Optical module, set of optical modules, method of designing an optical module, and method of performing spectroscopy

Abstract

The disclosure relates to optical modules for spectroscopy. In one arrangement, a transmission body (4) is provided that defines a sensing surface (10) configured to contact a sample (8) during a spectroscopic measurement of the sample (8) by attenuated total reflection using optical radiation (13). The sensing surface (10) comprises a meta-surface structure (16) configured to apply a wavelength-selective enhancement of absorption of the radiation (13) by the sample (8) during the measurement.

Description

[0001] OPTICAL MODULE, SET OF OPTICAL MODULES, METHOD OF DESIGNING AN OPTICAL MODULE, AND METHOD OF PERFORMING SPECTROSCOPY

[0002] The present disclosure relates to optical modules for spectroscopy and is particularly applicable to attenuated total reflection (ATR) spectroscopy.

[0003] ATR spectroscopy is a powerful analytical technique used to investigate the composition, structure, and properties of materials based on their interaction with infrared light. It can be applied to a wide range of samples, including solids, liquids, and thin films, without the need for extensive sample preparation. The technique finds applications in a range of fields, including chemistry, materials science, pharmaceuticals, forensics, and biochemistry. The technique enables researchers and analysts to gather valuable information about the molecular composition and properties of diverse substances.

[0004] ATR spectroscopy typically uses an internal reflection element (IRE) made from a material with a high refractive index, such as diamond, germanium, silicon or zinc selenide. The IRE is in direct contact with the sample being analysed. When infrared light is directed into the IRE at a specific angle, the light undergoes internal reflections within the material due to the difference in refractive indices between the IRE and the sample. The infrared light interacts with the sample by penetrating to a certain depth, known as the penetration depth, which can vary depending on the wavelength of light and the refractive indices of the materials involved. During this interaction, the infrared light is absorbed by the sample's chemical bonds, causing molecular vibrations that are characteristic of the sample's composition.

[0005] The depth of penetration of the infrared light into the sample allows for the analysis of the surface and subsurface layers, providing information about molecular structure, functional groups, and chemical composition. The intensity and frequencies of the absorbed infrared light are measured, resulting in an infrared spectrum that serves as a unique "fingerprint" for the sample.

[0006] IREs made from high refractive index materials can be costly and difficult to clean, which can impede widespread use and accessibility of spectroscopy using such elements. A solution to this challenge was proposed by Schumacher, H., Kunzelmann, U., Vasilev, B., Eichhorn, K. J. and Bartha, J. W., “Applications of microstructured silicon wafers as internal reflection elements in attenuated total reflection Fourier transform infrared spectroscopy,” Appl. Spectrosc. 64(9), 1022-1027 (2010), who introduced an IRE based on a microstructured (100) silicon wafer featuring v-shaped grooves on a side opposite to the sample. The grooves define facets that facilitate coupling of infrared light into and out of the IRE. The facets promote single-reflection ATR measurements on the sample side of the wafer.

[0007] DE102004045902B4 discloses an approach that refines the coupling of light into the IRE by guiding radiation through a thin, higher refractive index layer. The disclosed method augments the number of reflections per unit length within the IRE, thereby increasing sensitivity and allowing for shorter sensor lengths.

[0008] P. Huber-Walchli and H. H. Giinthard, “Interference Enhanced attenuated total reflection (IEATR). A new technique for i.r. matrix spectroscopy with high sensitivity,” Spectrochim. Acta Part A Mol. Spectrosc., vol. 34, no. 12, pp. 1253-1262, 1978, discloses that a thin layer of medium with refractive index n2, with condition ni>n2>ns, where m is the refractive index of an ATR crystal and ns is the refractive index of the surrounding medium, can enhance the sensitivity and reduce the amount of sample required to do the spectroscopy.

[0009] US10585040B2 discloses structuring of a sample-facing side of an IRE with micropillars, establishing an effective refractive medium with refractive index between that of the IRE and surrounding medium, which increases sensitivity.

[0010] It is an object of the present disclosure to provide further improvements in optical modules for spectroscopy and associated methods.

[0011] According to an aspect of the disclosure, there is provided an optical module for spectroscopy, comprising: a transmission body defining a sensing surface configured to contact a sample during spectroscopy of the sample using optical radiation, wherein: the sensing surface comprises a meta-surface structure configured to apply a wavelength- selective enhancement of absorption of the radiation by the sample during the spectroscopy.

[0012] The provision of a meta-surface structure that applies wavelength- selective enhancement makes it possible to improve measurement sensitivity and is particularly effective when the wavelength-selective enhancement is selected based on expected absorbance characteristics of entities of interest in the sample and / or other relevant sample characteristics. This allows high quality spectroscopy results to be obtained in a range of applications without excessively complex apparatus and / or measurement protocols.

[0013] Optionally, the module is configured to perform attenuated total reflection (ATR) spectroscopy. The enhancement of absorption may comprise enhancement of absorption of an evanescent field of radiation undergoing attenuated total reflection in the transmission body at the sensing surface. The meta-surface structure has been found to provide particularly effective enhancement for ATR spectroscopy.

[0014] Optionally, the meta-surface structure is configured to define the wavelength- selective nature of the enhancement of absorption predominantly via an influence of the meta-surface structure on Mie scattering of the radiation. Dimensioning features of the meta-surface structure to provide enhancement primarily via Mie scattering has been found to be particularly effective for providing useful wavelength-selective enhancement.

[0015] Optionally, the meta-surface structure comprises a plurality of protrusions extending along respective axes away from a local plane of the sensing surface. Providing a meta-surface structure comprising such protrusions has been demonstrated to provide strong wavelength-selective enhancement. The effects of protrusions on enhancement can also be simulated relatively easily and accurately, which facilitates design of meta-surface structures based on such elements (e.g., to choose spatial distributions and / or geometrical properties of the protrusions). The inventors have demonstrated how the wavelength- selective enhancement provided by a meta-surface structure comprising protrusions can be tuned effectively by varying parameters such as the protrusion heights, protrusion radii / diameters and / or protrusion separations.

[0016] Optionally, for each of at least a subset of the plurality of protrusions, an area of a first cross-section at a first height of the protrusion is greater than an area of a second cross-section at a second height of the protrusion. The first height is greater than the second height, areas of cross-sections vary smoothly between the first height and the second height, and cross-sections are perpendicular to the axis of the protrusion extending away from the local plane of the sensing surface. Optionally, an area of a third crosssection at a third height of the protrusion is greater than an area of the second crosssection. The third height is lower than the second height and areas of the cross sections varies smoothly between the second height and the third height. The enhancement from protrusions having such shapes has been found to provide particularly strong enhancements.

[0017] Optionally, at least a subset of the plurality of protrusions form a periodic pattern. Periodic patterns of protrusions have been found to provide particularly strong enhancements. The enhancement from such patterns can also be simulated relatively easily and accurately to support design of meta-surface structures based on such patterns.

[0018] Optionally, the meta-surface structure comprises a plurality of zones configured to provide different respective wavelength-selective enhancement of absorption by the sample. Providing such a plurality of zones makes it possible to use a single optical module to perform spectroscopy having enhanced sensitivity for a plurality of different target entities in the sample.

[0019] Optionally, the meta-surface structure comprises a coating and a sub-structure, the coating having a higher refractive index than the sub-structure. The enhancement of Mie resonance based scattering efficiency is closely tied to the refractive index of the material involved. As the refractive index increases, so does the scattering efficiency. Providing a high refractive index coating on a lower refractive index sub-structure allows scattering efficiency of Mie resonances to be enhanced without requiring the whole meta-surface structure to be formed from a high refractive index material. This provides greater flexibility for selecting the high refractive index material, allowing for higher refractive indices to be achieved than would otherwise be easily possible without excessively complicating the fabrication process (e.g., by deviating from what is compatible with standard cleanroom fabrication techniques). The sub-structure may, for example, comprise silicon, which is easily subjected to a range of well developed, precise and reliable fabrication techniques to define the shape of the meta-surface structure, while the coating can comprise a different material having a higher refractive index than silicon and that only needs to be provided as a coating (e.g., of uniform depth that does not need to define any geometrical features such as protrusions).

[0020] According to an alternative aspect of the disclosure, there is provided a set of optical modules for spectroscopy, wherein: the set comprises at least a first optical module and a second optical module; the first optical module comprises: a first transmission body defining a first sensing surface configured to contact a first sample during spectroscopy of the sample using optical radiation, wherein: the first sensing surface comprises a first metasurface structure configured to apply a first wavelength-selective enhancement of absorption of the radiation by the sample during the spectroscopy; the second optical module comprises a second transmission body defining a second sensing surface configured to contact the first sample or a second sample during spectroscopy of the sample using optical radiation, wherein: the second sensing surface comprises a second meta-surface structure configured to apply a second wavelength-selective enhancement of absorption of the radiation by the first sample or the second sample during the spectroscopy; and the first wavelength-selective enhancement has a different wavelength profile to the second wavelength-selective enhancement.

[0021] According to an alternative aspect of the disclosure, there is provided a method of designing an optical module for performing spectroscopy on a sample, wherein: the optical module comprises a transmission body defining a sensing surface configured to contact the sample during the spectroscopy of the sample; the sensing surface comprises a metasurface structure configured to apply a wavelength- selective enhancement of absorption of the radiation by the sample during the spectroscopy; and the method comprises selecting one or more parameters defining a shape of the meta-surface structure based on one or more expected absorption properties of one or more entities of interest in the sample.

[0022] Embodiments of the disclosure will be further described by way of example only with reference to the accompanying drawings.

[0023] Figure 1 schematic depicts an example optical module for spectroscopy. Figure 2 depicts an example optical module with prism structures.

[0024] Figure 3 depicts a further example optical module with prism structures.

[0025] Figure 4 is a schematic side sectional view of a portion of a meta-surface structure comprising a row of protrusions.

[0026] Figure 5 is a top view of a portion of the meta-surface structure of Figure 4. Figures 6-8 depict example protrusion cross-sections.

[0027] Figure 9 is a top view of an example meta-surface structure comprising zones providing different wavelength-selective enhancements.

[0028] Figure 10 is a sectional view of the plane marked X-X in Figure 9. Figure 11 is a top view of a further example of a portion of a meta-surface structure comprising protrusions having circular cross-sections of different diameter.

[0029] Figure 12 is a top view of a unit cell of a further example of a meta-surface structure comprising protrusions having obliquely aligned elongate cross-sections.

[0030] Figure 13 depicts an example set of optical modules.

[0031] Figure 14 is a graph showing the result of simulations of interaction between radiation and cylindrical protrusions of height 1.5 pm and different radii (see insert). The simulated scattering spectra show magnetic dipole (MD), electric dipole (ED) and magnetic quadrupole (MQ) resonances.

[0032] Figure 15 is a graph showing the result of simulations of interaction between radiation and protrusions of radius 1 pm and different heights.

[0033] Figure 16 is an intensity plot showing the result of further simulations of interaction between radiation and a protrusion.

[0034] Figure 17 is a graph showing a variation of the simulated scattering cross-section of a protrusion as a function of wavelength of radiation.

[0035] Figure 18 is a graph showing a variation of the simulated absorption enhancement factor associated with a protrusion as a function of wavelength of radiation.

[0036] Figure 19 shows the results of simulations demonstrating how the reflectivity of an array of protrusion of Figures 16-18 can be tuned by varying the radii of protrusions. The reflectance was calculated at angle of incidence of 30° to emulate ATR configuration of the device.

[0037] Figures 20 and 21 are intensity plots showing the results of simulations of an electric field respectively for a silicon protrusion with no coating and a silicon protrusion with a coating of Ge.

[0038] Figure 22 is a graph showing simulated reflectance and measured reflected intensity of a module having an array of protrusions of radius 1.25 pm. The measured intensity is not normalized.

[0039] Figure 23 is a graph showing the results of absorbance measurements of sodium acetate on modules with and without a meta-surface structure.

[0040] Figure 24 is a graph comparing absorbance and transmitted intensity of a module with a meta-surface structure. Figure 25 is a graph showing the absorbance of sodium acetate measured using modules with cylindrical protrusions of different radius at a fixed height of 1.5 pm.

[0041] Figure 26 is a graph showing the absorbance of sodium acetate measured using modules having protrusions of different heights, r is the radius of protrusion, d is the separation between protrusion and h, height of protrusions.

[0042] Figure 27 is a graph showing the absorbance of sodium acetate measured using modules having protrusions with different separations. The radius of the protrusion was 1.25 pm and the height of the protrusions was 1.5 pm.

[0043] Figure 28 schematically depicts an example optical module for spectroscopy comprising a coating.

[0044] Figure 29 schematically depicts an example optical module for spectroscopy comprising a coating including a cage molecule.

[0045] Figure 30 is a graph comparing the absorbance of silicon pillars with and without a coating of Ge.

[0046] Figure 31 is an SEM image of a portion of an example optical module for spectroscopy having a pillar dual layer structure comprising Si and Ge.

[0047] Figure 32 is a graph showing absorbance spectra recorded using various optical modules for spectroscopy according to the disclosure, and compared to a reference.

[0048] Figure 33 are intensity plots showing the results of simulations of an electric field respectively for protrusions of varying shapes according to the disclosure.

[0049] Figure 34 is a graph comparing the results of simulations of absorption enhancement factors for protrusions of varying shapes according to the disclosure compared to a flat sensing surface for different wavelengths.

[0050] Figure 35 is a graph comparing the absorbance for protrusions of varying shapes according to the disclosure compared to a flat sensing surface for different wavenumbers.

[0051] The present disclosure provides an optical module for performing spectroscopy and associated spectroscopy methods and methods of designing an optical module. The spectroscopy may comprise any transmission or reflection spectroscopy. The described approach is particularly well suited to attenuated total reflection (ATR) spectroscopy and is exemplified below mainly in this context.

[0052] Figure 1 schematically depicts an optical module 2 according to the disclosure. The optical module 2 is suitable for use in a spectroscopy method. The spectroscopy may comprise reflection spectroscopy. Alternatively, or additionally, the spectroscopy may comprise transmission spectroscopy. In one class of implementation, exemplified in detail below, the spectroscopy comprises ATR spectroscopy.

[0053] The optical module 2 comprises a transmission body 4. The transmission body 4 defines a sensing surface 6. In the example shown, the sensing surface 6 is an upper surface of the transmission body 4 in the orientation of the figure. Apart from surface structure or texture, the transmission body 4 may be substantially planar, although other general shapes could be used. The optical module 2 may further comprise a light source 12 configured to direct radiation into the transmission body 4. The radiation may be optical radiation, which is understood to encompass electromagnetic radiation in the IR, visible or UV range. The light source 12 may comprise any suitable arrangement for providing suitable radiation to the transmission body 4. The optical module 2 may further comprise a detection arrangement 14 configured to detect radiation output from the transmission body 4 after interaction with the sample 8. Any detection arrangement 14 capable of detecting the radiation in a way that allows the spectroscopic information to be extracted suitably accurately may be used.

[0054] In embodiments in which the module 2 is configured to perform ATR spectroscopy, the enhancement of absorption may comprise enhancement of absorption of an evanescent field of radiation undergoing ATR in the transmission body 4 at the sensing surface 6. The transmission body 4 may comprise an input surface 5. The input surface 5 may be configured to allow radiation to enter the transmission body 4 through the input surface 5 and undergo attenuated total reflection in the transmission body 4. The transmission body 4 further comprises an output surface 7. The output surface 7 and the input surface 5 may be different portions (separated from each other or overlapping) of the same surface. The output surface 7 is configured to allow the radiation to exit the transmission body 4 through the output surface 7 after undergoing the attenuated total reflection in the transmission body 4. The light source 12 may be configured to direct radiation onto the input surface 5. The detection arrangement 14 may be configured to detect radiation output from the output surface 7.

[0055] As exemplified in Figures 2 and 3, in some implementations the input surface 5 comprises a plurality of prism structures 40. The prism structures 40 may be configured to promote ATR in the transmission body 4. The prism structures 40 may be formed, for example, by wet etching of a substrate, such as a silicon substrate (e.g., a 100 silicon wafer or a 110 silicon wafer). Figure 2 depicts an example configuration in which individual prism structures 40 are relatively widely spaced. Figure 3 depicts a variation on the configuration of Figure 2 in which individual prism structures 40 are more closely spaced. Arrows labelled 13 and 15 schematically depict example trajectories of radiation into and out of the transmission body 4. Feature 17 schematically depicts an evanescent field penetrating into a sample region above the sensing surface 6.

[0056] The sensing surface 6 is configured to contact a sample 8 to be measured (e.g., during spectroscopy of the sample using optical radiation). The sensing surface 6 comprises a meta-surface structure 10. In Figure 1, the meta-surface structure 10 is depicted schematically as a solid layer for ease of illustration. The meta-surface structure 10 is configured to apply a wavelength-selective enhancement of absorption of radiation by the sample 8 during the spectroscopy. The inventors have demonstrated that a wide range of wavelength-selective enhancements are possible. The meta-surface structure 10 can be configured in different ways to provide enhancements that are suitable for different practical situations, for example to obtain information about different entities of interest in the sample 8 and / or to provide optimal results with different types of sample 8. Typically, the wavelength-selective enhancement will comprise an enhancement in one or more wavelength ranges in the range of about 2.5 micron to about 20 micron, although enhancements outside of this range may also be used. Enhancements having a range of different wavelength dependencies may be achieved, but typically the enhancement may include a wavelength dependence having a peak with a Q factor in the range of about 9 to about 15 for protrusions 16 having forms such as those shown in Figures 6-11 or similar. Higher Q factors may, however, be achieved, for example using protrusions 16 configured as shown in Figure 12.

[0057] In some implementations, as exemplified in Figures 2-12, the meta-surface structure 10 comprises a plurality of protrusions 16. The protrusions 16 are configured to provide the wavelength-selective enhancement of absorption and may take various forms and / or spatial relationships relative to each other in order to achieve this. The protrusions 16 may be cylindrical. Further details about example forms and spatial relationships are described below.

[0058] Referring to Figure 4, which is a schematic side sectional view through a row of example protrusions 16, the protrusions 16 may extend along respective axes 18 away from a local plane 20 of the sensing surface 6. An axial length of a protrusion 16 may be referred to as a height of a protrusion 16 (an example of a height is illustrated by the arrow labelled 25 in Figure 4). Unless stated to the contrary, any reference to a cross-section of a protrusion herein is intended to refer to a cross-section perpendicular to the axis along which the protrusion extends away from the local plane 20 of the sensing surface 6. The axis along which the protrusion extends away from the local plane 20 of the sensing surface 6 will typically be perpendicular to the local plane 20 of the sensing surface 6. The axis will typically be parallel to a height direction. A reference to the height direction may thus be understood as being equivalent to referring to an axial direction. The cross-sections represent views of notional slices through the protrusions 16 viewed along the height direction.

[0059] The meta-surface structure 10 may be manufactured in various ways. For example, protrusions 16 may be formed using reactive ion etching. For example, a layer of SiCh may be formed on the surface of a silicon wafer by oxidizing the wafer. A pattern defining the protrusions 16 may be transferred to the SiCh using photolithography. A subsequent etching step may selectively remove the SiCh in regions outside of desired locations of the protrusions 16. Remaining photoresist may be removed by etching, for example plasma etching. The protrusions 16 are formed by etching away material outside of the protrusions with the remaining SiCh acting as a hard mask. The height of the protrusions 16 is determined by the etching parameters. A longer etching time, for example, will typically provide protrusions 16 having a larger height. The SiCh layer may be removed at the end by etching, such as HF etching.

[0060] A meta-surface structure 10 (or a portion thereof) manufactured using the method above, or other methods where the meta-surface structure 10 is formed by removal of material from a substrate of the sensing surface, may be considered as being integrally formed with a substrate of the sensing surface, e.g. the meta-surface 10 and the substrate are formed from a single unitary material with no join between. The meta-surface structure 10 may be configured to define the wavelength- selective nature of the enhancement of absorption predominantly via an influence of the meta-surface structure 10 on Mie scattering of the radiation. This effect may be achieved by arranging the protrusions 16 to have a lateral size that is larger than a predetermined threshold size. For example, the protrusions 16 may be configured so that each protrusion 16 in at least a subset of the protrusions 16 has an average minimum lateral dimension (e.g., diameter for a cylindrical protrusion 16), averaged over the height (which may also be referred to as axial length) of the protrusion 16, that is greater than a threshold dimension, optionally greater than 1 micron.

[0061] The protrusions 16 may comprise individual localized features. For example, as illustrated in Figure 5, which is a top view of a portion of a meta-surface structure 10 containing the row of protrusions 16 depicted in Figure 4, each of at least a subset of the protrusions 16 may be surrounded by a recess 22 (see Figure 4) forming a closed loop 24 (indicated by the example broken line forming a closed loop 24 around an example one of the protrusions 16 in Figure 5).

[0062] The cross-sectional shape of each protrusion 16 may take various forms. For example, a cross-sectional shape of the protrusion 16 may be substantially circular, or substantially form a regular polygon, over at least a portion of the height of the protrusion 16. Non-limited examples of possible cross-sections are depicted in Figures 6-8. Figure 6 depicts a protrusion 16 having a circular cross-section. Figure 7 depicts a protrusion 16 having a square cross-section. Figure 8 depicts a protrusion 16 having an oval or elliptical cross-section. The cross-section may be uniform along the whole height (e.g., to form a cylinder) or may vary (e.g., to have a frustoconical form). For example, in some implementations, an area of the cross-section at a first height of each protrusion 16 may be greater than an area of the cross-section at a second height of each protrusion 16, where the first height is greater than the second height (e.g., the area of the cross section increases with increasing height). In such implementations, the area of the cross section may vary smoothly between the first height and the second height. In other words, the profile of each protrusion 16 may not be substantially stepped, and may form a trapezoidal or “necked” profile. This can be referred to as the protrusion 16 having negative side walls. In some implementations, an area of the cross-section at a third height of the protrusion 16 is greater than an area of the cross-section at the second height, wherein the third height is lower than the second height and the area of the cross section varies smoothly between the second height and the third height. In other words, the profile of each protrusion 16 may take on an hourglass-like shape. The variation of the cross-section of the protrusion 16 may take place over a portion of the height of each protrusion 16, or over substantially all of the height of each protrusion 16. For example, the protrusion 16 may have one or more portions where the cross section varies with height, and one or more portions where the cross section is constant with height. The variation of cross-section with height (e.g., the gradient of the wall of the protrusion 16) may be the same for each protrusion 16, or may vary between one or more subsets of protrusions 16. Performance of different shapes of protrusion 16 are discussed below with reference to Figures 33 to 35.

[0063] In one class of implementation, the protrusions 16 are relatively localized in all directions in plan view (e.g., in the view of Figure 5). For example, each of at least a subset of the protrusions 16 may be configured such that an average aspect ratio of a cross-section of the protrusion 16, averaged over the height of the protrusion 16, is less than 4, optionally less than 3, optionally less than 2, optionally less than 1.5, optionally less than 1.1. The aspect ratio may be defined as the ratio between a longest dimension 31 of a cross-section and a shortest dimension 32 of a cross-section, as labelled for illustrative purposes in Figure 8. The inventors have found that such localized features provide improved enhancement relative to more elongate features such as ridges that extend across a large portion of the sensing surface 6.

[0064] The height of each protrusion 16 is preferably in the range of about 0.5 micron to about 7 micron, although heights outside of this range may also be compatible with the desired functionality. The height will typically affect how sharply the enhancement is confined to a particular range of wavelengths. A larger height will typically be associated with an enhancement that is focussed within a narrower range of wavelengths than a smaller height and vice versa. Heights may therefore be selected based on expected absorption characteristics of entities of interest in the sample being measured. For example, where an entity of interest is known to absorb radiation in a relatively narrow band, it may be appropriate to configure protrusions 16 to have a relatively large height and focus the wavelength enhancement precisely on the expected absorption band. On the other hand, where a sample contains entities that are expected to absorb radiation over a wider range of wavelengths it may be desirable to use protrusions having a smaller height. In typical implementations, a ratio of a height of a protrusion to a maximum lateral dimension of a cross-section of the protrusion (e.g., the diameter of a circular protrusion) is in the range of about 0.5 to 5, optionally between about 1 and about 5, optionally between about 2 and about 5, optionally between about 2.5 and about 4.5.

[0065] At least a subset of the plurality of protrusions 16 are preferably provided in a periodic pattern. Providing protrusions 16 in a periodic pattern strengthens the desired wavelength-selective enhancement due to Mie scattering. The periodic pattern may be a one-dimensional periodic pattern (e.g., comprising protrusions 16 aligned with each other along an axis and regularly spaced along the axis). Alternatively, the periodic pattern may be a two-dimensional periodic pattern (e.g., comprising protrusions 16 aligned with each other and regularly spaced along multiple non-parallel axes).

[0066] In some implementations the plurality of protrusions 16 are provided in a regular grid. In such an arrangement, the protrusions 16 may be centred on respective grid points in a regular geometric grid, such as a square grid, a rectangular grid, or a hexagonal grid. The inventors have found that positioning protrusions 16 on a hexagonal grid provides particularly strong enhancement and is therefore preferred. Without wishing to be bound by theory, it is believed that the stronger enhancement may be caused by the higher number of electromagnetic hotspots available in hexagonal close-packing similar to the effects reported in plasmonic meta-surface as reported by ‘ G. Y. Yao and Z. Y. Zhao, “Exploring the modulation mechanism of the LSPR effect of Cu periodic nanosphere arrays to promote the performance of TiCh photoelectrodes,” Inorg. Chem. Front., vol. 9, no. 4, pp. 729-742, 2022, doi: 10.1039 / dlqiO 1430g’ .

[0067] Figure 5 shows an example of a plurality of protrusions 16 centred on respective grid points 34 of a square grid defined by vertical grid lines 36 and horizontal grid lines 38.

[0068] The pitch 40, 42 of the periodic pattern in at least one direction defining the periodicity (e.g., along an axis of a grid defining the periodicity, such as a square grid or a rectangular grid or a hexagonal grid) defines a range of wavelengths in which absorption is enhanced in the wavelength-selective enhancement. In the example of Figure 5, a direction defining a periodicity is the horizontal direction, which defines the pitch labelled 40, or the vertical direction, which defines the pitch labelled 42.

[0069] The pitch 40, 42 of the periodic pattern in a direction defining a periodicity may be in the range of about 0.5 micron to 7 micron. The pitch may be defined as the distance between geometrical centres of cross-sections of neighbouring protrusions 16 along the direction defining the periodicity. The cross-sections may be averaged cross-sections (averaged over the height of the protrusions 16) in cases where the protrusions 16 have cross-sections that vary as a function of axial position (height), although typically the geometrical centre will stay in the same position by symmetry even where cross-sections vary as a function of height (e.g., to form a frusto-cone etc.).

[0070] The plurality of protrusions, whether provided in a periodic pattern or a nonperiodic pattern, may have an average separation in the range of about 0.5 micron to 7 micron.

[0071] In some implementations, the meta-surface structure 10 is configured to have a plurality of zones 40A-C. An example of such an implementation is depicted schematically in Figures 9 and 10. Figure 9 is a top view of the meta-surface structure 10. Figure 10 is a sectional view of the plane marked X-X in Figure 9. The zones 40A-C may be non-overlapping zones 40A-C (e.g., spatially separated or directly adjacent to each other). The zones 40A-C may be configured to provide different respective wavelength- selective enhancement of absorption by the sample 8. In the example shown, the different zones 40A-C comprise groups of protrusions 16 having different respective spatial configurations. The different spatial configurations provide the different wavelength- selective enhancements of absorption.

[0072] A range of different spatial configurations may be used to provide the different wavelength-selective enhancements.

[0073] In one class of implementation, the different spatial configurations comprise protrusions 16 in one group (e.g., the group in zone 40A of Figure 9) having different average heights than in another group (e.g., the group in zone 40B in Figure 9). In the example shown, the protrusions 16 in one group (in zone 40 A) all have substantially the same height as each other and the protrusions 16 in the other group (in zone 40B) all have substantially the same height as each other.

[0074] Alternatively, or additionally, the different spatial configurations may comprise different minimum or maximum lateral dimensions of protrusions 16, optionally averaged over heights of the protrusions (e.g., for protrusions 16 having cross-sections that vary as a function of height). Protrusions having different lateral dimensions are exemplified in Figure 9, where the protrusions 16 in the group in zone 40C have a larger diameter than the protrusions in the groups in zones 40 A and 40B.

[0075] Figure 11 depicts a further example of a portion of a meta-surface structure 10, in which protrusions 16 are arranged in one or more linear arrays. Each linear array comprises protrusions 16 having different maximum lateral dimensions. In the example shown, four such linear arrays are shown. Each linear array extends horizontally within the plane of the page. In the example shown, each protrusion 16 has a circular crosssection, such that the maximum lateral dimension is equal to the diameter of the crosssection. Each linear array may comprise protrusions that increase progressively in size along the array (i.e., having progressively increasingly maximum dimensions). In the example shown, the linear arrays alternate in the direction perpendicular to the arrays between an array in which the protrusions 16 increase in size progressively in one direction and an array in which the protrusions 16 increase in size progressively in the opposite direction. Configurations having protrusions 16 with such a range of sizes provides absorption enhancement over a wide range of wavelengths. Arranging such protrusions in linear arrays, particularly in alternating arrays as described above, facilitates efficient packing of the protrusions 16.

[0076] Figure 12 depicts a unit cell of a further example of a portion of a meta-surface structure 10, in which a plurality of obliquely aligned elongated protrusions 16 are provided in the unit cell. The dimensions of the unit cell are indicated schematically by double-ended arrows 71 and 72. The oblique alignment may be such that at least a subset of the protrusions 16 in the unit cell have long axes that are obliquely aligned relative to each other. In the particular example shown, the unit cell comprises two protrusions 16 with long axes 74 that are aligned at an angle a relative to each other. The angle a is oblique (i.e., between 0 and 90 degrees, not inclusive). Arrangements of this type have been found to provide particularly strong absorption enhancement.

[0077] Alternatively, or additionally, the different spatial configurations may comprise different aspect ratios of cross-sections of protrusions, optionally averaged over heights of the protrusions.

[0078] Alternatively, or additionally, the different spatial configurations may comprise different cross-sectional shapes of protrusions.

[0079] As an alternative or in addition to providing a meta-surface structure 10 having different zones, as discussed above, a range of different wavelength-selective enhancements may be achieved by providing a set of optical modules 2A-2B, as depicted schematically in Figure 13. Each optical module 2 in the set 60 may take any of the forms described herein for a single optical module.

[0080] The set 60 may comprise at least a first optical module 2A and a second optical module 2B. The set 60 may comprise more than two optical modules, such as three, four or five optical modules. The optical modules in the set may be mechanically separable from each other (i.e., non-integrally formed with respect to each other).

[0081] The first optical module 2A comprises a first transmission body 341 defining a first sensing surface 361 configured to contact a first sample 381 to be measured. The first sensing surface 361 comprises a first meta-surface structure 3101. The first meta-surface structure 3101 is configured to apply a first wavelength-selective enhancement of absorption of radiation by the first sample 381, such as absorption by the first sample 281 of an evanescent field of radiation undergoing attenuated total reflection in the first transmission body 341 at the first sensing surface 361.

[0082] The second optical module 2B comprises a second transmission body 342 defining a second sensing surface 362 configured to contact the first sample 381 or a second sample 382 to be measured. The second sensing surface 362 comprises a second meta-surface structure 3102. The second meta-surface structure 3102 is configured to apply a second wavelength-selective enhancement of absorption of radiation by the first sample 381 or the second sample 382, such as absorption by the first sample 381 or the second sample 382 of an evanescent field of radiation undergoing attenuated total reflection in the second transmission body 342 at the second sensing surface 362. The first wavelength- selective enhancement has a different wavelength profile to the second wavelength-selective enhancement. The differences in wavelength profile may for example comprise differences in number, wavelengths and / or widths of enhancement peaks.

[0083] The enhancement of Mie resonance based scattering efficiency is closely tied to the refractive index of the material involved. As the refractive index increases, so does the scattering efficiency.

[0084] In some implementations, the meta-surface structure 10 is provided in the form of a coating and a sub-structure. The coating may be a uniform coating, for example of substantially uniform thickness. The sub-structure will typically be much thicker than the coating, such as at least 2 times thicker, optionally at least 5 times thicker, optionally at least 10 times thicker. Typically, the sub-structure will predominantly or completely define a shape of the meta-surface structure 10. For example, in cases where the metasurface structure 10 comprises protrusions 16, the protrusions 16 may be defined predominantly or completely by the sub-structure and not by the coating. An example configuration of a coating 15 on a sub-structure 13 defining a protrusion 16 is depicted in Figure 21.

[0085] In some implementations, the sub-structure 13 is integrally formed with a substrate of the sensing surface, and the coating is formed separately. For example, the substructure may be formed by removal of material from the substrate (e.g., by using reactive ion etching), and the coating 15may be formed by a different method (e.g. by subsequent deposition onto the sub-structure).

[0086] The coating 15 has a higher refractive index than the sub-structure 13. Enhancement of the scattering efficiency of Mie resonances can thereby be achieved without requiring the whole meta-surface structure 10 to be formed from a high refractive index material. This may provide greater flexibility for selecting the high refractive index material, allowing for higher refractive indices to be achieved than would otherwise be easily possible without excessively complicating the fabrication process (e.g., by deviating from what is compatible with standard cleanroom fabrication techniques). The substructure 13 may, for example, comprise silicon, which is easily subjected to a range of well developed, precise and reliable fabrication techniques (e.g., lithography and etching etc.). The coating 15 may comprise one or more of the following: germanium; ZnSe; a metal; a 2D Van der Waals material. Differences in field enhancement for a Ge coated silicon protrusion 16 in comparison to an uncoated protrusion 16 are discussed below with reference to Figures 20, 21 and 30. The coating 15 is seen to provide a significantly increased field enhancement and / or absorbance. The coating may cover substantially all of the meta-surface, or may be limited to only part of the meta-surface. For example, in some implementations, the recess 22 surrounding each protrusion may be free of the coating. An example of this implementation is depicted by Figure 28. In the example of Figure 28, the coating 15 does not exist in the recesses 22 between the protrusions 16. In other words, the coating 15 only covers the tops of the protrusions of the sub-structure 13. In some implementations, however, the coating 15 may occupy at least a portion of the recess 22, e.g. the coating exists in at least a portion of the recess, and may not exist in at least another portion of the recess 22. For example, the coating may be such that the portion of the recess 22 is free of the coating forming a closed loop surrounding the protrusion. In other words, the protrusion is surrounded by an area of the recess in which the coating does not exist. In some implementations, the coating may exist in substantially the entire recess 22. In some implementations, the meta-surface structure 10 comprises, consists substantially of, or consists of, germanium. Germanium is compatible with fabrication techniques developed for processing silicon, thereby facilitating precision and reliability in the manufacturing process.

[0087] In some implementations, the meta-surface may comprise a Ge coating layer on top of an Si sub-structure. Figure 31 shows an implementation with pillars comprising Si with height 3.25 pm and radius 1.5 pm, and Ge coating of height 1 pm on top of that. In this case, the top layer may act as a confinement and signal enhancement layer. In the example illustrated by Figure 31, the Si sub-structure takes the form of pillars, however any shape or form of sub-structure described herein may be used. In the example illustrated by Figure 31, the Ge coating layer only exists on top of the Si pillars, but the coating layer may take any of the forms described herein. Si pillars (or other sub-structure forms) may be etched from an Si substrate, for example, or manufactured using a different method, and the Ge coating layer deposited onto the Si before or after the etching of the substrate. The performance of pillars with and without Ge coatings will be discussed later with reference to Figures 30 and 32.

[0088] In some implementations, the coating may comprise a cage molecule suspended in a host material. In such implementations, the substructure may be made of silicon. Figure 29 illustrates a cage molecule suspended in a host material as a coating of the meta- structure. In such implementations, a molecule of interest can be immobilised in a host material coated onto the meta-surface. Examples of the cage molecules include but not limited to zeolites, cryptophanes, cucurbiturils, calixarenes, metal-organic frameworks, and polyhexamethylene biguanide. The host materials can be of polymers, glasses, ceramics etc. The immobilization layer can be used to immobilise gaseous molecules, liquids, solids, biomolecules or in some cases entire cells.

[0089] In some implementations, an optical module 2 may be provided as part of a system for spectroscopy. A system for spectroscopy may include a spectroscopy apparatus configured to perform spectroscopy using the module and provide output data and a data processing system configured to analyse the output data to obtain information about one or more properties of a sample comprising biological material. For example, the properties may include the chemical composition, molecular structure, concentrations, reaction rates, conformations, strand length, spectral profiles, pH, isoelectric points, degrees of dissociation / crystallinity, and / or isomerism. The sample may be aqueous, and / or contain or comprise live biological entities. The obtained information may include information about the live entities. Such a system for spectroscopy would therefore be appropriate for biomedical applications, among other applications.

[0090] Methods of designing an optical module 2 for performing spectroscopy on a sample 8 may be provided. The optical module 2 may take any of the forms described above. Thus, the optical module 2 may comprise a transmission body 4. The transmission body 4 defines a sensing surface 6 configured to contact the sample 8 during the spectroscopy. The spectroscopy may be optical spectroscopy using optical radiation. The sensing surface 6 comprises a meta-surface structure 10 configured to apply a wavelength-selective enhancement of absorption of the radiation by the sample 8 during the spectroscopy.

[0091] The method of designing the module 2 comprises selecting one or more parameters defining a shape of the meta-surface structure 10 based on one or more expected absorption properties of one or more entities of interest in the sample 8. The one or more parameters defining the shape of the meta-surface structure 10 may define any of the features of the meta-surface structure 10 discussed above with reference to Figures 1-13. The expected absorption properties of the one or more entities of interest may comprise one or more respective expected variations of absorbance as a function of wavelength of radiation.

[0092] For example, the meta-surface structure 10 may comprise a plurality of protrusions 16. The plurality of protrusions 16 may take any of the forms discussed above. The one or more parameters selected in the method of designing the optical module 2 may define one or more of the following: lateral dimensions (e.g., diameters) of the protrusions; heights of the protrusions; separations between the protrusions; a pitch or pitches (e.g., along different axes) of a periodic pattern comprising the protrusions; and shapes (e.g., cross- sectional shapes) of the protrusions. As described in the section headed “DEMONSTRATIONS OF EFFECTS AND FURTHER EXPLANATIONS”, the inventors have demonstrated how the wavelength-selective enhancement can be tuned particularly effectively by varying protrusion radii (and therefore diameters), protrusion heights and protrusion separations.

[0093] Once the module 2 has been designed, the module 2 may be fabricated using any suitable fabrication technique. The module 2 fabricated according to the design may then be used to perform spectroscopy on the sample 8 for which the module 2 was designed.

[0094] DEMONSTRATIONS OF EFFECTS AND FURTHER EXPLANATIONS

[0095] Figure 14 is a graph showing simulated scattering spectra from a cylindrical protrusion 16 for different radii. Different resonance modes are observed for each of the different radii. The simulations yield scattering cross-sections (vertical axis) against wavelength (horizontal axis). The curves labelled 1050, 1075, 1100, 1150 and 1175 respectively correspond to protrusions 16 having radii of 500 nm, 750 nm, 1 pm, 1.5 pm and 1.75 pm. The simulated protrusion 16 had a height of 1.5 pm for each different radius. The spectra show magnetic dipole (MD), electric dipole (ED) and magnetic quadrupole (MQ) modes. As indicated by broken lines marked ED, MD, and MQ, changing the radius of the protrusion 16 leads to shifts in the positions of peaks in the spectrum. Thus, by selecting appropriate radii of protrusions 16 it is possible to design an absorption spectrum that is optimised for a particular application, for example to position absorption peaks at locations that will provide the most useful information when a sample is analysed using a module comprising the protrusions.

[0096] Similar effects are seen when other geometrical features of the protrusions are varied, such as by changing the heights and / or pitches of the protrusions. For example, Figure 15 shows the shift of resonances as a function of height of a simulated protrusion 16. The calculation shows the scattering spectrum of a protrusion 16 with radius of 1pm for protrusion heights of 500 nm, 1 pm, 1.5 pm, 2 pm, 2.5 pm, 3 pm and 4 pm indicated respectively by labels 2005, 2010, 2015, 2020, 2025, 2030, and 2040. This method of tuning the resonance is useful during fabrication where the resonance can be tuned continuously by controlling the height of protrusions during fabrication.

[0097] Figure 16 shows the result of further simulations of interactions between radiation and a cylindrical protrusion 16. Figure 16 is an intensity plot showing field enhancement as a function of position around a cylindrical protrusion 16. There is a strong electromagnetic field generated due to dielectric dipole resonances. The field enhancement induces an absorption enhancement factor, T] given by the following expression: where the integrals sum over a volume of space around the protrusion 16. A higher electric field in sample 8 means higher absorption enhancement factor, which means increased absorption of radiation.

[0098] To calculate the absorption enhancement factor, the inventors simulated the scattering spectrum of a protrusion 16 formed from silicon on a silicon substrate 13. A sample would be located in the region labelled 8. An excitation beam was sent through the substrate 13 at an angle 25° in the simulation. The background electric field was calculated without the protrusion 16 on top of the substrate 13. The scattering spectrum was then recalculated with the protrusion 16 present.

[0099] Figure 17 is a graph showing a variation of the simulated scattering cross-section of the protrusion 16 (vertical axis) as a function of wavelength of radiation (horizontal axis).

[0100] Figure 18 is a graph showing a variation of the simulated absorption enhancement factor associated with the protrusion 16 (vertical axis) as a function of wavelength of radiation (horizontal axis). The simulated absorption enhancement factor resembles the scattering spectrum.

[0101] Figure 19 shows the results of simulations demonstrating how the behaviour described above with reference to Figures 16-18 can be tuned by varying the radius of the protrusion 16. Colour bar indicates the reflectance. The tuning changes the reflectance spectrum associated with the protrusion 16, for example such that features in the spectrum (e.g., peaks) change position and / or width as a function of protrusion radius. Figure 19 shows an intensity plot (main; bottom left) of simulated total reflectivity as a function of protrusion radius. Grayscale levels towards the left generally correspond to lower enhancement and grayscale levels towards the right and bottom-right generally correspond to higher enhancement. For the purpose of the simulation, the protrusions had a uniform height of 1.5 pm. The upper sub-plot shows the estimated reflectivity as a function of wavelength for an example protrusion radius of 1.25pm. The rightmost sub-plot shows the estimated reflectivity as a function of protrusion radius for an example wavelength of 6.1pm. Protrusions 16 with radii in the range of about 1pm to 4pm can cover resonances in the entire mid-infrared region and therefore selectively enhance molecular fingerprints from a wide range of molecules of interest.

[0102] The sensitivity can be enhanced by applying thin layers of materials (coatings) to the meta-surface structure, such as materials having high refractive indices, such as Ge or ZnSe. Figures 20 and 21 show the results of simulations of the electric field respectively for a silicon protrusion 16 (defined by a sub-structure 13) with no coating and a silicon protrusion 16 (defined by a sub-structure 13) with a coating 15 of Ge (50 nm on the sidewalls and 100 nm on the top surface). The simulations demonstrate that the electric field is significantly enhanced around the coated protrusion (Figure 21) in comparison to the uncoated protrusion (Figure 20). Other choices of materials for thin layer coatings include metals to leverage plasmonic properties, and lean on plasmonic photonic hybrid resonances to enhance the signal further. Metal coatings may also support use of the metasurface structures in chips in spectroelectrochemical platforms, for example for monitoring electrochemical reactions. Alternatively or additionally, the coating may comprise a 2D Van der Waals material such as graphene, rGO or h-BN. The enhancement of absorption using such materials is known as Surface Enhanced Infrared Absorption (SEIRA). The enhancement absorption in SEIRA happens due to two mechanisms: (a) the electromagnetic enhancement, where the increase in electric field intensity at the vicinity of these materials occurs due to optical or plasmonic resonances, (b) chemical enhancement. The chemical enhancement is not clearly understood and less prominent compared to electromagnetic enhancement.

[0103] Measurements characterising modules 2 of the present disclosure are described below.

[0104] A characterization was performed by comparing absolute measured reflected power to simulated values. This characterization involves assessing the performance of the modules 2 by directly comparing the absolute power of reflected light from the actual modules 2 to values obtained through simulation models. Thus, measured spectra obtained from experiments are compared with spectra generated through simulations. This comparison aims to validate the accuracy of the simulation models by ensuring that the trends, peaks, or features observed in the measured spectra closely align with those predicted by the simulations. Similar trends between the measured and simulated spectra indicate a good agreement between the experimental and theoretical data, bolstering confidence in the simulation's predictive capabilities. Figure 22 shows the simulated and measured reflectivity (curves 201 and 202 respectively) of a module 2 having an array of protrusions 16 of radius 1.25pm. The measured reflectivity is in absolute voltage measured because a single beam measurement was used. The simulated and measured spectra show a similar trend.

[0105] A further characterization was performed by measuring absorbance of modules 2 of the disclosure. Absorption was determined using sodium acetate. The absorbance measurements were conducted on modules 2 with and without the meta-surface structure 10. Figure 23 shows the absorbance measured with and without the meta-surface structure 10 (curves 301 and 302 respectively). Enhancement in absorbance by the meta-surface structure 10 is observed.

[0106] Figure 24 shows that the observed enhancement in absorption in the module 2 aligns with resonance of the protrusions 16 in the meta-surface structure 10. Curve 401 represents measured absorbance (left vertical axis) as a function of wavenumber (horizontal axis) for a module 2. Curve 402 represents transmitted intensity (right vertical axis) as a function of wavenumber (horizontal axis) for the same module 2. The curves 401 and 402 show that resonance dips align with the observed enhanced absorption.

[0107] A further characterization was performed to assess the effects of protrusion radius, protrusion height (etch depth), and separation between protrusions (related to array pitch) on the absorption enhancement.

[0108] Figure 25 shows the absorbance of sodium acetate measured using modules with cylindrical protrusions 16 of different radii (curves 501-503 respectively representing protrusion radii of 1.0pm, 1.25pm, and 1.5pm). The protrusion height (etch depth) and separation between protrusions was 1.5pm for all three cases. The absorbance measured using all three modules 2 were higher compared to a reference spectrum obtained from a module having no protrusions (curve 504). However, there was a slight decrease in observed absorbance (1550 cm’1) with increase in radius. This is due to a shift in the resonance. It was also possible to see that the module 2 with protrusion radius 1.5pm showed higher absorption around 1000 cm’1indicating shifting of resonance towards lower wavenumber with increase in radius. The enhancement can be calculated by estimating the ratio of the area under each absorption peak to that of the reference spectrum. For example, the reference spectrum has an area of 1.12 (around 1550 cm’1), and pillars with radius r = 1pm shows an area of 2.707 which corresponds to an enhancement of 2.4.

[0109] Figure 26 shows the absorbance spectrum of sodium acetate measured using modules 2 having protrusions 16 of different heights (different etch depths). Curve 601 represents absorbance for a module 2 having protrusions of radius of 1pm, a separation between protrusions of 1.5pm, and a protrusion height of 1.5pm. Curve 602 represents a module 2 having protrusions of radius of 1 pm, a separation between protrusions of 1.5pm, and a protrusion height of 2.5pm. The module 2 with higher etch depth shows larger absorbance. The enhancement estimated using the total peak area is 4. It is also possible to estimate the enhancement for individual peaks. The enhancement estimated for the peak at 1410cm-1, for example, is 6.30. Curve 603 shows results for a reference module without meta-surface structure 10.

[0110] Figure 27 shows the measured absorbance as function of protrusion separation with curves 701-703 respectively corresponding to protrusion separations of 1.75pm, 0.75pm, and 0.4pm, and curve 704 corresponding to a reference module 2 having no protrusions. Maximum enhancement was observed in a module chip with a protrusion separation of 0.75pm. The inventors’ simulations indicated that smaller protrusion separations provide higher absorption. The disparity with the measurements shown in Figure 26 is thought to be due to surface tension limiting the amount of fluid entering spaces between the protrusions for smaller separations.

[0111] A further characterization was performed by measuring absorbance of modules 2 of the disclosure with and without a coating of germanium on a silicon meta-structure. Figure 30 shows a spectrum of 250mM sodium acetate (NaOAc) absorption measured with and without 1 lOnm Ge deposition on an Si pillar. The coating of Ge in this characterisation was a coating existing substantially in the entire recesses surrounding the protrusions, similar to the illustration of Figure 21. The curve 801 corresponds to Si pillars with 1 lOnm Ge deposition. The curve 802 corresponds to Si pillars with no Ge deposition. As shown in Figure 30 the absorption at the peaks is improved for the module 2 with a Ge coating.

[0112] Figure 32 compares absorbance of modules 2 of the disclosure with and without a coating of germanium on a silicon meta-structure, where the sub-structure was formed of pillars with varying heights and the coating does not exist in the recesses surrounding the pillars (e.g. as illustrated in Figures 28 and 31). The curve 901 shows absorbance for a meta-structure comprising silicon pillars with a height of 3.25 pm and with a coating of 1 pm Ge on top of the Si pillars. The curve 902 shows absorbance for a meta-structure comprising silicon pillars with a height of 2 pm and with a coating of 1 pm Ge on top of the Si pillars. The curves 903 and 904 shows absorbance for silicon pillars with a height of 1.9 pm and 5.4 pm respectively, with no Ge coating. The curve 905 shows absorbance for a comparative example having a flat layer of Si used as a sensing surface, e.g. a sensing surface that is flat with no meta-surface. These results demonstrate that the pillars as a meta-surface provide an enhanced absorbance over the flat silicon comparative example, and that the absorbance is significantly enhanced when a Ge coating is used as part of the meta-structure.

[0113] Figure 33 shows the result of further simulations of interactions between radiation and protrusions 16 of different shapes. Figure 33a shows an intensity plot showing field enhancement as a function of position around a protrusion 16a with an hourglass shape. Figure 33b is a plot for a protrusion 16b having vertical side walls, e.g. a cylindrical protrusion with constant cross-section. Figure 33c is a plot for a protrusion 16c having a cross section that increases with increased height, e.g. the shape exhibits “necking”. There is a strong electromagnetic field generated due to dielectric dipole resonances. The angles and heights of different protrusions 16 structures were extracted directly from SEM images of the fabricated structures. The study focused on simulating how the enhancement factor varies with respect to the change in the shape of the protrusion. The enhancement factor was calculated with respect to a flat chip without any protrusions (E2 / Eo2). As shown, the hourglass shaped protrusion 16a of Figure 33a exhibited significant enhancement due to strong localization of the electromagnetic field around their periphery. The “necked” protrusion 16c of Figure 33c shows stronger field enhancement than the cylindrical protrusion 16b of Figure 33b.

[0114] Figure 34 shows simulation results for the protrusion shapes of Figure 33, plotting enhancement factor for different wavelengths compared to a flat silicon sensing surface (e.g., no meta-structure). As shown, the hourglass shape protrusion 16a of Figure 33a has the highest absorption enhancement factor across the majority of the wavelengths simulated.

[0115] Figure 35 shows absorption spectra recorded using sensing surfaces with a metasurface having hour-glass shaped protrusions like that of Figure 33a in curve 1101, negative sidewall protrusions like that of Figure 33c in curve 1102, and a flat sensing surface without any protrusions in curve 1103. The radius of protrusions for each of the hour-glass and negative sidewall protrusions was 1.25 pm and the separations between the protrusions were 0.75 pm (hourglass) and 0.7 pm (negative sidewall). The heights of the protrusions were 2.1 pm (hourglass) and 2.5 pm (negative sidewall). The hour-glass structures show significantly increased absorption compared to the other chips across the spectrum tested.

[0116] BIBLIOGRAPHIC DETAILS

[0117] This application claims priority to GB 2413653.3, which is incorporated by reference herein.

Claims

CLAIMS1. An optical module for spectroscopy, comprising: a transmission body defining a sensing surface configured to contact a sample during spectroscopy of the sample using optical radiation, wherein: the sensing surface comprises a meta-surface structure configured to apply a wavelength-selective enhancement of absorption of the radiation by the sample during the spectroscopy.

2. The module of claim 1, wherein the module is configured to perform attenuated total reflection spectroscopy.

3. The module of claim 2, wherein the enhancement of absorption comprises enhancement of absorption of an evanescent field of radiation undergoing attenuated total reflection in the transmission body at the sensing surface.

4. The module of any preceding claim, wherein the meta-surface structure is configured to define the wavelength-selective nature of the enhancement of absorption predominantly via an influence of the meta-surface structure on Mie scattering of the radiation.

5. The module of any preceding claim, wherein the meta-surface structure comprises a plurality of protrusions extending along respective axes away from a local plane of the sensing surface.

6. The module of claim 5, wherein for each of at least a subset of the plurality of protrusions: the protrusion is surrounded by a recess forming a closed loop.

7. The module of claim 5 or 6, wherein for each of at least a subset of the plurality of protrusions:an average minimum lateral dimension of the protrusion, averaged over a height of the protrusion, is greater than 1 micron; and / or a height of the protrusion is in the range of 0.5 micron to 7 micron; and / or an average aspect ratio of a cross-section of the protrusion, the cross-section being perpendicular to the axis of the protrusion extending away from the local plane of the sensing surface, averaged over the height of the protrusion, is less than 2; and / or a shape of the cross-section of the protrusion, the cross-section being perpendicular to the axis of the protrusion extending away from the local plane of the sensing surface, is substantially circular, or substantially forms a regular polygon, over at least a portion of the height of the protrusion.

8. The module of any of claims 5-7, wherein at least a subset of the plurality of protrusions form a periodic pattern.

9. The module of claim 8, wherein: a pitch of the periodic pattern in at least one direction defines a range of wavelengths in which absorption is enhanced in the wavelength-selective enhancement; and / or a pitch of the periodic pattern in at least one direction is in the range of 0.5 micron to 7 micron; and / or a separation between protrusions is in the range of 0.5 micron to 7 micron.

10. The module of any of claims 5-9, wherein the meta-surface structure comprises a plurality of zones configured to provide different respective wavelength-selective enhancement of absorption by the sample.

11. The module of claim 10, wherein the different zones comprise groups of protrusions having different respective spatial configurations.

12. The module of claim 11, wherein the different spatial configurations comprise protrusions in one group having different average heights than in another group.

13. The module of claim 12, wherein: the protrusions in the one group all have substantially the same height; and the protrusions in the other group all have substantially the same height.

14. The module of any of claims 11-13, wherein the different spatial configurations comprise one or more of the following: different minimum or maximum lateral dimensions of protrusions, optionally averaged over heights of the protrusions; different aspect ratios of cross-sections of protrusions, the cross-sections being perpendicular to axes along which the protrusions extend away from the local plane of the sensing surface, optionally averaged over heights of the protrusions; and different shapes of cross-sections of protrusions, the cross-sections being perpendicular to axes along which the protrusions extend away from the local plane of the sensing surface.

15. The module of any preceding claim, wherein the transmission body further comprises an input surface configured to allow radiation to enter the transmission body through the input surface and undergo attenuated total reflection in the transmission body.

16. The module of claim 15, wherein the transmission body further comprises an output surface configured to allow the radiation to exit the transmission body through the output surface after undergoing the attenuated total reflection in the transmission body.

17. The module of claim 16, further comprising: a light source configured to direct radiation onto the input surface; and / or a detection arrangement configured to detect radiation output from the output surface.

18. The module of any of claims 15-17, wherein the input surface comprises a plurality of prism structures configured to promote attenuated total reflection in the transmissionbody.

19. The module of any preceding claim, wherein the meta-surface structure comprises a coating and a sub-structure, the coating having a higher refractive index than the substructure.

20. The module of claim 19 when dependent on claim 6, wherein at least a portion of the recess is free of the coating.

21. The module of claim 20, wherein the portion of the recess that is free of the coating forms a closed loop surrounding the protrusion, optionally wherein the entire recess is free of the coating.

22. The module of any of claims 19-21, wherein the sub-structure comprises silicon.

23. The module of any of claims 19-22, wherein the coating comprises one or more of the following: germanium; ZnSe; a metal; a 2D Van der Waals material.

24. The module of any of claims 19-22, wherein the coating comprises a cage molecule suspended in a host material.

25. The module of claim 24, wherein the cage molecule comprises one or more of the following: zeolites; cryptophanes; cucurbiturils; calixarenes; metal-organic frameworks; polyhexamethylene biguanide.

26. The module of claim 24 or 25, wherein the host material comprises one or more of the following: a polymer; a glass; a ceramic.

27. The module of any preceding claim, wherein: the sensing surface comprises a substrate and the meta-surface structure; the meta-surface structure comprises a plurality of protrusions extending alongrespective axes away from a local plane of the substrate; and the substrate and at least a portion of the meta-surface structure are integrally formed with each other.

28. The module of claim 27, wherein the portion of the meta-surface structure has been formed by removal of material from the substrate, optionally wherein the portion of the meta-surface structure has been formed by etching of the substrate, further optionally wherein the etching is reactive ion etching.

29. The module of claim 27 or 28, wherein the sub-structure of the meta-structure and the substrate are integrally formed with each other.

30. The module of any preceding claim, wherein the meta-surface structure comprises germanium.

31. The module of any preceding claim, wherein the wavelength-selective enhancement comprises an enhancement in one or more wavelength ranges in the range of 2.5 micron to 20 micron.

32. The module of any of claims 5-31, wherein for each of at least a subset of the plurality of protrusions, an area of a first cross-section at a first height of the protrusion is greater than an area of a second cross-section at a second height of the protrusion, wherein the first height is greater than the second height, areas of cross-sections vary smoothly between the first height and the second height, and cross-sections are perpendicular to the axis of the protrusion extending away from the local plane of the sensing surface.

33. The module of claim 32, wherein for each of at least the subset of the plurality of protrusions, an area of a third cross-section at a third height of the protrusion is greater than an area of the second cross-section, wherein the third height is lower than the second height and areas of the cross sections varies smoothly between the second height and the third height.

34. The module of claim 32 or 33, wherein for each of at least the subset of theplurality of protrusions, areas of cross-sections increase with increased height between the first height and the second height.

35. The module of claim 33 or 34, wherein for each of at least the subset of the plurality of protrusions, areas of cross-sections decrease with increased height between the third height and the second height.

36. The module of any of claims 32-35, wherein the first height is the highest height of the protrusion, and one of the second height and the third height is the lowest height of the protrusion.

37. A set of optical modules for spectroscopy, wherein: the set comprises at least a first optical module and a second optical module; the first optical module comprises: a first transmission body defining a first sensing surface configured to contact a first sample during spectroscopy of the sample using optical radiation, wherein: the first sensing surface comprises a first meta-surface structure configured to apply a first wavelength-selective enhancement of absorption of the radiation by the sample during the spectroscopy; the second optical module comprises: a second transmission body defining a second sensing surface configured to contact the first sample or a second sample during spectroscopy of the sample using optical radiation, wherein: the second sensing surface comprises a second meta-surface structure configured to apply a second wavelength-selective enhancement of absorption of the radiation by the first sample or the second sample during the spectroscopy; and the first wavelength-selective enhancement has a different wavelength profile to the second wavelength-selective enhancement.

38. A system for spectroscopy comprising:the module or set of modules of any preceding claim; a spectroscopy system configured to perform spectroscopy using the module and provide output data; a data processing system configured to analyse the output data to obtain information about one or more properties of a sample comprising biological material.

39. A method of designing an optical module for performing spectroscopy on a sample, wherein: the optical module comprises a transmission body defining a sensing surface configured to contact the sample during the spectroscopy of the sample; the sensing surface comprises a meta-surface structure configured to apply a wavelength-selective enhancement of absorption of the radiation by the sample during the spectroscopy; and the method comprises selecting one or more parameters defining a shape of the meta-surface structure based on one or more expected absorption properties of one or more entities of interest in the sample.

40. The method of claim 39, wherein the meta-surface structure comprises a plurality of protrusions and the selected one or more parameters define one or more of the following: lateral dimensions of the protrusions; heights of the protrusions; separations between the protrusions; a pitch or pitches of a periodic pattern comprising the protrusions; and shapes of the protrusions.

41. The method of claim 39 or 40, wherein the one or more expected absorption properties comprise one or more respective expected variations of absorbance as a function of wavelength of radiation.

42. A method of performing spectroscopy on a sample, comprising:designing an optical module for performing spectroscopy on the sample using the method of any of claims 33-41; and performing spectroscopy on the sample using the designed optical module.

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