AU2016200064A1 - Optical sensor - Google Patents

Abstract

An optical sensor 1000 including a MicroElectroMechanicalSystems (MEMS) structure 1005, and a grating coupled resonating structure 1010 5 positioned adjacent to the MEMS structure 1005, the grating coupled resonating structure 1010 comprising an interrogating grating coupler 1020 wherein the interrogating grating coupler 1020 and the MEMS structure 1005 form an optical resonant cavity. 102 104 106 (PRIOR ART)

Description

1 OPTICAL SENSOR FIELD OF THE INVENTION The present invention relates to an optical sensor, and more 5 particularly to an optical sensor for detecting a deflection of a beam or a cantilever or any other suitable MicroElectroMechanicalSystems (MEMS) structure. BACKGROUND OF THE INVENTION 10 Different methods for detecting chemical and biological analytes have been used. Such technology has been used, for example, in process control, environmental monitoring, medical diagnostics and security. Mass spectroscopy is one approach to detect such analytes. The process begins with an ionized sample. The ionized sample is shot through a 15 vacuum that is subjected to an electromagnetic field. The electromagnetic field changes the path of lighter ions more than heavier ions. A series of detectors or a photographic plate are then used to sort the ions depending on their mass. The output of this process, which is the signal from the detectors or the photographic plate, can be used to determine the composition of the 20 analytes in the sample. A disadvantage of mass spectroscopy instruments is that they are generally high-cost instruments. Additionally, they are difficult to ruggedize, and are not useful for applications that require a sensor head to be remote from signal-processing electronics. 25 A more recent approach is to use Micro Electro Mechanical Systems (MEMS)-based microstructures, and more specifically micro-cantilevers. These are extremely sensitive systems, and several demonstrations of mass sensors that have detection limits as low 10 21 g, approximately the mass of a 2 single protein molecule, have been performed. While these experiments have been performed in idealised environments, practical cantilever-based systems have been demonstrated for the detection of a wide range of single analytes. 5 With these sensors a portion of the micro-cantilever is coated with an analyte selective coating to which the analyte is adsorbed. There are two common modes of operation of micro-cantilever sensors, namely static and dynamic. In the static mode, a stress differential is induced across the cantilever 10 due to preferential adsorption of an analyte onto the analyte selective coating causing the cantilever to bend. The extent of the bending is in direct relation to the amount of analyte adsorbed. The stress differential can be induced by the analyte causing swelling of an overlayer, or by changes in the Gibbs free energy of the surface. 15 In the dynamic mode, the adsorbed analyte changes the mass of the cantilever and hence its mechanical resonance frequency. The rate and size of the change in resonance frequency is then measured to estimate the analyte concentration. Active sensing using these structures is achieved by resonant excitation. 20 In general, long, compliant cantilevers are required for sensitive static sensors, while high sensitivity for dynamic sensors dictate that short, stiff beams with high Q-factor mechanical resonances are needed. The most sensitive MEMS-based sensors to date have been based on measurements of resonant frequency. 25 Readout technologies used with micro-cantilever sensors are primarily based on optical techniques developed for atomic force microscopy (AFM) analysis. Here, light is reflected from the cantilever tip to a distant quadrant detector, which process is referred to as optical leveraging. Electrical 3 sensing and optical sensing techniques are also used. Electrical sensing includes piezoresistive, piezoelectric, capacitive, Lorentz force/emf sensing and tunnelling current techniques. Optical sensing techniques include optical sensing based on optical interference, the optical interference being either in 5 an interferometer or in the use of diffraction from an optical grating formed by a line of cantilevers. This latter configuration using an optical grating formed by a line of cantilevers is often described as an array in literature, but is still effectively a sensor for a single analyte. Another approach to analyte detection is where large, compact, 10 integrated arrays of individual sensors are used, particularly for multi-analyte, multi-analysis applications. These are particularly useful when an unknown substance is to be identified or if there is a number of chemical species to be tested for simultaneously. Examples of such requirements can be found in the screening of food for pesticide residues where there are many different 15 potential contaminants, detection of different antibodies in a single blood sample, or the presence of any of the many possible illicit drugs or explosives in luggage. Additionally, an array of sensors can also give significantly improved statistics of detection (including fewer false-positives and false negatives) by averaging the response over a large number of sensors, and 20 allows the use of multivariate statistical chemometric techniques, as are typically applied in spectroscopic analysis. There are several disadvantages with the sensors of today. There is, for example, a lack of compact, robust and cost-effective read-out technology that combines high sensitivity with high dynamic range. Sensors that are 25 good at detecting small amounts of analyte typically have poor dynamic range which is especially noticeable when the levels of analyte are large. A problem with AFM-based cantilever systems is that they are very large as they incorporate bulky free space optics requiring a sensor for each cantilever 4 output. A problem with electrical cantilever systems is that they require extensive power on-chip electronics. The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the prior 5 art forms part of the common general knowledge. SUMMARY OF THE INVENTION In one embodiment, the invention resides in an optical sensor including: 10 a MicroElectroMechanicalSystems (MEMS) structure; and a grating coupled resonating structure positioned adjacent to the MEMS structure, the grating coupled resonating structure comprising an interrogating grating coupler; wherein the interrogating grating coupler and the MEMS structure form 15 an optical resonant cavity. Preferably, the MEMS structure is a cantilever. Preferably, the MEMS structure is a beam. Preferably, the interrogating grating coupler is one dimensional. Preferably, the interrogating grating coupler is two dimensional. 20 Preferably, the interrogating grating coupler includes a plurality of holes forming an array. Preferably, the array is a regular shape. Preferably, the array forms a square. Preferably, the array forms a rectangle. Preferably, the array is an irregular shape. 25 Preferably, the holes are etched in a Silicon on Insulator (SOI) layer. Preferably, each hole is cylindrical in shape.

5 Preferably, the SOI layer is formed on a Buried Oxide (BOX) layer. Preferably, the BOX layer is formed on a substrate. In an alternate embodiment, the SOI layer is formed on the substrate. Preferably, the grating coupled resonating structure includes an input 5 grating coupler for inputting light into the optical sensor. Preferably, the grating coupled resonating structure includes an output grating coupler for outputting light from the optical sensor. Preferably, the input grating coupler is one dimensional. Preferably, the input grating coupler is two dimensional. 10 Preferably, the input grating coupler includes a plurality of holes forming an array. Preferably, the output grating coupler is one dimensional. Preferably, the output grating coupler is two dimensional. Preferably, the output grating coupler includes a plurality of holes 15 forming an array. Preferably, the cantilever includes an analyte selective coating. In another form, the invention resides in a method of detecting a deflection of a MEMS structure, the method comprising the steps of: inputting an optical signal into an interrogating grating coupler, the 20 interrogating grating coupler being arranged to form an optical resonant cavity with the MEMS structure; and analyzing the optical signal output from the interrogating grating coupler to determine a deflection of the MEMS structure. In yet another embodiment, the invention resides in an apparatus for 25 detecting a movement of a cantilever, the apparatus comprising a first cantilever comprising an analyte selective coating that is selective to said one or more analytes, a first grating coupled resonating structure positioned adjacent to the cantilever and comprising a first interrogating grating coupler, 6 wherein the first interrogating grating coupler and the cantilever form an optical resonant cavity. Preferably, the cantilever is dynamic. Alternatively, the cantilever is static. 5 Preferably, the apparatus further comprises a second grating coupled resonating structure wherein the second grating coupled resonating structure comprises a second interrogating grating coupler; and the second interrogating grating coupler and the cantilever form an optical resonant cavity. 10 Preferably, the second grating coupled resonating structure is positioned adjacent to the first grating coupled resonating structure on an axis substantially parallel to the cantilever. Preferably, the apparatus further comprises a signal analyser for detection of the presence of one or more analytes in the sample. 15 Preferably, the signal analyser compares light modulated by the first grating coupled resonating structure and the cantilever with a plurality of predefined signals. Preferably, the first grating coupled resonating structure provides an initial measurement, and the second grating coupled resonating structure 20 provides a refinement of said initial measurement. Preferably, the first grating coupled resonating structure and the second grating coupled resonating structure are used to determine a shape of the cantilever. Optionally, the apparatus further comprises: 25 a second cantilever; a second grating coupled resonating structure comprising a second interrogating grating coupler; 7 wherein the second interrogating grating coupler and the second cantilever form an optical resonant cavity. Preferably, the first grating coupled resonating structure and the second grating coupled resonating structure are optically coupled in series. 5 Optionally, the first grating coupled resonating structure and the second grating coupled resonating structure are optically coupled in parallel. In another form, the invention resides in a method of detecting the presence of one or more analytes in a sample. The method comprises the steps of applying the sample to a cantilever, wherein the cantilever comprises 10 an analyte selective coating selective to the one or more analytes, passing an optical signal through a grating coupled resonating structure, wherein the grating coupled resonating structure is arranged to form a resonant cavity with the cantilever; and analyzing the optical signal. Preferably, the cantilever is dynamic, and the step of analyzing the 15 optical signal comprises determining the resonance frequency of the cantilever and comparing the resonance frequency to known resonant characteristics of the cantilever. Alternatively, the cantilever is static, and the analysis step comprises determining a deflection of the cantilever. 20 Preferably, the cantilever is dynamic, and the step of analyzing the optical signal comprises determining the resonance frequency of the cantilever and comparing the resonance frequency to known resonant characteristics of the cantilever. Preferably, the step of analyzing the optical signal comprises 25 comparing the optical signal to a plurality of predefined signals. Preferably, the method further comprises the steps of passing a second optical signal through a second grating coupled resonating structure, 8 wherein the second grating coupled resonating structure is arranged to form a resonant cavity with the cantilever, and analyzing the second optical signal. Preferably, the step of analysing the optical signal comprises estimating an initial cantilever deflection measurement, and the step of 5 analyzing the second optical signal comprises refining the initial cantilever deflection measurement Preferably, the method further comprises the step of estimating a shape of said cantilever, wherein the step of analysing the optical signal comprises estimating a cantilever deflection measurement at a first position, 10 and the step of analysing the second optical signal comprises estimating a cantilever deflection measurement at a second position. BRIEF DESCRIPTION OF THE DRAWINGS To assist in understanding the invention and to enable a person skilled 15 in the art to put the invention into practical effect, preferred embodiments of the invention will be described by way of example only with reference to the accompanying drawings, in which: FIG. 1 shows a side sectional view of an optical microcantilever waveguide, according to the prior art; 20 FIG. 2 shows a top perspective view of an optical microcantilever sensor according to an embodiment of the invention; FIG. 3 shows a front sectional view of the optical microcantilever sensor according to an embodiment of the invention; FIG. 4 shows a side sectional view of an optical microcantilever sensor 25 according to a second embodiment of the invention; FIG. 5 is a graph 500 showing the periodic nature of transmission power of a signal according to an embodiment of the invention; 9 FIG. 6 shows a top view of the embodiments of FIG. 2; FIG. 7 shows a schematic diagram of an optical microcantilever sensor according to a third embodiment of the invention; FIG. 8 shows a schematic diagram of an optical microcantilever sensor 5 according to a fourth embodiment of the invention; FIG. 9 shows a schematic diagram of an optical microcantilever sensor according to a fifth embodiment of the invention; FIG. 10 shows a perspective view of an optical microcantilever sensor according to a further embodiment of the present invention; 10 FIG. 11 shows a close-up perspective view of an interrogating grating coupler of FIG 10, according to an embodiment of the present invention; FIG. 12 shows a sectional view through section A-A of FIG. 11, according to an embodiment of the present invention; FIG. 13 shows a sectional view through section B-B of FIG. 11 15 according to an embodiment of the present invention; and FIG. 14 shows a side sectional view of another embodiment of a two dimensional interrogating grating coupler, according to an embodiment of the present invention. 20 DETAILED DESCRIPTION While the present invention is open to various modifications and alternative constructions, the example embodiments shown in the drawings will be described herein in detail. It is to be understood, however, there is no intention to limit the invention to the particular example forms disclosed. On 25 the contrary, it is intended that the invention cover all modifications, equivalences and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims.

10 In this specification, the terms "comprise", "comprises", "comprising", "include", "includes", "including" or similar terms are intended to mean a non exclusive inclusion, such that a system, method or apparatus that comprises a list of elements does not include those elements solely, but may well 5 include other elements not listed. FIG. 1 shows a side sectional view of an optical microcantilever waveguide 100, according to the prior art. The optical microcantilever waveguide 100 comprises a fixed component 102 and a dynamic component 104. The fixed component 102 is attached to an insulator 108 such as for 10 example SiO 2 or Si3N 4 . The insulator 108 is attached to a substrate 110 such as for example a Si substrate. This layered structure allows for the simple construction of the optical microcantilever waveguide 100 through layering of the substrate 110, the insulator 108 and the fixed component 102 and the dynamic component 104 of the optical microcantilever waveguide 100, and 15 by then etching away an area of the insulator 108 (and possibly also an area of the substrate 110) forming a void 112 under the dynamic component 104 of the optical microcantilever waveguide 100. The dynamic component 104 of the microcantilever waveguide 100 is optically coupled to a fixed waveguide 106. 20 The dynamic component 104 is free to move above the void 112 in the insulator 108. Upon adsorbtion of an analyte, the mass of the dynamic component 104 of the optical microcantilever waveguide 100 changes. This change in mass results in a change of a resonance frequency of the optical microcantilever waveguide 100. 25 Light enters at an end of the fixed component 102 of the optical microcantilever waveguide 100 and propagates along the waveguide 100 to the dynamic component 104. Light exits the dynamic component 104 in a direction towards the fixed waveguide 106.

11 In a dynamic mode, the light entering the fixed waveguide 106 is amplitude modulated as a result of a coupling loss between the dynamic component 104 and the fixed waveguide 106 that is in close proximity to the dynamic component 104, which loss occurs as the dynamic component 104 5 vibrates. The light entering the fixed waveguide 106 is nominally modulated at twice the vibration frequency of the dynamic component 104 for symmetric vibration. Alternatively, in a static mode, the dynamic component 104 of the optical microcantilever waveguide 100 may change shape upon adsorbtion of an analyte. In this case the light entering the fixed waveguide 106 has an 10 amplitude based upon the shape of the dynamic component 104 of the optical microcantilever waveguide 100. The light entering the fixed waveguide 106 is analysed to detect the presence of an analyte on the optical microcantilever waveguide 100. The light may be compared to light with known characteristics, such as for 15 example light modulated due to the presence of an analyte. Alternatively, the resonance frequency or shape of the optical microcantilever waveguide 100 may be estimated and compared to pre-determined characteristics. Some disadvantages of prior art designs is that they have a poor dynamic range which incorporate bulky free space optics and thus require a 20 sensor for each cantilever output and require more electrical power to operate. According to some embodiments, the present invention resides in an apparatus for detecting a presence of one or more analytes in a sample. The apparatus comprises a cantilever and a grating coupled resonating structure 25 positioned adjacent to the cantilever. The cantilever comprises an analyte selective coating that is selective to the one or more analytes. The grating coupled resonating structure comprises an interrogating grating coupler which forms an optical resonant cavity with the cantilever.

12 An advantage of the present invention is the ability to economically have a very large number of sensors on a small surface, enabling efficient detection on multiple analytes. Furthermore it does not require bulky free space optics or extensive power on-chip electronics. 5 FIG. 2 shows a top perspective view and FIG. 3 shows a front sectional view of an optical microcantilever sensor 200 according to an embodiment of the invention. As shown in FIG. 2 and FIG. 3, the optical microcantilever sensor 200 comprises a cantilever 205 and a grating coupled resonating structure 210. The grating coupled resonating structure 210 comprises an 10 input grating coupler 215, an interrogating grating coupler 220 and an output grating coupler 225. The interrogating grating coupler 220 is placed directly under and adjacent to the cantilever 205. The cantilever 205 comprises an analyte selective coating on a surface of the cantilever, and a reflective surface 207, where the reflective surface 207 is opposite the interrogating 15 grating coupler 220. However it should be appreciated that the cantilever 205 may be a beam, or a cantilever without an analyte selective coating. Thus the present invention may be used to detect a movement of any type of beam or cantilever. The input grating coupler 215 is optically connected to the 20 interrogating grating coupler 220 and the interrogating grating coupler 220 is optically connected to the output grating coupler 225. The output grating coupler 225 is optically connected to a signal analyser (not shown), for example through an optical fibre. Referring to FIG. 3, arrows illustrate the path of light through the optical 25 microcantilever sensor 200. Light is coupled to the input grating coupler 215 from a light source (not shown), via an optical waveguide or an optical fibre, for example. The light propagates along the grating coupled resonating structure 210 to the 13 interrogating grating coupler 220 and out of the interrogating grating coupler 220 in a near perpendicular direction towards the cantilever 205 and is then reflected back to the interrogating grating coupler 220, also in a near perpendicular direction. The light then propagates along the grating coupled 5 resonating structure 210 to the output grating coupler 225. The cantilever 205 and interrogating grating coupler 220 form an optical resonant cavity such that the amount and/or frequency of light coupled to the output grating coupler 225 is a function of the separation of the interrogating grating coupler 220 and the cantilever 205. 10 The light is output from the grating coupled resonating structure 210 via the output grating coupler 225 so that it may be analysed in real time or recorded in the analyser or on a computer, for example, for analysis at a later time. When a sample is applied to the cantilever 205, adsorbtion of an 15 analyte may occur depending on the analyte selective coating and a composition of the sample. A pattern or shape of the interrogating grating coupler 220, for example dimensions of grooves of the interrogating grating coupler 220, determines a modulation of light resonating between the interrogating grating 20 coupler 220 and the cantilever 205. Additionally, a change in distance between the cantilever 205 and the interrogating grating coupler 220 causes a change in the modulation of the light output from the output grating coupler 225 due to constructive or destructive interference with the light in the grating coupled resonating structure 210. 25 A change in mass of the cantilever 205 occurs upon adsorbtion of the analyte. In a dynamic mode of operation, the change in mass results in a change in resonance frequency of the cantilever 205 which may be compared to when the analyte is not present. The resonance frequency of the 14 cantilever can be determined at the output grating coupler 225 through resonant excitation of the cantilever 205. Alternatively, in a static mode of operation, the presence of an analyte causes a change in shape of the cantilever 205. The change in shape of the 5 cantilever 205 causes a change in the distance between the cantilever 205 and the interrogating grating coupler 220 and hence change in the light at the output grating coupler 225. The signal analyser, which indicates the presence and concentration of the analyte in the sample, uses analysis of the light to estimate the resonance 10 frequency of the cantilever 205, or in the case of a static cantilever the shape of the cantilever 205. The resonance frequency of the cantilever 205 in dynamic mode operation, or the shape of the cantilever 205 in static mode, may be compared to known characteristics of the cantilever 205 to determine whether 15 an analyte is present or not. Known characteristics of the cantilever 205 include resonance frequency without the presence of an analyte, resonance frequencies with the presence of a particular amount of analyte or concentration, shape without the presence of an analyte, and shapes with the presence of a particular amount of analyte or concentration. 20 In an embodiment of the invention, the resonance frequency, height or position need not be calculated or estimated explicitly for each cantilever and measurement. Predefined signals of the cantilever at, for example, different resonance frequencies, heights or positions may be compared directly to the signal in the analysis step. 25 FIG. 4 shows a side sectional view of an optical microcantilever sensor 400 according to a second embodiment of the invention. The optical microcantilever sensor 400 comprises a cantilever 405 and a first, second and third grating coupled resonating structure 410a, 410b and 410c, 15 respectively, which are each specific examples of the grating coupled resonating structure 210 of FIG. 2. Similarly, the cantilever 405 is a specific example of the cantilever 205 of FIG. 2. The first grating coupled resonating structure 410a, placed under a 5 distal end of the cantilever 405, can be used to measure fine changes in shape or fine movements in the cantilever 405. The second grating coupled resonating structure 410b is positioned adjacent to the first grating coupled resonating structure 410a on an axis substantially parallel to the cantilever 405. The second grating coupled 10 resonating structure 410b, placed under a central part of the cantilever 405, can be used when larger changes in shape or larger movements are to be measured, possibly in combination with the first grating coupled resonating structure 410a. In this case the second grating coupled resonating structure 410b provides a refinement of an initial measurement of the first grating 15 coupled resonating structure 410a. The third grating coupled resonating structure 410c is positioned adjacent to the second grating coupled resonating structure 410b on an axis substantially parallel to the cantilever 405. The third grating coupled resonating structure 410c is placed under a proximal end of the cantilever 20 405 and can be used when larger changes in shape or larger movements are to be measured, possibly in combination with the first and second grating coupled resonating structures 410a and 410b. In this case the third grating coupled resonating structure 410c provides a refinement of the initial measurement of the first grating coupled resonating structure 410a and the 25 refinement provided by the second grating coupled resonating structure 410b. As would be readily understood by those skilled in the art, any number of grating coupled resonating structures 410 may be placed under a single cantilever, and at any position, without deviating from the present invention.

16 The exemplary embodiments illustrated in FIG. 2, FIG. 3 and FIG. 4 are applicable to both static and dynamic cantilevers 205, 405, and in both gaseous and aqueous environments. Furthermore, the grating coupled resonating structure 210, 410a, 410b, 410c can be oriented arbitrarily with 5 respect to the cantilever 205, 405, and the design of the cantilever 205, 405 can be decoupled from the design of the grating coupled resonating structure 210, 410a, 410b, 410c. A further valuable capability of this approach is that the multiple grating coupled resonating structures 210, 410a, 410b, 410c under the single cantilever 205, 405, as described in FIG 4, allows for the 10 shape of the cantilever 205, 405 to be measured with greater precision. Since an analyte can initially be adsorbed anywhere along the analyte selective coating of the cantilever 205, 405, a change in shape of the cantilever 205, 405 can be used as an early indication of the presence of the analyte. Further, as is discussed further in FIG.5, it may be advantageous to 15 have multiple grating coupled resonating structures to enhance a dynamic range of the optical microcantilever sensor 200, 300, 400. FIG. 5 is a graph 500 showing the periodic nature of transmission power 510 of a signal 530 according to an embodiment of the invention, with respect to a separation 520 between the cantilever 205, 405 and the grating 20 coupled resonating structure 210, 410a, 410b, 410c. As can be seen in the figure, separations of 0.5, 1.25, 2 and 2.75 micrometres, for example, have similar transmission powers 510. This ambiguity can however be removed, while still maintaining high sensitivity, by measuring the displacement of the cantilever 205, 405 at multiple positions. FIG. 4 illustrates an example where 25 multiple grating coupled resonating structures 210, 410a, 410b, 410c are placed under a single cantilever. Such configurations allow for Vernier-like calculations to be made.

17 FIG. 6 shows a top view of the embodiments of FIG. 2. The optical microcantilever sensor 600 comprises a wavelength division de-multiplexer 605, the wavelength division de-multiplexer 605 comprising three optical outputs 610a, 610b, 610c, the three grating coupled resonating structures 5 410a, 410b, 410c, a cantilever 405 and a wavelength division multiplexer 625. An optical input is optically coupled to the wavelength division de multiplexer 605. The wavelength division de-multiplexer 605 processes light from the optical input and splits the light into a plurality of subsignals, each 10 subsignal having a particular wavelength or plurality of wavelengths. In this example, the wavelength division de-multiplexer 605 has the three optical outputs 610a, 610b, 610c, each carrying light corresponding to a different wavelength or wavelength band. The optical outputs 610a, 610b, 610c are optically coupled to the 15 grating coupled resonating structures 410a, 410b, 410c. Each grating coupled resonating structure 410a, 410b, 410c is connected in parallel and forms an optical resonance cavity with the cantilever 405. The wavelength division multiplexer 625 additively combines the light output from grating coupled resonating structures 410a, 410b, 41Cc such that an output signal of 20 the wavelength division multiplexer 625 comprises a single light signal comprising multiple wavelengths. By using different wavelengths changes a position of the peaks and nulls in Fig. 5 due to the constructive and destructive interference being dependent on the wavelength of operation. Analysis of an individual grating coupled resonating structure 410a, 25 410b, 410c, may be performed by using pre-known characteristics of the grating coupled resonating structure 410a, 410b, 410c. These characteristics include, for example, a wavelength throughput of the grating coupled resonating structure 410a, 410b, 41Cc.

18 FIG. 7 shows a schematic diagram of an optical microcantilever sensor 700 according to a third embodiment of the invention. The optical microcantilever sensor 700 comprises two cantilevers 705a, 705b and two grating coupled resonating structures 71 Ca, 71 Ob. 5 The grating coupled resonating structures 710a, 710b form resonant cavities with the cantilevers 705a, 705b. The grating coupled resonating structure 71 Ca is optically coupled to the grating coupled resonating structure 710b in series, i.e. an output of the first grating coupled resonating structures 710a is connected in an input of the second grating coupled resonating 10 structures 71 Ob. Cantilever and grating coupled resonating structure pairs, for example 705a and 710a, or 705b and 710b, may be analysed individually. This is advantageous as each pair may be sensitive to a different analyte. The analysis may be performed by using pre-known characteristics of the grating 15 coupled resonating structure 71 Ca, 71 Ob or the cantilever 705a, 705b. These characteristics include, for example, a resonance frequency of the cantilever 705a, 705b and a wavelength throughput of the grating coupled resonating structure 710a, 710b given a separation to the cantilever 705a, 705b. In addition each cantilever 705a, 705b can have different dimensions to shift a 20 natural resonant frequency of each cantilever 705a, 705b. FIG. 8 shows a schematic diagram of an optical microcantilever sensor 800 according to a fourth embodiment of the invention. The optical microcantilever sensor 800 comprises a wavelength division de-multiplexer 805, the wavelength division de-multiplexer 805 25 comprising two optical outputs 810a, 810b, two grating coupled resonating structures 815a, 815b, two cantilevers 820a, 820b and a wavelength division multiplexer 825.

19 An optical input is optically coupled to the wavelength division de multiplexer 805. The wavelength division de-multiplexer 805 processes light from the optical input and splits the light into a plurality of subsignals, each subsignal having a particular wavelength or plurality of wavelengths. In this 5 example, the wavelength division de-multiplexer 805 has the two optical outputs 810a, 810b, each carrying light corresponding to a different wavelength or wavelength band. The optical outputs 810a, 810b are optically coupled to the grating coupled resonating structures 815a, 815b respectively. The grating coupled 10 resonating structures 815a, 815b are similar to the grating coupled resonating structures 210, 410a, 410b, 410c. Each grating coupled resonating structure 815a, 815b forms an optical resonance cavity with the cantilevers 820a, 820b, respectively. The wavelength division multiplexer 825 additively combines the light output from grating coupled resonating structures 815a, 15 815b such that an output signal of the wavelength division multiplexer 825 comprises a single light signal comprising multiple wavelengths. Analysis of an individual cantilever grating coupled resonating structure combination, for example 815a/820a or 815b/820b, which are connected in parallel, may be performed by using pre-known characteristics 20 of the grating coupled resonating structure 815a, 815b or the cantilever 820a, 820b. These characteristics include, for example, a resonance frequency of the cantilever 820a, 820b and a wavelength throughput of the grating coupled resonating structure 815a, 815b. FIG. 9 shows a schematic diagram of an optical microcantilever sensor 25 900 according to a fifth embodiment of the invention. The optical microcantilever sensor 900 comprises a wavelength division de-multiplexer 905, the wavelength division de-multiplexer 905 comprising two optical outputs 910a, 910b, four grating coupled resonating 20 structures 915a, 915b, 915c, 915d, four cantilevers 920a, 920b, 920c, 920d and a wavelength division multiplexer 925. The optical microcantilever sensor 900 is similar to the embodiments described in FIG. 7 and FIG. 8, except for that the cantilevers 920a, 920b, 920c, 920d and grating coupled 5 resonating structures 915a, 915b, 915c, 915d are coupled in a series and parallel configuration. The terms 'series' and 'parallel' are used in this specification. Series refers to the case where an output of a first grating coupled resonating structure is optically connected to an input of a second grating coupled 10 resonating structure. Parallel refers to the case where an input is shared between a first and second grating coupled resonating structure. Parallel connections include the case where the first grating coupled resonating structure uses or modifies a first part of the input, and the second grating coupled resonating structure uses or modifies a second part of the input, 15 even where a series physical connection exists. Additionally, as is understood by a person skilled in the art, any number of parallel and series connections may exist on a single sensor. Referring to FIGs. 2 and 3, although the pattern or shape of the input grating coupler 215, the interrogating grating coupler 220, or the output 20 grating coupler 225 are shown as being grooves, it should be appreciated that the pattern or shape of the input grating coupler 215, the interrogating grating coupler 220, and the output grating coupler 225 may include other patterns or shapes. FIG. 10 shows a perspective view of an optical microcantilever sensor 25 1000 according to a further embodiment of the present invention. Similar to FIGs. 2 and 3, the optical microcantilever sensor 1000 includes a grating coupled resonating structure 1010. The grating coupled resonating structure 1010 comprises an input grating coupler 1015, an interrogating grating 21 coupler 1020 and an output grating coupler 1025. The interrogating grating coupler 1020 is placed directly under and adjacent to the cantilever 1005 such that the interrogating grating coupler 1020 and the cantilever 1005 form a resonant cavity. In order to form a resonant cavity, the cantilever 1005 must 5 be positioned sufficiently close to the interrogating grating coupler 1020. In some embodiments, a distance between a bottom surface of the cantilever 105, and the top surface of the interrogating grating coupler 1020 range between less than 1 pm and 30pm for light at infra-red wavelengths. However it should be appreciated that the distance will vary according to the 10 wavelength of the light, as would be understood by a person skilled in the art. Generally, the distance between the cantilever 105 and the interrogating grating coupler 120 is less than a wavelength of the wavelength of operation to a number of wavelengths of the wavelength of operation. An amplitude of light output (or a response) from the resonant cavity varies 15 cyclically through constructive and destructive interference as the separation changes through integer numbers of quarter wavelengths (for a round trip path length in half wavelengths). In some embodiments, the cantilever 1005 includes a reflective surface 1007, where the reflective surface 1007 is opposite the interrogating 20 grating coupler 1020. Unlike the embodiment in FIG. 2 which was formed from a plurality of grooves forming a one dimensional (1 D) grating coupler, in this embodiment, each of the input grating coupler 1015, the interrogating grating coupler 1020 and the output grating coupler 1025 are formed from an array of holes forming a two-dimensional (2D) grating coupler. 25 FIG. 11 shows a close-up perspective view of the interrogating grating coupler 1020 of FIG 10, FIG. 12 shows a sectional view through section A-A of FIG. 11, and FIG. 13 shows a sectional view through section B-B of FIG. 11 according to an embodiment of the present invention. However it should 22 be appreciated that the input grating coupler 1015, and the output grating coupler 1025 may be formed in a similar manner. In some embodiments, the input grating coupler 1015, the interrogating grating coupler 1020 and the output grating coupler 1025 are 5 etched in a Silicon on Insulator (SOI) layer 1001 using any suitable method known in the art. The SOI layer 1001 may be made from silicon, or silicon nitride; however it is not limited to such materials, but it should have a higher refractive index than the layer(s) above and the layer(s) below the SOI layer 1001. In some embodiments, the SOI layer 1001 is between 50nm and 10 800nm thick. More preferably, the SOI layer 1001 is between 220nm and 520nm thick. In some embodiments, the SOI layer 1001 is formed on a Buried Oxide (BOX) layer 1002, and the BOX layer 1002 is formed on a substrate 1003. The BOX layer 1002 is made from a lower refractive index material 15 than the SOI layer 1001 in order that the SOI layer 1001 functions as a waveguide. In some embodiments the BOX layer is also made from silicon dioxide and about 100nm to 2000nm in thickness. However a person skilled in the art will realise that other suitable thicknesses may be used, as can other suitable materials. 20 In some embodiments, the sensor 1000 may optionally include a Top Oxide (TOX) layer (not shown) formed on the SOI layer 1001, once the holes 1022 have been etched in the SOI layer 1001. The TOX layer aids the reduction ofinterface losses and back reflections. The TOX layer may be between 1 00nm and 2000nm thick, and made from silicon dioxide. However it 25 should be appreciated that other thicknesses and other materials may be used. In addition, some embodiments may include a bottom reflector (not shown) positioned between the BOX layer 1002 and the substrate 1003 to 23 improve the coupling efficiency of the input grating coupler 1015, the interrogating grating coupler 1020 and the output grating coupler 1025 by reflecting light. However it should be appreciated that in some applications the bottom reflector is not necessary. For example in Atomic Force 5 Microscopy applications, it might be beneficial to be able to visually see through the substrate. The bottom reflector may be made from Aluminium. However other high reflectance materials may be used. In addition, the bottom reflector may incorporate a dielectric mirror formed by alternating high and low refractive 10 index layers and tuned to the wavelength of operation, for example silicon dioxide may be used for the low refractive index layer (~1.45 at 1550nm) and silicon may be used as the high refractive index layer (-3.48 at 1550nm) or silicon nitride may be used as the high refractive index layer (~1.9 at 1550nm). In order to form a low loss waveguide in the SOI layer 1001, the 15 TOX layer and the BOX layer 1002 should have a lower refractive index than the SOI layer 1001 layer for a desired wavelength. Adjacent holes 1022 in the array of holes have a pitch P in the X axis and a pitch p in the Z axis. In addition, each hole 1022 has a radius R, and is etched to an etch depth E. 20 Although the array is shown in FIG. 11 as being rectangular in shape, in some embodiments, the array may be square in shape. However it should be appreciated that the array may form any suitable regular or irregular shape. Further, each hole 1022 has a same etch depth E, a same radius R, a same pitch p between adjacent holes 1022, and a same pitch P between 25 adjacent holes 1022, creating a two dimensional (2D) grating coupler. However it should be appreciated that pitch p may be different to pitch P, or the holes 1022 may be randomly distributed. It should be further appreciated 24 that the radius R of the holes 1022 may be different and that one or more holes 1022 may have different depths E. FIG. 14 shows a side sectional view of a further embodiment of a 2D interrogating grating coupler 1420, for example a section along A-A of FIG. 5 11, according to an embodiment of the present invention. As shown in FIG. 14, holes 1422 of the interrogating grating coupler 1420 are at different etch depths E. Although each hole 1022 in the array is shown in FIGs. 10-14 as being cylindrical, it should be appreciated that each hole 1022 may be any suitable 10 regular or irregular shape. As explained previously a 1 D grating coupler includes a series of grooves on a surface of the SOI layer which, as shown in FIG. 3, results in a series of notches in a surface of the SOI layer when a section is taken through a single axis, namely an X axis. 15 In the case of a 2D grating coupler, a section taken in an X axis results in a series of notches and a section taken in a Z axis results in a series of notches. Dimensions of the input grating coupler 1015, the interrogating grating coupler 1020 and the output grating coupler 1025 are chosen according to a 20 chosen wavelength of operation of light from the light source. The light may be at any suitable wavelength between infra-red wavelengths (about 700nm to 1mm) and ultra-violet wavelengths (about 100nm to 380nm), including visible wavelengths (about 380nm to 700nm). In particular, parameters such as the SOI layer 1001, the BOX layer 25 1002, the pitch P, the pitch p, the Radius R and the depth E may be tuned, for a wavelength of operation, by simulation. The process of tuning is described in more detail in a paper (Lee Carroll, Dario Gerace, Ilaria Cristiani, and Lucio C. Andreani, "Optimising polarization-diversity couplers for Si- 25 photonics: reaching the -1dB coupling efficiency threshold", Optical Society of America, Optics Express, Vol. 22, No. 12 (2014)) for tuning the coupling efficiency between a SOI and a fibre optic. The paper describes optimising a 2D grating coupler for coupling with a fibre optic waveguide used in the 5 telecoms industry, and provides an example for a wavelength of operation at infra-red wavelengths. The effect of varying the parameters of a grating coupler may be investigated by computer simulation in order to optimise the grating coupler to best suit a radiation pattern from different sources, such as an optical fibre or a laser diode, or light output from the grating coupler 10 Referring to section 3 of the paper, tuning the parameters of the 2D grating coupler of FIGs 10-13, may be performed using a three-dimensional finite-difference time-domain (3D-FDTD) simulation on the 2D grating coupler by experimenting with dimensions of the parameters. However a 3D-FDTD simulation can take 1000 times longer to run than a two-dimensional finite 15 difference time-domain (2D-FDTD) simulation. As such the 1D grating coupler is first optimised using a 2D-FDTD simulation, and some of the optimised parameters are used to perform the 3D-FDTD simulations of the 2D grating coupler. Referring again to section 3 of the paper, the 2D-FDTD simulations 20 performed on a 1 D grating coupler "... depend on the Si-layer [the SOI layer 1001] thickness (S), the etch-depth (E) [etch depth E], the BOX [the BOX layer 1002] thickness (B), the hole-radius (R) [radius R], and the grating-pitch (P) [pitch P and/or pitch p] of the SO/-PDC design [the input grating coupler 1015, the interrogating grating coupler 1020 or the output grating coupler 25 1025]. Imposing the boundary conditions of AP = 1550nm [the wavelength of operation], and e = 100 [an angle of incidence of the light on the input grating coupler 1015, the interrogating grating coupler 1020 or the output grating coupler 1025] , reduces the number of independent design parameters to four 26 - (i) the Si-layer thickness [the SOI layer 1001], (ii) the BOX [the BOX layer 1002] thickness, (iii) the normalized etch-depth (E/S) [the etch depth E divided by the thickness of the SOl layer 1001], and (iv) the normalized hole size (R/P) [radius R divided by the pitch P or pitch p]. ... The parameter 5 space around these starting values is explored by generating ~25 unique SOI-PDC [2D grating coupler] designs, each using the initial estimate of the Si-layer and BOX thickness, but spanned by different combinations of E/S and R/P values. The coupling efficiency of each design is calculated using 3D-FDTD, with the grating-pitch iteratively adjusted until AP of CET(9 = 45) 10 converges to 1550 ± 2nm. A contour plot of CET(9 = 450) spanned by E/S and R/P can then be built-up, from which the optimum combination of E/S and R/P (for the initial estimates of Si-layer and BOX thickness) can be immediately identified. Next, a small sweep of the BOX thickness around the initial estimate is performed for the SOI-PDC designs with the optimum pair of 15 E/S and R/P values. This identifies the optimized design parameters (E, R, P, and B) of the SOI-PDC with the initial estimate of the Si-layer thickness. When this procedure is repeated for different Si-layer thicknesses around the initial estimated value, i.e. when the design parameter of S is also allowed to vary, then the globally optimized set of all parameters can be identified. 20 "Each design has a unique combination of Si-layer thickness (from S = 160nm to 520nm, in 19 steps) [the SOI layer 1001], BOX-thickness (from B = 1800nm to 2100nm, in 7 steps) [the BOX layer 1002], etch-depth (from E = 0.2 x S to 0.8 x S, in 7 steps), and duty-cycle [duty-cycle (DC) of a 1 D grating structure which is a ratio between a width of a groove to a period of the 25 grooves, as would be understood by a person skilled in the art] (from DC = 0.2 to 0.8, in 7 steps). Each design is individually centred on AP = 1550 + 2nm [the wavelength of operation] by tuning the grating-pitch [pitch P and pitch p]. The hierarchy of the parameter sweep is S-B-ED,so while the duty- 27 cycle changes for each design, the Si-layer thickness only changes every 343 designs (343 = 7 x 7 x 7), etc." The simulations of FIG. 3(a) identified that the best performing grating coupler has a SOI layer 1001 thickness of 420nm, a BOX layer 1002 thickness of 1900nm, an etch depth E of 252nm, and DC = 5 0.7 at a wavelength of operation of 1550nm, Referring again to section 4.1 of the paper, "The design parameters of the best performing SO1 1D-GC design ... are used as the starting values for the optimization of the high performance SOI-PDC design. After following the procedure outlined in Section 3, the optimized SOl-PDC design parameters 10 are identified as S = 400nm, B = 1900nm, E/S = 291nm/400nm = 0.73, and R/P = 167nm/584nm = 0.29. As shown in the contour plot of Fig. 3(b), this SOl-PDC design offers a coupling efficiency of -1.9dB (65%), meaning that the performance gap with respect to the best S01 1D-GC is just 0.5dB. The coupling spectrum of this optimized S0-PDC design is given in Fig. 2(b), and 15 has a 1dB bandwidth of 38nm, which is adequate for multiplexed telecom applications. To establish if the performance gap can be closed for all Si-layer thicknesses, 3D-FDTD optimization was also carried-out for S0-PDCs with S = 220nm, 320nm, and 520nm." As a result, the paper found the optimal design of 2D grating coupler 20 without a reflector at a wavelength of 1550nm to have a SOI layer 1001 thickness of 400nm, an etch depth E of 291nm, a hole 1022 radius R of 167nm, a pitch P = pitch p of 584nm, and a BOX layer 1002 thickness of 1900nm. However it should be appreciated that the dimensions of the grating coupler will vary according to many variables including whether a BOX layer 25 1002 is used, and the wavelength of operation of a light source. As previously mentioned, a reflector may be positioned between the BOX layer 1002 and the substrate 1003 to improve the coupling efficiency of the input grating coupler 1015, the interrogating grating coupler 1020 and the 28 output grating coupler 1025, which generally improves the coupling efficiency by 1 dB. Referring to section 4.2 of the paper, "Figure 4(a) shows the coupling efficiency of 4704 unique SOI 1D-GC designs with bottom-reflector, as calculated by 2D-FDTD simulations. Each design has a unique combination 5 of Si-layer thickness (from S = 150nm to 290nm, in 8 steps), BOX-thickness (from B = 1550nm to 2100nm, in 12 steps), etch-depth (from E = 0.2 x S to 0.8 x S, in 7 steps), and duty-cycle (from DC = 0.2 to 0.8, in 7 steps). The range of the BOX thicknesses in this sweep spans 550nm (~1550nm/2nOX) to ensure the identification of a condition for perfectly constructive 10 interference. As was the case in Section 4.1, each 1D-GC design is individually tuned to AP = 1550 ± 2nm, and the sweep hierarchy is S-B-E-D, so that the Si layer thickness changes only once every 588 designs (588 = 12 x 7 x 7). The sweep identifies the best-performing uniform SOI 1D-GC with bottom-reflector as having a coupling efficiency of -0.6dB (8 7%) with S = 15 170nm, B = 1600nm (or 2150nm), E = 51nm, DC = 0.5, and P = 694nm. This is the highest reported calculated coupling-efficiency for a uniform SOI 1D GC design with bottom-reflector. However, it is somewhat less than the reported coupling efficiency from calculations of apodized SOI 1D-GC designs with bottom-reflector (-0.45dB = 92%) [9]. Both of these calculations 20 compare well with reports of measured coupling efficiencies of -0.62dB (87%) from apodized SOI 1D-GCs with bottom-reflectors. "Using the same optimization procedure as that outlined in the previous section [for the embodiment without a reflector], the parameters for the optimized SOI-PDC design with bottom-reflector are identified as S = 25 160nm, B = 2175nm, E/S = 80nm/160nm = 0.5, and R/P = 209nm/696nm = 0.3. As shown in the contour plot of Fig. 4(b), this SOI-PDC design offers a coupling efficiency of -0.95dB (80%). The coupling spectrum of this 29 optimized SOI-PDC with bottom-reflector is given in Fig. 2(c), and has a 1dB bandwidth of 42nm." As a result, the paper found the optimal design of a 2D grating coupler with a reflector at a wavelength of 1550nm to have a SOI layer 1001 5 thickness of 160nm, an etch depth E of 80nm, a hole 1022 radius R of nm, a pitch P = pitch p of 696nm, and a BOX layer 1002 thickness of 2175nm. However it should be appreciated that the dimensions of the grating coupler will vary according to many parameters including whether a BOX layer 1002 is used, and the wavelength of operation. 10 Referring back to FIG. 10, in use, a light source (not shown) is connected to the input grating coupler 1015 using a waveguide or optical fibre for example. Similarly, the output grating coupler 1025 is connected to an analyser (not shown), via a waveguide or optical fibre for example, for analysing light output from the grating coupled resonating structure 1010. The 15 light output from the grating coupled resonating structure 1010 may be analysed in real time or stored by the analyser or a computer for example, for analysis at a later time. Although the light is shown as being coupled via the input grating coupler 1015 it should be appreciated that the light may be coupled using any suitable method. Similarly, although the light is shown as 20 being output via the output grating coupler 1025, it should appreciated that the light may be coupled to the analyser via any suitable method. In some embodiments, a reflective surface, similar to the reflective surface 1007, 207, may be placed above the input grating coupler 1015, 215 and above the output grating coupler 1025, 225. In this embodiment, light is 25 coupled into the sensor 1000, 200 from under the input grating coupler 1015, 215 and light is coupled out of the sensor 1000, 200 from under the output grating coupler 1025, 225. In this case the substrate 1003 is substantially transparent to the wavelength of operation to allow light to penetrate through 30 the substrate 1003 and the BOX layer 1002 and into the SOI layer 1001. In another embodiment, there may be no output grating coupler 1025, and light may be coupled to a photodetector coupled directly to the SOI layer 1001. Solid arrows 1004 illustrate a path of light through the optical sensor 5 1000. Similar to the embodiment shown in FIGs. 2 and 3, light is input to the input grating coupler 1005. The light then propagates in the SOI layer 1001, to the interrogating grating coupler 1020. Light exits the interrogating grating coupler 1020 in a near perpendicular direction towards the cantilever 1005, and is reflected back by the cantilever 1005 allowing the light to resonate 10 between the cantilever 1005 and the interrogating grating coupler 1020. Light modulated by the resonant cavity then propagates along the grating coupled resonating structure 1010 in the SOI layer 1001 to the analyser via the output grating coupler 1025. The analyser analyses an amount, modulation and/or a frequency of 15 light coupled to the output grating coupler 1025, which is a function of the distance between the interrogating grating coupler 1020 and the cantilever 1005, in order to determine a separation between the interrogating grating coupler 1020 and the cantilever 1005. As explained previously a 1 D grating coupler includes a series of 20 grooves. The grooves are etched in a surface of the SOI layer 1001 which results in a series of notches in a surface of the SOI layer when a section is taken through a single axis. The notches that result may be similar to FIG. 12. In the case of a 2D grating coupler, a section taken in an X axis of FIG. 11 results in a series of notches in a surface of the SOI layer 1001 as shown 25 in FIG. 12. Similarly, a section taken in a Z axis of FIG. 11 results in a series of notches in a surface of the SOI layer 101 as shown in FIG. 13. Although the interrogating grating coupler 1020 may be used with a single cantilever, it should be appreciated that the interrogating grating 31 coupler 1020 or the interrogating grating coupler 1420 may be used as an alternative to the interrogating grating coupler used in multiple cantilever designs of FIGs. 4, 6, 7, 8 or 9. An advantage, of using the 2D grating coupler of FIGs. 10-13 for the 5 input grating coupler 1015 and the output grating coupler 1025 over the 1D grating coupler of FIGs. 2 and 3, is that the coupling efficiency of 1 D grating coupler depends on the polarisation of the fibre optic cable, and can be unknown in telecom fibre optic cables. However this problem may be overcome by using 2D grating couplers. 10 Although the invention has been described in relation to a cantilever or microcantilever, thus is fixed at a single end, it should be appreciated that the present invention may be applied to flexible beams fixed at opposing ends, a membrane or any suitable MicroElectroMechanicalSystems (MEMS) structure positioned above the interrogating grating coupler. In the case of a 15 beam, the interrogating grating coupler is positioned between the opposing ends of the beam. As will be understood by those having ordinary skill in the art, in light of the present description, advantages of the present invention include the ability to economically have a very large amount of sensors on a small 20 surface, enabling efficient detection on multiple analytes. Furthermore, the detection of analytes with high precision and fidelity is possible. These efficient sensors may be used for the efficient and economical detection of pesticides or other chemicals in food, for efficient detection of explosives, narcotics or other elicit substances just to name a few examples. 25 The above description of various embodiments of the present invention is provided for purposes of description to one of ordinary skill in the related art. It is not intended to be exhaustive or to limit the invention to a single disclosed embodiment. As mentioned above, numerous alternatives 32 and variations to the present invention will be apparent to those skilled in the art of the above teaching. Accordingly, while some alternative embodiments have been discussed specifically, other embodiments will be apparent or relatively easily developed by those of ordinary skill in the art. Accordingly, 5 this patent specification is intended to embrace all alternatives, modifications and variations of the present invention that have been discussed herein, and other embodiments that fall within the spirit and scope of the above described invention. Limitations in the patent claims should be interpreted broadly based on 10 the language used in the claims, and such limitations should not be limited to specific examples described herein. In this specification, the terminology ''present invention" is used as a reference to one or more aspects within the present disclosure. The terminology "present invention" should not be improperly interpreted as an identification of critical elements, should not be 15 improperly interpreted as applying to all aspects and embodiments, and should not be improperly interpreted as limiting the scope of any patent claims.

Claims (20)

1. An optical sensor including: a MicroElectroMechanicalSystems (MEMS) structure; and 5 a grating coupled resonating structure positioned adjacent to the MEMS structure, the grating coupled resonating structure comprising an interrogating grating coupler; wherein the interrogating grating coupler and the MEMS structure form an optical resonant cavity. 10

2. The optical sensor of claim 1 wherein the interrogating grating coupler is one dimensional.

3. The optical sensor of claim 1 wherein the interrogating grating coupler 15 is two dimensional.

4. The optical sensor of claim 3 wherein the interrogating grating coupler includes a plurality of holes forming an array. 20

5. The optical sensor of claim 4 wherein the array is a regular shape.

6. The optical sensor of claim 5 wherein the array forms a square.

7. The optical sensor of claim 5 wherein the array forms a rectangle. 25

8. The optical sensor of claim 4 wherein the holes are etched in a Silicon on Insulator (SOI) layer. 34

9. The optical sensor of claim 4 wherein each hole is cylindrical in shape.

10. The optical sensor of claim 8 wherein the SOI layer is formed on a Buried Oxide (BOX) layer. 5

11. The optical sensor of claim 10 wherein the BOX layer is formed on a substrate.

12. The optical sensor of claim 8 wherein the SOI layer is formed on a 10 substrate.

13. The optical sensor of claim 1 wherein the grating coupled resonating structure includes an input grating structure for inputting light into the optical sensor. 15

14. The optical sensor of claim 1 wherein the grating coupled resonating structure includes an output grating coupler for outputting light from the optical sensor. 20

15. The optical sensor of claim 13 wherein the input grating coupler is one dimensional.

16. The optical sensor of claim 13 wherein the input grating coupler is two dimensional. 25

17. The optical sensor of claim 13 wherein the input grating coupler includes a plurality of holes forming an array. 35

18. The optical microcantilever sensor of claim 14 wherein the output grating coupler is two dimensional.

19. The optical microcantilever sensor of claim 14 wherein the output 5 grating coupler includes a plurality of holes forming an array.

20. A method of detecting a deflection of a MEMS structure, the method comprising the steps of: inputting an optical signal into an interrogating grating coupler, the 10 interrogating grating coupler being arranged to form an optical resonant cavity with the MEMS structure; and analyzing the optical signal output from the interrogating grating coupler to determine a deflection of the MEMS structure.