for interfering output light which is output from the two different sensor regions of the same 1 D grating structure of the GMR sensor to thereby generate an interferogram. The interferogram depends on the phase shifts imposed on light incident on the two different sensor regions of the same 1 D grating structure of the GMR sensor as a result of excitation of the orthogonally polarised GMRs in the two different sensor regions, wherein the phase shifts are themselves dependent on a concentration of a target substance to which the sensor surface of the GMR sensor is exposed. However, such interferometric optical sensor systems rely upon the use of a laser to excite the orthogonally polarised GMRs in the two different sensor regions of the same 1 D grating structure, which may add to the cost and complexity of the optical sensor system, which may lead to speckle noise, and which may not be compatible with eye safety regulations. Furthermore, although it may be possible to use a low cost diode laser to excite the orthogonally polarised GMRs in the two different sensor regions, such a low cost diode laser may suffer from wavelength hopping, polarization instability and intensity noise. SUMMARY

According to an aspect of the present disclosure there is provided an interferometric optical sensor system for use in optical sensing of a target substance, the system comprising: an optical sensor element comprising first and second sensor regions for exposure to the target substance, wherein the first sensor region includes a first grating structure defining at least a first grating vector so that the first sensor region is configured to support at least a first guided mode resonance, wherein the first grating vector is oriented along a first direction, wherein the second sensor region includes a second grating structure defining at least a second grating vector so that the second sensor region is configured to support at least a second guided mode resonance, wherein the second grating vector is oriented along a second direction, wherein the first and second directions are different, wherein the first guided mode resonance has an associated first resonant spectral response and the second guided mode resonance has an associated second resonant spectral response, and wherein the first and second resonant spectral responses at least partially overlap; an optical source for illuminating the first and second sensor regions with input light so that a first polarisation component of the input light excites the first guided mode resonance in the first sensor region and a second polarisation component of the input light excites the second guided mode resonance in the second sensor region; a polarisation control arrangement for interfering a polarisation component of first output light output from the first sensor region and a corresponding polarisation component of second output light output from the second sensor region so as to generate an interferogram, the first output light being output from the first sensor region as a result of excitation of the first guided mode resonance and the second output light being output from the second sensor region as a result of excitation of the second guided mode resonance; and an image sensor for detecting an image of at least part of the interferogram.

One of ordinary skill in the art will understand that the term “grating vector” is a term of the art which refers to a vector associated with a grating structure and which defines a spatial frequency and a direction of a variation in refractive index and/or thickness of one or more materials from which the grating structure is formed. Accordingly, the first grating vector may define a spatial frequency of a variation in refractive index and/or thickness of one or more materials of the first grating structure along the first direction and the second grating vector may define a spatial frequency of a variation in refractive index and/or thickness of one or more materials of the second grating structure along the second direction.

Optionally, the first and second guide mode resonances (GM Rs) are degenerate or near-degenerate.

It should also be understood that the first grating structure may define one or more additional grating vectors (which are additional to the first grating vector) and that the first grating structure may support one or more additional GM Rs associated with the one or more additional grating vectors defined by the first grating structure. Similarly, the second grating structure may define one or more additional grating vectors (which are additional to the second grating vector) and the second grating structure may support one or more additional GM Rs associated with the one or more additional grating vectors. For example, one of the additional grating vectors defined by the first grating structure may be oriented along the second direction of the second grating vector defined by the second grating structure and/or one of the additional grating vectors defined by the second grating structure may be oriented along the first direction of the first grating vector defined by the first grating structure. However, it is only the first output light which is output from the first sensor region as a result of excitation of the first guided mode resonance in the first sensor region and only the second output light which is output from the second sensor region as a result of excitation of the second guided mode resonance in the second sensor region which is actually interfered on the image sensor i.e. it is only the output light resulting from the excitation of the GM Rs that are associated with the first and second grating vectors of the first and second grating structures that is interfered. As such, one of ordinary skill in the art will understand that the use of first and second grating structures like those described above enables the polarisation control arrangement to interfere output light which is output form the first and second sensor regions and which results from the excitation of degenerate or near-degenerate first and second GMRs in the first and second sensor regions.

Optionally, the optical sensor element comprises a substrate.

Optionally, the first grating structure is disposed on a surface of the substrate.

Optionally, the second grating structure is disposed on the surface of the substrate.

Optionally, a dominant electric field component of the first guided mode resonance and a dominant electric field component of the second guided mode resonance are both perpendicular to the surface of the substrate. Optionally, a dominant electric field component of the first guided mode resonance and a dominant electric field component of the second guided mode resonance are both parallel to the surface of the substrate.

Optionally, the first and second directions are orthogonal to one another.

Optionally, the surface of the substrate is flat or generally flat, or planar or generally planar.

Optionally, the optical sensor element is flat or generally flat, or planar or generally planar.

Optionally, the first and second grating structures are formed from the same material or the same materials.

Optionally, the first and second grating structures comprise a plurality of features of the same height or depth defined in a direction perpendicular to the surface of the substrate.

Optionally, the first grating structure has a first periodicity and a first fill factor in the first direction and the second grating structure has a second periodicity and a second fill factor in the second direction.

Optionally, the first and second periodicities differ by less than 10%, differ by less than 5%, differ by less than 1 %, are substantially equal, or are equal.

Optionally, the first and second fill factors differ by less than 10%, differ by less than 5%, or differ by less than 1%, are substantially equal, or are equal.

Optionally, each of the first and second grating structures comprises a corresponding constant periodicity and/or a corresponding constant fill factor.

Optionally, each of the first and second grating structures has a corresponding chirped periodicity and/or a corresponding chirped fill factor.

Optionally, each of the first and second grating structures comprises a corresponding plurality of different periodicities and/or a corresponding plurality of different fill factors.

Optionally, the optical sensor element comprises a 1 D or 2D array of sensor regions including a plurality of pairs of grating structures, each pair of grating structures comprising a first grating structure and a second grating structure, wherein the first and second grating structures of each pair of grating structures comprise, or are formed from, the same material(s) and have the same periodicity and the same fill factor, and wherein different pairs of grating structures comprise, or are formed from, the same material(s), but have different periodicities and/or different fill factors. Optionally, the first and second grating structures comprise first and second 1 D grating structures.

Optionally, the first and second 1 D grating structures are oriented differently to one another.

Optionally, the first and second 1 D grating structures are oriented perpendicular to one another.

Optionally, the first and second 1 D grating structures are formed from the same material or the same materials.

Optionally, the first 1 D grating structure has a first periodicity and a first fill factor in the first direction and the second 1 D grating structure has a second periodicity and a second fill factor in the second direction.

Optionally, the second direction is orthogonal to the first direction.

Optionally, the first and second periodicities differ by less than 10%, differ by less than 5%, differ by less than 1%, are substantially equal, or are equal.

Optionally, the first and second fill factors differ by less than 10%, differ by less than 5%, or differ by less than 1%, are substantially equal, or are equal.

Optionally, each of the first and second 1 D grating structures comprises a corresponding constant periodicity and/or a corresponding constant fill factor.

Optionally, each of the first and second 1 D grating structures has a chirped periodicity and/or a chirped fill factor.

Optionally, each of the first and second 1 D grating structures comprises a plurality of different periodicities and/or a plurality of different fill factors.

Optionally, each of the first and second 1 D grating structures comprises a corresponding plurality of grooves and ridges.

Optionally, the first and second 1 D grating structures comprise a plurality of ridges of the same height and a plurality of grooves of the same depth defined in a direction perpendicular to the surface of the substrate.

Optionally, the grooves and ridges of the first 1 D grating structure are oriented differently to the grooves and ridges of the second 1 D grating structure.

Optionally, the grooves and ridges of the first 1 D grating structure are oriented perpendicular to the grooves and ridges of the second 1 D grating structure.

Optionally, the first and second 1 D grating structures are configured so that each of the first and second sensor regions supports only one guided mode resonance for input light having a given spectral range, when the input light is incident on the optical sensor element at a given angle of incidence and the first and second sensor regions are exposed to a given fluid.

Optionally, each of the first and second grating structures comprises a plurality of holes or pillars.

Optionally, the first and second grating structures comprise different regions of the same 2D grating structure.

Optionally, the 2D grating structure has different grating vectors in different directions.

Optionally, the 2D grating structure has a primary periodicity and a primary fill factor in a primary direction and a secondary periodicity and a secondary fill factor in a secondary direction which is different to the primary direction.

Optionally, the secondary direction is orthogonal to the primary direction.

Optionally, the primary and secondary periodicities are the same or different.

Optionally, the primary and secondary fill factors are the same or different.

Optionally, the 2D grating structure has a corresponding constant periodicity and/or a corresponding constant fill factor along the primary direction. Optionally, the 2D grating structure has a corresponding constant periodicity and/or a corresponding constant fill factor along the secondary direction.

Optionally, the 2D grating structure has a corresponding chirped periodicity and/or a corresponding chirped fill factor along the primary direction. Optionally, the 2D grating structure has a corresponding chirped periodicity and/or a corresponding chirped fill factor along the secondary direction.

Optionally, the 2D grating structure comprises a plurality of corresponding different periodicities and/or a plurality of corresponding different fill factors along the primary direction. Optionally, the 2D grating structure comprises a plurality of corresponding different periodicities and/or a plurality of corresponding different fill factors along the secondary direction.

Optionally, the 2D grating structure comprises a 2D array of holes or pillars such as a regular 2D array of holes or pillars.

Optionally, the spatial arrangement and orientation of the plurality of holes or pillars in the different regions of the same 2D grating structure which form the first and second grating structures are the same.

Optionally, the 2D grating structure comprises a 2D square array of holes or pillars. Optionally, the 2D grating structure comprises a 2D rectangular array of holes or pillars.

Optionally, the 2D grating structure comprises a 2D hexagonal array of holes or pillars.

Optionally, the 2D grating structure comprises a 2D array of holes, wherein the holes of the first and second grating structures are of the same depth defined in a direction perpendicular to the surface of the substrate.

Optionally, the 2D grating structure comprises a 2D array of pillars, wherein the pillars of the first and second grating structures are of the same height defined in a direction perpendicular to the surface of the substrate.

Optionally, the first and second grating structures comprise first and second 2D grating structures.

Optionally, the first 2D grating structure has different grating vectors in different directions.

Optionally, the first 2D grating structure has a primary periodicity and a primary fill factor in a primary direction and a secondary periodicity and a secondary fill factor in a secondary direction which is different to the primary direction.

Optionally, the secondary direction is orthogonal to the primary direction.

Optionally, the primary and secondary periodicities of the first 2D grating structure are the same or different.

Optionally, the primary and secondary fill factors of the first 2D grating structure are the same or different.

Optionally, the second 2D grating structure has different grating vectors in different directions.

Optionally, the second 2D grating structure has a primary periodicity and a primary fill factor in a primary direction and a secondary periodicity and a secondary fill factor in a secondary direction which is different to the primary direction.

Optionally, the secondary direction is orthogonal to the primary direction.

Optionally, the primary and secondary periodicities of the second 2D grating structure are the same or different.

Optionally, the primary and secondary fill factors of the second 2D grating structure are the same or different.

Optionally, the first and second 2D grating structures have the same orientation.

Optionally, the first and second 2D grating structures have different orientations. Optionally, the first and second 2D grating structures are oriented perpendicular to one another.

Optionally, the first and second 2D grating structures are formed from the same material or the same materials.

Optionally, the first 2D grating structure has a first periodicity and a first fill factor in the first direction and the second 1 D grating structure has a second periodicity and a second fill factor in the second direction.

Optionally, the first direction corresponds to the primary or secondary direction of the first 2D grating structure.

Optionally, the second direction corresponds to the primary or secondary direction of the second 2D grating structure.

Optionally, the second direction is orthogonal to the first direction.

Optionally, the first and second periodicities differ by less than 10%, differ by less than 5%, differ by less than 1%, are substantially equal, or are equal.

Optionally, the first and second fill factors differ by less than 10%, differ by less than 5%, or differ by less than 1%, are substantially equal, or are equal.

Optionally, the first 2D grating structure comprises a corresponding constant periodicity and/or a corresponding constant fill factor along the primary direction of the first 2D grating structure. Optionally, the first 2D grating structure comprises a corresponding constant periodicity and/or a corresponding constant fill factor along the secondary direction of the first 2D grating structure.

Optionally, the second 2D grating structure comprises a corresponding constant periodicity and/or a corresponding constant fill factor along the primary direction of the second 2D grating structure. Optionally, the second 2D grating structure comprises a corresponding constant periodicity and/or a corresponding constant fill factor along the secondary direction of the second 2D grating structure.

Optionally, the first 2D grating structure has a corresponding chirped periodicity and/or a corresponding chirped fill factor along the primary direction of the first 2D grating structure. Optionally, the first 2D grating structure has a corresponding chirped periodicity and/or a corresponding chirped fill factor along the secondary direction of the first 2D grating structure.

Optionally, the second 2D grating structure has a corresponding chirped periodicity and/or a corresponding chirped fill factor along the primary direction of the second 2D grating structure. Optionally, the second 2D grating structure has a corresponding chirped periodicity and/or a corresponding chirped fill factor along the secondary direction of the second 2D grating structure.

Optionally, the first 2D grating structure comprises a corresponding plurality of different periodicities and/or a corresponding plurality of different fill factors along the primary direction of the first 2D grating structure. Optionally, the first 2D grating structure comprises a corresponding plurality of different periodicities and/or a corresponding plurality of different fill factors along the secondary direction of the first 2D grating structure.

Optionally, the second 2D grating structure comprises a corresponding plurality of different periodicities and/or a corresponding plurality of different fill factors along the primary direction of the second 2D grating structure. Optionally, the second 2D grating structure comprises a corresponding plurality of different periodicities and/or a corresponding plurality of different fill factors along the secondary direction of the second 2D grating structure.

Optionally, each of the first and second 2D grating structures comprises a 2D array of holes or pillars such as a regular 2D array of holes or pillars.

Optionally, the spatial arrangement and orientation of the plurality of holes or pillars of each of the first and second 2D grating structures are the same.

Optionally, the first 2D grating structure comprises a 2D square array of holes or pillars.

Optionally, the first 2D grating structure comprises a 2D rectangular array of holes or pillars.

Optionally, the first 2D grating structure comprises a 2D hexagonal array of holes or pillars.

Optionally, the second 2D grating structure comprises a 2D square array of holes or pillars.

Optionally, the second 2D grating structure comprises a 2D rectangular array of holes or pillars.

Optionally, the second 2D grating structure comprises a 2D hexagonal array of holes or pillars.

Optionally, the first and second grating structures comprise, or are formed from, a material which has a higher refractive index than a material of the substrate.

Optionally, the substrate comprises, or is formed from, silica (SiO2).

Optionally, the first and second grating structures comprise, or are formed from, silicon nitride (Sisl^ ). Optionally, the first and second 2D grating structures each comprise a corresponding 2D array of holes, wherein the holes of the first and second 2D grating structures are of the same depth defined in a direction perpendicular to the surface of the substrate.

Optionally, the first and second 2D grating structures each comprise a corresponding 2D array of pillars, wherein the pillars of the first and second 2D grating structures are of the same height defined in a direction perpendicular to the surface of the substrate.

Optionally, the target substance is contained in a fluid.

Optionally, the target substance comprises a fluid.

Optionally, the first sensor region comprises a coating disposed on the first grating structure.

Optionally, the second sensor region comprises a coating disposed on the second grating structure.

Optionally, the coatings of the first and second sensor regions are configured so that the target substance binds, bonds and/or adheres more effectively, more strongly, with a greater probability, and/or to a greater degree to the coating of the first sensor region than the coating of the second sensor region.

Optionally, the coating of the first sensor region comprises an antibody specific to the target substance.

Optionally, the coating of the second sensor region comprises an isotype or nonbinding antibody.

Optionally, the target substance comprises a protein such as immunoglobulin G (igG).

Optionally, the coatings of the first and second sensor regions have the same refractive index or different refractive indices.

Optionally, the coatings of the first and second sensor regions have the same thickness or different thicknesses.

Optionally, the second sensor region does not have a coating disposed on the second grating structure.

Optionally, the first resonant spectral response relates an intensity and a phase of the first output light as a function of wavelength to an intensity and a phase of the first polarisation component of the input light as a function of wavelength. Optionally, the second resonant spectral response relates an intensity and a phase of the second output light as a function of wavelength to an intensity and a phase of the second polarisation component of the input light as a function of wavelength.

Optionally, the first polarisation component of the input light is reflected from the first sensor region with a reflectance and a phase shift which depend on the first guided mode resonance and the first resonant spectral response comprises a first resonant reflectance spectrum and an associated first resonant phase shift spectrum. Optionally, the second polarisation component of the input light is reflected from the second sensor region with a reflectance and a phase shift which depend on the second guided mode resonance and the second resonant spectral response comprises a second resonant reflectance spectrum and an associated second resonant phase shift spectrum.

Optionally, the first polarisation component of the input light is transmitted through the first sensor region with a transmittance and a phase shift which depend on the first guided mode resonance and the first resonant spectral response comprises a first resonant transmittance spectrum and an associated first resonant phase shift spectrum. Optionally, the second polarisation component of the input light is transmitted through the second sensor region with a transmittance and a phase shift which depend on the second guided mode resonance and the second resonant spectral response comprises a second resonant transmittance spectrum and an associated second resonant phase shift spectrum.

Optionally, the first and second sensor regions are configured so that the first and second resonant spectral responses at least partially overlap before the first and second sensor regions are exposed to the target substance and so that the first and second resonant spectral responses move apart in wavelength as the first and second sensor regions are exposed to the target substance.

Optionally, the first and second sensor regions are deliberately detuned so that the first and second resonant spectral responses only partially overlap before the first and second sensor regions are exposed to the target substance, and so that, as the first and second sensor regions are exposed to the target substance, the first and second resonant spectral responses initially move together in wavelength so as to increase the overlap between the first and second resonant spectral responses until a maximum overlap between the first and second resonant spectral responses is reached whereupon the first and second resonant spectral responses move apart in wavelength so as to decrease the overlap between the first and second resonant spectral responses as the first and second sensor regions are further exposed to the target substance. Optionally, the system is configured so that the input light travels along an input optical path which extends from the optical source to the optical sensor element.

Optionally, the input optical path is arranged at a non-zero acute angle relative to a normal direction which is perpendicular to a surface of the substrate.

Optionally, the input optical path is arranged parallel to a normal direction which is perpendicular to the surface of the substrate.

Optionally, the system comprises a non-polarising beam splitter, wherein the non-polarising beam splitter is located on the input optical path.

Optionally, the optical source comprises an unpolarised optical source which is configured to generate unpolarised input light.

Optionally, the optical source is configured to generate light having a coherence length in the range of 10 .m to 1000 .m, 20 .m to 200 .m, or substantially equal to 100 .m.

Optionally, the optical source is configured to generate light having a full-width half maximum (FWHM) spectral width in the range of 0.01 nm to 10 nm, 0.5 nm to 5 nm, or 1 nm to 3 nm.

Optionally, the optical source comprises a light emitting diode (LED).

Optionally, the optical source comprises a super-luminescent LED (SLED) or a resonant cavity LED (RCLED).

Optionally, the system comprises a spectral filter, such as a bandpass spectral filter, for spectrally filtering light emitted by the optical source.

Optionally, the bandpass spectral filter has a full-width half maximum (FWHM) spectral width in the range 0.01 nm to 10 nm, 0.5 nm to 5 nm, or 1 nm to 3 nm.

Optionally, the optical source comprises a polarised optical source which is configured to generate polarised input light.

Optionally, the polarised optical source is oriented relative to the optical sensor element so as to illuminate the first and second sensor regions with the polarised input light so that a first polarisation component of the polarised input light excites the first guided mode resonance in the first sensor region and a second polarisation component of the polarised input light excites the second guided mode resonance in the second sensor region.

Optionally, wherein the polarised optical source comprises a coherent polarised optical source such as a laser, for example a laser diode

Optionally, the optical source comprises a coherent polarised optical source.

Optionally, the optical source comprises a laser, for example a laser diode. Optionally, the system comprises an aperture for spatially filtering light emitted by the optical source.

Optionally, there are no lenses or curved mirrors located on the input optical path.

Optionally, the input light is collimated.

Optionally, the optical source is configured to generate a collimated beam of input light.

Optionally, the system comprises one or more lenses or curved mirrors located on the input optical path for collimating light emitted by the optical source.

Optionally, the polarisation control arrangement comprises a polarisation shearing and analysing arrangement.

Optionally, the polarisation shearing and analysing arrangement comprises a polarisation shearing element and a polarisation analyser.

Optionally, the polarisation shearing element is configured to direct two orthogonal polarisations along two different optical paths.

Optionally, the optical sensor element and the polarisation shearing element are oriented relative to one another so that the polarisation shearing element directs orthogonally polarised components of the first and second output light along two different optical paths so that at least two of the orthogonally polarised components of the first and second output light at least partially overlap.

Optionally, the polarisation shearing element comprises a birefringent element which includes a birefringent material, for example a Wollaston prism, a Nomarski prism, or a Savart plate.

Optionally, the polarisation shearing element is flat or generally flat, or planar or generally planar.

Optionally, the polarisation shearing element has form-birefringence.

Optionally, the polarisation shearing element comprises a polarisation shearing metasurface.

Optionally, the polarisation shearing metasurface is configured to focus or image the first and second output light onto the image sensor.

Optionally, the polarisation analyser is configured to linearly polarise light along a polariser axis.

Optionally, the optical sensor element and the polarisation analyser are oriented relative to one another so that the polariser axis is oriented at a non-zero angle relative to a polarisation component of the first output light and the polariser axis is oriented at a non-zero angle relative to a polarisation component of the second output light. Optionally, the polariser axis is oriented at an angle of 45 degrees, or approximately 45 degrees, relative to a polarisation component of the first output light and at an angle of 45 degrees, or approximately 45 degrees, relative to a polarisation component of the second output light.

Optionally, the polarisation analyser is flat or generally flat, or planar or generally planar.

Optionally, the polarisation analyser comprises a polarisation analyser metasurface.

Optionally, the polarisation analyser metasurface is configured to focus or image the first and second output light onto the image sensor.

Optionally, the polarisation shearing element and the polarisation analyser are arranged generally parallel to one another and to the image sensor.

Optionally, the polarisation shearing and analysing arrangement comprises a single multi-function metasurface which is configured to direct two orthogonal polarisations along two different paths whilst also linearly polarising light along a polariser axis of the multi-function metasurface.

Optionally, the optical sensor element and the multi-function metasurface are oriented relative to one another so that the multi-function metasurface directs the first and second output light along respective output optical paths so that the first and second output light at least partially overlaps, and so that the polariser axis of the multi-function metasurface is oriented at a non-zero angle relative to a polarisation component of the first output light and the polariser axis of the multi-function metasurface is oriented at a non-zero angle relative to a polarisation component of the second output light.

Optionally, the optical sensor element and the multi-function metasurface are oriented relative to one another so that the polariser axis of the multi-function metasurface is oriented at an angle of 45 degrees, or approximately 45 degrees, relative to a polarisation component of the first output light and at an angle of 45 degrees, or approximately 45 degrees, relative to a polarisation component of the second output light.

Optionally, the multi-function metasurface is configured to focus or image the first and second output light onto the image sensor.

Optionally, the multi-function metasurface is flat or generally flat, or planar or generally planar.

Optionally, there are no lenses or curved mirrors located on the first or second output optical paths. Optionally, the polarisation control arrangement comprises an interferometer arrangement comprising at least one polarising beam splitter and a half-wave plate, wherein the at least one polarising beam splitter is configured to direct orthogonal polarisation components of the first and second output light along first and second optical paths and to recombine light which has travelled along the first and second optical paths, and wherein the half-wave plate is located in one of the first and second optical paths.

For example, the polarisation control arrangement may comprise a Mach- Zehnder interferometer arrangement which includes first and second polarising beam splitters, and a half-wave plate, wherein the first polarising beam splitter is configured to direct orthogonal polarisation components of the first and second output light along first and second optical paths, wherein the second polarising beam splitter is configured to recombine light which has travelled along the first and second optical paths, and wherein the half-wave plate is located in one of the first and second optical paths.

Alternatively, the polarisation control arrangement may comprise a Michelson interferometer arrangement which includes a polarising beam splitter and a half-wave plate, wherein the polarising beam splitter is configured to direct orthogonal polarisation components of the first and second output light along first and second optical paths, wherein the polarising beam splitter is configured to recombine light which has travelled along the first and second optical paths, and wherein the half-wave plate is located in one of the first and second optical paths.

Optionally, the system comprises a processing resource which is configured to determine, from the detected image, a value of a quantity representative of a concentration of the target substance to which the first and second sensor regions are exposed.

According to an aspect of the present disclosure there is provided an interferometric optical sensing method for use in optical sensing of a target substance, the method comprising: exposing first and second sensor regions of an optical sensor element to the target substance, wherein the first sensor region includes a first grating structure defining at least a first grating vector so that the first sensor region is configured to support at least a first guided mode resonance, wherein the first grating vector is oriented along a first direction, wherein the second sensor region includes a second grating structure defining at least a second grating vector so that the second sensor region is configured to support at least a second guided mode resonance, wherein the second grating vector is oriented along a second direction, wherein the first and second directions are different, wherein the first guided mode resonance has an associated first resonant spectral response and the second guided mode resonance has an associated second resonant spectral response, and wherein the first and second resonant spectral responses at least partially overlap; illuminating the first and second sensor regions with input light so that a first polarisation component of the input light excites the first guided mode resonance in the first sensor region and a second polarisation component of the unpolarised input light excites the second guided mode resonance in the second sensor region; interfering a polarisation component of first output light output from the first sensor region and a corresponding polarisation component of second output light output from the second sensor region so as to generate an interferogram, the first output light being output from the first sensor region as a result of excitation of the first guided mode resonance and the second output light being output from the second sensor region as a result of excitation of the second guided mode resonance; and detecting an image of at least part of the interferogram.

Optionally, the method comprises determining, from the detected image, a value of a quantity representative of a concentration of the target substance to which the first and second sensor regions are exposed.

Optionally, the first and second guided mode resonances (GMRs) are degenerate or near-degenerate.

Optionally, the target substance is contained in a fluid.

Optionally, the target substance comprises a fluid.

Optionally, the first sensor region comprises a coating disposed on the first grating structure and the second sensor region comprises a coating disposed on the second grating structure.

Optionally, the coatings of the first and second sensor regions are configured so that the target substance binds, bonds and/or adheres more effectively, more strongly, with a greater probability, and/or to a greater degree to the coating of the first sensor region than the coating of the second sensor region.

Optionally, the coating of the first sensor region comprises an antibody specific to the target substance.

Optionally, the coating of the second sensor region comprises an isotype or nonbinding antibody.

Optionally, the target substance comprises a protein such as immunoglobulin G

(igG). Optionally, the coatings of the first and second sensor regions have the same refractive index or different refractive indices.

Optionally, the coatings of the first and second sensor regions have the same thickness or different thicknesses.

Optionally, the first sensor region comprises a coating disposed on the first grating structure but the second sensor region does not have a coating disposed on the second grating structure.

Optionally, the method comprises exposing the first and second sensor regions to the target substance.

Optionally, the method comprises exposing the first sensor region to the target substance without exposing the second sensor region to the target substance.

Optionally, the optical sensor element comprises a substrate, wherein the first and second grating structures are disposed on a surface of the substrate.

Optionally, a dominant electric field component of the first guided mode resonance and a dominant electric field component of the second guided mode resonance are both perpendicular to the surface of the substrate.

Optionally, a dominant electric field component of the first guided mode resonance and a dominant electric field component of the second guided mode resonance are both parallel to the surface of the substrate.

Optionally, the surface of the substrate is flat or generally flat, or planar or generally planar.

Optionally, the optical sensor element is flat or generally flat, or planar or generally planar.

Optionally, the first resonant spectral response relates an intensity and a phase of the first output light as a function of wavelength to an intensity and a phase of the first polarisation component of the input light as a function of wavelength. Optionally, the second resonant spectral response relates an intensity and a phase of the second output light as a function of wavelength to an intensity and a phase of the second polarisation component of the input light as a function of wavelength.

Optionally, the first polarisation component of the input light is reflected from the first sensor region with a reflectance and a phase shift which depend on the first guided mode resonance and the first resonant spectral response comprises a first resonant reflectance spectrum and an associated first resonant phase shift spectrum. Optionally, the second polarisation component of the input light is reflected from the second sensor region with a reflectance and a phase shift which depend on the second guided mode resonance and the second resonant spectral response comprises a second resonant reflectance spectrum and an associated second resonant phase shift spectrum.

Optionally, the first polarisation component of the input light is transmitted through the first sensor region with a transmittance and a phase shift which depend on the first guided mode resonance and the first resonant spectral response comprises a first resonant transmittance spectrum and an associated first resonant phase shift spectrum. Optionally, the second polarisation component of the input light is transmitted through the second sensor region with a transmittance and a phase shift which depend on the second guided mode resonance and the second resonant spectral response comprises a second resonant transmittance spectrum and an associated second resonant phase shift spectrum.

Optionally, the input light is unpolarised or polarised.

It should be understood that any one or more of the features of any one of the foregoing aspects of the present disclosure may be combined with any one or more of the features of any of the other foregoing aspects of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

An interferometric optical sensor system and method will now be described by way of non-limiting example only with reference to the drawings of which:

FIG. 1A is a schematic plan view of a first interferometric optical sensor system;

FIG. 1 B is a schematic perspective view of an optical sensor element of the first interferometric optical sensor system of FIG. 1A;

FIG. 1C is a plan view electron microscope image of the optical sensor element of FIG. 1 B;

FIG. 1 D is a plan view electron microscope image of a metasurface polarisation shearing element of the first interferometric optical sensor system of FIG. 1 A;

FIG. 2A shows a resonant spectral response associated with a first guided mode resonance of a first sensor region of the optical sensor element of FIGS. 1 B and 1C; FIG. 2B shows a resonant spectral response associated with a second guided mode resonance of a second sensor region of the optical sensor element of FIGS. 1 B and 1C;

FIG. 3A is a schematic plan view of a second interferometric optical sensor system;

FIG. 3B is a schematic perspective view of an optical sensor element of the second interferometric optical sensor system of FIG. 3A;

FIG. 3C is a plan view electron microscope image of the optical sensor element of FIG. 3B;

FIG. 3D is a plan view electron microscope image of a metasurface polarisation shearing element of the second interferometric optical sensor system of FIG. 3A;

FIG. 4A is a schematic plan view of a third interferometric optical sensor system;

FIG. 4B is a schematic perspective view of an optical sensor element of the third interferometric optical sensor system of FIG. 4A;

FIG. 4C is a plan view electron microscope image of the optical sensor element of FIG. 4B;

FIG. 4D is a plan view electron microscope image of a metasurface polarisation shearing element of the third interferometric optical sensor system of FIG. 4A;

FIG. 5A is a plan view image of an optical sensor element which includes a 1 D array of sensor regions;

FIG. 5B is a plan view image of an optical sensor element which includes a 2D array of sensor regions;

FIG. 6A is a plan view image of an optical sensor element which includes a 2D array of sensor regions; FIG. 6B illustrates the dispensing of fluids containing different concentrations of target substance onto different sensor regions of the optical sensor element of FIG. 6A;

FIG. 6C is an image of a plurality of interferograms generated by exciting the 2D array of sensor regions of the optical sensor element of FIG. 6A with unpolarised light;

FIG. 6D shows the evolution of a phase shift as a function of time following application of a target substance onto one of the sensor regions of the optical sensor element of FIG. 6A during excitation of the 2D array of sensor regions of the optical sensor element of FIG. 6A with unpolarised light;

FIG. 7A is a schematic plan view of an alternative optical sensor element;

FIG. 7B is a schematic plan view of a further alternative optical sensor element;

FIG. 8A is a perspective exploded view of an alternative interferometric optical sensor system which includes the interferometric optical sensor system of FIG. 4A;

FIG. 8B is an image of the alternative interferometric optical sensor system of FIG. 8A;

FIG. 9A is a plan view of a first alternative optical sensor element for use in the interferometric optical sensor system of FIG. 1A, 3A or 4A;

FIG. 9B is a plan view of a second alternative optical sensor element for use in the interferometric optical sensor system of FIG. 1A, 3A or 4A;

FIG. 10A is a plan view of a third alternative optical sensor element for use in the interferometric optical sensor system of FIG. 1A, 3A or 4A;

FIG. 10B is a plan view of a fourth alternative optical sensor element for use in the interferometric optical sensor system of FIG. 1A, 3A or 4A;

FIG. 10C is a plan view of a fifth alternative optical sensor element for use in the interferometric optical sensor system of FIG. 1 A, 3A or 4A; FIG. 11A is a plan view of a sixth alternative optical sensor element for use in the interferometric optical sensor system of FIG. 1A, 3A or 4A;

FIG. 11 B is a plan view of a seventh alternative optical sensor element for use in the interferometric optical sensor system of FIG. 1A, 3A or 4A;

FIG. 11C is a plan view of an eighth alternative optical sensor element for use in the interferometric optical sensor system of FIG. 1A, 3A or 4A;

FIG. 12A illustrates additional grating vectors in a second sensor region of the sixth alternative optical sensor element of FIG. 11 A;

FIG. 12B illustrates additional grating vectors in a second sensor region of the seventh alternative optical sensor element of FIG. 11 B; and

FIG. 12C illustrates additional grating vectors in a second sensor region of the eighth alternative optical sensor element of FIG. 11C.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring initially to FIG. 1A there is shown a first interferometric optical sensor system generally designated 2 for use in sensing a target substance 3 such as a protein contained in a fluid. The interferometric optical sensor system 2 includes an optical sensor element 4 having first and second sensor regions 4a and 4b respectively for exposure to the target substance 3, an optical source 6 for illuminating the sensor regions 4a and 4b with input light, a polarisation control arrangement 8 for interfering a polarisation component of first output light output from the first sensor region 4a and a polarisation component of second output light output from the second sensor region 4a so as to generate an interferogram, an image sensor 10 for detecting an image of at least part of the interferogram, and a processing resource 12 which is configured to determine, from the detected image, a value of a quantity representative of a concentration of the target substance 3 to which the first and second sensor regions 4a, 4b are exposed.

As will be described in more detail below, the first sensor region 4a includes a first grating structure defining at least a first grating vector so that the first sensor region 4a is configured to support at least a first guided mode resonance, wherein the first grating vector is oriented along a first direction, and the second sensor region 4b includes a second grating structure defining at least a second grating vector so that the second sensor region 4b is configured to support at least a second guided mode resonance, wherein the second grating vector is oriented along a second direction, wherein the first and second directions are different, wherein the first guided mode resonance has an associated first resonant spectral response and the second guided mode resonance has an associated second resonant spectral response, and wherein the first and second resonant spectral responses at least partially overlap. One of ordinary skill in the art will understand that the term “grating vector” is a term of the art which refers to a vector associated with a grating structure and which defines a spatial frequency and a direction of a variation in refractive index and/or thickness of one or more materials from which the grating structure is formed. Accordingly, the first grating vector defines a spatial frequency of a variation in refractive index and/or thickness of one or more materials of the first grating structure along the first direction and the second grating vector defines a spatial frequency of a variation in refractive index and/or thickness of one or more materials of the second grating structure along the second direction.

As shown in FIGS. 1 B and 1 C, the optical sensor element 4 includes a substrate 20. The first sensor region 4a includes a first grating structure in the form of a first 1 D grating structure 22 disposed on a surface of the substrate 20, and a coating 24 disposed on the first 1 D grating structure 22. Similarly, the second sensor region 4b includes a second grating structure in the form of a second 1 D grating structure 32 disposed on the surface of the substrate 20 and a coating 34 disposed on the second 1 D grating structure 32. Each of the first and second 1 D grating structures 22, 32 defines a corresponding plurality of grooves and ridges, wherein the grooves and ridges of the first 1 D grating structure 22 are oriented perpendicular to the grooves and ridges of the second 1 D grating structure 32.

The first and second 1 D grating structures are formed from the same material. The first and second 1 D grating structures 22, 32 comprise, or are formed from, a material which has a higher refractive index than a material of the substrate 20. For example, the substrate 20 comprises, or is formed from, silica (SiC>2) and the first and second 1 D grating structures 22, 32 comprise, or are formed from, silicon nitride (SislSk). The depths of the grooves/heights of the ridges of the first and second 1 D grating structures 22, 32 are the same.

The first 1 D grating structure 22 has a first periodicity and a first fill factor in the first direction and the second 1 D grating structure 32 has a second periodicity and a second fill factor in the second direction which is the orthogonal to the first direction, wherein the first and second periodicities are the same, and wherein the first and second fill factors are the same. From the foregoing description, one of ordinary skill in the art will understand that the first and second 1 D grating structures 22, 32 are configured so that the first and second grating vectors have the same magnitude but are oriented orthogonally and are, therefore, oriented differently. Moreover, one of ordinary skill in the art will understand that the first and second 1 D grating structures 22, 32 are configured so that the first and second resonant spectral responses at least partially overlap before the first and second sensor regions 4a, 4b are exposed to the target substance 3.

The first and second 1 D grating structures 22, 32 are configured so that each of the first and second sensor regions 4a, 4b supports only one guided mode resonance for input light having a given spectral range when the input light is incident on the optical sensor element 4 at a given angle of incidence and the first and second sensor regions 4a, 4b are exposed to a given fluid. Moreover, one of skill in the art will understand that the optical sensor element 4 is configured for operation with input light which has a predetermined wavelength or range of wavelengths and which is directed towards the optical sensor element 4 at a predetermined angle of incidence or over a predetermined range of angles of incidence. In particular, the material(s), the depths of the grooves/heights of the ridges, the periodicity and fill factor of each of the first and second 1 D grating structures 22, 32 are selected for operation with input light which has a predetermined wavelength or range of wavelengths and which is directed towards the optical sensor element 4 at a predetermined angle of incidence or over a predetermined range of angles of incidence.

As illustrated in FIG. 1A, the coatings 24, 34 of the first and second sensor regions 4a, 4b are configured so that the target substance 3 binds, bonds and/or adheres more effectively, more strongly, with a greater probability, and/or to a greater degree to the coating 24 of the first sensor region 4a than to the coating 34 of the second sensor region 4b. For example, the coating 24 of the first sensor region 4a comprises an antibody specific to the target substance 3 and the coating 34 of the second sensor region 4b comprises an isotype or non-binding antibody.

The optical source 6 is configured to generate unpolarised light having a coherence length in the range of a few tens of .m to several hundred .m or a full-width half maximum (FWHM) spectral width in the range of 0.01 nm to 10 nm, 0.5 nm to 5 nm, or 1 - 3 nm. For the example, the optical source 6 may include an LED such as a resonant cavity LED (RCLED) or a super luminescent LED (SLED). The polarisation control arrangement 8 comprises a polarisation shearing and analysing arrangement which includes a polarisation shearing element 40 and a polarisation analyser 42.

The polarisation shearing element 40 is flat or generally flat, or planar or generally planar. The polarisation shearing element 40 is configured to direct two orthogonal polarisations along two different optical paths. The polarisation shearing element 40 has form-birefringence. The polarisation shearing element 40 comprises a polarisation shearing metasurface shown in FIG. 1 D. The metasurface takes the form of a planar structure that imparts an arbitrary polarisation or phase-distribution onto an optical beam. The specific function of the metasurface is determined by the nature of the individual elements, called “meta-atoms”, which are typically arranged on a lattice of a period smaller than the wavelength of the light with which the metasurface is designed to interact. As shown in FIG. 1 D, the meta-atoms are designed as ellipses that act differently on the light incident on the metasurface.

The polarisation analyser 42 is flat or generally flat, or planar or generally planar. The polarisation analyser 42 is configured to linearly polarise light along a polariser axis.

The polarisation shearing element 40 and the polarisation analyser 42 are arranged generally parallel to one another and to the image sensor 10.

In use, the first and second sensor regions 4a, 4b are exposed to the fluid containing the target substance 3 and the target substance 3 binds preferentially to the antibody of the coating 24 of the first sensor region 4a. The optical source 6 illuminates the first and second sensor regions 4a, 4b with unpolarised input light 50 at a non-zero acute angle of incidence relative to a normal to the surface of the substrate 20 so that a first polarisation component of the unpolarised input light 50 excites the first guided mode resonance in the first sensor region 4a and a second polarisation component of the unpolarised input light 50 excites the second guided mode resonance in the second sensor region 4b, wherein the first and second polarisation components of the unpolarised input light 50 are orthogonal. Specifically, the unpolarised input light 50 is incident on a first side of the substrate 20 and is transmitted through the substrate 20 to the 1 D grating structures 22, 32 where the first polarisation component of the unpolarised input light 50 excites the first guided mode resonance in the first sensor region 4a and a second polarisation component of the unpolarised input light 50 excites the second guided mode resonance in the second sensor region 4b.

The first and second 1 D grating structures 22, 32 are configured so that the unpolarised input light 50 excites only one guided mode resonance in each of the first and second sensor regions 4a, 4b, wherein a dominant electric field component of the first guided mode resonance in the first sensor region 4a and a dominant electric field component of the second guided mode resonance in the second sensor region 4b are both perpendicular to the surface of the substrate 20.

Excitation of the first guided mode resonance in the first sensor region 4a results in light being output from the first sensor region 4a through the substrate 20 and out of the first side of the substrate 20 as first output light 52a. Similarly, excitation of the second guided mode resonance in the second sensor region 4b results in light being output from the second sensor region 4b through the substrate 20 and out of the first side of the substrate 20 as second output light 52b.

The polarisation control arrangement s interferes a polarisation component of the first output light 52a and a polarisation component of the second output light 52b so as to generate an interferogram 54. Specifically, the optical sensor element 4 and the polarisation shearing element 40 are oriented relative to one another so that the polarisation shearing element 40 directs orthogonally polarised components of the first and second output light 52a, 52b along two different optical paths so that two of the orthogonally polarised components of the first and second output light 52a, 52b at least partially overlap. Moreover, the optical sensor element 4 and the polarisation analyser 42 are oriented relative to one another so that the polariser axis of the polarisation analyser 42 is oriented at an angle of 45 degrees, or approximately 45 degrees, relative to a polarisation component of the first output light 52a and at an angle of 45 degrees, or approximately 45 degrees, relative to a polarisation component of the second output light 52b so as to generate the interferogram 54.

The image sensor 10 detects an image of at least part of the interferogram 54.

The processing resource 12 then determines, from the detected image, a value of a quantity representative of a concentration of the target substance 3 to which the first and second sensor regions 4a, 4b are exposed. Specifically, as mentioned above, the first and second sensor regions 4a, 4b are configured so that the first and second resonant spectral responses associated with the first and second GM Rs which are supported by the first and second sensor regions 4a, 4b at least partially overlap before the first and second sensor regions 4a, 4b are exposed to the target substance 3. One of ordinary skill in the art will understand that the first and second GMRs which are supported by the first and second sensor regions 4a, 4b and which are associated with the differently oriented grating vectors in the first and second sensor regions 4a, 4b before the first and second sensor regions 4a, 4b are exposed to the target substance 3 are degenerate or near-degenerate. FIGS. 2A and 2B show the first and second resonant spectral responses respectively after exposure of the first and second sensor regions 4a, 4b to the target substance 3. On exposure of the first and second sensor regions 4a, 4b to the target substance 3, the greater degree of binding of the target substance 3 to the coating 24 of the first sensor region 4a compared with the degree of binding of the target substance 3 to the coating 34 of the second sensor region 4b causes the first and second resonant spectral responses to shift progressively apart in wavelength. More specifically, the first polarisation component of the unpolarised input light 50 is reflected from the first sensor region 4a with a reflectance and a phase shift which depend on the first guided mode resonance and the first resonant spectral response comprises a first resonant reflectance spectrum and an associated first resonant phase shift spectrum as shown in FIG. 2A. Similarly, the second polarisation component of the unpolarised input light 50 is reflected from the second sensor region 4b with a reflectance and a phase shift which depend on the second guided mode resonance and the second resonant spectral response comprises a second resonant reflectance spectrum and an associated second resonant phase shift spectrum as shown in FIG. 2B. As shown in FIG. 2B, unpolarised input light 50 having a centre or peak wavelength Ao experiences a phase shift of <|)o on reflection from the second sensor region 4b as a result of the excitation of the second guided mode resonance in the second sensor region 4b. As shown in FIG. 2A, unpolarised input light 50 having a centre or peak wavelength Ao experiences a phase shift of <t>o-A<|) on reflection from the first sensor region 4a as a result of the excitation of the first guided mode resonance in the first sensor region 4a. The difference A<|) between the phase shifts in the first and second sensor regions 4a, 4b determines the spatial position of the interferogram 54 on the image sensor 10, which spatial position depends on the concentration of the target substance 3. Moreover, the processing resource 12 is configured to determine, from the spatial position of the interferogram 54 on the image sensor 10, a value of a quantity representative of the concentration of the target substance 3 to which the first and second sensor regions 4a, 4b are exposed.

From the foregoing description, one of skill in the art will understand that the interferometric optical sensor system 2 provides several advantages over conventional optical sensor systems. For example, the first and second guided mode resonances may be excited in the first and second sensor regions 4a, 4b using unpolarised input light 50 from an LED, which is lower cost and less noisy than a diode laser. Moreover, LEDs are intrinsically more stable optical sources than diode lasers, especially low-cost diode lasers, which suffer from wavelength hopping, polarization instability and intensity noise. Use of an LED may also at least partially suppress speckle noise. Use an LED may also avoid any eye-safety issues making regulatory approval easier to achieve for the interferometric optical sensor system 2.

Use of first and second sensor regions 4a, 4b which are configured to support guided mode resonances which at least partially overlap and which are both spectrally narrow may result in the interferometric optical sensor system 2 having a higher sensitivity than known interferometric optical sensor systems which rely upon the interference of guided mode resonances wherein at least one of the guided mode resonances is spectrally broader than the other.

The absence of any apertures between the optical sensor element and the image sensor in the interferometric optical sensor system 2 means that the contrast of the interferogram generated using the interferometric optical sensor system 2 may be greater than the contrast of an interferogram generated using an interferometric optical sensor system which relies upon the use of apertures aligned with different sensor regions of a GMR sensor such that the interferometric optical sensor system 2 may be less susceptible to noise than such known interferometric optical sensor systems. Moreover, the interferometric optical sensor system 2 does not require the alignment of any apertures with different sensor regions of a GMR sensor, thereby simplifying the assembly of the interferometric optical sensor system 2 and eliminating any issues associated with changes in alignment due to vibrations or over time.

The interferometric optical sensor system 2 is inherently self-referencing in the sense that a phase shift results in a change in a spatial position of the interferogram 54 which can be detected and translated into a value of a quantity representative of a concentration of the target substance 3. The interferometric optical sensor system 2 may have a lower measurement noise compared with known multi-channel optical sensor systems which rely on post-processing to subtract a reference channel interferogram from a signal channel interferogram because such subtraction, while being effective at removing background effects, increases noise (because the noise of two signals is additive when the signals are subtracted). The interferometric optical sensor system 2 is therefore a fully self-referencing optical sensor system in the sense that the resulting phase only shifts according to differential binding of the target substance 3 in the first and second sensor regions 4a, 4b.

In contrast to some known interferometric optical sensor systems, the interferometric optical sensor system 2 does not require any bulky and costly optical elements. In particular, the interferometric optical sensor system 2 may be a nonimaging, lens-free system. Moreover, unlike some known interferometric optical sensor systems, the interferometric optical sensor system 2 does not require a Wollaston prism for polarisation shearing. Instead, in the interferometric optical sensor system 2, polarisation shearing is achieved using a relatively low cost dielectric metasurface polarisation shearing element 40. Moreover, the metasurface polarisation shearing element 40 and the polarisation analyser 42 may both be ‘flat’ and arranged parallel to one another to provide a compact configuration. Consequently, it may be easier to simplify and miniaturize the interferometric optical sensor system 2 relative to known interferometric optical sensor systems. As a consequence of the simpler, more compact arrangement of the interferometric optical sensor system 2 and the use of fewer optical elements than known interferometric optical sensor systems, the interferometric optical sensor system 2 may be less susceptible to background light or may reduce scattering relative to known interferometric optical sensor systems resulting in the interferometric optical sensor system 2 having better SNR performance than known interferometric optical sensor systems.

As a consequence of the above-mentioned advantages of the interferometric optical sensor system 2, the interferometric optical sensor system 2 may be simplified, miniaturized and operated in a non-controlled environment. The simplification and miniaturization may be important for translating the interferometric optical sensor system 2 into a low-cost technology for use by non-experts, or to integrate the interferometric optical sensor system 2 into a portable device such as a smartphone or a wearable device.

Referring now to FIGS 3A-3D, there is shown a second interferometric optical sensor system 102 for use in sensing a target substance 103 such as a protein contained in a fluid. Like the interferometric optical sensor system 2 of FIGS. 1A-1 D, the interferometric optical sensor system 102 includes an optical sensor element 104 having first and second sensor regions 104a and 104b respectively for exposure to the target substance 103, an optical source 106 for illuminating the sensor regions 104a and 104b with input light, a polarisation control arrangement 108 for interfering a polarisation component of first output light output from the first sensor region 104a and a polarisation component of second output light output from the second sensor region 104a so as to generate an interferogram, an image sensor 110 for detecting an image of at least part of the interferogram, and a processing resource 112 which is configured to determine, from the detected image, a value of a quantity representative of a concentration of the target substance 103 to which the first and second sensor regions 104a, 104b are exposed.

As will be described in more detail below, the first sensor region 104a includes a first grating structure defining at least a first grating vector so that the first sensor region 104a is configured to support at least a first guided mode resonance, wherein the first grating vector is oriented along a first direction, and the second sensor region 104b includes a second grating structure defining at least a second grating vector so that the second sensor region 104b is configured to support at least a second guided mode resonance, wherein the second grating vector is oriented along a second direction, wherein the first and second directions are different, wherein the first guided mode resonance has an associated first resonant spectral response and the second guided mode resonance has an associated second resonant spectral response, and wherein the first and second resonant spectral responses at least partially overlap.

Like the optical sensor element 4, the optical sensor element 104 includes a substrate 120. The first sensor region 104a also includes a first grating structure. However, unlike the optical sensor element 4, the first grating structure takes the form of a first 2D grating structure 122 disposed on a surface of the substrate 120, and a coating 124 disposed on the first 2D grating structure 122. Similarly, the second sensor region 104b includes a second grating structure in the form of a second 2D grating structure 132 disposed on the surface of the substrate 120 and a coating 134 disposed on the second 2D grating structure 132. Each of the first and second 2D grating structures 122, 132 defines a corresponding plurality of holes in the form of a regular 2D array of holes, wherein the spatial arrangement and orientation of the 2D array of holes of the first 2D grating structure 122 is the same as the spatial arrangement and orientation of the 2D array of holes of the second 2D grating structure 132.

The first and second 2D grating structures 122, 132 are formed from the same material. The first and second 2D grating structures 122, 132 comprise, or are formed from, a material which has a higher refractive index than a material of the substrate 120. For example, the substrate 120 comprises, or is formed from, silica (SiC>2) and the first and second 2D grating structures 122, 132 comprise, or are formed from, silicon nitride (SisN4). The depths of the holes of the first and second 1 D grating structures 122, 132 are the same.

The first 2D grating structure 122 has a first periodicity and a first fill factor in the first direction and the second 2D grating structure 132 has a second periodicity and a second fill factor in the second direction which is the orthogonal to the first direction, wherein the first and second periodicities are the same, and wherein the first and second fill factors are the same. From the foregoing description, one of ordinary skill in the art will understand that the first 2D grating structure 122 is configured so that the first sensor region 104a defines a primary grating vector in the X direction indicated in FIG. 3C and a secondary grating vector in the Y direction indicated in FIG. 3C. Similarly, the second sensor region 104b defines a primary grating vector in the X direction indicated in FIG. 3C and a secondary grating vector in the Y direction indicated in FIG. 3C. Moreover, one of skill in the art will understand that the primary grating vector of the first sensor region 104a in the X direction and the secondary grating vector of the second sensor region 104b in the Y direction have the same magnitude but are oriented orthogonally and are, therefore, oriented differently. Conversely, the secondary grating vector of the first sensor region 104a in the Y direction and the primary grating vector of the second sensor region 104b in the X direction have the same magnitude but are oriented orthogonally and are, therefore, oriented differently. Consequently, one of ordinary skill in the art will understand that the first and second 2D grating structures 122, 132 are configured so that each of the first and second sensor regions 104a, 104b supports a primary guided mode resonance associated with a corresponding primary grating vector and a secondary guided mode resonance associated with a corresponding secondary grating vector for input light having a given spectral range when the input light is incident on the optical sensor element 104 at a given angle of incidence and the first and second sensor regions 104a, 104b are exposed to a given fluid.

Moreover, one of skill in the art will understand that the optical sensor element 104 is configured for operation with input light which has a predetermined wavelength or range of wavelengths and which is directed towards the optical sensor element 104 at a predetermined angle of incidence or over a predetermined range of angles of incidence. In particular, the material(s), the depths of the holes, the periodicity, and fill factor of each of the first and second 2D grating structures 122, 132 are selected for operation with input light which has a predetermined wavelength or range of wavelengths and which is directed towards the optical sensor element 104 at a predetermined angle of incidence or over a predetermined range of angles of incidence.

As illustrated in FIG. 3A, the coatings 124, 134 of the first and second sensor regions 104a, 104b are configured so that the target substance 103 binds, bonds and/or adheres more effectively, more strongly, with a greater probability, and/or to a greater degree to the coating 124 of the first sensor region 104a than to the coating 134 of the second sensor region 104b. For example, the coating 124 of the first sensor region 104a comprises an antibody specific to the target substance 103 and the coating 134 of the second sensor region 104a comprises an isotype or non-binding antibody.

The optical source 106 is configured to generate unpolarised light having a coherence length in the range of a few tens of .m to several hundred .m or a full-width half maximum (FWHM) spectral width in the range of 0.01 nm to 10 nm, 0.5 nm to 5 nm, or 1 - 3 nm. For the example, the optical source 106 may include an LED such as a resonant cavity LED (RCLED) or a super luminescent LED (SLED).

Like the polarisation control arrangement 8 of the interferometric optical sensor system 2 of FIGS. 1A-1 D, the polarisation control arrangement 108 comprises a polarisation shearing and analysing arrangement which includes a polarisation shearing element 140 and a polarisation analyser 142. The polarisation shearing element 140 comprises a flat or generally flat, or planar or generally planar polarisation shearing metasurface shown in FIG. 3D. The polarisation analyser 142 is flat or generally flat, or planar or generally planar. The polarisation analyser 142 is configured to linearly polarise light along a polariser axis.

The polarisation shearing element 140 and the polarisation analyser 142 are arranged generally parallel to one another and to the image sensor 210.

In use, the interferometric optical sensor system 102 is similar to the interferometric optical sensor system 2 described above with reference to FIGS. 1 A-1 D. The first and second sensor regions 104a, 104b are exposed to the target substance 103 and unpolarised input light from the optical source 106 is directed towards the optical sensor element 104 at a non-zero acute angle of incidence. However, unlike the first and second 1 D grating structures 22, 32 of the interferometric optical sensor system 2, the first and second 2D grating structures 122, 132 are each configured so that the unpolarised input light excites a primary guided mode resonance and a secondary guided mode resonance in each of the first and second sensor regions 104a, 104b. Output light resulting from excitation of the primary and secondary GM Rs in each of the first and second sensor regions 104a, 104b is output from the optical sensor element 104. The polarisation shearing element 140 then shears the output light resulting from the excitation of the primary and secondary GMRs in the first sensor region 104a and shears the output light resulting from the excitation of the primary and secondary GMRs in the second sensor region 104b so that the polarisation analyser 142 only interferes output light resulting from excitation of the primary GMR in the first sensor region 104a with output light resulting from excitation of the secondary GMR in the second sensor region 104b or so that the polarisation analyser 142 only interferes output light resulting from excitation of the secondary GMR in the first sensor region 104a with output light resulting from excitation of the primary GMR in the second sensor region 104b so as to form an interferogram on the image sensor 110. Put another way, the polarisation analyser 142 only interferes output light resulting from excitation of a first GMR in the first sensor region 104a with output light resulting from excitation of a second GMR in the second sensor region 104b, wherein the first and second GM Rs are associated with grating vectors which have different orientations, wherein the first guided mode resonance has an associated first resonant spectral response and the second guided mode resonance has an associated second resonant spectral response, and wherein the first and second resonant spectral responses at least partially overlap. As such, one of ordinary skill in the art will understand that the use of such 2D grating structures 122, 132 enables the polarisation control arrangement 108 to interfere output light which is output from the first and second sensor regions 104a, 104b and which results from the excitation of degenerate or near-degenerate first and second GM Rs in the first and second sensor regions 104a, 104b.

From the foregoing description, one of skill in the art will understand that the interferometric optical sensor system 102 described with reference to FIGS. 3A-3D has many of the same advantages as the interferometric optical sensor system 2 described above with reference to FIGS. 1A-1 D.

Referring now to FIGS 4A-4D, there is shown a third interferometric optical sensor system 202 for use in sensing a target substance 203 such as a protein contained in a fluid. Like the interferometric optical sensor system 2 of FIGS. 1A-1 D, the interferometric optical sensor system 202 includes an optical sensor element 204 having first and second sensor regions 204a and 204b respectively for exposure to the target substance 203, an optical source 206 for illuminating the sensor regions 204a and 204b with input light, a polarisation control arrangement 208 for interfering a polarisation component of first output light output from the first sensor region 204a and a polarisation component of second output light output from the second sensor region 204a so as to generate an interferogram, an image sensor 210 for detecting an image of at least part of the interferogram, and a processing resource 212 which is configured to determine, from the detected image, a value of a quantity representative of a concentration of the target substance 203 to which the first and second sensor regions 204a, 204b are exposed. However, unlike the interferometric optical sensor system 2 of FIGS. 1A-1 D, the interferometric optical sensor system 202 includes a non-polarising beam splitter 214 located on an input optical path which extends from the unpolarised optical source 206 to the optical sensor element 204.

As will be described in more detail below, the first sensor region 204a includes a first grating structure defining at least a first grating vector so that the first sensor region 204a is configured to support at least a first guided mode resonance, wherein the first grating vector is oriented along a first direction, and the second sensor region 204b includes a second grating structure defining at least a second grating vector so that the second sensor region 204b is configured to support at least a second guided mode resonance, wherein the second grating vector is oriented along a second direction, wherein the first and second directions are different, wherein the first guided mode resonance has an associated first resonant spectral response and the second guided mode resonance has an associated second resonant spectral response, and wherein the first and second resonant spectral responses at least partially overlap.

Like the optical sensor element 4, the optical sensor element 204 includes a substrate 220. The first sensor region 204a also includes a first grating structure. However, unlike the optical sensor element 4, the first grating structure takes the form of a first 2D grating structure 222 disposed on a surface of the substrate 220, and a coating 224 disposed on the first 2D grating structure 222. Similarly, the second sensor region 204b includes a second grating structure in the form of a second 2D grating structure 232 disposed on the surface of the substrate 220 and a coating 234 disposed on the second 2D grating structure 232. Each of the first and second 2D grating structures 222, 232 defines a corresponding plurality of holes in the form of a regular 2D array of holes, wherein the spatial arrangement and orientation of the 2D array of holes of the first 2D grating structure 222 is the same as the spatial arrangement and orientation of the 2D array of holes of the second 2D grating structure 232.

The first and second 2D grating structures 222, 232 are formed from the same material. The first and second 2D grating structures 222, 232 comprise, or are formed from, a material which has a higher refractive index than a material of the substrate 220. For example, the substrate 220 comprises, or is formed from, silica (SiC>2) and the first and second 2D grating structures 222, 232 comprise, or are formed from, silicon nitride (SisN4). The depths of the holes of the first and second 1 D grating structures 222, 232 are the same.

The first 2D grating structure 222 has a first periodicity and a first fill factor in a first direction and the second 2D grating structure 232 has a second periodicity and a second fill factor in a second direction which is the orthogonal to the first direction, wherein the first and second periodicities are the same, and wherein the first and second fill factors are the same. From the foregoing description, one of ordinary skill in the art will understand that the first 2D grating structure 222 is configured so that the first sensor region 204a defines a primary grating vector in the X direction indicated in FIG. 4C and a secondary grating vector in the Y direction indicated in FIG. 4C. Similarly, the second sensor region 204b defines a primary grating vector in the X direction indicated in FIG. 4C and a secondary grating vector in the Y direction indicated in FIG. 4C. Moreover, one of skill in the art will understand that the primary grating vector of the first sensor region 204a in the X direction and the secondary grating vector of the second sensor region 204b in the Y direction have the same magnitude but are oriented orthogonally and are, therefore, oriented differently. Conversely, the secondary grating vector of the first sensor region 204a in the Y direction and the primary grating vector of the second sensor region 204b in the X direction have the same magnitude but are oriented orthogonally and are, therefore, oriented differently. Consequently, one of ordinary skill in the art will understand that the first and second 2D grating structures 222, 232 are configured so that each of the first and second sensor regions 204a, 204b supports a primary guided mode resonance associated with a corresponding primary grating vector and a secondary guided mode resonance associated with a corresponding secondary grating vector for input light having a given spectral range, when the input light is incident on the optical sensor element 204 at normal incidence and the first and second sensor regions 204a, 204b are exposed to a given fluid.

Moreover, one of skill in the art will understand that the optical sensor element 204 is configured for operation with input light which has a predetermined wavelength or range of wavelengths and which is directed towards the optical sensor element 204 at normal incidence or over a predetermined range of angles around normal incidence. In particular, the material(s), the depths of the holes, the periodicity, and fill factor of each of the first and second 2D grating structures 222, 232 are selected for operation with input light which has a predetermined wavelength or range of wavelengths and which is directed towards the optical sensor element 204 at normal incidence or over a predetermined range of angles around normal incidence.

As illustrated in FIG. 4A, the coatings 224, 234 of the first and second regions 204a, 204b are configured so that the target substance 203 binds, bonds and/or adheres more effectively, more strongly, with a greater probability, and/or to a greater degree to the coating 224 of the first sensor region 204a than to the coating 234 of the second sensor region 204b. For example, the coating 224 of the first sensor region 204a comprises an antibody specific to the target substance 203 and the coating 134 of the second sensor region 204b comprises an isotype or non-binding antibody.

The optical source 206 is configured to generate unpolarised light having a coherence length in the range of a few tens of .m to several hundred .m or a full-width half maximum (FWHM) spectral width in the range of 0.01 nm to 10 nm, 0.5 nm to 5 nm, or 1 - 3 nm. For the example, the optical source 206 may include an LED such as a resonant cavity LED (RCLED) or a super luminescent LED (SLED).

Like the polarisation control arrangement 8 of the interferometric optical sensor system 2 of FIGS. 1A-1 D, the polarisation control arrangement 208 comprises a polarisation shearing and analysing arrangement which includes a polarisation shearing element 240 and a polarisation analyser 242. The polarisation shearing element 240 comprises a flat or generally flat, or planar or generally planar polarisation shearing metasurface shown in FIG. 4D. The polarisation analyser 242 is flat or generally flat, or planar or generally planar. The polarisation analyser 242 is configured to linearly polarise light along a polariser axis.

The polarisation shearing element 240 and the polarisation analyser 242 are arranged generally parallel to one another and to the image sensor 210.

In use, the interferometric optical sensor system 202 is similar to the interferometric optical sensor system 2. The first and second sensor regions 204a, 204b are exposed to the target substance 203. However, unlike the interferometric optical sensor system 2, the unpolarised light from the optical source 206 is split by the nonpolarising beam splitter 214 and a portion of the unpolarised light is directed towards the optical sensor element 204 as unpolarised input light at normal incidence. Moreover, unlike the first and second 1 D grating structures 22, 32 of the interferometric optical sensor system 2, the first and second 2D grating structures 222, 232 are each configured so that the unpolarised input light excites a primary guided mode resonance and a secondary guided mode resonance in each of the first and second sensor regions 204a, 204b. Output light resulting from excitation of the primary and secondary GMRs in each of the first and second sensor regions 204a, 204b is output from the optical sensor element 204. The non-polarising beam splitter 214 splits the output light and a portion of the output light is directed towards the polarisation control arrangement 208. The polarisation shearing element 240 then shears the output light resulting from the excitation of the primary and secondary GMRs in the first sensor region 204a and shears the output light resulting from the excitation of the primary and secondary GMRs in the second sensor region 204b so that the polarisation analyser 242 only interferes output light resulting from excitation of the primary GMR in the first sensor region 204a with output light resulting from excitation of the secondary GMR in the second sensor region 204a or so that the polarisation analyser 242 only interferes output light resulting from excitation of the secondary GMR in the first sensor region 204a with output light resulting from excitation of the primary GMR in the second sensor region 204b so as to form an interferogram on the image sensor 210. Put another way, the polarisation analyser 242 only interferes output light resulting from excitation of a first GMR in the first sensor region 204a with output light resulting from excitation of a second GMR in the second sensor region 204b, wherein the first and second GMRs are associated with grating vectors which have different orientations, wherein the first guided mode resonance has an associated first resonant spectral response and the second guided mode resonance has an associated second resonant spectral response, and wherein the first and second resonant spectral responses at least partially overlap. As such, one of ordinary skill in the art will understand that the use of such 2D grating structures 222, 232 enables the polarisation control arrangement 208 to interfere output light which is output from the first and second sensor regions 204a, 204b and which results from the excitation of degenerate or near-degenerate first and second GMRs in the first and second sensor regions 204a, 204b.

Use of a non-polarising beam splitter 214 may simplify the alignment and/or manufacturing of the interferometric optical sensor system 202, but may increase the size and/or the cost of the interferometric optical sensor system 202 relative to the interferometric optical sensor system 2 of FIGS. 1A-1 D. Use of a non-polarising beam splitter 214 may also increase the optical losses or scattering thereby reducing the SNR of the interferometric optical sensor system 202 compared with the interferometric optical sensor system 2 of FIGS. 1 A-1 D.

From the foregoing description, one of skill in the art will understand that the interferometric optical sensor system 202 described with reference to FIGS. 4A-4D has many of the same advantages as the interferometric optical sensor system 2 described above with reference to FIGS. 1A-1 D.

In a variant of the interferometric optical sensor system 202 of FIGS. 4A-4D, an optical sensor element comprising first and second sensor regions comprising orthogonally oriented 1 D grating structures (like the optical sensor element 4 described above with reference to FIGS. 1A-1C) may be used in place of the optical sensor element 204. It should be understood that such an optical sensor element would need to be configured for operation with input light which has a predetermined wavelength or range of wavelengths and which is directed towards the optical sensor element at normal incidence or over a predetermined range of angles around normal incidence. In particular, the material(s), depths of the grooves/heights of the ridges, the periodicity and the fill factor of each of the orthogonally oriented first and second 1 D grating structures would need to be selected for operation with input light which has a predetermined wavelength or range of wavelengths and which is directed towards the optical sensor element at normal incidence or over a predetermined range of angles around normal incidence.

Referring to FIG. 5A there is shown an image of an optical sensor element 304 which includes a 1 D array of sensor regions, each sensor region labelled “A” or “B” to denote first and second sensor regions having different functionalisation. For example, the “A” and “B” sensor regions may be configured so that the target substance binds selectively to the “A” sensor regions in preference to the “B” sensor regions. Alternatively, only the “A” sensor regions are functionalised so that the target substance binds only to the “A” sensor regions. The first and second sensor regions may comprise orthogonally oriented 1 D grating structures which are formed from the same material(s), which have grooves of the same depth/ridges of the same height, and which have the same periodicity and fill factor. Alternatively, the first and second sensor regions may comprise 2D square arrays of holes which are formed from the same material(s), which have the same depths, and which have the same spatial arrangement and orientation. In use, a fluid containing a corresponding concentration of target substance is applied, dispensed or spotted onto each of the “A” sensor regions and unpolarised input light 350 is used to illuminate one pair of sensor regions 304a, 304b at a time. A value of a quantity representative of a concentration of the target substance to which each “A” sensor region is exposed is determined from the corresponding interferogram which is generated using one of the interferometric optical sensor systems and methods described above.

Referring to FIG. 5B there is shown an image of an optical sensor element 404, which includes a 2D array of sensor regions, each sensor region having a label including the letter “A” or “B” to denote first and second sensor regions having different functionalisation. For example, the “A” and “B” sensor regions may be configured so that the target substance binds selectively to the “A” sensor regions in preference to the “B” sensor regions. Alternatively, only the “A” sensor regions are functionalised so that the target substance binds only to the “A” sensor regions. The first and second sensor regions may comprise orthogonally oriented 1 D grating structures which are formed from the same material(s), which have grooves of the same depth/ridges of the same height, and which have the same periodicity and fill factor. Alternatively, the first and second sensor regions may comprise 2D square arrays of holes which are formed from the same material(s), which have the same depths, and which have the same spatial arrangement and orientation. In use, a fluid containing a target substance is applied, dispensed or spotted onto each of the “A” sensor regions and unpolarised input light 450 is used to illuminate a plurality of pairs of sensor regions simultaneously. A value of a quantity representative of a concentration of the target substance for each “A” sensor region is determined from the corresponding interferogram which is generated using one of the interferometric optical sensor systems and methods described above.

For example, FIG. 6A shows an optical sensor element 504 comprising a 2D array of sensor regions including a plurality of pairs of “A” and “B” sensor regions. Each sensor region comprises a 2D square array of holes of the same depth which has the same spatial arrangement and orientation, which is formed from the same material(s), and which has the same periodicity and fill factor as the 2D square array of holes of each of the other sensor regions. As shown in FIG. 6B, fluids containing different concentrations of immunoglobulin G are applied, dispensed or spotted onto each “A” sensor region. Unpolarised input light is used to illuminate a plurality of pairs of sensor regions simultaneously and the plurality of corresponding interferograms shown in FIG. 6C are generated using the interferometric optical sensor system 202 of FIG. 4A. The white bar in FIG. 6C indicates the interferogram corresponding to the pair of sensor regions located under the white bar in FIG. 6A. FIG. 6D shows the evolution of the phase shift as a function of time following application of the immunoglobulin G onto the “A” sensor region of the pair of sensor regions located under the white bar in FIG. 6A as determined from a change in spatial position of the corresponding interferogram.

Referring back to FIGS. 2A and 2B, one of skill in the art will understand that when the reflectance and phase shift spectra include resonant features which are sharp or narrow and when the first sensor region 4a is exposed to a concentration of the target substance 3 which exceeds a predetermined threshold concentration, the shift in wavelength of the resonant features corresponding to the first guided mode resonance in the first sensor region 4a may be so large that very little light is actually output from the first sensor region 4a at the operating wavelength of Ao for interference with light output from the second sensor region 4b at the operating wavelength of Ao with the result that the concentration measurement range may be limited.

Accordingly, in a variant of the interferometric optical sensor system 2, the first and second 1 D grating structures 22, 32 may be configured so that the unpolarised input light 50 excites only one guided mode resonance in each of the first and second sensor regions 4a, 4b, wherein a dominant electric field component of the first guided mode resonance in the first sensor region 4a and a dominant electric field component of the second guided mode resonance in the second sensor region 4b are both parallel to the surface of the substrate 20. Such first and second guided mode resonances may have a broader resonant spectral response than first and second guided mode resonances which have a dominant electric field component perpendicular to the surface of the substrate 20. Consequently, although use of such first and second guided mode resonances may result in reduced sensitivity when compared with the use of first and second guided mode resonances which have a dominant electric field component perpendicular to the surface of the substrate 20, use of such first and second guided mode resonances may provide a greater concentration measurement range when compared with the use of first and second guided mode resonances which have a dominant electric field component perpendicular to the surface of the substrate 20.

In the optical sensor elements described above, first and second sensor regions are configured (e.g. by forming the first and second sensor regions from appropriate materials, selecting appropriate depths/heights, periodicities and/or appropriate fill factors for the features of the corresponding grating structures) so that the first and second resonant spectral responses are at least partially overlapping before the first and second sensor regions are exposed to the target substance and so that the first and second resonant spectral responses move apart in wavelength as the first and second sensor regions are exposed to the target substance. In a variant of any of the optical sensor elements described above, the first and second sensor regions are deliberately detuned (e.g. by forming the first and second sensor regions from appropriate materials, selecting appropriate depths/heights, periodicities and/or appropriate fill factors for the features of the corresponding grating structures) so that the first and second resonant spectral responses only partially overlap before the first and second sensor regions are exposed to the target substance and so that, as the first and second sensor regions are exposed to the target substance, the first and second resonant spectral responses initially move together in wavelength so as to increase the overlap between the first and second resonant spectral responses until a maximum overlap between the first and second resonant spectral responses is reached whereupon the first and second resonant spectral responses move apart in wavelength so as to decrease the overlap between the first and second resonant spectral responses as the first and second sensor regions are further exposed to the target substance. Deliberately detuning the first and second sensor regions in this way may provide a greater concentration measurement range.

In a variant of any of the optical sensor elements described above, the first and second sensor regions may comprise chirped first and second grating structures having a chirped periodicity and/or a chirped fill factor to extend the concentration measurement range. In use, input light is used to illuminate the chirped first and second grating structures. Light is output from one or more regions of the chirped first grating structures for interference with light output from one or more regions of the chirped second grating structures, wherein the one or more regions of the first and second grating structures from which the light is output depends on the concentration of the target substance applied to the first and second grating structures.

As an alternative to first and second sensor regions comprising chirped first and second grating structures, the first and second sensor regions may comprise first and second grating structures having a plurality of different periodicities and/or a plurality of different fill factors to extend the concentration measurement range. In use, input light is used to illuminate the first and second grating structures. Light is output from one or more regions of the first grating structures for interference with light output from one or more regions of the second grating structures, wherein the one or more regions of the first and second grating structures from which the light is output depends on the concentration of the target substance applied to the first and second grating structures.

As an alternative to the first and second sensor regions comprising first and second grating structures having a plurality of different periodicities and/or a plurality of different fill factors, the optical sensor element may comprise an array of sensor regions including a plurality of pairs of grating structures, each pair of grating structures comprising a first grating structure and a second grating structure, wherein the first and second grating structures of each pair of grating structures comprise, or are formed from, the same material(s) and have the same periodicity and the same fill factor, and wherein different pairs of grating structures comprise, or are formed from, the same material(s), but have different periodicities and/or different fill factors. In use, the first grating structure of each pair of grating structures is exposed to the same concentration of target substance and input light is used to illuminate the plurality of pairs of grating structures simultaneously. Light is output from one or more of the first grating structures for interference with light output from one or more corresponding second grating structures, wherein the one or more of the first grating structures from which the light is output depends on the concentration of the target substance applied to the first grating structures. For example, FIG. 7A shows an optical sensor element 604 comprising a 2D array of sensor regions including a plurality of pairs of 1 D grating structures, each pair of 1 D grating structures comprising a first 1 D grating structure and a second 1 D grating structure, wherein the first and second 1 D grating structures are oriented perpendicular to one another. The first and second 1 D grating structures of each pair of 1 D grating structures comprise, or are formed from, the same material(s), have the same depths of grooves/heights of the ridges, and have the same periodicity and the same fill factor, and wherein different pairs of 1 D grating structures comprise, or are formed from, the same material(s), have the same depths of grooves/heights of the ridges, but have different periodicities and/or different fill factors. In use, the first 1 D grating structure of each pair of 1 D grating structures is exposed to the same concentration of target substance and unpolarised input light 650 is used to illuminate the plurality of pairs of 1 D grating structures simultaneously. Light is output from one or more of the first 1 D grating structures for interference with light output from one or more corresponding second 1 D grating structures, wherein the one or more of the first 1 D grating structures from which the light is output depends on the concentration of the target substance applied to the first 1 D grating structures. Similarly, FIG. 7B shows an optical sensor element 704 comprising a 2D array of sensor regions including a plurality of pairs of 2D grating structures, each pair of 2D grating structures comprising a first 2D square array of holes and a second 2D square array of holes, wherein the first and second 2D square arrays of holes have the same spatial arrangement and orientation. The square arrays of holes of different pairs of 2D grating structures comprise, or are formed from, the same material(s), have the same hole depths, but have different periodicities and/or different fill factors in 2D. In use, the first 2D square arrays of holes of each pair of 2D grating structures is exposed to the same concentration of target substance and unpolarised input light 750 is used to illuminate the plurality of pairs of 2D grating structures simultaneously. Light is output from one or more of the first 2D square arrays of holes for interference with light output from one or more corresponding second 2D square arrays of holes, wherein the one or more of the first 2D square arrays of holes from which the light is output depends on the concentration of the target substance applied to the first 2D square arrays of holes.

FIGS. 8A and 8B show images of part of an alternative interferometric optical sensor system which includes the interferometric optical sensor system 202 described above with reference to FIGS. 4A-4D together with an additional collimating lens 250 located on an optical path between the unpolarised optical source 206 and the optical sensor element 204 and a 3D printed housing 260 for mounting the optical components of the interferometric optical sensor system in a fixed spatial relationship relative to one another.

One of ordinary skill in the art will also understand that various modifications are possible to any of the optical sensor elements, the interferometric optical sensor systems, and the interferometric optical sensing methods described above. For example, it is possible to use an optical sensor element having first and second sensor regions which have different grating structures to those described above. For example, FIG. 9A is a plan view of a first alternative optical sensor element for use in the interferometric optical sensor system of FIG. 1A, 3A or 4A in place of the optical sensor element of FIGS. 1 B and 1C, the optical sensor element of FIGS. 3B and 3C, or the optical sensor element of FIGS. 4B and 4C. A 1 D grating structure in a first sensor region of the optical sensor element defines a first grating vector v1 so that the first sensor region is configured to support a first GMR. A 1 D grating structure in a second sensor region of the optical sensor element defines a second grating vector v2 so that the second sensor region is configured to support a second GMR. As may be appreciated from FIG. 9A, the first and second grating vectors v1 , v2 have the same magnitude but different directions. In use, the first GMR is excited in the first sensor region and the second GMR is excited in the second sensor region and light output from the first and second sensor regions is interfered on the image sensor, wherein the light is output from the first and second sensor regions as a result of the excitation of the first and second GMRs in the first and second sensor regions.

FIG. 9B is a plan view of a second alternative optical sensor element for use in the interferometric optical sensor system of FIG. 1A, 3A or 4A in place of the optical sensor element of FIGS. 1 B and 1 C, the optical sensor element of FIGS. 3B and 3C, or the optical sensor element of FIGS. 4B and 4C. A 1 D grating structure in a first sensor region of the optical sensor element defines a first grating vector v1 so that the first sensor region is configured to support a first GMR. A 1 D grating structure in a second sensor region of the optical sensor element defines a second grating vector v2 so that the second sensor region is configured to support a second GMR. As may be appreciated from FIG. 9A, the first and second grating vectors v1 , v2 have the same magnitude but different directions. In use, the first GMR is excited in the first sensor region and the second GMR is excited in the second sensor region and light output from the first and second sensor regions is interfered on the image sensor, wherein the light is output from the first and second sensor regions as a result of the excitation of the first and second GMRs in the first and second sensor regions.

FIGS. 10A-10C show alternative optical sensor elements for use in the interferometric optical sensor system of FIG. 1A, 3A or 4A in place of the optical sensor element of FIGS. 1 B and 1C, the optical sensor element of FIGS. 3B and 3C, or the optical sensor element of FIGS. 4B and 4C. In each of FIGS. 10A-10C, a 2D grating structure in a first sensor region of the optical sensor element comprises a rectangular array of holes which defines primary and secondary grating vectors v11 , v12 so that the first sensor region is configured to support primary and secondary GMRs, wherein the primary and secondary grating vectors v11 , v12 have different magnitudes and different directions. Similarly, a 2D grating structure in a second sensor region of the optical sensor element comprises a rectangular array of holes which defines primary and secondary grating vectors v21 , v22 so that the second sensor region is configured to support primary and secondary GMRs, wherein the primary and secondary grating vectors v21 , v22 have different magnitudes and different directions. Moreover, the primary grating vector v11 in the first sensor region has the same magnitude but a different direction to the secondary grating vector v22 in the second sensor region. Conversely, the secondary grating vector v12 in the first sensor region has the same magnitude but a different direction to the primary grating vector v21 in the second sensor region. In use, primary and secondary GMRs are excited in each of the first and second sensor regions and light output from the first and second sensor regions is interfered on the image sensor, wherein the light which is interfered on the image sensor is the light output as a result of excitation of either the primary GMR in the first sensor region and the secondary GMR in the second sensor region or the light output as a result of excitation of the secondary GMR in the first sensor region and the primary GMR in the second sensor region.

FIGS. 11A-11C show alternative optical sensor elements for use in the interferometric optical sensor system of FIG. 1A, 3A or 4A in place of the optical sensor element of FIGS. 1 B and 1C, the optical sensor element of FIGS. 3B and 3C, or the optical sensor element of FIGS. 4B and 4C. In each of FIGS. 11A-11C, a 2D grating structure in a first sensor region of the optical sensor element comprises a hexagonal array of holes which defines primary and secondary grating vectors v11 , v12 so that the first sensor region is configured to support primary and secondary GMRs, wherein the primary and secondary grating vectors v11 , v12 have different magnitudes and different directions. Similarly, a 2D grating structure in a second sensor region of the optical sensor element comprises a hexagonal array of holes which defines primary and secondary grating vectors v21 , v22 so that the second sensor region is configured to support primary and secondary GMRs, wherein the primary and secondary grating vectors v21 , v22 have different magnitudes and different directions. The primary grating vector v11 in the first sensor region has the same magnitude and direction as the primary grating vector v21 in the second sensor region, and the same magnitude as, but a different direction to, the secondary grating vector v22 in the second sensor region. Similarly, the secondary grating vector v12 in the first sensor region has the same magnitude and direction as the secondary grating vector v22 in the second sensor region, and the same magnitude as, but a different direction to, the primary grating vector v21 in the second sensor region. In use, primary and secondary GMRs are excited in each of the first and second sensor regions and light output from the first and second sensor regions is interfered on the image sensor, wherein the light which is interfered on the image sensor is the light output as a result of excitation of either the primary GMR in the first sensor region and the secondary GMR in the second sensor region or the light output as a result of excitation of the secondary GMR in the first sensor region and the primary GMR in the second sensor region.

FIG. 12A shows the same alternative optical sensor element as FIG. 11A but illustrates tertiary and quaternary grating vectors v23, v24 defined by the 2D grating structure of the second sensor region for supporting tertiary and quaternary guided mode resonances in the second sensor region and which have different magnitudes and different directions to the primary and secondary grating vectors v11, v12 of the first sensor region. FIG. 12B shows the same alternative optical sensor element as FIG. 11 B but illustrates tertiary and quaternary grating vectors v23, v24 defined by the 2D grating structure of the second sensor region for supporting tertiary and quaternary guided mode resonances in the second sensor region and which have different magnitudes and different directions to the primary and secondary grating vectors v11, v12 of the first sensor region. FIG. 12C shows the same alternative optical sensor element as FIG. 11C but illustrates tertiary and quaternary grating vectors v23, v24 defined by the 2D grating structure of the second sensor region for supporting tertiary and quaternary guided mode resonances in the second sensor region and which have different magnitudes and different directions to the primary and secondary grating vectors v11, v12 of the first sensor region. In use, the primary and secondary GMRs are excited in the first sensor region and the tertiary and quaternary GMRs may be excited in the second sensor region and light output from the first and second sensor regions is interfered on the image sensor, wherein the light which is interfered on the image sensor is the light output as a result of excitation of the primary GMR in the first sensor region and excitation of either the tertiary or quaternary GMR in the second sensor region or wherein the light which is interfered on the image sensor is the light output as a result of excitation of the secondary GMR in the first sensor region and excitation of either the tertiary or quaternary GMR in the second sensor region.

One of skill in the art will understand that the 2D grating structure of the first sensor region of FIGS. 11A-11C also defines tertiary and quaternary grating vectors (not shown) for supporting tertiary and quaternary guided mode resonances in the first sensor region and which have different magnitudes and different directions to the primary and secondary grating vectors v21 , v22 of the second sensor region. Moreover, in use, the tertiary and quaternary GM Rs may be excited in the first sensor region and the primary and secondary GM Rs may be excited in the second sensor region and light output from the first and second sensor regions is interfered on the image sensor, wherein the light which is interfered on the image sensor is the light output as a result of excitation of the tertiary GMR in the first sensor region and excitation of either the primary or secondary GMR in the second sensor region or wherein the light which is interfered on the image sensor is the light output as a result of excitation of the quaternary GMR in the first sensor region and excitation of either the primary or secondary GMR in the second sensor region.

It should be understood that in each of the alternative optical sensor element examples of FIG. 9A-12C, the first grating structure of the first sensor region is configured so as to define at least a first grating vector so that the first sensor region is configured to support at least a first guided mode resonance, wherein the first grating vector is oriented along a first direction, and the second grating structure of the second sensor region is configured so as to define at least a second grating vector so that the second sensor region is configured to support at least a second guided mode resonance, wherein the second grating vector is oriented along a second direction, wherein the first and second directions are different, wherein the first guided mode resonance has an associated first resonant spectral response and the second guided mode resonance has an associated second resonant spectral response, and wherein the first and second resonant spectral responses at least partially overlap. For example, the first grating structure of the first sensor region may be configured so as to define at least the first grating vector by forming the first sensor region from appropriate materials, and/or by selecting appropriate depths/heights of grating structure features, selecting appropriate periodicities and/or selecting appropriate fill factors for the first grating structure. Similarly, the second grating structure of the second sensor region may be configured so as to define at least the second grating vector by forming the second sensor region from appropriate materials, and/or by selecting appropriate depths/heights of grating structure features, selecting appropriate periodicities and/or selecting appropriate fill factors for the second grating structure. As such, one of ordinary skill in the art will understand that the use of grating structures like those of FIGS. 9A-12C enables a polarisation control arrangement of the interferometric optical sensor system to interfere output light which is output from the first and second sensor regions and which results from the excitation of degenerate or near-degenerate first and second GMRs in the first and second sensor regions.

The interferometric optical sensor system may comprise a spectral filter, such as a bandpass spectral filter, for spectrally filtering light emitted by the unpolarised optical source. The bandpass spectral filter may have a full-width half maximum (FWHM) spectral width in the range 0.01 nm to 10 nm, 0.5 nm to 5 nm, or 1 nm to 3 nm.

The interferometric optical sensor system may comprise an aperture for spatially filtering light emitted by the unpolarised optical source.

Rather than comprising an unpolarised optical source such as an LED, the interferometric optical sensor system may comprise a polarised optical source which is configured to generate polarised input light. For example, the interferometric optical sensor system may comprise a coherent polarised optical source. For example, the interferometric optical sensor system may comprise a laser such as a laser diode. The polarised optical source may be oriented relative to the optical sensor element so as to illuminate the first and second sensor regions with the polarised input light so that a first polarisation component of the polarised input light excites the first guided mode resonance in the first sensor region and a second polarisation component of the polarised input light excites the second guided mode resonance in the second sensor region.

The polarisation shearing metasurface may be configured to focus or image the first and second output light onto the image sensor.

The polarisation analyser may comprise a polarisation analyser metasurface.

The polarisation analyser metasurface may be configured to focus or image the first and second output light onto the image sensor. The polarisation shearing element may comprise a birefringent element which includes a birefringent material. For example, the polarisation shearing element may comprise a Wollaston prism, a Nomarski prism, or a Savart plate.

The polarisation shearing and analysing arrangement may comprise a single multi-function metasurface which is configured to direct two orthogonal polarisations along two different paths whilst also linearly polarising light along a polariser axis of the multi-function metasurface.

The optical sensor element and the multi-function metasurface may be oriented relative to one another so that the multi-function metasurface directs the first and second output light along respective output optical paths so that the first and second output light at least partially overlaps, and so that the polariser axis of the multi-function metasurface is oriented at a non-zero angle relative to a polarisation component of the first output light and the polariser axis of the multi-function metasurface is oriented at a non-zero angle relative to a polarisation component of the second output light.

The optical sensor element and the multi-function metasurface may be oriented relative to one another so that the polariser axis of the multi-function metasurface is oriented at an angle of 45 degrees, or approximately 45 degrees, relative to a polarisation component of the first output light and at an angle of 45 degrees, or approximately 45 degrees, relative to a polarisation component of the second output light.

The multi-function metasurface may be configured to focus or image the first and second output light onto the image sensor.

As described above, the polarisation control arrangement comprises a polarisation shearing and analysing arrangement.

Alternatively, the polarisation control arrangement may comprise an interferometer arrangement comprising at least one polarising beam splitter and a halfwave plate, wherein the at least one polarising beam splitter is configured to direct orthogonal polarisation components of the first and second output light along first and second optical paths and to recombine light which has travelled along the first and second optical paths, and wherein the half-wave plate is located in one of the first and second optical paths. For example, the polarisation control arrangement may comprise a Mach-Zehnder interferometer arrangement which includes first and second polarising beam splitters, and a half-wave plate, wherein the first polarising beam splitter is configured to direct orthogonal polarisation components of the first and second output light along first and second optical paths, wherein the second polarising beam splitter is configured to recombine light which has travelled along the first and second optical paths, and wherein the half-wave plate is located in one of the first and second optical paths. Alternatively, the polarisation control arrangement may comprise a Michelson interferometer arrangement which includes a polarising beam splitter and a half-wave plate, wherein the polarising beam splitter is configured to direct orthogonal polarisation components of the first and second output light along first and second optical paths, wherein the polarising beam splitter is configured to recombine light which has travelled along the first and second optical paths, and wherein the half-wave plate is located in one of the first and second optical paths.

As described above, the first and second grating structures of the optical sensor element may comprise first and second 2D grating structures, wherein the first and second 2D grating structures have the same spatial arrangement and orientation. Each of the first and second 2D grating structures may comprise a plurality of holes or a plurality of pillars. Each of the first and second 2D grating structures may comprise a 2D array of holes or pillars such as a regular 2D array of holes or pillars.

Alternatively, the first and second grating structures of the optical sensor element may comprise different regions of the same 2D grating structure. The 2D grating structure may comprise a plurality of holes or pillars. The 2D grating structure may comprise a 2D array of holes or pillars such as a regular 2D array of holes or pillars.

As described above, the first sensor region comprises a first coating disposed on the first grating structure and the second sensor region comprises a second coating disposed on the second grating structure, wherein the materials of the first and second coatings are configured so that the target substance binds, bonds and/or adheres more effectively, more strongly, with a greater probability, and/or to a greater degree to the first coating than to the second coating. Alternatively, the first sensor region may comprise a first coating disposed on the first grating structure the second sensor region may comprise no coating disposed on the second grating structure.

As described above, the target substance may be contained in a fluid. Alternatively, the target substance may comprise a fluid.

Each of the optical sensor elements described above are configured for sensing in reflection - i.e. in an epi-detection geometry - wherein illumination of the optical sensor element with input light and collection of the output light resulting from the excitation of GMRs in the optical sensing element take place on the same side of the optical sensor element. As described above, a first polarisation component of the input light is reflected from the first sensor region with a reflectance and a phase shift which depend on the first guided mode resonance, and a second polarisation component of the input light is reflected from the second sensor region with a reflectance and a phase shift which depend on the second guided mode resonance.

Alternatively, each of the optical sensor elements may be configured for sensing in transmission. For example, a first polarisation component of the input light may be transmitted through the first sensor region with a transmittance and a phase shift which depend on the first guided mode resonance, and a second polarisation component of the input light may be transmitted through the second sensor region with a transmittance and a phase shift which depend on the second guided mode resonance.

Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives to the described embodiments in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in any embodiment, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein. In particular, one of ordinary skill in the art will understand that one or more of the features of the embodiments of the present disclosure described above with reference to the drawings may produce effects or provide advantages when used in isolation from one or more of the other features of the embodiments of the present disclosure and that different combinations of the features are possible other than the specific combinations of the features of the embodiments of the present disclosure described above.

The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘above’, ‘along’, ‘side’, etc. are made with reference to conceptual illustrations, such as those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to an object when in an orientation as shown in the accompanying drawings.

Use of the term "comprising" when used in relation to a feature of an embodiment of the present disclosure does not exclude other features or steps. Use of the term "a" or "an" when used in relation to a feature of an embodiment of the present disclosure does not exclude the possibility that the embodiment may include a plurality of such features.

The use of reference signs in the claims should not be construed as limiting the scope of the claims.