Optical sensor element

FIELD OF THE INVENTION

The invention relates to an optical sensor element. The invention further relates to an optical sensor matrix. The invention further relates to an imaging method. The invention further relates to an imaging system. The invention further relates to uses of the optical sensor element.

BACKGROUND OF THE INVENTION

Optical sensor elements are known in the art. For instance, Kumar et al., “Enhanced Sensitivity of Silicon-Photonics-Based Ultrasound Detection via BCB Coating”, IEEE Photonics Journal, 2019, describes that ultrasound detection via silicon waveguides relies on the ability of acoustic waves to modulate the effective refractive index of the guided modes, and that the sensitivity of silicon waveguides to ultrasound may be significantly enhanced by replacing a silica over-cladding with bisbenzocyclobutene (BCB).

W02020039436A1 describes an apparatus including an acoustic sensor having an optical waveguide. The optical waveguide includes a waveguide core having a waveguide core refractive index and a waveguide core photo-elastic coefficient, and an over-cladding layer coupled to the waveguide core and including an optically transparent polymer having an overcladding refractive index and an over-cladding photo-elastic coefficient. The waveguide core refractive index is greater than the over-cladding refractive index, and the over-cladding photoelastic coefficient is greater than the waveguide core photo-elastic coefficient.

Mistry et al., “Isotropic Liquid Crystal Elastomers as Exceptional Photoelastic Strain Sensors”, Macromolecules, May 2020, describes a family of acrylate-based isotropic Liquid Crystal Elastomers (LCEs) that exhibit stress- and strain-optic coefficients orders of magnitude greater than conventional polymeric and photoelastic materials.

Tabib-Azar et al., “Fiber optic electric field sensors using polymer-dispersed liquid crystal coatings and evanescent field interactions”, Sensors and actuators A: Physical, August 2000, describes an evanescent field fiber optic electric field sensor constructed by coating the exposed fiber optic core with a polymer-dispersed liquid crystal (PDLC).

SUMMARY OF THE INVENTION Photoacoustic and ultrasound imaging methods may be used in medical diagnostics and also in other industries like industrial non-destructive-testing and material testing. State-of-the-art high-end ultrasound detector systems may typically use a 1D/2D matrix of piezo-electric elements as ultrasound sensors. However, piezo-electric elements may have a relatively low sensitivity, and may typically require a coaxial wire per sensor element, or bulky digitization electronics directly on a sensor head. In particular, the low sensitivity may result in a low ultrasonic imaging depth and/or resolution.

Optical ultrasound sensors may be substantially more sensitive than piezoelectric ultra-sound sensors. However, it may be challenging to scale prior art sensors to a matrix of optical ultrasound sensors.

The prior art may further describe optomechanical ultrasound sensor in silicon photonics based on a membrane with an elongated waveguide. However, this system may not be scalable to higher ultrasound frequencies (tens of MHz) for high-resolution ultrasound imaging.

A concept is to use on-chip silicon photonic waveguides as ultrasound sensors. Incident ultrasound waves may deform the waveguide, which deformation may cause a change in the propagation speed of light through the waveguide, which can be measured in real-time. Moreover, on-chip integrated photonic technology, such a silicon photonic technology, may allow to integrate complex optical functionality, such as resonators and multiplexers, and electro-optics, such as photodetectors or even CMOS electronics. This may facilitate fabricating matrices of sensors. However, the ultrasound-induced deformation of silicon waveguides - optionally cladded with a polymer - may be small, which may result in a low measurement range and/or sensitivity.

The prior art may further describe optical sensor matrices based on the firee- space Fabry -Perot Method. However, such systems may be bulky, may be restricted to photoacoustic imaging, and may be unsuitable for capturing an ultrasound time-trace.

Further, prior art systems may be cost-prohibitive, and/or may have a low bandwidth.

Hence, prior art systems may be insufficiently sensitive, may be inconvenient/bulky, may have scalability issues (with regards to a matrix and/or multiplexing), may have a small measurement range, may have a low detection limit (noise equivalent pressure), may be incompatible with well-established and reliable fabrication methods, may be incompatible with passive and/or active photonic and electronic components, may be incompatible with magnetic resonance imaging, may be unsuitable to interface with optical fibers, may be (substantially) affected by electromagnetic interference, may be unsuitable for capturing ultrasound time-traces, and may be prohibitively expensive.

In particular, for photo-acoustic imaging, the sensitivity of an ultrasound sensor may be particularly important as pressures in photo-acoustic imaging may generally be substantially lower than for conventional ultrasound methods.

Hence, it is an aspect of the invention to provide an alternative optical sensor element, which preferably further at least partly obviates one or more of above-described drawbacks. The present invention may have as object to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

Hence, in a first aspect, the invention may provide an optical sensor element for imaging, especially for ultrasound imaging. The optical sensor element may comprise an optical waveguide which may comprise of a waveguide core and a waveguide cladding. In embodiments, the optical sensor element may comprise a (transparent or translucent) elasto- optic material, which may especially be arranged on the optical waveguide core. In further embodiments, the elasto-optic material may comprise liquid crystals. Especially, in embodiments, the elasto-optic material may have a stress-optic coefficient > 500 Brewsters.

In particular, exposing the optical sensor element to an ultrasound pressure may result in a deformation of the optical sensor element, especially of the elasto-optic material. The elasto-optic material may provide a relatively large change in its refractive index upon such a deformation, which may result in a large change in the (effective) propagation speed of optical radiation through the optical waveguide core. Hence, by arranging the elasto-optic material on the waveguide, a small ultrasound pressure may give a large change in optical propagation speed through the waveguide, which may be accurately measured. The sensor may thus be sensitive to small differences in ultrasound pressures.

The term “ultrasound radiation” may herein especially refer to acoustic radiation, such as a sound wave, with a frequency of at least (about) 20 kHz, such as acoustic radiation with a frequency in the range of 20 kHz - 200 MHz.

The term “ultrasound pressure” may herein especially refer to a local pressure deviation from an ambient atmospheric pressure caused by ultrasound radiation.

Further, the optical sensor element may be relatively simple/cheap to produce, and may be interrogated through a small piece of a waveguide, especially of an optical fiber.

In specific embodiments, the invention may provide an optical sensor element for imaging, wherein the optical sensor element comprises an optical waveguide core, and a elasto-optic material arranged on the optical waveguide core, wherein the elasto-optic material comprises liquid crystals, and wherein the elasto-optic material has a stress-optic coefficient > 500 Brewsters.

Hence, the invention may provide an optical sensor element. The term “optical sensor element” may herein especially refer to an element suitable for an optical sensor. In particular, the optical sensor element may comprise an optical waveguide, especially an optical waveguide core, and especially one or more coating (or “cladding”) materials arranged on the optical waveguide core. The optical sensor element may especially be configured to photoacoustic or ultrasound imaging. In particular, the optical sensor element may be configured to modulate optical radiation passing through the optical sensor element in dependence on an ultrasound pressure the optical sensor element is exposed to.

In embodiments, the optical sensor element may comprise an optical waveguide. The term “optical waveguide” may herein especially refer to a physical structure configured to guide optical radiation. In embodiments, the optical waveguide may be configured to guide radiation with a wavelength in the range of (about) 100 - 2000 nm, especially 380-2000 nm, such as 1100 - 1700 nm, especially 1250 - 1650 nm. In further embodiments, the optical waveguide may be configured to guide radiation in the range of 1260 - 1360 nm. In further embodiments, the optical waveguide may be configured to guide radiation in the range of 1530 - 1565 nm. In further embodiments, the optical waveguide may be configured to guide radiation in the range of 1565 - 1625 nm.

The optical sensor element may, in embodiments, further comprise an elasto- optic material. The term “elasto-optic material” may herein refer to a material exhibiting a photoelastic effect (also referred to as “elasto-optic effect”), i.e., a material that provides a (substantial) change in its refractive index in response to a stress, here especially in response to an ultrasound pressure.

In embodiments, the elasto-optic material may comprise liquid crystals. The term “liquid crystal” may refer to a material with state of matter between a liquid and a solid (a “mesophase”). In particular, a liquid crystal may change shape similar to a fluid, but may have a molecular arrangement akin to a solid, i.e., the molecules forming the liquid crystal may be collectively oriented and may show some level of positional long range order analogous to structures observed in crystalline materials.

In particular, whereas liquid may be (essentially) unordered, and solids, such as crystals, may be ordered, liquid crystals may have an orientational order parameter between those of a liquid, which may have an orientational order parameter of 0, and a well-ordered crystal, which may have an orientational order parameter of 1. The orientational order parameter may especially be defined according to wherein the angular brackets imply an ensemble average, and herein 9 is the angle of the molecules with respect to an average direction of the (long) molecular axes of all molecules in the liquid crystal. Liquid crystals may typically have an orientational order parameter selected from the range of 0.3 - 0.8.

Hence, in embodiments, the liquid crystals may have an orientational order parameter selected from the range of 0.05 - 0.95, especially from the range of 0.25 - 0.85, such as from the range of 0.25 - 0.8. The liquid crystals may especially have an orientational order parameter corresponding to a liquid crystal elastomer.

In further embodiments, the liquid crystals may be in an isotropic state. Hence, the liquid crystals may, in embodiments, have an orientational order parameter selected from the range of < 0.1, such as < 0.05, including 0. In particular, deformation, such as due to strain, of isotropic liquid crystals may result in a symmetry break, which may contribute to the photoelastic effect.

Specifically, the alignment (or “ordering”) of the molecules in the liquid crystal may relate to the macroscopic properties of the liquid crystal. In particular, in relation to the present invention, the liquid crystals may have a high elasto-optic effect, i.e., a small deformation of the (elasto-optic material comprising the) liquid crystal may result in a relatively large change of the refractive index of the material. Also, liquid crystals may have a low elastic modulus, i.e., a high compliance.

In embodiments, the liquid crystals may have an strain-optic coefficient of at least 0.0001, such as at least 0.01. In embodiments, the liquid crystals may have a compliance of at least 0.1 inverse GPa, such as at least 1 inverse GPa. In particular, the liquid crystals may, in embodiments, have a compliance selected from the range of IE-10 (i.e., 10' ¹⁰ ) Pa' ¹ - 1 Pa' ¹ , especially from the range of 3E-10 Pa' ¹ - 1 Pa' ¹ , such as from the range of IE-9 Pa' ¹ - IE-2 Pa' especially from the range of IE-7 Pa' ¹ - IE-4 Pa' ¹ . In further embodiments, the liquid crystals may have a compliance of at least 2E-10 Pa' ¹ , such as at least 3E-10 Pa' ¹ , especially at least 1E- 9 Pa' ¹ , such as at least IE-8 Pa' ¹ , especially at least IE-7 Pa' ¹ . In further embodiments, the liquid crystals may have a compliance of at most 1 Pa' ¹ , especially at most IE-2 Pa' ¹ , such as at most IE-4 Pa' ¹ , especially at most IE-5 Pa' ¹ .

In embodiments, the liquid crystals may be (essentially) uniformly distributed in the elasto-optic layer. In particular, in further embodiments, the liquid crystals may be (essentially) uniformly distributed in the elasto-optic layer around the optical waveguide core, such as in the central region of the elasto-optic layer.

In further embodiments, the elasto-optic material may have a stress-optic coefficient > 100 Brewsters, such as > 500 Brewsters, especially > 800 Brewsters, such as > 1000 Brewsters. In further embodiments, the elasto-optic material may have a stress-optic coefficient > 1200 Brewsters, such as > 1500 Brewsters, especially > 2000 Brewsters.

The term “stress-optic coefficient” may herein especially refer to the birefringence that is introduced per applied stress. Birefringence may refer to the difference between refractive indices that describe the propagation of light through an anisotropic material for two different polarizations. The Brewster is a non-SI unit used to measure the susceptibility of a material to photoelasticity, or the value of the stress-optic coefficient of the material. The unit has dimensions reciprocal to those of stress. One Brewster is defined to be equal to ten to the power of minus 12 (IO ^-12 ) square meters per newton. Hence, one Brewster may be equal to 10 ^-12 * m * s ² /kg . The stress-optic coefficient may (approximately) be the strain-optic coefficient multiplied by the compliance (or the strain-optic coefficient divided by the Young’s modulus). Specifically, the elasto-optic (photoelastic) effect may be described by an elasto- optic tensor. Typically, not the full elasto-optic tensor may be characterized, but rather only the difference between two polarizations: the birefringence. The term “strain-optic coefficient” may herein especially refer to the birefringence that is introduced per strain. Strain may refer to a definition as used in the study of mechanics, thus strain may represents the displacement between particles in the body relative to a reference length. The “strain-optic coefficient” may relate to this birefringence and may be a single number that can be computed from the difference between two elements of the full elasto-optic tensor. The strain per applied stress may be described by the Young’s modulus or the compliance of the material. The “stress-optic coefficient” may describe the change in birefringence per applied stress. Hence, the stress-optic coefficient of an (elasto-optic) material may relate to and be indicative of the elasto-optic effect of the material.

The stress-optic coefficient may especially be determined using the method described in Mistry, D. et al. (2018) ‘New insights into the nature of semi-soft elasticity and “mechanical-Freedericksz transitions” in liquid crystal elastomers’, Soft Matter, 14(8), pp. 1301-1310 (incl. supplementary material), which is hereby herein incorporated by reference. In embodiments, the elasto-optic material may be arranged on the optical waveguide core. In particular, the elasto-optic material may be provided on the optical waveguide core as a coating (or cladding). In particular, the elasto-optic material may be arranged on the optical waveguide core such that an effective refractive index of the optical waveguide core depends in part on the refractive index of the elasto-optic material.

In particular, in embodiments, the optical sensor element may comprise an optical waveguide, especially wherein the optical waveguide comprises the optical waveguide core and the elasto-optic material arranged on the optical waveguide core.

The phrase “an elasto-optic material arranged on the optical waveguide core” and similar phrases may herein refer to the elasto-optic material being arranged directly on the optical waveguide core or being arranged on the optical waveguide core with an intermediate layer being arranged between the optical waveguide core and the elasto-optic material. Hence, in embodiments, the elasto-optic material may be arranged directly on the optical waveguide core. In further embodiments, the elasto-optic material may be arranged on an intermediate layer, wherein the intermediate layer is arranged on the optical waveguide core. In particular, the location of the elasto-optic material may be configured such that the optical wave traveling through the optical waveguide is significantly affected by the refractive index of the elasto- optic material.

In further embodiments, the intermediate layer may comprise silicon-dioxide.

In further embodiments, the intermediate layer may have a thickness selected from the range of < 300 nm, such as < 150 nm, especially < 50 nm. Hence, the elasto-optic material and the optical waveguide core may, in embodiments, be arranged at a distance < 300 nm, such as < 150 nm, especially < 50 nm. In further embodiments, the intermediate layer may have a thickness selected from the range of < 20 nm, such as < 10 nm, especially < 5 nm. Hence, the elasto-optic material and the optical waveguide core may, in embodiments, be arranged at a distance < 20 nm, such as < 10 nm, especially < 5 nm.

In particular, a wave of optical radiation passing through the optical waveguide may pass both through the optical waveguide core and through a cladding (of elasto-optic material); hence, both the optical waveguide core and the (optical waveguide) cladding may influence the effective refractive index of the optical waveguide, and thus the propagation speed of optical radiation through the optical waveguide.

Hence, in embodiments, the optical waveguide may comprise an optical waveguide core and a cladding, wherein the cladding comprises the elasto-optic material. In further embodiments, the optical waveguide core may comprise a material selected from the group comprising silicon, silicon-nitride, germanium, indium-gallium-arsenide, galliumarsenide, indium-phosphide, lithium-niobate, diamond, silicon-carbide, silicon-dioxide, and glass. Hence, imposing an ultrasound pressure on the optical sensor element may provide a deformation in the elasto-optic material (due to compressibility), which may result in a change in the refractive index of the elasto-optic material (due to the elasto-optic effect), which may result in a change of the effective refractive index of the optical waveguide, and may thus result in a change in the propagation speed of optical radiation through the optical waveguide, which may be measured. Thereby, the properties, especially the phase, or especially the wavelength distribution, of the optical radiation that has passed through the optical waveguide may be indicative of characteristics of ultrasound radiation, especially of ultrasound pressure.

In embodiments, the elasto-optic material may be (essentially) translucent.

As optical radiation passes through the optical waveguide core (and through the elasto-optic material), attenuation (or “transmission loss”) may occur, which may have a detrimental effect for determining a characteristic of ultrasound radiation using the optical sensor element. Hence, in embodiments, the elasto-optic material may be configured to provide (acceptably) low, especially negligible, scattering of optical radiation passing through the waveguide, i.e., the elasto-optic material may be (essentially) transparent. In particular, in embodiments, the elasto-optic material may have an attenuation < 20 dB/cm, especially < 5 dB/cm.

In embodiments, the optical sensor element may comprise a waveguide layer comprising the optical waveguide core, and an elasto-optic layer arranged on the waveguide layer, wherein the elasto-optic layer comprises the elasto-optic material. In further embodiments, the optical sensor element may comprise two (or more) elasto-optic layers, especially wherein each of the elasto-optic layers is arranged on the waveguide layer. For instance, the optical sensor element may comprise a bottom layer, a middle layer, and a top layer with respect to an element axis, wherein the bottom layer and the top layer comprise elasto-optic layers, and wherein the middle layer comprises the waveguide layer.

In further embodiments, the optical sensor element may have an element axis A. The element axis A may especially be (essentially) perpendicular to the waveguide layer, or especially (essentially) perpendicular to the elasto-optic layer.

In specific embodiments, the elasto-optic layer may be sufficiently thin to reduce the attenuation of acoustic (especially ultrasonic) waves traveling through this layer. The thickness of the elasto-optic layer may be smaller than 1 mm, especially smaller than 100 pm, in particular smaller than 10 pm. In further embodiments, the elasto-optic layer may comprises first voids having a first length LI perpendicular to the element axis A. Thereby, a compressibility of the elasto- optic layer may be improved, i.e., the deformation for a given ultrasound pressure may be increased, which may facilitate providing an sensitivity and/or an increased sensing range.

In particular, LI may be selected to be smaller than an acoustical wavelength the optical sensor element is exposed to during use, such as at most half of the acoustical wavelength, especially at most a quarter of the acoustical wavelength, such as at most an eight of the acoustical wavelength.

In further embodiments, the first length LI may be selected from the range of 0.1-300 pm, especially from the range of 1 - 100 pm. In further embodiments, LI may be at least 1 pm, such as at least 5 pm, especially at least 10 pm. In further embodiments, LI may be at least 20 pm, such as at least 30 pm. In further embodiments, LI may be at most 300 pm, such as at most 200 pm, especially at most 100 pm. In further embodiments, LI may be at most 80 pm, such as at most 60 pm, especially at most 50 pm.

As indicated above, the elasto-optic material may have a high elasto-optic coefficient (large change in optical proper-ties for given deformation). The addition of voids may further increase compressibility of the elasto-optic material, i.e., the deformation for a given (ultrasound) pressure. In particular, by processing the material at a length scale below the ultrasound wavelength, this material may act as a compressible meta-material with a good change in deformation per applied pressure for ultrasound wave-lengths down to, for example, 30 pm. Moreover, the elasto-optic material may be locally homogeneous around the optical waveguide core, reducing, especially preventing, optical scattering loss due to the voids.

Hence, in further embodiments, the elasto-optic layer may comprise a central region and a peripheral region, especially wherein the peripheral region comprises the first voids. In particular, the central region may be devoid of first voids. The central region may be arranged on the optical waveguide core. Especially, the central region may cover at least part of the optical waveguide core. The peripheral region, however, may be arranged at a first distance from the optical waveguide core, especially wherein the first distance dl is perpendicular to the element axis A. In particular, it may be preferable for the first distance dl to be as small as possible (while non-negative), such that the compressibility of the material can be most improved, while at the same time being sufficiently large to avoid high optical scattering loss. Hence, In further embodiments, the first distance dl may be at most 15 pm, such as at most 10 pm, especially at most 5 pm. In further embodiments, the first distance may be at most 3 pm, such as at most 1 pm, especially at most 500 nm. In further embodiments, the first distance dl may especially be at least 100 nm, such as at least 200 nm, especially at least 500 nm. In further embodiments, the first distance may be at least 1 pm, such as at least 2 pm. In particular, thereby the elasto-optic material may comprise a compressible acoustical metamaterial while maintaining homogeneous material locally around the optical waveguide core. The first may improve the sensitivity of the waveguide and the latter may reduce, especially prevent, optical scattering loss due to first voids.

In further embodiments the distance dl may especially refer to the distance between the optical waveguide core and the closest first void, measuring in a direction perpendicular to the element axis.

In particular, the distance dl may be selected such that the electromagnetic wave associated with the optical waveguide core is not significantly affected by the voids, while at the same time the size and distribution of the voids is such that they facilitate deformation of the elasto-optic layer upon incident (ultrasound) pressure.

In further embodiments, the peripheral region may comprise at most 99 vol.% of first voids, such as at most 95 vol.%, especially at most 90 vol.%. In further embodiments, the peripheral region may comprise at most 80 vol.% of first voids, such as at most 50 vol.%, especially at most 20 vol.%, such as at most 15 vol.%. In further embodiments, the peripheral region may comprise at least 0.1 vol.% of first voids, such as at least 0.3 vol.% of first voids, especially at least 0.6 vol.% of first voids. In further embodiments, the peripheral region may comprise at least 1 vol.% of first voids, such as at least 3 vol.% of first voids, especially at least 5 vol.% of first voids. In further embodiments, the peripheral region may comprise at least 10 vol.% of first voids, such as at least 15 vol.%.

In embodiments, the peripheral region may comprise a plurality of (spatially separated) first voids. In such embodiments, the first voids may be separated by a first void distance smaller than an acoustical wavelength, such as, in embodiments, a first void distance selected from the range of < 300 pm, such as from the range of < 30 pm, especially from the range of < 10 pm. In further embodiments, the first void distance may be selected from the range of > 0.5 pm, especially > 1 pm, such as > 2 pm. In further embodiments, the first void distance may be selected from the range of > 3 pm, especially > 10 pm.

In further embodiments, the first void distance may be selected based on an (ultrasound) wavelength the optical sensor element is to be exposed to during use. In particular, the first void distance may be selected from the range of i/10 - i/6, wherein i is an (acoustical) wavelength the optical sensor element is (to be) exposed to during use. In further embodiments, the elasto-optic material may comprise second (microscale) voids. These second voids may (further) increase the compressibility of the elasto- optic material. In particular, the second voids should have a sufficient size to (substantially) improve the compressibility of the elasto-optic material, but should be small enough that optical scattering loss is kept (acceptably) low, especially completely avoided. Hence, in embodiments, the elasto-optic material may comprise second voids, especially wherein the second voids have spherical equivalent diameters (independently) selected from the range of 0.005 - 2 pm, such as from the range of 0.01 - 1 pm. In further embodiments, the second voids may have spherical equivalent diameters (independently) selected from the range of < 1500 nm, such as < 750 nm, especially < 500 nm. In further embodiments, the second voids may have spherical equivalent diameters (independently) selected from the range of < 200 nm, such as < 100 nm, especially < 50 nm. In further embodiments, the second voids may have spherical equivalent diameters (independently) selected from the range of > 50 nm, such as > 100 nm, especially > 150 nm, such as > 200 nm. In further embodiments, the second voids may have spherical equivalent diameters (independently) selected from the range of 100 - 1000 nm, such as from the range of 200 - 750 nm.

In further embodiments, the peripheral region may comprise second voids. In further embodiments, the central region may comprise second voids. In yet further embodiments, both the peripheral region and the central region may comprise second voids.

In further embodiments, the elasto-optic material may comprise at most 10 vol.% second voids, such as at most 5 vol.% second voids, especially at most 3 vol.% second voids. In further embodiments, the elasto-optic material may comprise at least 0.1 vol.% second voids, such as at least 0.5 vol.% second voids, especially at least 1 vol.% second voids.

The second voids may, in the peripheral region, be (essentially) uniformly distributed. In particular, in embodiments, second voids may be separated from one another by a second void distance of at most LI. In further embodiments, the second voids may be separated from one another by a second void distance of at most 1.5 pm, such as at most 1 pm, especially at most 0.5 pm.

In embodiments, the elasto-optic material comprises a polymer dispersed liquid crystal, i.e., a polymer matrix with a liquid crystal dispersed therein. In particular, the polymer may, in embodiments, have a compliance selected from the range of 2E-10 Pa' ¹ - 1 Pa' ¹ , especially from the range of 3E-10 Pa' ¹ - 1 Pa' ¹ , such as from the range of IE-9 Pa' ¹ - IE-2 Pa' especially from the range of IE-7 Pa' ¹ - IE-4 Pa' ¹ . In further embodiments, the polymer may be selected from the group comprising (crosslinked) polyacrylate, silicone rubbers, and (crosslinked) polyether. In further embodiments, the polymer matrix may be an UV-curable optically transparent adhesive.

In particular, the liquid crystals may comprise an (essentially) fully miscible plasticizing agent or a non-volatile solvent, especially selected based on the (selected) polymers.

In further embodiments, the elasto-optic material may comprise a liquid-phase liquid crystal which may be held in place by other materials that prevent the liquid crystal from dissolving in the medium to which the optical sensor element is connected.

In further embodiments, the elasto-optic material may comprise a side-chain liquid crystal polymer. The side-chain liquid crystal polymer may have a polymer backbone and liquid crystal side chains. In further embodiments, the elasto-optic material may comprise a polymer dispersed liquid crystal.

In embodiments, the elasto-optic material comprises an elastomer, especially a liquid crystalline elastomer. In further embodiments, the elastomer may comprise one or more of siloxane and methacrylate. The elasto-optic material may especially be LCE1, LCE2, or LCE3, especially LCE1, or especially LCE2, or especially LCE3 as described in Mistry et al., “Isotropic Liquid Crystal Elastomers as Exceptional Photoelastic Strain Sensors”, Macromolecules, 2020, which is hereby herein incorporated by reference. In particular, in embodiments, the elasto-optic material may comprise a monofunctional mesogenic acrylate selected from the group comprising 6-(4-Cyano-biphenyl-4'-yloxy)hexyl acrylate (a6OCB), 4- (6-acryloyloxy)hexyloxy)phenyl 4-(trans-4-propylcyclohexyl)benzoate (Ml), and 4- methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl (M2). In further embodiments, the elasto-optic material may comprise a Non-mesogenic monofunctional acrylate, especially 2- Ethylhexyl acrylate (EHA). In further embodiments, the elasto-optic material may comprise a bifunctional acrylate cross-linking agent selected from the group comprising l,4-Bis-[4-(6- acryloyl oxy hexyl oxy) benzoyloxy]-2 -methylbenzene (RM82) and 1,6-hexanediol diacrylate (HDDA). In further embodiments, the elasto-optic material comprise a nonreactive nematic compound such as 4-Cyano-4'- hexyloxybiphenyl (6OCB). In further embodiments, the elasto- optic material may comprise a photoinitiator, such as UV-reactive photoinitiator methyl benzoylformate (MBF). In particular, the composition of LCE1, LCE2, and LCE3 may be: component (mol%)

A6OCB 6OCB HDDA Ml M2 RM82 EHA MBF

LCE1 14.6 55.9 7.1 20.9 1.5

LCE2 27.4 35.3 10.2 25.6 1.5

LCE3 9.6 29.0 10.1 50.3 1.0 The birefringence (An = n _e - n ₀ ) is the difference in the material refractive index as seen by light with polarizations along the extraordinary optical axis of the material (n _e ) and the ordinary optical axis of the material (n ₀ ), as for example defined in chapter 6, section 6.3 of B.A.E. Saleh & M.C. Teich, Fundamentals of Photonics, 2 ^nd edition, John Wiley & Sons : Hoboken New Jersey USA, 2007, which is hereby herein incorporated by reference.

In further embodiments, the elasto-optic material may comprise an organogel, such as a supramolecular organogel, i.e., a self-assembled molecular system cross-linked via self-assembling optionally including a (non-volatile) organic solvent, such as an organic solvent selected from the group comprising glycerin, (long-chain) alkanes. In further embodiments, the organogel may (further) comprise one or more of 4-tertbutyl-l-aryl cyclohexanol derivatives, polymeric organogel ators (e.g. poly(ethylene glycol), polycarbonate, polyesters, and poly(alkylene)), gemini gelators (e.g. N-lauroyl-L-lysine ethyl ester), Boc- Ala(l)-Aib(2)-B-Ala(3)-OMe (synthetic tripeptide), low molecular weight gelators (e.g. fatty acids and n-alkanes).

In embodiments, the liquid crystals may comprise a mesogenic group, especially wherein the mesogenic group comprises a molecular structure selected from the group comprising (partially substituted) phenyl, pyridine, cyclohexane, naphthalene, and dicyclooctane. In further embodiments, the mesogenic group may comprise two or more molecular structures selected from the group comprising (partially substituted) phenyl, pyridine, cyclohexane, naphthalene, and dicyclooctane, especially wherein the two or more molecular structure are linked directly or via one or more of an ester, an amide, -C=C-, -C=N, -N=N-, or acetylene. As will be clear to the person skilled in the art, in embodiments, the mesogenic group may comprise a substitution group (replacing a -H group) selected from the group comprising a methyl group and a halogen. In further embodiments, the molecular structure, especially one or more of the two or more molecular structures, may comprise a heterocyclic ring.

In further embodiments, the mesogenic group may have an aspect ratio > 2, especially > 3, such as > 5. In further embodiments, the mesogenic group may have an aspect ratio < 20, especially < 15, such as < 10.

In further embodiments, the liquid crystals may be selected from the group comprising mesogenic liquid crystals. In further embodiments, the liquid crystals may comprise calamitic mesogens or discotic mesogens, especially calamitic mesogens, or especially discotic mesogens. In further embodiments, the liquid crystals may comprise an aromatic compound, especially an aromatic compound selected from the group comprising biphenyls, terphenyls, phenyl-cyclohexane, dicyclooctanes, stilbenes, azo-compounds, aromatic esters, such as phenylbenzoate esters, or especially an aromatic compound selected from the group comprising benzene or naphthalene. In further embodiments, the liquid crystals may comprise one or more of a cyanobiphenyl, such as 6OCB, a stilbene, and an azo-compound.

In further embodiments, the liquid crystals may comprise nematic liquid crystals, such as E7.

In further embodiments, the liquid crystals may comprise room temperature (nematic) liquid crystals, i.e., the (nematic) liquid crystals may be crystalline at room temperature.

In further embodiments, the elasto-optic material may have a glass transition temperature T _g , wherein T _g is below an application temperature. In particular, in embodiments, T _g < 45 °C, such as < 35 °C, especially < 25 °C, such as < 20 °C. In further embodiments, T _g > -10 °C, such as > 0 °C, especially > 10 °C.

In embodiment, the elasto-optic material may comprise liquid crystal elastomers.

In embodiments, the liquid crystals may comprise isotropic liquid crystals, which may provide one or more of the following benefits: easier fabrication because there is no need for alignment layers to control the liquid crystal orientation, the photoelastic response may have minimal temperature dependence, the mechanical response to stress may be preferable such as a classical isotropic rubber response, and the nearby liquid crystalline state may cause an anomalously large opto-mechanical response in the isotropic state of the liquid crystal. In other embodiments, the liquid crystal may comprise a nematic phase, which may provide the benefit that the liquid crystal can be approximately aligned parallel or orthogonal to an electric field of the waveguide mode which may cause the effective refractive index of the waveguide to become sensitive to a change in the liquid crystal that is caused by stress, strain, or pressure. In further embodiments, the liquid crystals may be thermotropic, which may provide the benefit that the change in the refractive index tensor of the liquid crystal material may be relatively large when a given strain, stress, or pressure is applied, possibly also making use of the temperature dependence of the order parameter.

In further embodiments, the liquid crystal may be close to a phase transition, for example between an isotropic phase and a nematic phase, or for example between a crystalline phase and a fluid phase, which may cause the refractive index tensor of the liquid crystal to become exceptionally sensitive to an applied strain, stress, or pressure.

In further embodiments, the liquid crystal may have a lyotropic phase, which may provide the benefit that the orientation of the liquid crystal may be relatively easier to achieve in fabrication because the liquid crystal may be layered and may (therefore) prefer alignment with the surface of the waveguide core.

In particular, in embodiments, the liquid crystal may be of any type, such as of a type selected from the group comprising thermotropic, lyotropic, and metallotropic. In further embodiments, the liquid crystal may have any phase, such as a phase selected from the group comprising nematic, smectic, chiral, twisted, discotic, conic, discontinuous cubic, micellar cubic, hexagonal, lamellar, bicontinuous cubic, reverse hexagonal columnar, inverse cubic, and combinations thereof, including combinations with one or more other phases.

Hence, in embodiments, the liquid crystals may have a thermotropic phase. In further embodiments, the liquid crystals may have a lyotropic phase. In further embodiments, the liquid crystals may have a metallotropic phase.

In embodiments, the elasto-optic material may comprise 5 - 100 wt.% of liquid crystals. In further embodiments, the elasto-optic material may comprise at least 5 wt.% of liquid crystals, such as at least 10 wt.%, especially at least 20 wt.%. In further embodiments, the elasto-optic material may comprise at least 30 wt.% of liquid crystals, especially at least 50 wt.%, such as at least 80 wt.%. In further embodiments, the elasto-optic material may comprise at most 100 wt.% of liquid crystals, such as at most 99 wt.%, especially at most 95 wt.%. In particular, a (relatively) low wt.% of liquid crystals may result in a (too) low birefringence. However, a (relatively) high wt.% of liquid crystals may complicate, especially prevent, the formation of a polymer network. Hence, in further embodiments, the elasto-optic material may comprise 25-90 wt.% of liquid crystals, especially 30-80 wt.% of liquid crystals.

In particular, the elasto-optic material may comprise 25-90 wt.% of mesogenic core (or “mesogenic groups”), especially 30-80 wt.%. The term “mesogenic core” may herein refer to the part of the liquid crystals that initiates the molecular rod-shape, and comprises the rigid part of the liquid crystalline compound(s). If the wt.% of mesogenic core is (too) low, liquid crystal formation may be inhibited, especially absent. However, if the wt.% of the mesogenic core is (too) high, the elasto-optic material may be inflexible.

Further, the optical sensor element of the invention may be scalable to an optical sensor matrix. In particular, in embodiments, the optical waveguide may comprise an integrated optical waveguide, especially a silicon integrated optics (or “optical”) waveguide, i.e. the optical waveguide may comprise an integrated optics waveguide comprising a silicon or silicon-nitride waveguide core. In further embodiments, the optical waveguide may comprise an indium-phosphide integrated photonic waveguide, i.e. the optical waveguide core may comprise a material that comprises indium and phosphide. In further embodiments, the optical waveguide may comprise a silicon-nitride photonic waveguide, i.e., the optical waveguide core may comprise silicon-nitride and silicon-oxide. By using “integrated optics” many different components can be fabricated on a chip. These components may be sensor elements, but may also comprise part(s) of an interrogator (read-out) system, such as lasers, detectors, etc. Further, integrated optics may facilitate more complex interrogation (read-out) architectures.

In embodiments, the optical sensor matrix may be configured for wavelength division multiplexing. In such embodiments, the optical sensor matrix (on a photonic chip) may be interfaced through a (limited) number of optical fibers. In wavelength division multiplexing, different wavelengths of light may be used simultaneously, such that a plurality of optical sensor elements of the optical sensor matrix may be addressed simultaneously, especially wherein each wavelength addresses a specific optical sensor element of the optical sensor matrix.

In further embodiments, the optical sensor matrix may comprise an active integrated photonic chip, especially a silicon photonic chip. In such embodiments, many different active and passive photonic components may be integrated in the active silicon photonic chip. This would e.g. facilitate tuning of the resonance wavelengths of the sensor resonators to a fixed laser wavelength. In this configuration, one laser may (be configured to) address many sensor elements. Moreover, each sensor element may be connected to a photodetector on the same chip, which may facilitate providing large scale matrices.

In embodiments, the optical sensor element may comprise a resonator, especially a resonator selected from the group comprising a ring resonator, a disk resonator, and a photonic crystal resonator. In further embodiments, the resonator may comprise a Fabry- Perot cavity or a Bragg reflector. In particular, in further embodiments, the resonator may comprise the optical waveguide core.

The term “ring” resonator should be construed as a closed-loop waveguide resonator of any shape and is not limited to a circular shape.

In further embodiments, the optical sensor element may comprises a waveguide element, especially a bus waveguide. In further embodiments, the optical sensor element may comprise at least one bus waveguide, such as (at least) two bus waveguides. In further embodiments, the resonator may be functionally coupled with the bus waveguide. In such a configuration, optical radiation may be provided to the waveguide element, especially the bus waveguide, (at a first location), and, as the optical radiation passes through the waveguide element, the optical radiation may interact with the resonator in a wavelength-dependent manner. Specifically, (part of) the optical radiation may couple with the resonator due to an evanescent field. The coupling of the optical radiation into the resonator may (effectively) result in a property change of one or more wavelengths of the optical radiation due to interference and/or an increased travel path of the one or more wavelengths of the optical radiation, such as resulting in a phase change of one or more wavelengths of the optical radiation. In particular, the affected wavelengths may depend on the propagation speed, and thus on the refractive index, in the optical waveguide, which may vary as a function of the (ultrasound) pressure the optical sensor element is exposed to.

Hence, in embodiments, the resonator and the waveguide element may be arranged (sufficiently close) to facilitate optical coupling. In further embodiments, the resonator and the waveguide element may be arranged at a (closest) distance selected from the range of 50 nm to 1.5 pm, such as from the range of 100 nm to 1 pm, especially from the range of 200 - 800 nm.

In other embodiments, for example with regards to a Bragg reflector or a Fabry- Perot resonator, the resonator may be physically connected to the waveguide.

Hence, in embodiments, the resonator and the waveguide element may be arranged at a distance selected from the range of < 1 pm, such as < 500 nm, especially < 100 nm. In further embodiments, the resonator and the waveguide element may be arranged abutted, i.e., in (direct) physical contact. In further embodiments, light may be coupled from a bus waveguide to the resonator using a multi-mode interference (MMI) coupler.

In further embodiments, the optical sensor element, especially the waveguide element, may comprise an interferometer. The interferometer may comprise two (or more) waveguide arms, especially wherein one of the two (or more) waveguide arms comprises or is functionally coupled to the optical waveguide core, especially wherein the elasto-optic material is arranged on the optical waveguide core. In further embodiments, one of the two (or more) waveguide arms may comprise the optical waveguide core. In such embodiments, (same) optical radiation may be (separately) provided to the two waveguide arms and may be measured after passing through the two waveguide arms. As the one of the two waveguide arms comprises the optical waveguide core, the one of the two waveguide arms may (substantially) modulate the optical radiation in dependence of an (ultrasound) pressure, whereas the other of the two waveguide arms may modulate the optical radiation in an essentially (ultrasound) pressure- independent manner. Thereby, the optical sensor element may facilitate determining an ultrasound pressure.

In a further aspect, the invention may provide an optical sensor matrix. The optical sensor matrix may especially comprise a plurality of the optical sensor elements of the invention.

In a further aspect, the invention may provide an (imaging) method for determining a characteristic of acoustic radiation, such as of ultrasound radiation, especially using the optical sensor element according to the invention. The imaging method (or “method”) may comprise exposing the optical sensor element to the acoustic radiation, such as to the ultrasound radiation, which, as indicated above, may affect a propagation speed of optical radiation through the optical waveguide of the optical sensor element. The imaging method may further comprise passing optical radiation through a waveguide element to provide modified (or “adjusted” or “modulated”) optical radiation. Especially, the waveguide element may comprise or be functionally coupled to the optical waveguide (core) (of the optical sensor element), especially comprise the optical waveguide, or especially be functionally coupled to the optical waveguide. Hence, the optical radiation may be modified while passing through the waveguide element in dependence of an ultrasound pressure (see above), such as when the optical sensor element comprises a resonator and/or an interferometer. The imaging method may further comprise detecting the modified optical radiation and providing a related sensor signal. In further embodiments, the imaging method may comprise determining the characteristic of the acoustic radiation, especially the ultrasound radiation, based on the related sensor signal. Especially, the imaging method may comprise determining an acoustic pressure (of the acoustic radiation), especially an ultrasound pressure (of the ultrasound radiation), based on the related sensor signal, i.e., the characteristic may especially comprise an acoustic pressure, especially an ultrasound pressure, more especially an acoustic pressure (or ultrasound pressure) as a function of time.

The phrase “the waveguide element may be functionally coupled to the optical waveguide” may herein especially refer to the waveguide element and the optical waveguide being optically coupled, i.e., the waveguide element and the optical waveguide may be configured, especially arranged, such that optical radiation from the waveguide element can couple into the optical waveguide (and vice versa).

The term “related sensor signal” may herein refer to a signal that is related to the detected modified optical radiation. In particular, the related sensor signal may comprise raw and/or processed data related to the (detected) modified optical radiation. Hence, in specific embodiments, the imaging method may comprise exposing the optical sensor element to ultrasound radiation; passing optical radiation through a waveguide element to provide modified optical radiation, wherein the waveguide element comprises or is functionally coupled to the optical waveguide; detecting the modified optical radiation and providing a related sensor signal; and determining the characteristic of the ultrasound radiation based on the related sensor signal.

In embodiments, the imaging method may comprise providing optical radiation at a first location of the waveguide element, especially to pass the optical radiation through the waveguide element to provide modified optical radiation. In further embodiments, the imaging method may comprise detecting the modified optical radiation at a second location and providing a related sensor signal, wherein the second location is arranged downstream from the first location.

The terms “upstream” and “downstream” relate to an arrangement of items or features relative to the propagation of the optical radiation from a radiation generating means (here the especially the optical radiation source), wherein relative to a first position within a travel path of the optical radiation from the optical radiation source, a second position along the path closer to the optical radiation source is “upstream”, and a third position along the path further away from the optical radiation source is “downstream”.

Hence, in further embodiments, the first location and the second location may spatially coincide, i.e., the optical radiation may be provided at the same location as where (reflected) modified optical radiation is measured. In such embodiments, a circulator may be used to separate the optical radiation that is provided to the waveguide element and the (modified) optical radiation that comes back.

In further embodiments, the first location and the second location may be spatially separated.

In embodiments, the method may comprise passing the optical radiation through at least 50 nm, especially at least 100 nm, such as at least 200 nm, of the waveguide element to provide the modified optical radiation. Hence, the first location and the second location may - along a path traveled by the optical radiation - have a distance of at least 50 nm, especially at least 100 nm, such as at least 200 nm.

In embodiments, the imaging method may comprise providing the ultrasound radiation to the optical sensor element from a hosting space configured for hosting a subject. In particular, the method may comprise providing first radiation to the subject. In further embodiments, the first radiation may comprise ultrasound radiation or may cause the generation of ultrasound radiation in the subject.

Hence, in embodiments, the imaging method may comprise providing (initial) ultrasound radiation to a subject in a hosting space, and the method may comprise exposing the optical sensor element to (reflected) ultrasound radiation from the subject. In particular, the ultrasound radiation may be reflected by the subject, such as by tissues of an animal subject, and the optical sensor element may be exposed to the (reflected) ultrasound radiation. Hence, the method may comprise providing ultrasound radiation to the optical sensor element via the hosting space, especially via the subject in the hosting space.

The optical waveguide core may be configured in any direction (or position) with respect to the hosting space, including the cases that the hosting space is “above” the optical waveguide core, “below” the optical waveguide core, and “parallel” to the optical waveguide core.

In further embodiments, the imaging method may comprise providing first radiation to the subject in the hosting space, wherein the first radiation causes the generation of ultrasound radiation in the subject, and wherein the imaging method comprises exposing the optical sensor element to the ultrasound radiation. In particular, the first radiation may comprise (non-ionizing) laser radiation (pulses). The subject may absorb (part of) the laser radiation, which may be converted to heat, which may lead to transient thermoelastic expansion and (thus) to the emission of ultrasound radiation. Hence, in embodiments, the imaging method may be a photoacoustic imaging method.

In particular, to generate a photoacoustic image, a short pulse of laser light may be shone into a subject, such as into an (animal) body, which gets locally absorbed by tissue, causes tissue to heat and expand, thereby generating ultrasound waves. These ultrasound waves are then picked up at the skin by an imaging system comprising the optical sensor element, which may simultaneously record the ultrasonic wavefield at a large number (100 - 10000) of locations. From these pressure-versus-time recordings, a full two-dimensional (2D) or three- dimensional (3D) image can be reconstructed.

The subject may especially comprise an animal, such as a human, or such a nonhuman animal, especially a mouse or a rat. In particular, the subject may comprise a part of the animal, especially a part selected from the group comprising a head, a lymph node, a breast, a joint, a heart, a blood vessel, and a prostate.

In embodiments, the imaging method may be a non-medical method, such as a method for one or more of non-destructive-testing of materials, structural integrity monitoring, non-destructive-testing of structures, including of welds, inspecting welds, locating welding defects, and inspection of corrosion.

As indicated above, in embodiments, the waveguide element may comprise a bus waveguide, wherein the optical waveguide comprises a resonator, and wherein the bus waveguide and the resonator may be functionally coupled.

In further embodiments, the waveguide element may comprise an interferometer, wherein the interferometer comprises two (or more) waveguide arms, and wherein one of the two waveguide arms comprises the optical waveguide (of the optical sensor element).

In further embodiments, the imaging method may comprise providing an (ultrasound) image of the subject based on the characteristic of the ultrasound radiation. Hence, the imaging method may comprise reconstructing an image based on the characteristic of the ultrasound radiation, such as based on the ultrasound pressure, and especially providing the image.

In a further aspect, the invention may provide an imaging system (or “system”), especially an ultrasound imaging system, or especially a photoacoustic imaging system. In embodiments, the imaging system may comprise the optical sensor element according to the invention, or especially the optical sensor matrix according to the invention. The imaging system may further comprise an optical radiation source, especially an electro-optic interrogator, an optical radiation sensor, and an optical waveguide. The optical radiation source may be configured to provide optical radiation to the optical radiation sensor via the waveguide element, especially wherein the optical radiation sensor is configured to detect the optical radiation. In embodiments, the optical sensor element may comprise the waveguide element. In further embodiments, the waveguide element may comprise or be functionally coupled to the optical waveguide (of the optical sensor element).

Hence, in specific embodiments, the imaging system may comprise the optical sensor element according to the invention, an optical radiation source, an optical radiation sensor, and a waveguide element, wherein the optical radiation source is configured to provide optical radiation to the optical radiation sensor via a waveguide element, wherein the waveguide element comprises or is functionally coupled to the optical waveguide, and wherein the optical radiation sensor is configured to detect the optical radiation. The term “optical radiation sensor” may also refer to a plurality of optical radiation sensors. Similarly, the terms “optical sensor element” and waveguide element” may also refer to pluralities of optical sensor elements and wave guide elements, respectfully. Hence, in embodiments, the imaging system may be configured to interrogate a plurality of optical sensor elements, especially via a plurality of waveguide elements and optical radiation sensors.

In embodiments, the imaging system may comprise an ultrasound generator configured to provide ultrasound radiation to the optical sensor element, such as to an optical sensor matrix comprising the optical sensor element. In further embodiments, the ultrasound generator may be configured to provide the ultrasound radiation to the optical sensor element via a hosting space configured for hosting a subject.

In further embodiments, the imaging system may comprise a laser source configured to provide laser radiation to a hosting space configured for hosting a subject, especially (during use) to provide laser radiation to the subject in the hosting space. In embodiments, the optical sensor element may be configured to receive ultrasound radiation from the hosting space, especially (during use) from the subject in the hosting space.

The laser radiation may especially have a modulation in the laser intensity, especially a continuous modulation. In embodiments, the laser radiation may be pulsed laser radiation. In particular, the pulse duration of the laser pulses may be in the (few) nanoseconds timescale. The pulse duration of the laser may shorter than a nanosecond. For example, in embodiments, the pulse duration of the laser radiation may be selected from the range of 0.01 - 20 ns, such as from the range of 5-10 ns. The laser radiation may further comprise a laser wavelength chosen such that the laser radiation is (at least partially) absorbed by a specific types of (target) tissue, for example by a (target) tissue comprising one or more of oxygenated and de-oxygenated hemoglobin.

In particular, in embodiments, the laser source may be configured to provide pulsed laser radiation. In further embodiments, the laser source may be configured to provide non-ionized laser radiation.

In embodiments, the system may (be configured to) detect acoustic (ultrasound) pressure originating from the hosting space that was generated in other means than with the system, for example from movement, flow, or temperature in the hosting space.

In embodiments, the imaging system may comprise a control system. The control system may be configured to control the imaging system, especially one or more of the optical radiation source, and the optical radiation sensor.

The term “controlling” and similar terms herein may especially refer at least to determining the behavior or supervising the running of an element. Hence, herein “controlling” and similar terms may e.g. refer to imposing behavior to the element (determining the behavior or supervising the running of an element), etc., such as e.g. measuring, displaying, actuating, opening, shifting, changing temperature, etc.. Beyond that, the term “controlling” and similar terms may additionally include monitoring. Hence, the term “controlling” and similar terms may include imposing behavior on an element and also imposing behavior on an element and monitoring the element. The controlling of the element can be done with a control system. The control system and the element may thus at least temporarily, or permanently, functionally be coupled. The element may comprise the control system. In embodiments, the control system and the element may not be physically coupled. Control can be done via wired and/or wireless control. The term “control system” may also refer to a plurality of different control systems, which especially are functionally coupled, and of which e.g. one master control system may be a control system and one or more others may be slave control systems.

In further embodiments, the imaging system, especially the control system, may have an operational mode. The operational mode may comprise exposing the optical sensor element to ultrasound radiation (or acoustic radiation), i.e., the optical sensor element being exposed to ultrasound radiation (or acoustic radiation). The operational mode may further comprise the optical radiation source (being configured for) passing optical radiation through the waveguide element to provide modified optical radiation to the optical radiation sensor. The operational mode may further comprise the optical radiation sensor (being configured for) detecting the modified optical radiation and to provide a related sensor signal to the control system. The operational mode may further comprise the control system (being configured for) determining a characteristic of the ultrasound radiation (or acoustic radiation) based on the related sensor signal, especially determining an ultrasound pressure (or acoustic pressure) of the ultrasound radiation, more especially an acoustic pressure (or ultrasound pressure) as a function of time.

The system, especially the control system, may have an operational mode. The term “operational mode” may also be indicated as “controlling mode”. The system, or apparatus, or device (see further also below) may execute an action in a “mode” or “operational mode” or “mode of operation”. Likewise, in a method an action, stage, or step may be executed in a “mode” or “operation mode” or “mode of operation”. This does not exclude that the system, or apparatus, or device may also be adapted for providing another operational mode, or a plurality of other operational modes. Likewise, this does not exclude that before executing the mode and/or after executing the mode one or more other modes may be executed. However, in embodiments a control system (see further also below) may be available, that is adapted to provide at least the operational mode. Would other modes be available, the choice of such modes may especially be executed via a user interface, though other options, like executing a mode in dependence of a sensor signal or a (time) scheme, may also be possible. The operational mode may in embodiments also refer to a system, or apparatus, or device, that can only operate in a single operation mode (i.e. “on”, without further tunability).

In particular, in embodiments, the imaging system of the invention may be configured to execute the imaging method of the invention, especially the control system may be configured to (have the imaging system) execute the imaging method of the invention.

In embodiments, the control system may further be configured to control the ultrasound generator and/or the laser source, especially the ultrasound generator, or especially the laser source.

In further embodiments, the operational mode may comprise the ultrasound generator (being configured to) provide ultrasound radiation to the optical sensor element, especially via the hosting space.

In further embodiments, the operational mode may comprise the laser source (being configured to) provide laser radiation to the optical sensor element, especially via the hosting space.

The imaging system may, in embodiments, further comprise a circulator. The circular may especially be functionally coupled with the waveguide elements. In embodiments, the circulator may be configured to separate optical radiation that is provided to the waveguide element (by the optical radiation source) and the modified optical radiation, and especially to provide the modified optical radiation to the optical radiation sensor.

In further embodiments, the circulator may be used to separate the optical radiation that is provided to the waveguide element and the (modified) optical radiation that comes back.

The imaging system may, in embodiments, especially comprise a plurality of optical radiation sensors. For instance, in embodiments wherein the optical sensor element comprises an interferometer, the imaging system may comprise a plurality of optical radiation sensors configured to detect optical radiation at different arms of the interferometer.

In a further aspect, the invention may provide a use of the optical sensor element according to the invention to detect acoustic radiation, especially ultrasound radiation.

In a further aspect, the invention may provide a use of the optical sensor element according to the invention to detect acoustic pressure as a function of time, especially the ultrasound pressure of ultrasound radiation, or especially the acoustic pressure of acoustic radiation having a frequency in the range of 20 - 20000 Hz. In embodiments, the optical waveguide may be selected from the group comprising a buried waveguide, a diffused waveguide, a strip waveguide, a wire waveguide, a rectangular waveguide, a ridge waveguide, a rib waveguide, a strip-loaded waveguide, an arrow waveguide, a slot waveguide, a subwavelength grating waveguide.

In a further aspect, the invention may provide a production method for providing the optical waveguide. In further embodiments, the production method may comprise providing the optical waveguide with (corresponding) photonic integrated circuit and/or (corresponding) electrooptic components.

In further embodiments, the production method may comprise arranging the elasto-optic material on the optical waveguide core.

For example, in embodiments, the production method may comprise providing the optical waveguide (core) (and optionally the corresponding photonic integrated circuit and/or optionally the corresponding electrooptic components) in a photonic platform. In embodiments, the photonic platform may be obtainable from a semiconductor manufacturing plant, for example through multi -project- wafer fabrication. In further embodiments, the production method may comprise providing the optical waveguide core, and especially a supporting structure, and/or especially an (associated) photonic integrated circuit, in a photonic platform using wafer-scale fabrication. In particular, this fabrication may use equipment that was originally developed for the fabrication of electronic circuits, for example for the fabrication of CMOS electronic circuits. The optical waveguide core and its supporting structure may, for instance, be fabricated in a semiconductor fabrication plant. In particular, lithography may be used to pattern the optical waveguide and photonic integrated circuit on the chip. Hence, in embodiments, the production method may comprise patterning of the optical waveguide (core) and/or the photonic integrated circuit (on a chip) by lithography. In further embodiments, the optical waveguide and photonic integrated circuit may be patterned on smaller chips and with other means than described above, for example using methods like electron-beam lithography, direct laser writing, imprinting, or stamping techniques.

In embodiments, the elasto-optic material may be added to an integrated optical chip that was previously fabricated to have optical and electro-optical functions. In embodiments, the elasto-optic material may be chemically synthesized on the optical chip, in particular, liquid crystal formation may be done on the optical chip. In further embodiments, the elasto-optic material may be added as a coating, in particular using spin-coating. In other embodiments, the electro-optic material may be added using sputtering, deposition, or material growth. Hence, in embodiments, the production method may comprise arranging the elasto-optic material on an optical chip, especially by providing the elasto-optic material as a coating, or especially by providing the elasto-optic material using one or more of sputtering, deposition, and material growth.

In embodiments, the production method may comprise providing first voids, especially by selectively removing the elasto-optic material (at predefined locations). In further embodiments, for the formation of the first voids, the production method may comprise removing the elasto-optic material using a laser, especially using one or more of laser micromachining, laser ablation, and laser melting. In further embodiments, the production method may comprise defining the first voids using lithography and removing the elasto-optic material (at the defined sites) using development and/or etching. In further embodiments, the required accuracy for the fabrication of the first voids may be (relatively) course, such as larger than 1 pm, especially larger than 10 pm, such as larger than 100 pm. This may provide the advantage that fabrication methods can be used that are relatively course such as some of the aforementioned methods.

In other embodiments, the production method may comprise providing the first voids by preparing the surface of the chip with an optical and/or electro-optical functionality such that the technique for addition of the materials naturally generates first voids at the desired locations. In particular, chip topology (or height differences at the surface of a chip) may be used such that addition techniques such as spin-coating, deposition, sputtering, or transfer naturally generates first voids around the chip topology. This may provide the advantage that the fabrication of the first voids does not require additional fabrication steps. This may additionally provide the advantage that the locations of the first voids can be accurately defined in a material of choice.

In embodiments, an integrated optical chip that was previously fabricated to have optical and electro-optical functions may first be prepared to receive the elasto-optic material in the vicinity of the waveguide core, such as closer than 1 pm, in particular closer than 500 nm, in particular closer than 100 nm, in particular in direct contact. This may be achieved by removing material to what is referred to as the “front-side” of the chip or alternatively to what is referred to as the “back-side” of the chip. In further embodiments, the production method may comprise adding the elasto-optic material (exclusively) in the vicinity of the waveguide core of the optical sensor element, especially without adding the elasto-optic material in the vicinity of the waveguide core of other optical and electrooptical components of the chip. This may have the advantage that the performance or behavior of the other components on the chip may (essentially) be maintained. This may also be used to fabricate an aforementioned interferometer such that only one of its arms is sensitive to acoustic radiation.

The invention may herein primarily be described in the context of the detection and characterization of ultrasound radiation. It will be clear to the person skilled in the art that the invention is not limited to such embodiments, and may further be applied to the detection and characterization of acoustic radiation with a frequency < 20kHz.

The embodiments described herein are not limited to a single aspect of the invention. For example, an embodiment describing the (imaging) method may, for example, further relate to the system, especially to an operational mode of the system, or especially to the control system. Similarly, an embodiment of the system describing an operation of the system may further relate to embodiments of the method. In particular, an embodiment of the method describing an operation (of the system) may indicate that the system may, in embodiments, be configured for and/or be suitable for the operation. Similarly, an embodiment of the system describing actions of (a stage in) an operational mode may indicate that the method may, in embodiments, comprise those actions. Embodiments of the optical sensor element may further relate to embodiments of the method (or the system) using (or comprising) the optical sensor element.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: Fig.lA-B schematically depict embodiments of the optical sensor element, Fig. 1C schematically depicts an embodiment of the optical sensor matrix, Fig. 2A-C schematically depict further embodiments of the optical sensor element, and Fig. 3 schematically depicts an embodiment of the imaging system. The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 A-2C schematically depict embodiments of the optical sensor element 100.

Specifically, Fig. 2A schematically depicts an embodiment of the optical sensor element 100 for acoustic imaging, especially for ultrasound imaging. The optical sensor element 100 comprises an optical waveguide core 111, and a (transparent) elasto-optic material 121 arranged on the optical waveguide core 111. The elasto-optic material 121 may especially comprise liquid crystals, and may especially have a stress-optic coefficient > 100 Brewsters, such as > 500 Brewsters, especially > 800 Brewsters.

In the depicted embodiment, the optical sensor element 100 comprises a waveguide layer 110 comprising the optical waveguide core 111. The waveguide layer 110 further comprises a waveguide cladding region 112. Further, in the depicted embodiment, the optical sensor element 100 comprises an elasto-optic layer 120 arranged on the waveguide layer 110, especially wherein the elasto-optic layer 120 comprises the elasto-optic material 121.

In the depicted embodiment, the waveguide layer 110 is arranged on a support layer 140, and the support layer is arranged on a substrate 150.

In particular, the optical sensor element 100 may comprise an optical waveguide comprising the waveguide layer 110, especially the optical waveguide core 111 and the waveguide cladding region 112, the elasto-optic layer 120, and the support layer 140.

The optical sensor element 100 may further have an element axis A, especially wherein the element axis A is perpendicular to the waveguide layer 110.

In the depicted embodiment, the layers are arranged stacked along the element axis A.

In further embodiments, the elasto-optic material may also be arranged on a side of the optical waveguide core 111. Hence, in embodiments, the waveguide layer, especially the waveguide cladding region 112, may (also) comprise the elasto-optic material.

Fig. 2B schematically depicts an embodiment wherein the elasto-optic layer 120 comprises first voids 131 having a first length LI perpendicular to the element axis A, especially wherein the first length LI is selected from the range of 1-100 pm.

In particular, in the depicted embodiment, the elasto-optic layer 120 comprises a central region 125 and a peripheral region 129, wherein the central region 125 is arranged on the optical waveguide core 111, and wherein the peripheral region 129 is arranged at a first distance from the optical waveguide core 111. The first distance dl may especially be at least 1 pm, and may especially be perpendicular to the element axis A. In further embodiments, the peripheral region 129 may comprise the first voids 131, whereas the central region 125 may especially be devoid of the first voids 131. Thereby, the first voids 131 may increase the compressibility of the elasto-optic layer 120, without (substantially) increasing scattering loss.

Further, in the depicted embodiments, such as in Fig. 2A, the elasto-optic material 121 may comprise second voids, wherein the second voids have spherical equivalent diameters (independently) selected from the range of 0.01 - 1 pm. Thereby, the compressibility of the elasto-optic material 121, especially the elasto-optic layer 120, may be improved. Fig. 2C schematically depicts an embodiment wherein the optical sensor element 100 further comprises an auxiliary layer 160, especially a stack of layers comprising part of the integrated photonic and/or electronic circuit, optionally including thermo-optic, electro-optic and electronic components, materials, and/or functionality. Especially, auxiliary layer 160 may include an active silicon photonic layer (stack). In embodiments, the optical sensor element, especially the optical waveguide core 111, may additionally be tunable with other means, e.g. thermal tuning or electro-optic tuning, especially via the active silicon photonic layer.

In the depicted embodiment, the support layer and the waveguide layer 120 are (at least partially) arranged at the same position with respect to the element axis A. In particular, in the depicted embodiment, the support layer 140 may comprise the waveguide layer 120.

In embodiments, the optical waveguide core 111 may comprise a silicon integrated optics waveguide.

In the embodiments depicted in Fig. 2A-C, the elasto-optic layer 120, especially the elasto-optic material 121, may be arranged directly on the waveguide layer 110, especially on the optical waveguide core 111. In further embodiments, the elasto-optic layer 120, especially the elasto-optic material 121, may be arranged on an intermediate layer, wherein the intermediate layer is arranged on the waveguide layer 110, especially on the optical waveguide core 111. In further of such embodiments, the intermediate layer may have a thickness of < 300 nm, such as < 150 nm, especially < 50 nm. In further embodiments, the intermediate layer may have a thickness of < 20 nm, such as < 10 nm, especially < 5 nm. In further embodiments, the intermediate layer may have a thickness of > 0.5 nm, especially > 1 nm, such as > 3 nm, especially > 5 nm.

Fig. 1A schematically depicts an embodiment of the optical sensor element 100, wherein the optical sensor element 100 comprises a resonator 50, especially a ring resonator, and a waveguide element 400. In particular, the resonator 50 comprises the optical waveguide core 111 and the elasto-optic material 121 (not depicted for visualization purposes; see Fig. 2A-C) arranged on the optical waveguide core 111. In the depicted embodiment, the waveguide element comprises a bus waveguide 410, wherein the resonator 50 is functionally coupled with the bus waveguide 410. Hence, optical radiation 20 provided to the waveguide element 400, especially at a first location 401, may pass through the waveguide element 400, and may couple in to the resonator 50 in a waveguide-dependent manner, which may result in a modification of the optical radiation 20 and modified optical radiation 21 may be provided from a second location 402 of the waveguide element. In particular, the modification of the optical radiation 20, especially affected wavelengths of the optical radiation 20, may depend on an ultrasound pressure the resonator 50, especially the elasto-optic material 121, is exposed to.

In further embodiments, the resonator may comprise a disk resonator or a photonic crystal resonator.

Fig. IB schematically depicts an embodiment of the optical sensor element 100, wherein the optical sensor element 100 comprises a waveguide element 400, wherein the waveguide element comprises an interferometer 420. Specifically, the depicted interferometer comprises two waveguide arms 421,422, wherein one of the two waveguide arms 421 comprises (or is functionally coupled to) the optical waveguide core 111 and the elasto-optic material 121 (not depicted for visualization purposes; see Fig. 2A-C) arranged on the optical waveguide core 111. In the depicted embodiment the one of the two waveguide arms 421 comprises the optical waveguide core 111 and the elasto-optic material 121 arranged thereupon.

In the depicted embodiment, the two waveguide arms 421, 422 converge and optical radiation 20 from both waveguide arms 421, 422 is provided to a (single) optical radiation sensor 1020 (also see below). In further embodiments, however, the waveguide arms 421, 422 may not converge, and may especially provide (respective) optical radiation 20 to different optical radiation sensors 1020.

Fig. 1C schematically depicts an embodiment of the optical sensor matrix 200 comprising a plurality of the optical sensor elements 100.

Fig. 1A-B, Fig 2C and Fig. 3A-B schematically depict embodiments of the imaging method for determining a characteristic of acoustic radiation, especially of ultrasound radiation 10, using the optical sensor element 100. The imaging method may comprise providing acoustic radiation, especially ultrasound radiation 10, to the optical sensor element 100, i.e., exposing the optical sensor element 100 to acoustic radiation. The imaging method may further comprise passing optical radiation 20 through a waveguide element 400 to provide modified optical radiation 21, especially wherein the waveguide element 400 comprises or is functionally coupled to the optical waveguide core 111 of the optical sensor element 100. The imaging method may further comprise detecting the modified optical radiation 21 and providing a related sensor signal, and determining the characteristic, especially a pressure, of the acoustic radiation based on the related sensor signal.

In embodiments, the method may comprise providing optical radiation 20 to a first location 401 of the waveguide element 400, and especially passing optical radiation 20 through the waveguide element 400 (from the first location 401 to a second location 402 of the waveguide element). In further embodiments, the imaging method may comprise detecting the optical radiation 20, especially the modified optical radiation 21, at a second location 402 and providing a related sensor signal, especially wherein the second location 402 is arranged downstream from the first location 401 of the waveguide element 400.

Fig. 3A schematically depicts an embodiment of the imaging method comprising providing ultrasound radiation 10 to the optical sensor element 100 via a hosting space 1200 configured for hosting a subject, especially configured for hosting an animal subject, such as a human subject.

Fig. 3B schematically depicts an embodiment of the imaging method comprising providing laser radiation 30 to the hosting space 1200 configured for hosting a subject. The subject, following exposure to the laser radiation 30, may generate ultrasound radiation 10. Hence, in the depicted embodiment, the method may further comprise exposing the optical sensor element 100 the ultrasound generation 10 from the hosting space 1200.

In further embodiments, the imaging method may comprise providing laser radiation to the hosting space for hosting the subject, and exposing the optical sensor element 100 to acoustic radiation, especially ultrasound radiation 10, from the hosting space, especially to acoustic radiation generated in the hosting space.

In further embodiments, the imaging method may comprise detecting acoustic pressure, especially ultrasound pressure, as a function of time.

Fig. IB and 3A-B further schematically depict embodiments of the imaging system 1000, especially of an ultrasound imaging system, or especially of a photoacoustic imaging system. In the depicted embodiments, the imaging system comprises the optical sensor element 100, an optical radiation source 1010, especially an electro-optic interrogator, an optical radiation sensor 1020, and a waveguide element 400. The optical radiation source 1010 may be configured to provide optical radiation 20 to the optical radiation sensor 1020 via the waveguide element 400, wherein the optical radiation sensor 1020 is configured to detect the optical radiation 20, especially modified optical radiation 21. In embodiments, the waveguide element 400 may comprise or be functionally coupled to the optical waveguide core 111.

In the embodiment depicted in Fig. 3 A, the imaging system 1000 further comprises an ultrasound generator 1100 configured to provide ultrasound radiation 10 to the optical sensor element 100 via a hosting space 1200 configured for hosting a subject. In particular, during use, the ultrasound generator 1100 may (be configured to) provide the ultrasound radiation 10 to a subject in the hosting space, and the optical sensor element 100 may be configured to receive ultrasound radiation 10 reflected by the subject. However, in further embodiments, the imaging system 1000 may be devoid of an ultrasound generator 1100, i.e., the imaging system 1000 may not comprise an ultrasound generator 1100.

Fig. 3B schematically depicts an embodiment, wherein the imaging system 1000 comprises a laser source 1300 configured to provide laser radiation 30 to a hosting space 1200 configured for hosting a subject, and wherein the optical sensor element 100 is configured to receive ultrasound radiation 10 from the hosting space 1200.

In further embodiments, the imaging system may comprise a control system 300, especially wherein the imaging system 1000, or especially the control system, has an operational mode. The operational mode may comprise: the system exposing the optical sensor element 100 to ultrasound radiation 10; the optical radiation source 1010 passing optical radiation 20 through the waveguide element 400 to provide modified optical radiation 21 to the optical radiation sensor 1020; the optical radiation sensor 1020 detecting the modified optical radiation 21 and providing a related sensor signal to the control system 300; and the control system 300 determining a characteristic of the ultrasound radiation 10 based on the related sensor signal.

In further embodiments, the operational mode may comprise the ultrasound generator 1100 providing ultrasound radiation 10 to the optical sensor element 100 via the hosting space.

In further embodiments, the operational mode may comprise the laser source 1300 providing laser radiation 30 to the hosting space 1200 to generate ultrasound radiation 10 in the hosting space, wherein the optical sensor element 100 is configured to receive the ultrasound radiation 10 from the hosting space 1200.

In further embodiments, the operational mode may comprise the control system 300 providing an image based on the related sensor signal, or especially based on the determined characteristic of the ultrasound radiation 10.

In further embodiments, the control system 300 may be configured to have the imaging system 1000 execute the method of the invention.

In further embodiments, the operational mode may comprise detecting acoustic pressure, especially ultrasound pressure, as a function of time.

Fig. 3A-B further schematically depicts a use of the optical sensor element 100 to detect acoustic radiation, especially ultrasound radiation 10. In further embodiments, the use may comprise detecting acoustic radiation, especially ultrasound radiation 10, with a plurality of optical sensor elements 100, especially with an optical sensor matrix 200 comprising a plurality of optical sensor elements 100.

Fig. 3A-B further schematically depict a use of the optical sensor element 100 to detect acoustic pressure, especially as a function of time.

The term “plurality” refers to two or more. Furthermore, the terms “a plurality of’ and “a number of’ may be used interchangeably.

The terms “substantially” or “essentially” herein, and similar terms, will be understood by the person skilled in the art. The terms “substantially” or “essentially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially or essentially may also be removed. Where applicable, the term “substantially” or the term “essentially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. Moreover, the terms ’’about” and “approximately” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. For numerical values it is to be understood that the terms “substantially”, “essentially”, “about”, and “approximately” may also relate to the range of 90% - 110%, such as 95%-105%, especially 99%-101% of the values(s) it refers to.

The term “comprise” also includes embodiments wherein the term “comprises” means “consists of’.

The term “and/or” especially relates to one or more of the items mentioned before and after “and/or”. For instance, a phrase “item 1 and/or item 2” and similar phrases may relate to one or more of item 1 and item 2. The term "comprising" may in an embodiment refer to "consisting of' but may in another embodiment also refer to "containing at least the defined species and optionally one or more other species".

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

The devices, apparatus, or systems may herein amongst others be described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation, or devices, apparatus, or systems in operation. The term “further embodiment” and similar terms may refer to an embodiment comprising the features of the previously discussed embodiment, but may also refer to an alternative embodiment.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, “include”, “including”, “contain”, “containing” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim, or an apparatus claim, or a system claim, enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The invention also provides a control system that may control the device, apparatus, or system, or that may execute the herein described method or process. Yet further, the invention also provides a computer program product, when running on a computer which is functionally coupled to or comprised by the device, apparatus, or system, controls one or more controllable elements of such device, apparatus, or system.

The invention further applies to a device, apparatus, or system comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. Moreover, if a method or an embodiment of the method is described being executed in a device, apparatus, or system, it will be understood that the device, apparatus, or system is suitable for or configured for (executing) the method or the embodiment of the method, respectively. The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined. Furthermore, some of the features can form the basis for one or more divisional applications.