SPECTROSCOPY

Technical Field of the Invention

The present invention relates to improvements in or relating to microsphere- enhanced spectroscopy.

Background to the Invention

A number of spectroscopy techniques involve illuminating a sample with laser radiation, typically in the form of a spatially limited beam. Radiation from the illuminated area is collected. This radiation can include scattered, absorbed, emitted or reflected radiation depending on the type of spectroscopy technique employed and the interactions being studied. Raman spectroscopy is typically used to investigate radiation scattered from a sample, whereas IR and UV spectroscopy are typically used for investigating radiation absorbed, emitted or reflected by a sample.

In the case of Raman spectroscopy, the scattered radiation will include elastic scattered radiation at the wavelength corresponding to the laser line (Rayleigh scattering) which is typically filtered out. The scattered radiation will also include Raman scattering which results from inelastic scattering of a photon by molecules which are excited to higher vibrational or rotational energy levels. This Raman scattering may be passed to a suitable detector for analysis. Raman spectroscopy thus provides a non-destructive technique to observe vibrational, rotational, and other low-frequency modes in molecules and provide structural fingerprints of materials for identification. Raman scattering signals are typically relatively weak, particularly in relation to the incident light, and are thus difficult to detect. In order to enhance the Raman signal intensity, various enhancement techniques have been developed based on various physical/chemical mechanisms. These techniques include, for example, surface-enhanced Raman scattering (SERS), tip-enhanced Raman scattering (TERS), interference-enhanced Raman scattering (IERS), resonance Raman scattering (RRS), coherent anti- Stokes Raman scattering (CARS), and stimulated Raman scattering (SRS), etc.

IR and UV spectroscopy also provide non-destructive techniques to observe vibrational and rotational-vibrational modes in molecules and provide structural fingerprints of materials for identification. In some implementations, IR spectroscopy can be enhanced by use of a Fourier-transform spectrometer, which can allow a broadband illumination source to be used and can also improve the signal to noise ratio.

One particular enhancement technique is known as microsphere-enhanced spectroscopy. This technique requires the provision of one or more microspheres held on or just above a surface of a sample. Typically, the microspheres may be held in position mechanically, for instance within a film or attached to a suitable frame or member. Alternatively, other techniques such as the optical tweezer effect may be utilised. A radiation collection arrangement, typically a microscope can then be used to collect radiation and direct it to a suitable detector. Nevertheless, such techniques suffer from significant drawbacks. Firstly, positioning the microspheres directly on or just above a surface limits the resolution and signal strength that can be achieved. The microsphere positioning also limits the range of focus possible, thus limiting collection of radiation to the surface of a sample. Another drawback is that using these techniques, it is only possible to collect radiation from a discrete selection of points corresponding to the position of the microspheres.

It is therefore an object of the present invention to provide an improved method and apparatus for microsphere-enhanced spectroscopy.

Summary of the Invention

According to a first aspect of the present invention, there is provided an apparatus for microsphere-enhanced spectroscopy, the apparatus comprising: an illumination source for illuminating a sample with radiation; a radiation collection arrangement for collecting radiation scattered by the sample; and a spectral analyser for analysing collected radiation; wherein the radiation collection arrangement comprises a base lens and a microsphere lens mounted in fixed relation to the base lens.

According to a second aspect of the present invention there is provided a method of microsphere-enhanced spectroscopy, the method comprising the steps of: illuminating a sample with radiation; collecting radiation scattered by the sample for analysis; wherein the radiation is collect using an arrangement comprising a base lens and a microsphere lens mounted in fixed relation to the base lens.

Carrying out microsphere-enhanced spectroscopy where the microsphere is mounted to a base lens rather held on or just above the sample provides improved signal strength and resolution. The present invention also allows for variation of the microsphere position relative to the sample. This can enable imaging of a full sample rather than discrete points and imaging of subsurface layers, as outlined further below. The method may be carried out with a fixed separation between the sample and the microsphere lens. In some embodiments, the method may be carried out with a variable separation between the sample and the microsphere lens. The variable separation may enable the variation of the focus of the microsphere lens, thus enabling imaging of subsurface layers of the sample.

The illumination source may be operable to illuminate the sample with a range of wavelengths of output radiation. This range of wavelengths may be a continuous band of wavelengths of output radiation within an operating range, or multiple discrete wavelengths of output radiation within an operating range. The illumination source may be operable to illuminate the sample with a single wavelength of output radiation. The illumination radiation may be monochromatic. The illumination source may be a laser. The illumination source may be operable to illuminate a sample with ultraviolet (UV) radiation, visible radiation or infrared (IR) radiation. The illumination source may be operable to enable variation in the wavelength of the output radiation. This may be achieved by adjustment of the illumination source or by replacement by an alternative illumination source operable to output illumination of a different wavelength. The illumination wavelength may be varied dependent upon the nature of the sample. For example, UV wavelengths may be preferred for analysis of inorganic samples and IR wavelengths may be preferred for organic samples.

The apparatus may be used for microsphere-enhanced Raman spectroscopy. This technique can provide information of the polarizability of the electrons in a molecule. When the illumination source is operable to produce IR radiation, the apparatus may be used for microsphere-enhanced IR spectroscopy. This technique can provide additional information about a sample. This technique can be beneficial in situations where Raman spectroscopy is ineffective, such as when a molecule does not exhibit a change in its polarizability. Molecular transitions between rovibronic energy states which produce strong IR intensities may have weak Raman intensities, and vice versa.

When the illumination source is operable to produce UV radiation, the illumination source may be used for microsphere-enhanced UV spectroscopy. The illumination source is preferably focussed on to the sample. This may be achieved by use of a focussing arrangement. The focussing arrangement may comprise one or more lenses.

Illumination from the illumination source may be directed on to the sample through the radiation collection arrangement or through at least part of the radiation collection arrangement.

The method may include the steps of scanning the sample relative to the illumination source and/or radiation collection arrangement. Scanning can enable the radiation to be collected over a wide area of a sample rather than only from discrete points. Subsequent to scanning the method may include the step of processing collected radiation to generate an image of the sample. The scanning may be achieved by mounting the sample on a scanning stage or by integrating the radiation collection arrangement into a scanning stage. The scanning stage may be operable to vary the separation between the microsphere lens and the sample. The radiation collection arrangement may comprise one or more lenses. In a preferred embodiment, the radiation collection arrangement may comprise a microscope. In such cases, the base lens may be an objective lens of the microscope. The radiation collection arrangement may comprise one or more filters. The or each filter may be operable to filter out radiation with a wavelength equal to that of the illumination source.

The radiation collection arrangement may comprise one or more adjustable pinholes. The radiation collection arrangement may comprise a shutter. The spectral analyser may comprise an optical grating and a detector operable to collect radiation from the grating. The grating may comprise a holographic grating. The detector may comprise any suitable device including CCD detectors, photomultipliers detectors, spectrometers or monochromators.

In some embodiments, fluid may be provided between the microsphere lens and the sample. In such embodiments, the method may include the step of applying a fluid to the surface of the sample or to the microsphere lens. In some embodiments the fluid may be water. In other embodiments, the fluid may be an oil.

The microsphere lens may comprise a microsphere or a truncated microsphere. The use of a microsphere rather than a truncated microsphere increases resolution but also increases distortion. For the avoidance of doubt, a truncated microsphere comprises a microsphere truncated by a plane perpendicular to the optical axis. In some embodiments, the truncated microsphere may comprise a hemisphere.

The microsphere comprising the microsphere lens may have a diameter of in the range 1-1000 μπι. In one embodiment, the microsphere may have a diameter in the range 90-106 μιη. In particular, the microsphere may have a diameter of around ΙΟΟμιη. In another embodiment, the microsphere may have a diameter in the range 5- 15 μιη.

The microsphere comprising the microsphere lens may have a refractive index in the range of 1.5 - 4. In one embodiment, the microsphere lens may have a refractive index in the range 1.55-2.4. In particular, the microsphere lens may have a refractive index of around 1.9 - 2.2.

The microsphere comprising the microsphere lens may be formed from any suitable material, including but not limited to Barium Titanate (BaTi0 ₃ ), BaTi0 ₃ - Si0 ₂ -Ti0 ₂ polystyrene, silica (SiC ), diamond, sapphire (A1 ₂ 0 ₃ ), titanium dioxide, cubic zirconia, zinc oxide, silicon, germanium, gallium phosphide, cerium oxide and gallium arsenide or the like.

The microsphere and lenses in the radiation pathway between the sample and the collection apparatus are preferably substantially optically clear. This ensures minimum attenuation of the desired scattering signal. Whilst it is preferable that these components substantially do not absorb or scatter radiation from the illumination source, in practice, due to the illumination being focussed on the sample, any such signal is very weak compared to the signal form the sample.

The microsphere lens may be mounted directly to the base lens. In alternative embodiments, the microsphere lens may be provided on an attachment for mounting to the base lens or a housing for the base lens.

In embodiments where the microsphere lens is mounted directly to the base lens this may be achieved by use of a column of optically clear material extending from a front surface of the base lens to the microsphere lens. Such an arrangement is described in detail in our co-pending application GB1710324.3.

The optically clear material may comprise glass or a suitable plastic. In such embodiments, the optically clear material may be performed into a column by a suitable technique including but not limited to moulding or machining. In such embodiments, the preformed column may be affixed to the base lens and the microsphere lens by suitable adhesive. Suitable adhesives may include NOA 81, MY- 132, MY132A, NOA63, PDMS, PMMA based adhesives or the like.

The choice of adhesive may depend on various structural or material considerations. The choice of adhesive may depend on the microsphere lens or base lens material. The choice of adhesive may depend on the wavelength of the illumination source that is to be used. For example, an adhesive that is suitable for use at visible wavelengths may not be suitable at IR or UV wavelengths, and vice versa.

The optically clear material may comprise an adhesive or resin. Preferably, the optically clear material is UV curable. In the case that the optically clear material is an adhesive it may comprise an adhesive such as NOA 81, MY- 132, MY132A NOA63, PDMS, PMMA based adhesives or the like.

The geometry of the column of optically clear material is determined by the relative optical properties and dimensions of the microsphere lens and base lens. In particular, the geometry of the column of optically clear material is selected such that the microsphere lens as a whole is capable of focussing light from a sample for imaging or focussing light on a sample for machining purposes. In preferred embodiments, the column extends from the edges of the base lens to the edges of the microsphere lens. As the microsphere lens is typically narrower than the base lens, the column may be a tapered column. The tapering of the column may be constant or may be variable.

In embodiments where the microsphere lens is provided on an attachment for mounting to the base lens or a housing for the base lens this may be achieved by use of an attachment comprising: a cap locatable in relation to an outer housing of the base lens; a support sheet affixed to the cap; an adhesive layer provided on the support sheet; and a microsphere lens affixed to the support sheet by the adhesive layer, the microsphere lens aligned to the optical axis of the base lens. Such an arrangement is described in detail in our co-pending application PCT/GB2017/052060.

The cap may comprise a substantially tubular body and a top. The cap may be releasably attachable to the housing of the objective lens. Releasable attachment may be facilitated by provision of an attachment formation on the inner surface of the body. The relative displacement of the cap from the objective lens along the optical axis may be adjustable. In embodiments wherein the relative displacement of the cap from the objective lens along the optical axis is adjustable, a graded index optical element may be provided between the support sheet and the objective lens.

Where the objective lens is intended for immersion in fluid during use, the cap may be provided with a seal so as to retain said fluid between the support sheet and the objective lens. In such embodiments, the cap may be provided with valve allowing for the introduction of fluid between the cap and objective lens and/or the removal of fluid between the cap and objective lens. The top of the cap may be provided with an aperture. The cap aperture may be provided within a surrounding contact surface. The support sheet may be affixed to the contact surface. The support sheet may be fitted to the cap by an adhesive. The cap and recess may be such that the support sheet either abuts or is substantially adjacent to at least part of the surface of the objective lens.

The support sheet may be formed from any suitable transparent material. In one embodiment, the support sheet is formed from glass. In alternative embodiments, the support sheet may comprise an alternative transparent material such as Poly (methyl methacrylate) (PMMA), Polydimethylsiloxane (PMDS) or the like. The support sheet may have a thickness in the range of 50 - 200 μπι. In one embodiment, the support sheet may have a thickness in the range 80-100μπι. In particular, the support sheet may have a thickness of around 100 μπι.

The adhesive layer may comprise an optical adhesive applied to the spacing layer and spun to a desired thickness. In such embodiments, the adhesive layer may comprise a UV curable adhesive such as NO A 81, MY- 132, MY132A NOA63, PDMS, PMMA based adhesives or the like. Alternatively, the adhesive layer may comprise an optically clear double sided adhesive tape of a known thickness applied to the spacing layer. Suitable adhesive tapes include but are not limited to OCA8146- 2, OCA8146-3 or the like. The adhesive layer may have a thickness in the range 30 -150 μπι. In one embodiment, the adhesive layer may have a thickness in the range 50-75 μπι. In particular, the adhesive layer may have a thickness of around 75μπι. The attachment may comprise a surface coating layer. The surface coating may be applied over the adhesive layer and the microsphere lens. In one embodiment, the surface coating layer is an adhesive. In such embodiments, the surface coating layer may comprise a UV curable adhesive such as NOA 81, MY- 132, MY132A NOA63, PDMS, PMMA based adhesives or the like. In another embodiment, the surface coating layer may be metallic. A combination of the two may be used.

The surface coating layer may be significantly thinner than the adhesive layer. In one embodiment, the surface coating layer may have a thickness in the range 1 nm- 20μπι. In particular, the surface coating layer may have a thickness of around 5-10 nm μπι in case of a metallic coating. In another embodiment, the surface coating layer thickness is around 5-20 μπι in case of the use of UV curable adhesive.

Detailed Description of the Invention

In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a schematic illustration of an apparatus for microsphere-enhanced spectroscopy according to the present invention;

Figure 2 shows an embodiment of a microsphere lens assembly for use in the present invention; Figure 3 shows an alternative embodiment of a microsphere lens assembly for use in the present invention; and Figure 4 shows a further alternative embodiment of a microsphere lens assembly for use in the present invention.

Turning now to figure 1, an apparatus 100 for carrying out microsphere- enhanced spectroscopy, in this particular example Raman spectroscopy, on a sample 15 is shown. The sample 15 is mounted on an XYZ scanning stage, enabling the sample 15 to be scanned relative to the apparatus 100. This enables illumination and subsequent collection of radiation to take place over a wide area of the sample. The skilled man will appreciate that in alternative arrangements, the sample 15 may be fixed in position and active elements of the apparatus 100 may be scanned past the sample.

Illumination of the sample 15 is provided by an illumination laser 11 operable to output a beam of illumination radiation at a desired wavelength. The beam passes through an ND filter 10 to a dichroic mirror 4 and is thus directed into the objective lens assembly 2 of a microscope. Optionally, an adjustable pinhole 3 may be provided between the dichroic mirror 4 and the objective lens assembly 2.

In the example shown in figure 1, a microsphere 1 is attached to the front lens of the objective lens assembly 2 by a column 13 of optically clear material, thus defining a microsphere lens assembly 20. This allows the illumination radiation to be focused on the sample 15. Optionally, as is shown in figure 1, a fluid 14 such as oil or water can be applied between the sample 15 and the microsphere 1.

Scattered radiation from the illuminated area is collected via the microsphere 1 and the objective lens assembly 2. The scattered radiation passes through the objective lens assembly 2, optional adjustable pinhole 3 and dichroic mirror 4. Subsequently, the scattered radiation is directed to a notch filter 8. The filter 8 is operable to block the passage of elastic scattered radiation (Rayleigh scattering) at the wavelength corresponding to the illumination radiation.

In the example shown, the scattered radiation is directed by a mirror 9. The skilled man will of course appreciate that alternative arrangements may alternatively be used to direct this radiation as required.

The filtered scattered radiation is incident upon an optical grating 6. Optionally, a shutter 5 may be interposed between the filter 8 and the grating 6.

Scattered radiation from the grating 6 is then incident on a detector. The scattered radiation at the detector 7 will include radiation resulting from Raman scattering by molecules within the sample 15.

The provision of the microsphere 1 on a column 13 of optically clear material ensures that it can be retained in a desired alignment and at a desired separation from the front lens of the objective lens assembly 2. This allows the alignment and separation to be maintained at optimal values whilst conducting Raman spectroscopy.

The fixed relation between the microsphere 1 and objective lens assembly 2 also allows for the sample 15 to be scanned relative to the microsphere 1 and the objective lens assembly 2. This can allow the detector 7 to build up an image of Raman scattering across the full scanned area of the sample 15 rather than being limited to sampling discrete points within the sample 15.

The fixed relation between the microsphere 1 and objective lens assembly 2 also allows for the separation between the microsphere 1 and the sample 15 to be varied. This can enable the effective focus of the microsphere 1 to be adjusted. Accordingly, scattered radiation can be collected from the surface of the sample 15 or from layers of the sample 15 beneath the surface.

Turning now to figure 2, an embodiment of a microsphere lens assembly 20 for use in the present invention is shown in more detail. The microsphere lens assembly 20 comprises a microsphere lens 1 and a base lens 21 connected together by a column 13 of optically clear material. In the example shown in figure 1, the microsphere lens 1 is in the form of a single microsphere. The column of optically clear material 13, holds the microsphere lens 1 in a fixed position relative to the base lens 21 For practical implementations, the base lens 21 either comprises the objective lens of a microscope or the front lens of an objective lens arrangement 2. In a preferred embodiment, the base lens 21 is a removable front lens of an objective lens assembly 2, as indicated in dotted lines in figure 2. Such a lens assembly 2 typically comprises a series of selected lenses fitted at fixed separation with relation to one another within a housing, the housing provided with a releasably attachable front lens mount. This enables the front lens 21 to be removed for cleaning and/or for the front lens 21 to be replaced in the event that it is damaged.

Turning now to figure 3, another embodiment of a microsphere lens assembly 20 for use in the present invention is shown in more detail. The microsphere lens assembly 20 of figure 3 differs from that of figure 2 in that the microsphere lens 1 is a truncated microsphere.

The assembly 20 of figure 3 can be effectively utilised with a shorter separation between sample and the microsphere lens 1 than the assembly 20 of figure 2. The reduction in separation distance is related to the amount of truncation of the microsphere.

Turning now to figure 4, a further embodiment of a microsphere lens assembly 20 for use in the present invention is shown in more detail. In this embodiment, the assembly 20 comprises an attachment for the exterior housing 40 of the objective lens assembly 2, operable to position the microsphere lens 1 between the objective lens assembly 2 and sample 15 at a desired alignment and separation.

The objective lens attachment 20 comprises a cap 34 having a substantially tubular body 35 and a top 36. The housing 40 is provided with an end section 43 within which is mounted the objective lens 2. The housing 40 is provided with a rotary adjustment collar 42, enabling adjustment of the objective lens position relative to the microscope body and hence the focus of the objective lens 2. The housing 40 also comprises a fitting 41 for enabling secure attachment of the housing 40 to the microscope body. Where the objective lens assembly 2 is designed in use to be immersed in a fluid, the cap 34 may be adapted to retain fluid. In particular, the interior of the tubular body 35 may be provided with a seal to help retain said fluid and/or a valve facilitating the introduction or removal of said fluid.

The top 36 is provided with a contact surface 37 surrounding an aperture (not shown), the aperture being aligned with the axis of the objective lens assembly 2 when the cap 34 is fitted to the objective lens housing 40. A support sheet 31 formed from optically clear material is affixed to the cap 34 by use of adhesive provided on the contact surface 37. Upon at least the centre of the support sheet 31 is provided an adhesive layer 32, which may be an optical adhesive or an optically clear adhesive tape. Affixed to the adhesive layer 32 is a microsphere lens 1.

In the example shown, the microsphere lens 1 is in the form of a single microsphere. The skilled man will of course appreciate that the microsphere may be replaced by a truncated microsphere if desired or if required.

The microsphere lens 1 is aligned with the optical axis of the objective lens 2. In order to further secure the microsphere lens 1 in position, a surface coating layer (not shown) may be applied over the microsphere lens, the adhesive layer 32 and the support sheet 31. The surface coating layer may comprise an optical adhesive. The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.