FIELD OF THE INVENTION

[0001 ] The present invention relates to an optical sensor and a method for determining a deflection of a beam or a cantilever or any other suitable MicroElectroMechanical Systems (MEMS) structure.

BACKGROUND OF THE INVENTION

[0002] MicroElectroMechanical Systems (MEMS)-based microstructures, and more specifically micro-cantilevers, are commonly used in applications such as Atomic Force Microscopy and analyte detection.

[0003] In the application of Atomic Force Microscopy, an Atomic Force Microscope (AFM) includes a cantilever with a pointed tip or probe at its end that is used to scan a sample surface. A laser light reflected from the back of the cantilever measures the deflection of the cantilever. This information is fed back to a computer, which generates a map of topography and/or other properties of interest.

[0004] Various measurements can be made including measuring either the deflection of the cantilever (static mode) or a vibration frequency of the cantilever (dynamic mode). In some applications, the tip is coated with a thin film of ferromagnetic material that reacts to magnetic areas on the sample surface.

[0005] A problem with current AFMs is that the sensitivity is limited by shot noise in the optical detection system. Although Brownian motion of the cantilever is a contributor to the noise, in practice it is not a factor as the shot noise is substantially greater than the noise induced by Brownian motion. While noise induced by Brownian motion may be reduced by cooling the cantilever, this is not practical for current AFMs as the cooling process may interfere with the alignment of the optical system. A further problem is that the process of measuring an entire surface of a sample is time consuming, as the probe tip must make many passes over the sample in order to build up an image.

[0006] Yet a further problem with current AFMs is that the probe often needs to be replaced, and each time the probe is replaced the optical detection system needs to be re-calibrated, which is a time consuming process.

[0007] Mass spectroscopy is a common technique used to detect analytes.

However, a disadvantage of mass spectroscopy instruments is that they are generally very 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.

[0008] A more recent approach is to use MEMS-based microstructure sensors, and in particular microcantilevers coated with an analyte selective coating to detect analytes. These are extremely sensitive sensors, and several demonstrations of mass sensors that have detection limits as low 10 ^"21 g, approximately the mass of a single protein molecule, have been performed.

[0009] When the analyte is adsorbed, the cantilever bends, and a deflection is measured using free space optics, for example by focussing a light on the cantilever and measuring a reflection.

[0010] Although these sensors are good at detecting small amounts of analyte, they typically have poor dynamic range, which is especially noticeable when the concentrations of analyte are large.

[001 1 ] There is therefore a need for an improved optical sensor.

[0012] 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 art forms part of the common general knowledge. OBJECT OF THE INVENTION

[0013] It is an object of some embodiments of the present invention to provide consumers with improvements and advantages over the above described prior art, and/or overcome and alleviate one or more of the above described disadvantages of the prior art, and/or provide a useful commercial choice.

SUMMARY OF THE INVENTION

[0014] In one form, but not necessarily the only or the broadest form, the invention resides in an optical sensor including a grating coupled resonating structure, the grating coupled resonating structure including:

a MicroElectroMechanicalSystems (MEMS) structure; and

an interrogating grating coupler positioned under the MEMS structure; wherein the interrogating grating coupler has a two dimensional grating; and

the MEMS structure and the interrogating grating coupler form an optical resonant cavity.

[0015] Preferably the MEMS structure is a cantilever.

[0016] Preferably, MEMS structure is a beam. Preferably, the beam is secured at opposing ends.

[0017] Preferably, the two dimensional grating is in the form of a two dimensional array. Preferably, the interrogating grating coupler includes an array of holes. Preferably the holes are arranged uniformly. Preferably the holes are aligned in a grid.

[0018] Preferably, the holes are round in cross-sectional shape. In another form the holes may be polygonal in cross-sectional shape such as, for example, square, hexagonal, or octagonal. Preferably the holes are all substantially the same shape. Preferably the holes have the same depth. In another form the holes may have different depths. [0019] Preferably, the array is a regular shape. Preferably, the array forms a square. Preferably, the array forms a rectangle.

[0020] Preferably, the array is an irregular shape.

[0021 ] Preferably, the holes are etched in a Silicon on Insulator (SOI) layer. [0022] Preferably, each hole is cylindrical in shape.

[0023] 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.

[0024] Preferably, a Top Oxide (TOX) layer is formed on the SOI layer.

[0025] Preferably, the grating coupled resonating structure includes an input grating structure for inputting light into the optical sensor.

[0026] Preferably, the grating coupled resonating structure includes an output grating coupler for outputting light from the optical sensor.

[0027] Preferably, the input grating coupler is one dimensional. Preferably, the input grating coupler is two dimensional.

[0028] Preferably, the input grating coupler includes a plurality of holes forming an array.

[0029] Preferably, the output grating coupler is one dimensional. Preferably, the output grating coupler is two dimensional.

[0030] Preferably, the output grating coupler includes a plurality of holes forming an array.

[0031 ] Preferably, the MEMS structure, e.g. cantilever, includes an analyte selective coating. [0032] 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 interrogating grating coupler being arranged to form a resonant cavity with the MEMS structure; and analysing the optical signal output from the interrogating grating coupler to determine a deflection of the MEMS structure; wherein

the interrogating grating coupler has a two dimensional grating.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] To assist in understanding the invention and to enable a person skilled 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 perspective view of an optical sensor according to an embodiment of the present invention;

FIG. 2 shows a close-up perspective view of an interrogating grating coupler of the optical sensor of FIG 1 , according to an embodiment of the present invention;

FIG. 3 shows a sectional view through section A-A of FIG. 2, according to an embodiment of the present invention;

FIG. 4 shows a sectional view through section B-B of FIG. 2 according to an embodiment of the present invention;

FIG. 5 shows a side sectional view of another embodiment of a two dimensional interrogating grating coupler, according to an embodiment of the present invention; and FIG. 6 shows a graph of transmitted power against the distance between the cantilever and the interrogating grating structure.

DETAILED DESCRIPTION

[0034] Elements of the invention are illustrated in concise outline form in the drawings, showing only those specific details that are necessary to understanding the embodiments of the present invention, but so as not to clutter the disclosure with excessive detail that will be obvious to those of ordinary skill in the art in light of the present description.

[0035] In this patent specification, adjectives such as first and second, left and right, front and back, top and bottom, etc., are used solely to define one element from another element without necessarily requiring a specific relative position or sequence that is described by the adjectives. Words such as "comprises" or "includes" are not used to define an exclusive set of elements or method steps. Rather, such words merely define a minimum set of elements or method steps included in a particular embodiment of the present invention. It will be appreciated that the invention may be implemented in a variety of ways, and that this description is given by way of example only.

[0036] FIG. 1 shows a perspective view of an optical sensor 100 according to an embodiment of the present invention. The optical sensor 100 includes a grating coupled resonating structure 1 10. The grating coupled resonating structure 1 10 comprises an input grating coupler 1 15, an interrogating grating coupler 120 and an output grating coupler 125. The interrogating grating coupler 120 is placed directly under and adjacent to a cantilever 105 such that the interrogating grating coupler 120 and the cantilever 105 form a resonant cavity. In order to form a resonant cavity, the cantilever 105 must be positioned sufficiently close to the interrogating grating coupler 120. In some embodiments, a distance between a bottom surface of the cantilever 105, and the top surface of the interrogating grating coupler 1 20 may range from less than one micron to up to 30 microns, for use at infra-red wavelengths such as at 1550nm. However the person skilled in the art will appreciate that the distance between the interrogating grating coupler 120 and the cantilever 105 depends of various factors including a wavelength of operation of light input into the interrogating grating coupler 120.

[0037] 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 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).

[0038] In some embodiments, the cantilever 105 includes a reflective surface 107 on a bottom surface of the cantilever 105, where the reflective surface 107 is opposite the interrogating grating coupler 120. In some embodiments, each of the input grating coupler 1 15, the interrogating grating coupler 120 and the output grating coupler 125 are formed from an array of holes 122 forming a two-dimensional (2D) grating coupler. However, it should be appreciated that the input grating coupler 1 15, and the output grating coupler 125 may be a one dimensional (1 D) grating coupler as described in Australian Patent Application number 201 1200815, which describes a 1 D grating coupler in the form of a series of grooves formed in the SOI layer 101 .

[0039] FIG. 2 shows a close-up perspective view of the interrogating grating coupler 120 of FIG 1 . FIG. 3 shows a sectional view through section A-A of FIG. 2, and FIG. 4 shows a sectional view through section B-B of FIG. 2 according to an embodiment of the present invention. However it should be appreciated that the input grating coupler 1 15, and the output grating coupler 125 may be formed in a similar manner.

[0040] In some embodiments, the input grating coupler 1 15, the interrogating grating coupler 120 and the output grating coupler 125 are etched in a Silicon on Insulator (SOI) layer 101 using any suitable method known in the art. The SOI layer 101 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 101 . The SOI layer 101 acts as a waveguide in order to optically couple the input grating coupler 1 15, the interrogating grating coupler 120 and the output grating coupler 125. In some embodiments, the SOI layer 101 is between 50nm and 800nm thick. More preferably, the SOI layer 101 is between 220nm and 520nm thick.

[0041 ] In some embodiments, the SOI layer 101 is formed on a Buried Oxide (BOX) layer 102, and the BOX layer 102 is formed on a substrate 103. The BOX layer 102 is made from a lower refractive index material than the SOI layer 101 in order that the SOI layer 101 functions as a waveguide. In some embodiments the BOX layer 102 is also made from silicon dioxide and is 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.

[0042] In some embodiments, the sensor 100 may optionally include a Top Oxide (TOX) layer (not shown) formed on the SOI layer 101 , once the holes 122 have been etched in the SOI layer 101 . The TOX layer aids the reduction of interface losses and back reflections. The TOX layer may be between 100nm and 2000nm thick, and may be made from silicon dioxide. However it should be appreciated that other thicknesses and other materials may be used.

[0043] In addition, some embodiments may include a bottom reflector (not shown) positioned between the BOX layer 102 and the substrate 103 to improve the coupling efficiency of the input grating coupler 1 15, the interrogating grating coupler 120 and the output grating coupler 125 by reflecting light. However it should be appreciated that in some applications the bottom reflector is not necessary. For example in Atomic Force Microscopy applications, it might be beneficial to be able to visually see through the substrate. In some embodiments, the bottom reflector is limited to areas beneath the input grating coupler 1 15, the interrogating grating coupler 120 and the output grating coupler 125.

[0044] 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 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). Alternatively 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 101 , the TOX layer and the BOX layer 102 should have a lower refractive index than the SOI layer 101 layer for a desired wavelength of operation.

[0045] Adjacent holes 122 in the array of holes have a pitch P in an X axis and a pitch p in a Z axis. In addition, each hole 122 has a radius R, and is etched to a depth E. Although the array is shown in FIG. 1 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 122 has a same depth E, a same radius R, a same pitch p between adjacent holes 122, and a same pitch P between adjacent holes 122, creating a two dimensional (2D) grating coupler. However it should be appreciated that pitch p may be different to pitch P. It should also be appreciated that the radius R of the holes 122 may be different. Furthermore, it should be appreciated that the depth E of some of the holes, may be different. The effect of varying the parameters forming the 1 D grating coupler or the 2D 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 a grating coupler.

[0046] FIG. 5 shows a side sectional view of another embodiment of a two dimensional interrogating grating coupler, according to an embodiment of the present invention. As shown in FIG. 5, some holes 522 of interrogating grating coupler 520 are etched to different depths E. [0047] Although each hole 122 in the array is shown in FIGs. 1 and 2 as being cylindrical, it should be appreciated that each hole 122 may be any suitable regular or irregular shape.

[0048] Parameters of the input grating coupler 1 15, the interrogating grating coupler 120 and the output grating coupler 125 are chosen according to a 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 1 mm) and ultra-violet wavelengths (about 100nm to 380nm), including visible wavelengths (about 380nm to 700nm).

[0049] In particular, parameters such as the SOI layer 101 , the BOX layer 102, 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, Maria Cristiani, and Lucio C. Andreani, "Optimising polarization-diversity couplers for Si-photonics: reaching the -1 dB coupling efficiency threshold", Optical Society of America, Optics Express, Vol. 22, No. 12 (2014)) which addresses tuning the coupling efficiency between a SOI and a fibre optic. The paper describes optimising a 2D grating coupler for coupling with fibre optic waveguides used for the telecoms industry. Thus the paper provides an example of operation at infrared wavelengths.

[0050] Referring to section 3 of the paper, tuning the parameters of the 2D grating coupler of FIGs 1 -3 may be performed using a three-dimensional finite-difference time-domain (3D-FDTD) simulation on the 2D grating coupler by experimenting with the parameters of the grating coupler. However a 3D- FDTD simulation can take 1000 times longer to run than a two-dimensional finite-difference time-domain (2D-FDTD) simulation. As such a one- dimensional (1 D) 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. An exemplary 1 D grating coupler is described in Australian Patent Application number 201 1200815. 51 ] Referring again to section 3 of the paper, the 2D-FDTD simulations performed on a 1 D grating coupler "... depend on the Si-layer [the SOI layer 101 ] thickness (S), the etch-depth (E) [depth E], the BOX [the BOX layer 102] thickness (B), the hole-radius (R) [radius R], and the grating-pitch (P) [pitch P and/or pitch p] of the SOI-PDC [2D grating coupler] design [the input grating coupler 1 15, the interrogating grating coupler 120 or the output grating coupler 125]. Imposing the boundary conditions of AP = 1550nm [the wavelength of operation], and Θ = 10° [an angle of incidence of the light on the input grating coupler 1 15, the interrogating grating coupler 120 or the output grating coupler 125], reduces the number of independent design parameters to four - (i) the Si-layer thickness [the SOI layer 101 ], (ii) the BOX [the BOX layer 102] thickness, (Hi) the normalized etch-depth (E/S) [the depth E divided by the thickness of the SOI layer 101 ], and (iv) the normalized hole- size (Pi/P) [radius R divided by the pitch P or pitch p]. ... The parameter-space around these starting values is explored by generating -25 unique SOI-PDC 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(cp = 45°) converges to 1550 ± 2nm. A contour plot of CET(q> = 45°) 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 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.

[0052] "Each design has a unique combination of Si-layer [the SOI layer 101 ] thickness (from S = 160nm to 520nm, in 19 steps), BOX [the BOX layer 102] - thickness (from B = WOOnm to 2100nm, in 7 steps), 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 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-cycle changes for each design, the Si-layer thickness only changes every 343 designs (343 = 7 * 7 * 7), etc." The simulations of FIG. 3(a) of the paper identified the best performing 2D grating coupler has a SOI layer 101 thickness of 420nm, a BOX layer 102 thickness of WOOnm, an depth E of 252nm, and DC = 0.7) at a wavelength of operation of 1550nm.

[0053] Referring again to section 4.1 of the paper, "The design parameters of the best performing SOI 1D-GC [1 D grating coupler] 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 SOI-PDC design parameters are identified as S = 400nm, B = WOOnm, E/S = 291nm/400nm = 0.73, and R/P = 167nm/584nm = 0.29. As shown in the contour plot of Fig. 3(b), this SOI-PDC design offers a coupling efficiency of -1.9dB (65%), meaning that the performance gap with respect to the best SOI 1D-GC is just 0.5dB. The coupling spectrum of this optimized SOI-PDC design is given in Fig. 2(b), and 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 SOI-PDCs with S = 220nm, 320nm, and 520nm."

[0054] As a result, the paper found the optimal design of a 2D grating coupler without a reflector at a wavelength of 1550nm to have a SOI layer 101 thickness of 400nm, a depth E of 291 nm, a hole 122 radius R of 167nm, a pitch P = pitch p of 584nm, and a BOX layer 102 thickness of 1900nm. However it should be appreciated that the dimensions of the structure will vary according to many variables including whether a BOX layer 102 is used, and the wavelength of operation.

[0055] As previously mentioned, a reflector may be positioned between the BOX layer 102 and the substrate 103 to improve the coupling efficiency of the input grating coupler 1 15, the interrogating grating coupler 120 and the output grating coupler 125, 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 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 χ 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 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 (87%) with S = 170nm, B = 1600nm (or 2150nm), E = 51 nm, 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 form calculations of apodized SOI 1D-GC designs with bottom-reflector (-0.45dB = 92%) [9]. Both of these calculations compare well with reports of measured coupling efficiencies of -0.62dB (87%) from apodized SOI 1D-GCs with bottom-reflectors.

[0056] "Using the same optimization procedure as that outlined in the previous section [for the embodiment without a bottom reflector], the parameters for the optimized SOI-PDC design with bottom-reflector are identified as S = WOnm, 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 optimized SOI- PDC with bottom-reflector is given in Fig. 2(c), and has a 1dB bandwidth of 42nm.."

[0057] 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 thickness of the SOI layer 101 of 160nm, a depth E of 80nm, a radius R of hole 122 of 209nm, a pitch P = pitch p of 696nm, and a thickness of the BOX layer 102 of 2175nm. However it should be appreciated that the dimensions of the structure will vary according to many parameters including whether a BOX layer 102 is used, and the wavelength of operation.

[0058] Referring back to FIG. 1 , in use, a light source (not shown) is connected to the input grating coupler 1 15 using a waveguide or optical fibre, for example. Similarly, the output grating coupler 125 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 1 10. The light output from the grating coupled resonating structure 1 10 may be analysed in real time or recorded on the analyser or a computing device for analysis at a later time.

[0059] Although the light is shown as being coupled via the input grating coupler 1 15 to the interrogating grating coupler 120 it should be appreciated that the light may be coupled into the SOI layer 101 using any suitable method. Similarly, although the light is shown as being output from the interrogating coupling structure 120 via the output grating coupler 125, it should appreciated that the light may be coupled from the SOI layer 101 to the analyser via any suitable method.

[0060] In some embodiments, a reflective surface, similar to the reflective surface 107, may be placed above the input grating coupler 1 15 and above the output grating coupler 125. In this embodiment, light is coupled into the sensor 100 from under the input grating coupler 1 15 and light is couple out of the sensor 100 from under the output grating coupler 125. In this case the substrate 103 is substantially transparent to the wavelength of operation to allow light to penetrate through the substrate 103 and the BOX layer 102 and into the SOI layer 101 . In another embodiment, there may be no output grating coupler 125, and light may be coupled to a photodetector coupled directly to the SOI layer 101 .

[0061 ] Solid arrows 104 illustrate a path of light through the optical sensor 100.

Light is input to the input grating structure 105. The light then propagates in the SOI layer 101 , to the interrogating grating coupler 120. Light exits the interrogating grating coupler 120 in a near perpendicular direction towards the cantilever 105, and is reflected back by the cantilever 105 allowing the light to resonate between the cantilever 105 and the interrogating grating coupler 120. Light modulated by the resonant cavity then propagates along the grating coupled resonating structure 1 10 in the SOI layer 101 to the analyser via the output grating coupler 125.

[0062] The analyser analyses an amplitude, a modulation and/or a frequency of light coupled to the output grating coupler 125, which is a function of the distance between the interrogating grating coupler 120 and the cantilever 105, in order to determine a separation between the interrogating grating coupler 120 and the cantilever 105. [0063] An example of light output from the grating coupled resonating structure 1 10 of FIG. 1 , and analysed by the analyser, is shown in FIG. 6. FIG. 6 shows a graph of transmitted power 610 against the distance between the cantilever 105 and the interrogating grating structure 120. As shown in FIG. 6, an amplitude 630 of light output from the grating coupled resonating structure 1 10 changes as the distance between the cantilever 105 and the interrogating grating structure 120 changes. As previously mentioned, and as shown in FIG. 6, the amplitude resonates due to the cavity formed between the cantilever 105 and the interrogating coupling structure 120.

[0064] As explained previously a 1 D grating coupler includes a series of grooves.

The grooves are etched in a surface of the SOI layer 101 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. 3.

[0065] In the case of a 2D grating coupler, a section taken in an X axis of FIG. 2 results in a series of notches in a surface of the SOI layer 101 as shown in FIG. 3. Similarly, a section taken in a Z axis of FIG. 2 results in a series of notches in a surface of the SOI layer 101 as shown in FIG. 4.

[0066] Although the grating coupled resonating structure 1 10 may be used with a single cantilever, it should be appreciated that the grating coupled resonating structure 1 10 may be used in arrays of cantilevers as discussed in Australian Patent Application number 201 1200815.

[0067] In addition, although the invention has been described in relation to a cantilever, where it is fixed at a single end, it should be appreciated that the present invention also may be applied to a beam fixed at opposing ends or any other suitable MEMS structure. In the application of a beam, the interrogating coupling structure may be positioned under a centre of the beam in order to measure a deflection of the beam as it flexes. [0068] In some embodiments, the cantilever 105 may include an analyte selective coating applied to all or part of a surface of the cantilever 105 such that the cantilever 105 bends on a presence of an analyte. In this embodiment, the sensor 100 may be used, for example, to detect illicit drugs used in drug detection apparatus.

[0069] It should be appreciated that the present invention may be used with cantilevers 105 operating in a static mode, where the cantilever 105 bends due to an external force or adsorption of an analyte, for example. In addition, the present invention may be used in a dynamic mode, where the cantilever 105 oscillates, and changes in mass of the cantilever 105 change a frequency of oscillation, for example.

[0070] An advantage, of using the 2D grating coupler of FIGs. 1 -3 for the input grating coupler 1 15, the interrogating grating coupler 120 and the output grating coupler 125, is that the coupling efficiency does not depend on the polarisation of light from the fibre optic cable.

[0071 ] A further advantage of the sensor according to the present invention is that the resonant cavity, formed between the cantilever and the interrogating grating coupler, results in a larger amplitude of light output from the sensor, and thus a better signal to noise ratio, than compared with free space optics. Better signal to noise ration leads to faster or more accurate measurements or more sensitive measurements for very small displacements. In addition, due to its compact nature, the sensor may be incorporated into arrays of cantilevers.

[0072] The term 'two dimensional' with respect to gratings is intended to mean a grating having a periodicity in two dimensions. A one dimensional grating is typically formed from a plurality of parallel gratings which may physically have two dimensions but in the context of gratings is considered to be one dimensional. A two dimensional grating is typically formed by combining two one dimensional gratings at an angle to each other, typically perpendicularly, such that the grating forms a grid or array of gratings. 73] 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 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, 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.