GB2579031A

GB2579031A - Acousto-optic device

Info

Publication number: GB2579031A
Application number: GB1818611.4A
Authority: GB
Inventors: Coimbatore Balram Krishna; Valle Stefano
Original assignee: University of Bristol
Current assignee: University of Bristol
Priority date: 2018-11-15
Filing date: 2018-11-15
Publication date: 2020-06-10
Anticipated expiration: 2038-11-15
Also published as: GB201818611D0; WO2020099851A1; GB2579031B

Abstract

An acousto-optic (AO) device, 10, operates by setting up an acoustic standing wave and an optical standing wave in the acousto-optic device. Both acoustic and optical standing waves are set up to be partially co-located within the acousto-optic device so that the optical standing wave interacts with the acoustic standing wave within the acousto-optic device to generate output light which is output from the acousto-optic device. The acoustic and optical standing waves may be internally reflected within the device at a common reflective interface, 14b. Also disclosed is an acousto-optic device comprising first and second electrodes, 12, 14, with a piezoelectric material, 16, between the two electrodes and an acoustic driver which generates an electric field across the piezoelectric material. In one embodiment the second electrode is optically transparent. In a further embodiment the second electrode comprises transparent metal or a doped semi-conductor, such as doped silicon.

Description

Acousto-Optic Device

FIELD OF THE INVENTION

The present invention relates to an acousto-optic device. BACKGROUND OF THE INVENTION Two major challenges facing acousto-optic devices are low operating frequencies (< 1 GHz due to the difficulty of efficient motion transduction at high frequencies, the increased acoustic decay and the reduced modulation strength) and cost (due to the usage of non-standard materials like Lithium Tantalate, Lithium Niobate and Tellurium Dioxide which prevents manufacture in a CMOS foundry). In addition, lack of CMOScompatibility results in reduced integration and platform scalability.

A. M. Siddiqui, J. Moore, M. Tomes, P. Stanfield, R. Camacho, and M. Eichenfield, "Direct RF to Optical Link Based on Film Bulk Acoustic Wave Resonators (FBAR)," in Conference on Lasers and Electro-Optics, OSA Technical Digest (2016) (Optical Society of America, 2016), paper SF2G.8 (referred to below as Siddiqui et al) demonstrates conversion of a lOGHz radio frequency signal directly to fiber optics.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a method of operating an acousto-optic device, the method comprising: setting up an acoustic standing wave in the acousto-optic device; and setting up an optical standing wave which is at least partially co-located with the acoustic standing wave within the acousto-optic device so that the optical standing wave interacts with the acoustic standing wave within the acousto-optic device to generate output light which is output from the acousto-optic device.

The co-location of the standing waves enables a resonant mode of the optical standing wave to interact with a resonant mode of the acoustic standing wave to generate the output light.

Typically the acoustic standing wave is bounded within an acoustic cavity and the optical standing wave is bounded within an optical cavity. The boundaries of the acoustic cavity and the optical standing wave may be different, or they may share a common boundary.

Optionally the acoustic standing wave and the optical standing wave are internally reflected at a common reflective interface, which may form a common boundary of both cavities.

Optionally the acousto-optic device comprises: first and second electrodes; and a piezoelectric material between the electrodes, and the method comprises: generating an electric field between the electrodes and across the piezoelectric material to set up the acoustic standing wave; and directing input light through the second electrode and into the piezoelectric material to set up the optical standing wave, wherein the optical standing wave interacts with the acoustic standing wave within the piezoelectric material and/or within the second electrode to generate the output light which is output from the acousto-optic device through the second electrode.

Optionally each electrode has an inner face which faces towards the piezoelectric material and an outer face which faces away from the piezoelectric material, and the acoustic standing wave is reflected at the outer face of each electrode Optionally the second electrode has an inner face which faces towards the piezoelectric material and an outer face which faces away from the piezoelectric material, and the second electrode reflects the optical standing wave at its outer face, which optionally may have a metal coating.

Optionally the second electrode has an inner face which faces towards the piezoelectric material and an outer face which faces away from the piezoelectric material, and the second electrode reflects the optical standing wave and the acoustic standing wave at its outer face, which may have a metal coating.

Optionally each electrode has an inner face which faces towards the piezoelectric material and an outer face which faces away from the piezoelectric material, the first electrode reflects the optical standing wave at its inner face, and the second electrode reflects the optical standing wave at its outer face, which may have a metal coating.

Optionally the optical standing wave and the acoustic standing wave interact within the piezoelectric material and within the second electrode.

Optionally the optical standing wave and the acoustic standing wave are co-linear where they interact with each other.

Optionally the acoustic standing wave has a frequency greater than 1 GHz or greater than 5 GHz or greater than 10 GHz.

Optionally the method further comprises receiving the output light at a detector.

Optionally the output light comprises a Stokes sideband and an anti-Stokes sideband.

These sidebands may have the same amplitude, but more preferably they have different amplitudes.

Optionally the optical standing wave is set up by directing input light into the device.

Preferably the input light has a wavelength selected such that the Stokes sideband and anti-Stokes sideband have different amplitudes.

A second aspect of the invention provides an acousto-optic device comprising: an acoustic resonant cavity; an acoustic driver arranged to set up an acoustic standing wave in the acoustic cavity; an optical resonant cavity which is at least partially co-located with the acoustic cavity; and a light source arranged to set up an optical standing wave in the optical resonant cavity.

A third aspect of the invention provides an acousto-optic device comprising: first and second electrodes; a piezoelectric material between the electrodes; and an acoustic driver arranged to generate an electric field between the electrodes and across the piezoelectric material, wherein one of the electrodes is electrically coupled to the acoustic driver, another one of the electrodes is electrically coupled to a voltage reference, and the second electrode is at least partially optically transparent.

The second electrode is both electrically conductive and at least partially optically transparent. The electrically conductivity enables it to operate as an electrode, and the optical transparency enables input light to pass through the second electrode into the device to set up an optical standing wave.

Optionally the electrodes and the piezoelectric material together provide an acoustic resonant cavity; and the second electrode and the piezoelectric material together provide an optical resonant cavity.

Optionally the second electrode comprises an optically transparent material, such as an optically transparent metal (for example indium tin oxide) or a doped semiconductor.

The second electrode may be fully transparent -for instance it may consist of an uncoated layer of an optically transparent material. Alternatively the second electrode may be only partially transparent -for instance it may comprise a layer of an optically transparent material coated in a thin metal layer, the thin metal layer having a reflectivity less than 70%. Optionally the metal layer has a thickness less than 50 nm.

A fourth aspect of the invention provides an acousto-optic device comprising: first and second electrodes; a piezoelectric material between the electrodes; and an acoustic driver arranged to generate an electric field between the electrodes and across the piezoelectric material, wherein one of the electrodes is electrically coupled to the acoustic driver, the other one of the electrodes is electrically coupled to a voltage reference, and the second electrode comprises a doped semiconductor.

Doped semiconductor material is both electrically conductive and at least partially optically transparent. The electrically conductivity of the doped semiconductor material enables it to operate as an electrode, and the optical transparency enables input light to pass through the second electrode into the device to set up an optical standing wave.

Optionally the second electrode comprises doped silicon.

The following comments apply to the device or method of any aspect of the invention.

Optionally the second electrode is thicker than the piezoelectric material and/or thicker than the first electrode.

Optionally the second electrode has a thickness greater than a combined thickness of the piezoelectric material and the first electrode.

Optionally the second electrode has a thickness greater than 2 pm or greater than 5 Pm.

Optionally the second electrode has a thickness less than 50 pm or less than 20 pm or less than 15 pm.

Optionally the second electrode has a thickness between 5 pm and 15 pm.

Optionally the device further comprises a light source arranged to direct input light through the second electrode and into the piezoelectric material to set up an optical standing wave in the piezoelectric material and/or the second electrode.

Optionally the acoustic driver is arranged to set up an acoustic standing wave in the piezoelectric material and the second electrode.

Optionally the second electrode has an inner face which faces towards the piezoelectric material, and an outer face which faces away from the piezoelectric material and has a metal coating. The metal coating optionally has a thickness less than 100 nm or less than 50 nm.

The voltage reference may be electrical ground On the case of a single-ended signalling process) or the voltage reference may carry a time-varying complementary signal On the case of a differential signalling process).

Typically the acoustic driver comprises a radio frequency (RF) driver. The RF driver may comprise an RF source, which may be for instance a radio telescope, a magnetic resonance imaging (MRI) device or an RF source with an amplitude-modulated output carrying data for transmission via the output light.

Optionally the second electrode is coupled to the voltage reference, although in a less preferred alternative the first electrode may be coupled to the voltage reference.

Preferably the first electrode is not optically transparent.

A further aspect of the invention provides an acousto-optic device array comprising an array of acousto-optic devices according to any aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a cross section of an acousto-optic device according to an embodiment of the invention; Figure 2 is a graph showing overlapping standing waves (both acoustic and optic) within the device; Figure 3 shows the device of Figure 1 in further detail; Figure 4 is an S11 plot showing measured multiple acoustic resonances of the acoustic cavity; Figure 5 is an S11 plot showing one acoustic resonance of the acoustic cavity; Figure 6 is an S11 plot showing multiple optical resonances of the optical cavity; Figure 7 is an 511 plot showing an optical resonance of the optical cavity; Figure 8 shows an 511 and 521 plot demonstrating amplitude modulation; Figure 9 shows an RF driver coupled to the device; Figure 10 shows the device on a chip along with apparatus for generating and detecting light; and Figure 11 shows an array of devices.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Figure 1 shows an Acousto-Optic (AO) device 10. The device operates by setting up an acoustic standing wave bounded within an acoustic resonant cavity 18 and an optical standing wave bounded within an optical resonant cavity 19 which is located within the acoustic cavity 18. Figure 2 shows the acoustic standing wave 18a and the optical standing wave 19a. As demonstrated by Figure 2, the acoustic and optical standing waves 18a, 19a are mostly co-located (i.e. spatially overlapped) within the AO device (i.e. the device provides a coupled cavity system). This co-location enables the optical standing wave 18a to interact with the acoustic standing wave 19a within the acousto-optic device to generate output light 17b which is output from the AO device as shown in Figures 1 and 10.

The AO device 10 comprises an acoustic transducer comprising a metal first (upper) electrode 12, a silicon second (lower) electrode 14; and a piezoelectric material 16 between the electrodes 12, 14. The first electrode 12 is electrically coupled to an acoustic or SF driver 44, 46 by means of an electrical connection 35, 35a shown in Figures 9 and 10, and the second electrode 14 is electrically coupled to electrical ground by means of electrical connections 33, 33a also shown in Figure 9.

To set up the acoustic standing wave 18a, an electric field between the electrodes 12, 14 and across the piezoelectric material 16 is generated by the SF driver. The SF driver produces an SF signal which drives the first electrode 12, and the second electrode 14 acts as a ground plane.

The first electrode 12 has an inner face 12a which faces towards and is in contact with the piezoelectric material 16, and an outer face 12b which faces away from the piezoelectric material 16. The second electrode 14 also has an inner face 14a which faces towards and is in contact with the piezoelectric material 16 and an outer face 14b which faces away from the piezoelectric material 16. The outer face 14b of the piezoelectric material 16 optionally has a thin metal coating 15 (for example gold or aluminium) to improve the optical quality factor of the optical cavity 19.

The acoustic cavity 18 is bounded by the outer faces 12b, 14b of the electrodes 12, 14, which reflect the acoustic standing wave 18a between them. The optical cavity 19 is bounded by the outer face 14b of the second electrode 14 and the inner face 12a of the first electrode 12a, which reflect the optical standing wave 19a between them. Thus the acoustic cavity 18 and the optical cavity are partially co-located, in other words they occupy the same volume of space between the outer face 14b of the second electrode 14 and the inner face 12a of the first electrode 12a. The reflective faces 12a, 12b, 14b forming the cavities are mutually parallel so that the optical standing wave 19a and the acoustic standing wave 18a are co-linear where they interact with each other along an acoustic-optic axis 11 of the device.

The first electrode 12, piezoelectric material 16 and second electrode 14 have respective widths W1, W2, W3 transverse to the acousto-optic axis 11 of the device.

These widths decrease gradually so they satisfy the relationship: W1<VV2< W3.

The upper boundaries of the acoustic cavity 18 and the optical cavity 19 are different, but they share a common lower boundary -namely the outer face 14b of the second electrode 14.

A pump light source 50 shown in Figure 10 is arranged to direct pump or input optical light 17a through the metal coating 15 and the second electrode 14, and into the piezoelectric material 16. The metal coating 15 is sufficiently thin (for instance 30nm thick) to ensure that it is partially optically transparent. The metal coating 15 typically has a reflectivity between 50% and 70% at the wavelength of the input light 17a (1500-1600nm).

The input light 17a is then reflected by the inner face 12a of the first electrode 12 and partially reflected back into the optical cavity 18 at the outer face 14b of the second electrode 14 (which optionally may have the metal coating 15). This creates the optical standing wave 19a.

The light being reflected inside the optical cavity 19 is only partially reflected at the outer face 14b of the second electrode 14, and light which is not reflected exits as the output light 17b.

As shown most clearly in Figure 2, the acoustic standing wave 18a and the optical standing wave 19a are reflected at a common reflective interface: the outer face 14b of the second electrode 14. The acoustic and optical fields also reflect partially at other internal interfaces: for instance the internal interface between the piezoelectric material 16 and the second electrode 14, but these reflections will be much weaker.

The standing waves 18a, 19a interact within the piezoelectric material 16 and within the second electrode 14 to generate the output light 17b which is output from the AO device through the second electrode 14 as shown in Figure 1.

The first electrode 12 is 1 pm thick metal, such as aluminium. This makes the first electrode 12 sufficiently thick to reflect the optical standing wave 19 and provide a robust electrical connection to the RE driver, unlike the metal coating 15 which is much thinner to make it partially optically transparent. The metal coating 15 has no intended electrical use. It is there just to boost the optical quality factor.

The second electrode 14 is silicon, doped with phosphorus or arsenic to make it electrically conductive, and is 10 pm thick. Silicon is almost fully transparent at the wavelength of the input light 17a (1500-1600nm), and has a reflectivity of approximately zero. So the second electrode 14 and the metal coating 15, taken as a whole, are partially optically transparent.

The second electrode 14 is doped from above, and the dopant concentration may reduce through the thickness of the silicon material so only a thin top layer of the second electrode (for instance 1 pm or so) is electrically conductive and provides an electrical ground plane. Alternatively the dopant concentration may be constant through a full thickness of the second electrode 14.

The piezoelectric material 16 is made of c-axis oriented Aluminium Nitride, and is 0.5 pm thick.

The second electrode 14 is thicker than the combined thickness of the piezoelectric material 16 and the first electrode 12, so that it forms the majority of both cavities 18,19 and acts as a substrate for the device.

Acoustic standing waves 18a are created at specific frequencies which represent the longitudinal acoustic modes of the acoustic cavity 18. The inventors have measured multiple discrete acoustic resonances from 300 MHz to 12 GHz using the AO device 10. Figure 4 shows acoustic resonances of the AO device 10 from 0.3 to 5 GHz. A zoomed in snapshot of a single acoustic resonance of Figure 4 is shown in Figure 5.

The device can be operated at any of the resonant frequencies shown in Figure 4, or other resonant frequencies up to 16 GHz.

The device described in Siddiqui et al, has only one acoustic resonance in the 10GHz range, whereas the device 10 has a total effective thickness of 10.5 pm and hence can couple to many more longitudinal modes. The next mode in Siddiqui et al would be at 17 GHz but cannot be seen because of the weak coupling.

Likewise, optical standing waves 19a are created at specific frequencies. Figure 6 shows three optical resonances of the AO device 10 from 1500 -1600nm.

The 0-factor is a standard dimensionless parameter that indicates the energy losses within a resonant element. The 0-factor of the optical cavity 19 is defined as Qopt. The optical field inside the OA device 10 travels back and forth due to the optical cavity 19, and at frequencies that the optical fields are in phase or an integral multiple of 2*11, create an optical standing wave proportional to the magnitude of the electric field Qapt times the input optical field. The phase relation is fixed by the dimension (thickness) of the resonator and by the wavelength.

Advantageously, the geometry of the AO device 10 produces an extended frequency range well above conventional acousto-optic modulator devices. The minimum resonance can be about 300 MHz and the highest resonance frequency can be above 12 GHz.

As shown in Figure 2, the acoustic standing wave 18a has multiple cycles within the acoustic cavity 18 (in this case about six cycles, this will vary dependent on wavelength) and the optical standing wave 19a has multiple cycles within the optical cavity 19 On this case about twenty four cycles, this will vary dependent on wavelength) each of which overlaps and interacts with the acoustic standing wave 18a.

As the resonant acoustic field interacts with the optical field at a specific wavelength, two sidebands are created known as Stokes and anti-Stokes which are related to the frequency shift created by the acousto-optic interaction. The magnitude of the frequency shift is defined by the RF frequency applied to the piezoelectric material 16 by the RF driver, and therefore the acoustic standing wave 18a. As an example, if the input (pump) light frequency is ft), and the acoustic resonant frequency is fa, then the Stokes sideband will be at a frequency fo -fa and the anti-Stokes sideband will be at a frequency [0 +J.

Figure 7 shows a graph of amplitude against wavelength for one optical resonance of the optical cavity 19. The input light 17a is input into the optical cavity 19 at a pump wavelength A,. The Stokes sideband 24 and the anti-Stokes sideband 26 are produced by the acousto-optic interaction. If the wavelength A, of the input light 17a is close to an optical resonance, as shown in Figure 7, then the Stokes sideband 24 and the anti-Stokes sideband 26 will have different amplitudes, the difference being indicated at 29 in Figure 7. In the example of Figure 7, the anti-Stokes sideband 26 is further from the bottom 22 of the resonant dip than the Stokes sideband 24 and thus has a smaller amplitude.

By choosing a wavelength Au which corresponds to the point of maximum gradient in the graph of Figure 7, the maximum amplitude difference 29 between the Stokes and anti-Stokes sidebands 24, 26 will be produced.

In this case the wavelength A, of the input light 17a is off-resonance -in other words it is offset from the bottom 22 of the resonant dip in Figure 7. In another embodiment, the input light may have a wavelength which coincides with the bottom 22 of the resonant dip in Figure 7 so the optical cavity is driven at resonance. This provides higher efficiency, but since the sidebands 24, 26 will have the same amplitude an interferometer will be required to provide discrimination.

If there was no optical cavity 19, then both sidebands 24, 26 would have the same 10 amplitude.

Figure 8 shows a resonant frequency 30 of the device 10 identified by measuring the Si 1 parameter using a Vector Network Analyser which scans the RF frequency in a defined range and measure the return losses of the system (how much RF power is reflected back to the RF source due to the electrical impedance mismatch). Thus at the resonant frequency the RF source is able to couple more RF power inside the device 10. The S21 plot 32 in Figure 8 shows the amplitude modulation of the optical field by the acoustic field.

Vector Network Analysers measure amplitude and phase properties of electrical networks defined as scattering parameters or S -parameters. The 511 parameter is a measurement of the reflected electrical signal due to the electrical impedance mismatch between the source and the device. The 521 parameter measures the transmission between the source connected to port one and the detector connected to port two. The doubly resonant nature of the OA device 10 means that a weak RF signal can easily be measured by current standard receiver devices due to the signal enhancement of the dual resonator configuration.

The device of Figure 3 can be fabricated in a CMOS foundry process, such as the PiezoMumps process described at: (http:/(www.rriernscap.comiproductsim um p sipi e.zo m umps).

A first silicon layer is etched from below to form a pair of blocks 30a, 30b. A second layer of silicon is formed, separated from the first silicon layer by an oxide etch-stop layer (not shown). The second silicon layer is doped from above, then etched to provide silicon layers 31a, 31b on top of the blocks 30a,30b, a pair of bridges 32a, 32b and the second electrode 14 which has a circular profile as shown in Figure 3. The bridges physically suspend the AO device 10 between the blocks, and electrically connect the second electrode 14 to the silicon layers 31a, 31b. Metal ground connection pads 33 are formed on top of the silicon layer 31a, and electrically connected to electrical ground. Thus the second electrode 14 is connected to ground via the bridge 32a, the silicon layer 31a, and the ground connection pads 33.

A layer of Aluminium Nitride is formed on top of the doped silicon and etched to form the piezoelectric material 16 on top of the second electrode 14 and an insulating pad 16a on top of the silicon layers 31a.

Finally, a layer of aluminium is formed on top of the Aluminium Nitride layer and etched to form the first electrode 12, an electrical connection line 34, and a connection pad 35 on top of the insulating pad 16a. Thus the first electrode 12 is connected to the RF driver via the connection line 34 and the connection pad 35.

The compact size and planar architecture shown in Figure 3 enables a high density pixelated array of AO devices 10 to be engineered, or a large number of the AO devices 10 to be manufactured on a high density "chip".

The teachings of this disclosure can be used in several applications ranging from radio astronomy to medical applications, to data transmission for 5G applications and beyond.

The AO device 10 can operated at large frequency ranges (from 300 MHz to at least 12 GHz) with high efficiency. The efficiency is achieved by the dual resonance structure (both optical and mechanical modes are resonant in the device). The higher frequency operation is primarily enabled by the fact that the device has a small physical footprint, which allows excitation of high frequency resonances (in general, fr,s--1/L where L is the device length).

The device can be produced in a CMOS foundry, tailoring the device architecture so that a standard foundry process can implement it. This allows the device to be very cost effective. CMOS fabrication brings with it repeatability, reliability and the promise of scale The CMOS implementation allows the device to be implemented in large 2D arrays (which can approach a million pixels), something never before achieved with AOMs (or any other kind of modulator).

The use of resonant acousto-optical interactions can enhance modulation efficiency, and further the interactions increase the figure of merit of these devices (V" , L). One figure of merit 177r is the external voltage which results in a u radians phase difference between two specific orthogonally polarized light beams. Reducing 14, to be less than 10V, less than 5V or less than 1V even at frequencies up to 3 GHz, allows massive energy reduction and increased functionality.

Figure 9 shows how the device 10 can be driven acoustically by an RF driver comprising an RF source 44 and an RF matching network 46. Note that the RF matching network 46 is optional, so it is indicated in dashed lines. The RF source 44 generates an RF signal on a co-axial cable 45, which is input to the RF matching network 46. The RF matching network 46 has an output line 35a which carries the RF signal, and a pair of voltage reference lines 33a which provide an electrical ground reference. The output line 35a is connected to the connection pad 35 and the voltage reference lines 33a are connected to the ground connection pads 33. In an alternative embodiment there may be only a single voltage reference line 33a and a single ground connection pad 33.

Figure 10 shows how the device 10 can be integrated into a chip 42. The RF matching network 46 has a co-axial cable attached to a connector 48 which has signal and ground lines connected to the device 10 as shown.

A laser 50 generates vertically polarized input light 17a which is input to the chip via an optical fibre 52, a connector 54 and a lens 56. The input light 17a passes through a polarizing beam splitter 58, then a quarter-wave plate 60 which makes the input light circularly polarized. A lens or micro-lens system 62 directs the circularly polarized input light into the device 10. The output light from the device 10 is circularly polarized and is converted into horizontally polarized light by the quarter-wave plate 60. The horizontally polarized output light 17b is directed onto a photodetector 70 via a lens 64, connector 66 and optical fibre 68.

By way of example, the device 10 in Figure 10 may operate as an acousto-optic modulator in a communication application. The RF signal from the RF source 44 is amplitude modulated to carry data. For instance the RF signal may be amplitude modulated by being turned on and off to carry binary data. The device 10 modulates the output light 17b accordingly, and the amplitude-modulated output light is detected by the photodetector 70.

In another example, the RF source 44 may be a radio telescope which picks up very weak RF signals which are amplified by the device 10.

Figure 11 shows an acousto-optic device array 80 comprising an array of acousto-optic devices 10 as described above. Figure 11 shows an array with sixty-four pixels but there may be more than 0.5 million pixels in the array 80 due to the compact geometry of the devices 10. The array 80 may form part of a spatial light modulator for example.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS1 A method of operating an acousto-optic device, the method comprising: setting up an acoustic standing wave in the acousto-optic device; and setting up an optical standing wave which is at least partially co-located with the acoustic standing wave within the acousto-optic device so that the optical standing wave interacts with the acoustic standing wave within the acousto-optic device to generate output light which is output from the acousto-optic device.
2 The method of claim 1 wherein the acoustic standing wave and the optical standing wave are internally reflected at a common reflective interface.
3 The method of claim 1 or 2 wherein the acousto-optic device comprises: first and second electrodes; and a piezoelectric material between the electrodes, and the method comprises: generating an electric field between the electrodes and across the piezoelectric material to set up the acoustic standing wave; directing input light through the second electrode and into the piezoelectric material to set up the optical standing wave, wherein the optical standing wave interacts with the acoustic standing wave within the piezoelectric material and/or within the second electrode to generate the output light which is output from the acousto-optic device through the second electrode.
4 The method of claim 3 wherein each electrode has an inner face which faces towards the piezoelectric material and an outer face which faces away from the piezoelectric material, and the acoustic standing wave is reflected at the outer face of each electrode.
The method of claim 3 or 4 wherein the second electrode has an inner face which faces towards the piezoelectric material and an outer face which faces away from the piezoelectric material, and the second electrode reflects the optical standing wave at its outer face.
6 The method of any of claims 3 to 5 wherein the second electrode has an inner face which faces towards the piezoelectric material and an outer face which faces away from the piezoelectric material, and the second electrode reflects the optical standing wave and the acoustic standing wave at its outer face.
7 The method of any of claims 3 to 6 wherein each electrode has an inner face which faces towards the piezoelectric material and an outer face which faces away from the piezoelectric material, the first electrode reflects the optical standing wave at its inner face, and the second electrode reflects the optical standing wave at its outer face.
8 The method of any of claims 3 to 7 wherein the optical standing wave and the acoustic standing wave interact within the piezoelectric material and within the second electrode.
9. An acousto-optic device comprising: first and second electrodes; a piezoelectric material between the electrodes; and an acoustic driver arranged to generate an electric field between the electrodes and across the piezoelectric material, wherein one of the electrodes is electrically coupled to the acoustic driver, another one of the electrodes is electrically coupled to a voltage reference, and the second electrode is at least partially optically transparent.
The acousto-optic device of claim 9 wherein the second electrode comprises a transparent metal or a doped semiconductor.
11 An acousto-optic device comprising: first and second electrodes; a piezoelectric material between the electrodes; and an acoustic driver arranged to generate an electric field between the electrodes and across the piezoelectric material, wherein one of the electrodes is electrically coupled to the acoustic driver, the other one of the electrodes is electrically coupled to a voltage reference, and the second electrode comprises a doped semiconductor.
12 The acousto-optic device of any of claims 9 to 11 wherein the second electrode comprises doped silicon.
13. The acousto-optic device of any of claims 9 to 12 wherein the second electrode is thicker than the piezoelectric material and/or thicker than the first electrode.
14 The acousto-optic device of any of claims 9 to 13 further comprising a light source arranged to direct input light through the second electrode and into the piezoelectric material to set up an optical standing wave in the piezoelectric material and/or the second electrode.
15. The acousto-optic device of any of claims 9 to 14 wherein the acoustic driver is arranged to set up an acoustic standing wave in the piezoelectric material and the second electrode.
16 The acousto-optic device of any of claims 9 to 15 wherein the second electrode has an inner face which faces towards the piezoelectric material, and an outer face which faces away from the piezoelectric material and has a metal coating.
17 The acousto-optic device of any of claims 9 to 16 wherein the second electrode is coupled to electrical ground.
18 The acousto-optic device of any of claims 9 to 17 wherein the first electrode is not optically transparent.
19 An acousto-optic device comprising: an acoustic resonant cavity an acoustic driver arranged to set up an acoustic standing wave in the acoustic cavity; an optical resonant cavity which is at least partially co-located with the acoustic cavity; and a light source arranged to set up an optical standing wave in the optical resonant cavity.
20 An acousto-optic device array comprising an array of acousto-optic devices according to any of claims 9 to 19.