Optical device

Abstract

An optical device 1 configured to modulate light having a design wavelength is described. The optical device comprises a liquid crystal layer 2 disposed between, or in contact with, a first electrode

Description

Field The invention relates to an optical device for modulating light, in particular to 5 integration of subwavelength structures in liquid crystal-based light modulating devices. Background Liquid crystal (LC)-based light modulating devices, such as liquid crystal on silicon io (LCoS), modulate information carried by incident light. The information can take the form of amplitude, phase, polarization, propagation direction, angular momentum, and the like. In LC-based light modulating devices, incident light propagates through the LC layer and is modulated by the rotation of LC molecule director (the alignment of which causes birefringence). The rotation of LC molecule director is actuated by applying 15 electrical voltages across the LC layer or by changing the temperature of the LC material. LC material is widely used in a broad range of applications such as projectors, displays, smart glasses, and the like. However, LC-based devices generally have limited 20 resolution and response time. These limitations are fundamentally rooted in the LC material properties. Conventional LC-based light modulating devices rely on the propagation of incident light inside the LC layer for a certain distance so that sufficient modulation is 25 accumulated. This means that, in conventional LC-based light modulating devices, it is required that the LC layer has a thickness equal to or greater than a minimum thickness over which sufficient modulation is accumulated. In some applications, such as phase-modulating devices, greater LC layer thickness is 30 required for larger wavelengths of incident light. This can increase the response time and reduce the response speed. For devices like LCoS, pixellated backplanes are used to introduce spatial control over LC. Greater LC thickness can make the spatial selectivity of LC less accurate due to electrical fringing fields. For example, LC from nearby pixels can be actuated even the corresponding pixels are not activated. This can lead to the 35 degradation of resolution. LC elast icity also leads to the slow response speed of LC to electrical driving signals. In practical applications such as wavelength selective switching (WSS) using LCoS devices, the continuous behaviour of LC introduces a flyback region, which leads to an 5 inevitable gradient ‘phase reset’ profile and an accompanying reduction in diffraction efficiency. X. Chang et al.: “Electrically tuned active metasurface towards metasurface-integrated liquid crystal on silicon (meta-LCoS) devices”, Optics Express, volume 31, page 5378 10 (2023) describes that immersing a metasurface formed by a single layer of periodic nanostructures with subwavelength dimension and subwavelength period in LC can remove the requirement for the LC layer to have a minimum thickness over which sufficient modulation is accumulated. This is because the light modulation is carried out by the metasurface instead of by the LC layer. 15 20 Reference is also made to US 2017 / 15630396 A, US 2019 / 16505687 A, US 2017 / 15630456 A, US 2019 / 16357288 A, and US 202016 / 882662 A. To better address the aforementioned problems, it is desirable to provide an improved LC-based light modulating device. 18 09 23 Summary According to a first aspect of the invention, there is provided an optical device configured to modulate light having a design wavelength. The optical device comprises a liquid crystal layer disposed between a first electrode and one or more second 5 electrodes. The optical device also comprises a subwavelength metasurface disposed within, or in physical contact with, the liquid crystal layer. The metasurface is formed from a two-dimensional lattice combined with a motif comprising a first region and a second region which is separate from the first region. io The metasurface may take the form of a single layer of periodic nanostructures with subwavelength dimension and subwavelength period. The periodic nanostructures may include, or be formed from, “metaatoms”. Herein, the term “metaatom” may refer to a distinct region capable of interacting with light, for example by modulating the one or more of the amplitude, phase, polarisation or orbital momentum of light, in a way 15 determined by the shape, geometry, size, orientation, and / or arrangement of the metaatom. The metasurface may be oriented parallel to the liquid crystal layer. The liquid ciystal layer may also be referred to as “liquid crystal director”. Light modulation may include changing one or more properties of incident light. 20 Properties of light which may be modulated include, without being limited to, amplitude, phase, polarization, propagation direction, and angular momentum. The first electrode may be transparent. The first electrode may by formed of a conductive oxide. The first electrode may be formed of a conductive, transparent 25 polymer. The first electrode may be formed of metal sufficiently thin to permit passage of light, for example silver less of than or equal to 10 nm thickness. The first electrode may be formed of metal patterned to form a mesh to permit transparency. The second electrode may be transparent. The second electrode may by formed of a 30 conductive oxide. The second electrode may be formed of a conductive, transparent polymer. The second electrode may be formed of metal sufficiently thin to permit passage of light, for example silver less of than or equal to 10 nm thickness. The second electrode may be formed of metal patterned to form a mesh to permit transparency. 35 The liquid crystal layer may be nematic. The liquid crystal layer may be chiral. The liquid crystal layer may be ferroelectric. The thickness of the liquid crystal layer may be 18 09 23 maintained by spacers. For example, using glass spacers in the same way as in prior art liquid crystal display devices. Alternatively, regions of patterned photoalignment material may be used as spacers. The liquid crystal layer may be encapsulated by glues / adhesive layers. 5 The first and second regions may be regarded as separate if they do not form a continuously connected region when viewed parallel to a thickness direction of the optical device. Equivalently, the first and second regions may be regarded as separate if they do not form a continuously connected region when viewed perpendicular to the io metasurface. The first and second regions may take the form of first and second “metaatoms”. The first and second regions may be separated by a region filled by the liquid crystal layer. The first and second regions may be separated by a region filled by a filler material. The filler material may also be referred to as “planarizing material”. The filler material may have a different dielectric constant from the metaatoms forming 15 the metasurface. The optical device may be an active device. In other words, the modulation of light by the optical device may be varied by application of a voltage between the first electrode and the one or more second electrodes. The optical device may be a passive device. 20 The metasurface may be configured to provide an anchoring force which acts to cause alignment of the liquid crystal molecules of the liquid crystal layer. In other words, the metasurface maybe configured such that, in the absence of an electric field applied between the first electrode and second electrode(s), the free energy of the liquid cry stal 25 in contact with the metasurface is minimised by the alignment of liquid crystal molecules along the interface with the metasurface. One or more surfaces of the first and second regions of the metasurface may be coated with a functionalization layer which is configured to modify the anchoring force for 30 alignment of the liquid crystal layer. The metasurface may be anisotropic and configured to provide an anchoring force for alignment of the liquid crystal layer. The metasurface may be anisotropic in that each of the first and second regions are not identical in all directions when considered 35 individually. The metasurface may be anisotropic in that each of the first and second regions have no more than two-fold rotational symmetry-. The metasurface may be 18 09 23 anisotropic in that each of the first and second regions have no more than one-fold rotation symmetry. The metasurface may be anisotropic in that the metasurface has no more than two-fold rotational symmetry. The metasurface may be anisotropic in the sense that the metasurface has no more than one-fold rotation symmetry. 5 The first region may be offset from the second region. The second region may differ from the first region by one or more of shape, size, orientation relative to the lattice, and material. io The first region may have a first shape and the second region may have a second shape. The first shape may be symmetric or asymmetric. The first shape may be regular or irregular. The first shape may have one or more vertices. The first shape may take the form of a rectangle, a square, a circle, a triangle, an annulus, or an arc of an annulus. 15 The second shape be of any type described in relation to the first shape, and may be the same as, or different to, the second shape. The second region may take the form of a cut-out of the first region and first. Second region(s) may be formed from different material(s) to the first region(s). The second 20 region may be wholly enclosed by the first region within the plane of the metasurface. The second region may take the form of a void filled by the liquid crystal layer. For example, the first region may take the form of a metaatom, whilst the second region takes the form of a void formed within (in the plane of the metasurface), and wholly enclosed by, the metaatom first region. 25 According to a second aspect of the invention, there is provided an optical device configured to modulate light having a design wavelength. The optical device comprises a liquid crystal layer disposed between a first electrode and one or more second electrodes. The optical device also comprises a subwavelength metasurface disposed 30 within, or in physical contact with, the liquid crystal layer. The metasurface is anisotropic and configured to provide an anchoring force for alignment of the liquid crystal layer. The second aspect may include features corresponding to any features of the first 35 aspect. Definitions applicable to the first aspect (or features thereof) may be equally applicable to the second aspect (or features thereof). 18 09 23 The following optional features are equally applicable to the optical device of the first aspect and the optical device of the second aspect. 5 The metasurface may take the form of a one-dimensional lattice. The metasurface may take the form of a subwavelength grating. The metasurface may take the form of a two-dimensional lattice. The metasurface may be formed from a two-dimensional lattice combined with a motif comprising a single io region. The single regions of adjoining unit cells may be connected along a direction. The single regions may be separated transverse to the direction by regions filled by the liquid crystal layer. The single regions may be separated transverse to the direction by regions filled by a filler material. 15 The metasurface may be formed from a two-dimensional lattice combined with a motif comprising a first region and a second region which is separate from the first region. The first region and the second region may be configured as described in relation to the optical device of the first aspect. The first regions of adjoining unit cells may be connected along a direction. The second regions of adjoining unit cells may be 20 connected along the direction. In other words, the metasurface may take the form of a subwavelength grating in which the thicknesses of the elements forming the grating are periodic along their lengths. The first and second regions may be separated by regions filled by the liquid crystal layer. The first and second regions may be separated a regions filled by a filler material. 25 The liquid crystal layer may be confined between a first surface and a second surface, wherein the second surface is parallel to the first surface. The first electrode may be supported on the first surface. The second electrode may be 30 supported on the second surface. The metasurface may be supported on the first surface or the second surface. The metasurface may be supported on the first surface and a second metasurface may be supported on the second surface. 18 09 23 The metasurface may be a first metasurface. When the second metasurface is used and the first metasurface is anisotropic, the second metasurface may be anisotropic and oriented at an angle to the first anisotropic metasurface. In this way, twisting of the liquid crystal director may be induced between the first and second surfaces. 5 The optical device may be configured for operation in reflection. The optical device may be configured for operation in transmission. The metasurface may be formed from metallic material(s) such as gold (Au), silver io (Ag), and aluminium (Al). The metasurface may be formed from dielectric material(s) such as titanium dioxide (Ti02) and silicon nitride (Si3N4). The metasurface may be formed from semiconductor materials such as silicon (Si) and gallium nitride (GaN). The metasurface may be manufactured by a method comprising depositing a thin film 15 layer on the first surface and / or the second surface, patterning the thin film layer, and etching the thin film layer. Depositing the thin film layer may comprise one or more of chemical vapour deposition and physical vapour deposition. Patterning the think film layer may comprise lithography such as UV lithography, electron beam lithography and nanoimprint lithography. Etching the thin film layer may comprise one or more of dry 20 etching and wet etching. The optical device may further comprise: an anti-reflective layer disposed on the first electrode side of the liquid crystal layer; and / or an anti-reflective layer disposed on the second electrode side of the liquid crystal layer. 25 A thickness of the liquid crystal layer required to obtain a given phase shift of light at the design wavelength may be reduced relative to a thickness of a liquid crystal layer in a device which is identical except for omitting the metasurface. 30 The one or more second electrodes may take the form of an array of individually addressable second electrodes. The individually addressable second electrodes may also be referred to as “pixels”. Apparatus may be provided which comprises the optical device and a power supply 35 configured to apply potential differences between the first electrode and each of the one or more second electrodes. 18 09 23 The apparatus maybe configured to control the optical device as a filter for a bandwidth centred on the design wavelength. The apparatus may be configured to control the optical device as a wavefront shaper for a bandwidth centred on the design 5 wavelength. The apparatus may be configured to control the optical device to perform beamsteering for a bandwidth centred on the design wavelength. The apparatus may be configured to control the optical device as an amplitude modulator. The apparatus may be configured to control the optical device as a display. The apparatus may be configured to control the optical device as a switch. io According to a third aspect of the invention, there is provided a method comprising modulating light using: an optical device according to the first aspect or the second aspect; or an apparatus comprising an optical device according to the first aspect or the second aspect and a power supply configured to apply potential differences between the 15 first electrode and each of the one or more second electrodes. The method may, when using an apparatus comprising an optical device according to the first aspect or the second aspect and a power supply configured to apply potential differences between the first electrode and each of the one or more second electrodes, 20 further comprise controlling the modulation of the light by applying potential differences between the first electrode and at least one of the one or more second electrodes of the apparatus. The third aspect may include features corresponding to any features of the first aspect 25 or the second aspect. Definitions applicable to the first aspect (or features thereof) or the second aspect (or features thereof) may be equally applicable to the third aspect (or features thereof). According to a fourth aspect of the invention, there is provided a method comprising 30 manufacturing the optical device of the first aspect or manufacturing an apparatus incorporating the optical device of the first aspect. Definitions applicable to the first aspect (or features thereof), the second aspect (or features thereof), or the third aspect (or features thereof) may be equally applicable to the fourth aspect (or features thereof). 18 09 23 Brief description of drawings Figure 1A is a schematic side view of a first optical device in accordance with the present specification. Figure 1B is an exploded view of the first optical device; 5 Figure 1C is a schematic showing side (top) and plan (bottom) views of the metasurface of the first optical device; Figure 1D is a plot showing the reflection amplitude and reflected phase as a function of LC director angle for the first optical device; Figures 1E and 1F are photographs of the liquid crystal layer of the first optical device; io Figures 1G and 1H are photographs of a liquid crystal layer of an optical device which is identical to the first optical device apart from that it does not include a metasurface; Figure 2A is a schematic side view of a second optical device; Figure 2B is a plot showing the reflection amplitude and reflected phase as a function of LC director angle for the second optical device; 15 Figure 3A is a schematic showing side (top) and plan (bottom) views of the metasurface of a third optical device; Figure 3B is a plot showing the reflection amplitude and reflected phase as a function of LC director angle for the third optical device; Figure 4A is a schematic side view of a fourth optical device; 20 Figure 4B is a schematic showing side (top) and plan (bottom) views of the metasurface of the fourth optical device; Figure 4C is a plot showing the reflection amplitude and reflected phase as a function of LC director angle for the fourth optical device; Figure 5A is a schematic showing side (top) and plan (bottom) views of the metasurface 25 of a fifth optical device; Figure 5B is a plot showing the reflection amplitude and reflected phase as a function of LC director angle for the fifth optical device; Figure 6A is a schematic showing side (top) and plan (bottom) views of the metasurface of a sixth optical device; 30 Figure 6B is a plot showing the reflection amplitude and reflected phase as a function of LC director angle for the sixth optical device; Figure 7A is a schematic showing side (top) and plan (bottom) views of the metasurface of a seventh optical device; Figure 7B is schematic plan view of the metasurface of the seventh optical device; 35 Figure 7C is a plot showing the reflection amplitude and reflected phase as a function of LC director angle for the sixth optical device; 18 09 23 Figure 8 is schematic plan view of an eighth optical device; and Figure 9 is a schematic side view of a ninth optical device. Detailed description of certain embodiments 5 Metasurfaces, for example formed using periodic arrays of metaatoms, can be engineered to interact with light in a specific manner. For instance, metaatoms forming a metasurface can be designed to resonate with light at certain wavelength, leading to an enhanced electromagnetic effect such as scattering, reflection, and phase shift. Through proper design of a metasurface built up from such metaatoms, these combined io effects can be utilised to provide a desired modulation of light. Such metasurfaces may also be built up with other elements such as subwavelenth grating structures formed of nano-wires. The description and figures of the present specification descried a variety of examples of metasurfaces useful for modulating one or more properties of transmitted or reflected light. 15 Carrying out light modulation with a metasurface, compared to a liquid crystal layer, of a LCoS derice can reduce the minimum liquid crystal layer thickness over which a sufficient modulation is accumulated. In X. Chang et al.: “Electrically tuned active metasurface tow ards metasurface-integrated liquid crystal on silicon (meta-LCoS) 20 devices”, Optics Express, volume 31, page 5378 (2023) (hereinafter “Chang2O23”), the inventors reported on using metasurfaces formed using primitive unit cells and single metaatom motifs, in order to reduce the thickness of liquid crystal layer needed for a given phase shift. The results of Chang2O23 did also display significant suppression of reflected light intensity, coupled with a transition across the phase range which was 25 relatively sharp. In the present specification, improved metasurface designs are discussed. For example, two-region motifs for a metasurface unit cell, which have resulted in improvements (i.e. increases) in the reflected / transmitted intensity. Additionally, in some examples 30 described herein the transition of the phase of the modulated light is a more gradual function of voltage applied between the first and second electrodes (and hence LC director angle), providing improved control. In particular, see Figures 1D, 2B, 3B, 5B, 6B, and 7C, and compare against Figure 4B corresponding to an optical device incorporating the metasurface of Chang2O23. 18 09 23 Moreover, in prior art LC-based devices, LC molecules are aligned through a two-step process: firstly, a thin layer of organic alignment material is applied to a surface by spin-coating or spray-coating, then an alignment material is aligned by a treatment such as mechanical rubbing or photoalignment. Inorganic alignment material such as 5 silicon dioxide deposited via vacuum deposition can also be used to align LC. As described herein, the inventors have discovered that these alignment processes are not required for some metasurface-integrated LC devices, as described in detail hereinafter (see Figures 1B, 1C, 3A, 5A, 6A, 7A, 8, 9, and 10). This can lead to an 10 improvement in the performance of metasurface-integrated LCoS devices, since the LC can be in direct contact with the metasurface without an intervening alignment layer. This can also lead to a simplification of the device fabrication process, as well as an accompanying reduction in cost, since there is no longer a need to include a separate alignment process in the device fabrication process. 15 In the following, like parts are denoted by like reference numerals. First example of an optical device A first optical device 1 shall be discussed in more detail with reference to Figures 1A to 20 1H. Figures 1A and 1B show a schematic side view and an exploded view of the first optical device 1 respectively. The first optical device 1 includes a liquid crystal layer 2 interposed between a first electrode 3 of a superstrate 4 and second electrodes 5 of a 25 backplane 6. A subwavelength metasurface 7 is disposed within in the liquid crystal layer 2. Figure 1C is a schematic showing side (top) and plan (bottom) views of a unit cell of the metasurface 7 of the first optical device 1. The metasurface 4 is formed from a two-dimensional lattice combined with a motif 30 comprising first regions and second regions which are separate from the first regions. In the example of the first optical device, the first and second regions take the form of respective first metaatoms 14 and second metaatoms 15 which are separate from (and different to) the first metaatoms 14. In this example, the first metaatoms have a first shape which is different to a second shape of the second metaatoms. Specifically, the 35 first shape extends further along the y direction than the second shape (different aspect ratio of the rectangular first 14 and second 15 metaatoms). In other examples, the first 18 09 23 shape may be the same as the second shape, or the first shape may be different to the second shape in a different way, such as being formed of materials having different dielectric constants. 5 The superstate 4 includes a first anti-reflective layer 8 and a glass substrate 2. In other examples, however, this need not be the case. For example, the anti-reflective layer 8 may be omitted, and the substrate may be formed from an electrically insulating transparent material which is not glass. The backplane 6 includes an alignment layer io, a second anti-reflective layer n, and a protective layer 12. 10 Including a first anti-reflective layer 8 and / or a second anti-reflective layer 11 can help to enhance the optical transmission of certain wavelengths of incident light. For instance, the transmission may be enhanced for visible light having a wavelength equal to or greater than 400 nm and equal to or less than 700 nm and / or for near infrared 15 light having a wavelength equal to or greater than 800 nm and equal to or less than 2500 nm, depending on the applications. The AR coating may be a single layer or multi-layer stack, and may be formed from dielectric material(s) such as titanium dioxide (Ti02) and silicon nitride (Si3N4). 20 The metasurface 7 is anisotropic and configured to provide an anchoring force for alignment of the liquid crystal layer 5. This means that the alignment layer 10 may be omitted. In examples in which an alignment layer 10 is present, it can take the form of organic 25 material (prepared by mechanical rubbing or photoalignment) or inorganic material (prepared by vacuum deposition). For instance, the alignment layer 10 may take the form of polyimide-based materials (for the case of alignment via mechanical rubbing), azebenzene (for the case of alignment via photoalignment), or silicon oxide deposited by vacuum deposition on top of the second anti-reflective layer 11 at a tilted angle. 30 The second electrodes 5 takes the form of a pixel array which is coupled to integrated addressing circuits 13. Different voltages can be applied to different second electrodes 5 in the pixel array. This means that different LC director angles can be provided in different parts of the liquid crystal layer 2 in x-z plane (as illustrated in Figure 1A) 35 and / or the y-z plane. Consequently, metaatoms corresponding to different pixels can be tuned differently so as to interact with light differently, depending on the voltage 18 09 23 applied to the corresponding pixel. In other examples, there may be only one second electrode to provide global control over the liquid crystal layer 2. The one or more second electrodes 5 are reflective and may be formed from aluminium 5 or other reflective conducting material(s). A protective layer 12 is disposed on the pixel array and the integrated addressing circuits 13. The protective layer may be formed from silicon dioxide (Si02) deposited by physical vapour deposition. The integrated addressing circuits 13 may be based on static random-access memory (SRAM) circuit or dynamic random-access memory (DRAM) circuit, adapted from thin film transistor 10 (TFT) technologies in LC displays. The integrated circuit may be further electrically bonded to a driving board, providing the hardware interface to computer(s). Such integrated control is not required, and other approaches to controlling voltages applied to the LC layer 2 are equally compatible. For instance, in other examples, 15 particularly examples with few pixels (compared to LC displays) and / or with coarse pixels or other regions, an external driver for the one or more second electrodes may be provided instead of using integrated control by a TFT layer. Example of suitable materials for implementing the first optical device 1 include 20 forming the glass substrate 9 from borosilicate glass with a thickness of 1 mm, forming the first electrode 3 from indium tin oxide (ITO), forming the first and second anti-reflective layers 8,11 for operation at a wav elength of 1550 nm, and using a nematic LC as the liquid crystal layer 2. 25 The first optical device 1 is designed to be operated in the reflection mode in which incident light is modulated by metaatoms immersed in the liquid crystal layer 2 and further reflected by the reflective pixels, forming the output light. In other examples, the backplane may alternatively take the form of transmissive circuitry7, or transparent electrodes driven externally, permitting such a modified first optical device 1 to be 30 operable in a transmissive mode. The metasurface 7 of the first optical device 1 has a rectangular lattice, with a motif comprising the first 14 and second 15 metaatoms. Incident light is linearly polarized in the y direction, as indicated by the arrow labelled E. The incident direction is indicated 35 by the arrow labelled k. In the absence of a electric field applied between the first 3 and second 5 electrodes, the LC director is aligned along the y-direction. 18 09 23 The metaatoms 14,15 are made of silicon. A silicon thin film is first coated on top of the superstrate 6 through thin film deposition technique(s) (such as chemical vapour deposition and physical vapour deposition). The silicon thin film is then patterned 5 using nanofabrication technique(s) (such as UV lithography, electron beam lithography and nanoimprint lithography). The pattern is purposely designed, optimized and generated by dedicated computer software for desired wavelengths of operation. The modelling may be based on one or more of modelling Mie scattering, modelling operation as a dielectric waveguide, and modelling operation as a dielectric resonator. 10 Finally, the metaatoms are formed by etching technique(s) (such as dry etching and wet etching). As mentioned hereinbefore, the metaatoms are periodically arranged in a single layer of 2D rectangular lattice, and Figure 1C only depicts a single unit cell. In this example, the 15 metasurface 7 is formed from a two-dimensional lattice combined with a motif comprising a first region in the form of the first metaatom 14 and a second region in the form of a second metaatom 15. The first metaatom 14 is separate from the second metaatom 15. Although the lattice structure is shown as rectangular in Figures 1B and 1C, the lattice structure can take a different form. For example, the lattice structure may 20 be rectangular or hexagonal. In general, the lattice structure may be based on any of the five possible 2D Bravais lattices. More specifically, the metaatom unit cell of the Figure 1C example is composed by two silicon bars. The particular dimensions of the unit cell may be varied according to a 25 desired wavelength of operation (or “design wavelength”. For example, for modulation of light at and around 1550 nm, suitable geometrical parameters are: height H = 190 nm, the width of two bars Xi = X2 = 170 nm, the length of two bars Yi = 610 nm, Y2 - 490 nm, period in x and y direction Px - 705 nm, Py - 740 nm. 30 The theoretically calculated reflection amplitude and reflected phase at 1550 nm are shown in Figure 1D with solid lines and dashed lines, respectively. The LC director angle is measured in the y-z plane relative to the y-axis as shown in Figures 1A to 1C. Compared to the metasurface of Chang2O23, which is used in the optical device for 35 which comparable data is shown in Figure 4C, the minimum reflectance of the first 18 09 23 optical device i configured for 1550 nm is greatly increased, from about 0.2 to about 0.85. Moreover, the anisotropic patterning of the metasurface 7 of the first optical device 1 5 also results in an unexpected ability to align LC directors without the need for a photoalignment material and photoalignment process. Figures 1E and 1F are photographs of the liquid crystal layer 2 of the first optical device 1. Figure 1E shows the metasurface observed at an angle of o° relative to crossed 10 polarizers, and Figure 1F shows the metasurface 7 observed at an angle of 450 relative to the crossed polarizers. The square represents 1000 x 1000 unit cells. The images are taken with a microscope through crossed polarizers. The alignment effect of the metasurface 7 is indicated by the change in intensity when it is observed at o° and 450 relative to the crossed polarizers, which is clearly shown in Figures 1E and 1F. The 15 average intensity ratio between Figure 1F and Figure 1E is 3.2. Figures 1G and 1H are photographs of a liquid crystal layer of an optical device which is identical to the first optical device 1 apart from that it does not include a metasurface 7. The intensity variation is not observed in this optical device, with the average intensity 20 ratio between Figure 1H and Figure 1G being 1.0. This experimental verification demonstrates the alignment effect of the metasurface 7 on the liquid crystal layer 2, which eliminates the need for photoalignment material and photoalignment process. 25 Second example of an optical device A second optical device 16 shall be discussed with reference to Figures 2A and 2B. The second optical device 16 is the same as the first optical device, except that the metasurface 17 is supported on a surface of the backplane 6 instead of on a surface of 30 the superstrate 4. Figure 2Ais a schematic side view of the second optical device 16. The protective layer 11 is formed from silicon dioxide (Si02) and coated with an antireflection layer lOi formed from silicon nitride (Si3N4). The first electrode 3 is coated with an alignment 35 layer io2, which is optional. 18 09 23 The theoretically calculated reflection amplitude and reflected phase at 1550 nm are shown in Figure 2B with solid lines and dashed lines, respectively. Whilst the reflectance is reduced so at to be comparable with that of Chang2O23 (see Figure 4C), the transition of phase as a function of LC director angle is broader, allowing for more 5 accurate and sensitive control of phase. Moreover, the anisotropic patterning of the metasurface 7 of the second optical device also results in an unexpected ability to align LC directors without the need for a photoalignment material and photoalignment process. 10 Third example of an optical device A third optical device shall be discussed with reference to Figures 3A and 3B. The third optical device is the same as the first optical device 1, except that the metasurface 7 of the third optical device has a different structure. In the third optical device, the metasurface 7 is formed from a two-dimensional lattice rectangular combined with a 15 motif comprising a first region which is a disk-shaped first metaatom 14a, and a second region which is formed as a sector void 14b in the disk-shaped first metaatom 14a. Figure 3A is a schematic showing side (top) and plan (bottom) views of the metasurface 7of the third optical device. For example, for modulation of light at and around 1550 20 nm, suitable geometrical parameters are: height H = 190 nm, disk diameter D = 500 nm, the inner and outer radius of the sector Ri = 75 nm and R2 = 150 nm, the period Px = 750 nm and Py = 750 nm. The sector angle 0 is 1200. The theoretically calculated reflection amplitude and reflected phase at 1550 nm are 25 shown in Figure 3B with solid lines and dashed lines, respectively. Compared to Chang2O23, comparable data of which is reproduced in Figure 4C, the minimum reflectance of the third optical device configured for 1550 nm is greatly increased, from about 0.2 to about 0.5. 30 Fourth example of an optical device (comparative optical device) The device of Chang2O23 is not susceptible of direct comparison because it used a different layer structure. In order to provide direct comparison, a fourth optical device 18 (or “comparative optical device”) shall be discussed with reference to Figures 4A, 4B, and 4C. The fourth optical device 18 includes all the same layers as the other optical 35 devices described herein, in combination with a metasurface 7 configured as described in Chang2O23. 18 09 23 Figure 4A is a schematic side view of the fourth optical device 18. Transparent glass substrates 19,20 are both coated with indium tin oxide (ITO) 21, 23. A metasurface 23 is coated with photoalignment material (not shown). The substrate 20 is coated with an 5 alignment layer 24 taking the form of photoalignment material. The metasurface 23 is immersed in a liquid crystal layer 25. Figure 4B is a schematic showing side (top) and plan (bottom) views of the metasurface of the fourth optical device. In contrast to the preceding examples, in the fourth optical 10 device 18 the metaatoms are squat cylinders and each have the same dimensions. Thus, a motif of the unit cell includes only single region. The values for metaatom height (H), diameter (D) and periodicity (P) are H = 190 nm, D = 530 nm, P = 890 nm. The reflection and reflected phase are plotted against LC director angle In Figure 4C. 15 The reflection has a minimum when LC director angle is around 450. The phase shift of the reflected light spans over 2n when LC director angle is swept from to0 to 8o°. An alignment process is necessary7 in this example. Metasurface 23 can be aligned by photoalignment. The backplane 6 can be aligned by rubbing or vacuum deposition. 20 Fifth example of an optical device A fifth optical device shall be discussed with reference to Figures 5A and 5B. The fifth optical device is the same as the first optical device 1, except for using a different metasurface structure. In the fifth optical device, the metasurface 7 is formed from a two-dimensional rectangular lattice combined with a motif comprising a first region in 25 the form of an elliptically-shaped first metaatom 14 having a rotation angle of 0 of +100 and a second region in the form of an elliptically-shaped second metaatom 15 having a rotation angle of 0 of -10°. Figure 5A is a schematic showing side (top) and plan (bottom) views of the metasurface 30 of the fifth optical device. For example, for modulation of light at and around 1550 nm, suitable geometrical parameters are: height H = 190 nm, short axis length Di = 180 nm, long axis length D2 = 500 nm, period in x direction and y direction Px = 860 nm and Py = 680 nm. 35 The theoretically calculated reflection amplitude and reflected phase at 1550 nm are shown in Figure 5B with solid lines and dashed lines, respectively. Compared to the 18 09 23 data for the comparative fourth optical device shown in Figure 4C, the minimum reflectance of the fifth optical device configured for 1550 nm is greatly increased, from about 0.2 to about 0.8. Furthermore, the transition of reflected phase as a function of LC director angle is broader, allowing for more accurate and sensitive control of phase. 5 Moreover, the anisotropic patterning of the metasurface 7 of the fifth optical device also results in an unexpected ability to align LC directors without the need for a photoalignment material and photoalignment process. Sixth example of an optical device 10 A sixth optical device shall be discussed with reference to Figures 6A and 6B. The sixth optical device is the same as the first optical device 1 except that a differently structured metasurface 7 is used. In the sixth optical device, the metasurface 7 is formed from a two-dimensional rectangular lattice combined with a motif comprising a first region which is a bar-shaped first metaatom 14 and a second region in the form of circular 15 disk-shaped second metaatom 15. Figure 6A is a schematic showing side (top) and plan (bottom) views of the metasurface of the sixth optical device. For example, for modulation of light at and around 1550 nm, suitable geometrical parameters are: height H = 190 nm, bar width Xi = 600 nm and 20 bar length ¥1 = 200 nm, disk diameter = 380 nm, period in x direction Px = 780 nm and in y direction Py = 780 nm. The centres of bar and disk are overlapped along the illustrated x-axis, with the centre of the top half and the bottom half of the unit cell, respectively. 25 The theoretically calculated reflection amplitude and reflected phase at 1550 nm are shown in Figure 6B with solid lines and dashed lines, respectively. Compared to the data of the comparative (fourth) optical device shown in Figure 4C, the minimum reflectance of the sixth optical device configured for 1550 nm is increased, from about 0.2 to about 0.4 (roughly doubled reflected intensity). Moreover, the anisotropic 30 patterning of the metasurface 7 of the sixth optical dev ice also results in an unexpected ability to align LC directors without the need for a photoalignment material and photoalignment process. Seventh example of an optical device 35 A seventh optical device shall be discussed with reference to Figures 7A to 7C. The seventh optical device the same as the first optical component 1 except that the 18 09 23 metasurface 7 is structured differently. In the seventh optical device the metasurface 7 takes the form of a subwavelength grating 14 which is anisotropic and configured to provide an anchoring force for alignment of the liquid crystal layer 2. 5 Figure 7A is a schematic showing side (top) and plan (bottom) views of the subwavelength grating. Figure 7B is a larger schematic plan view of the subwavelength grating 14, in which the plan view of Figure 7A is indicated with the chained rectangle. In this example, the subwavelength grating 14 is perturbed periodically. In this example, a circular structure is overlapped with the grating and shifted in y direction. 10 Without wishing to be bound by theory, the mirror asymmetry7 of each grating element along its length is believed to introduce perturbations giving rise to sharp spectral features. For example, for modulation of light at and around 1550 nm, suitable geometrical parameters are: height H = 190 nm, grating width W = 200 nm and circle radius R = 210 nm, shift in y direction dy’ = 50 nm, period in x direction Px = 750 nm 15 and in y direction Py = 800 nm. The theoretically calculated reflection amplitude and reflected phase at 1550 nm are shown in Figure 7C with solid lines and dashed lines, respectively. Compared to Chang2O23, comparable data of which is reproduced in Figure 4C, the minimum 20 reflectance of the seventh optical device configured for 1550 nm is greatly increased, from about 0.2 to about 0.4. Moreover, the anisotropic patterning of the metasurface 7 of the seventh optical device also results in an unexpected ability to align LC directors without the need for a photoalignment material and photoalignment process. 25 Eighth example of an optical device An eighth optical device shall be discussed in more detail with reference to Figure 8. The eighth optical device is a modified version of the seventh optical device in which the subwavelength grating 14 has a different shape. 30 Figure 8 is schematic plan view of the eighth optical device. The black regions indicate the first regions and the white parts indicate the liquid crystal layer 2. The dashed rectangle is the unit structure. The modulation on the sides is optional. In other examples in which the gratings are straight, there may be two grating bars in 35 each unit cell, in other words two or more sub-gratings may be interleaved to generate a grating unit supercell. Like the first, second, fifth, sixth, and seventh optical devices, the 18 09 23 eighth optical device is configured to provide an anchoring force for alignment of the liquid crystal layer 2. Thus, the anisotropic patterning of the metasurface 7 of the eighth optical device also results in an unexpected ability to align LC directors without the need for a photoalignment material and photoalignment process. 5 Ninth example of an optical device A ninth optical device shall be discussed in more detail with reference to Figure 9. The ninth optical device is a specific implementation of the first optical device 1 in which 10 areas between the metaatoms of the metasurface 7 include a filler (or “planarising”) material 26 which is not liquid crystal. The filler material 26 may have a different dielectric constant from the metasurface 7, for example, the first and second metaatoms 14,15 or, in examples with a subwavelength grating, the subwavelength grating 14. 13 Modifications It will be appreciated that various modifications maybe made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design and use of optical devices for modulating light, and which may be used instead of or in addition to features already described herein. 20 Features of one embodiment may be replaced or supplemented by features of another embodiment. The first to third and fifth to ninth optical devices have been described and illustrated as using motifs / grating elements which are formed from a relatively small number of 25 basic shapes. However, the principles described herein are more generally applicable, and more complex shapes of motif and / or grating element shape may be used. For example, the principles of the first optical device 1 may be implemented using a unitcell motif having a “freeform” geometry- which may be iteratively generated and / or optimised. For example, during each iteration the reflection and phase for the current 30 shape of “freeform” geometry may be modelled, and the shape modified for the subsequent iteration. In this way, a motif satisfying the principles explained in relation to the preceding first to first to third and fifth to ninth optical devices (for example numbers of distinct regions, asymmetry- to rotations and so forth) may be generalised and optimized for particular applications, design wavelengths and so forth. 18 09 23 In some examples, the geometry of the metasurface may be generated and / or optimised using machine learning / deep learning (for example trained using prior optimised shapes), or any other suitable topology optimization approach. 5 A machine learning model may be trained on outputs of the fully modelled iterative optimisation described hereinbefore. The trained machine learning model may subsequently be used to generate starting geometries for the fully modelled iterative optimisation process when applied to different applications, design wavelengths and so forth. io When a motif includes first and second regions which are separate, this applies to the unit cell internally. First regions of adjacent unit cells may be connected to one another. Additionally or alternatively, second regions of adjacent unit cells may be connected to one another. 15 Particular options and alternatives are specified in the dependent claims appended hereto, and in the preceding summary section. Although claims have been formulated in this application to particular combinations of 20 features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. 25 The applicants hereby give notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

1. An optical device configured to modulate light having a design wavelength, comprising:5 a liquid crystal layer disposed between a first electrode and one or more secondelectrodes;a subwavelength metasurface disposed within, or in contact with, the liquid crystal layer, wherein the metasurface is formed from a two-dimensional lattice combined with a motif comprising a first region and a second region which is separate io from the first region.

2. The optical device of claim i, wherein the wherein the metasurface is anisotropic and configured to provide an anchoring force for alignment of the liquid crystal layer.

153. The optical device of claims i or 2, wherein the first region is offset from the second region.

4. The optical device of any one of claims 1 to 3, wherein the second region differs20 from the first region by one or more of:shape;size;orientation relative to the lattice; and material.

255. The optical device of any one of claims 1 to 4, wherein the second region takesthe form of a cut-out of the first region and wherein first and second regions are formed from different materials.30 6. An optical device configured to modulate light having a design wavelength,comprising:a liquid crystal layer disposed between a first electrode and one or more second electrodes;a subwavelength metasurface disposed within, or in contact with, the liquid35 crystal layer, wherein the metasurface is anisotropic and configured to provide an anchoring force for alignment of the liquid crystal layer.

7. The optical device of claim 6, wherein the metasurface takes the form of a onedimensional lattice.5 8. The optical device of claim 6, wherein the metasurface takes the form of a two-dimensional lattice.

9. The optical device of any one of claims 1 to 8, wherein the liquid crystal layer is confined between a first surface and a second surface, wherein the second surface is 10 parallel to the first surface.

10. The optical device of any one of claims 1 to 9, wherein the metasurface is supported on the first surface or the second surface.15 11. The optical device of any one of claims 1 to 9, wherein the metasurface issupported on the first surface and a second metasurface is supported on the second surface.

12. The optical device of any one of claims 1 to 11, configured for operation in20 reflection.

13. The optical device of any one of claims 1 to 11, configured for operation in transmission.25 14. The optical device of any one of claims 1 to 13 further comprising:an anti-reflective layer disposed on the first electrode side of the liquid crystal layer; and / oran anti-reflective layer disposed on the second electrode side of the liquid crystal layer.3015. The optical device of any one of claim 1 to 14, wherein a thickness of the liquid crystal layer required to obtain a given phase shift of light at the design wavelength is reduced relative to a thickness of a liquid crystal layer in a device which is identical except for omitting the metasurface.

16. The optical device of any one of claims 1 to 15, wherein the one or more second electrodes take the form of an array of individually addressable second electrodes.

17. Apparatus comprising:5 the optical device according to any one of claims 1 to 16; anda power supply configured to apply potential differences between the first electrode and each of the one or more second electrodes.

18. The apparatus of claim 17, configured to control the optical device as a filter for 10 a bandwidth centred on the design wavelength.

19. The apparatus of claim 17, configured to control the optical device as a wavefront shaper for a bandwidth centred on the design wavelength.15 20. The apparatus of claims 17 or 19, configured to control the optical device toperform beamsteering for a bandwidth centred on the design wavelength.

21. The apparatus of any one of claims 17 to 20, configured to control the optical device as an amplitude modulator.2022. The apparatus of any one of claims 17 to 21, configured to control the optical device as a display.

23. The apparatus of any one of claims 17 to 22, configured to control the optical25 device as a switch.

24. A method comprising modulating light using an optical device of any one of claims 1 to 16 or an apparatus of any one of claims 17 to 23.30 25. The method of claim 24, when using an apparatus of any one of claims 17 to 23,further comprising controlling the modulation of the light by applying potential differences between the first electrode and at least one of the one or more second electrodes of the apparatus.

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