Liquid crystal devices

Abstract

A liquid crystal-based optical device (100) comprises: a first liquid crystal device comprising: a first optical cell (110) comprising a first front substrate (112) and a first rear substrate (114); a first liquid crystal layer (120) disposed between the first front substrate (112) and the first rear substrate (114), the first liquid crystal layer (120) comprising: a non-polymerised liquid crystal portion (122); and a polymerised liquid crystal portion (124) forming a first three-dimensional polymer structure; and a first pair of electrodes (116a, 116b) configured to apply an electric field (E1) to the first liquid crystal layer (120); and a second liquid crystal device having a similar structure to the first liquid crystal device, wherein the first liquid crystal device is coupled to the second liquid crystal device at an interface comprising the first rear substrate (114) and a second front substrate (132) of the second liquid crystal device.

Description

[0001] LIQUID CRYSTAL DEVICES

[0002] TECHNICAL FIELD

[0003] The present invention relates to liquid crystal devices and methods of manufacturing the same.

[0004] BACKGROUND

[0005] Liquid crystals (LC) can be utilised in a wide range of applications due to their electrical, optical, and / or magnetic properties. Typical LC devices comprise a layer of LC material arranged between a pair of electrodes. When a voltage, commonly referred to as a drive voltage, is applied across the electrodes, an electric field is generated across the LC material. If the electric field is sufficient, the molecules of the LC material can arrange themselves into a stable state. In particular, the LC molecules can align themselves along the direction of the electric field. This arrangement of the LC molecules can impart a phase shift to light passing through the LC layer. Thus, by varying the applied drive voltage / electric field, and consequently the arrangement of the molecules, the LC device can be configured to modify light accordingly.

[0006] The optical properties of LCs can be further enhanced and adjusted through the use of polymer networks. By polymerising portions of the LC material within the LC layer, polymer structures can be formed. The LC molecules of the polymer structures are fixed or ‘frozen’ at a particular alignment, and can interfere with or complement the alignment of the LC molecules that are not part of the polymer structures. Thus, by configuring the polymerisation and then selecting an appropriate drive voltage, the modification of light that is enabled by the LC layer may be varied.

[0007] However, ultimately, the LC’s optical properties depend upon the turning range (that is, the range of angles over which the LC molecules can arrange themselves in response to an electric field), and this is limited by the thickness of the LC layer and the birefringence of the LC material. Further improvements in the operational range or capability of LC devices are therefore desirable and could enable their use in further applications.SUMMARY

[0008] According to a first aspect of the invention, there is provided a liquid crystal-based (LC-based) optical device comprising:

[0009] a first liquid crystal device comprising:

[0010] a first optical cell comprising a first front substrate and a first rear substrate;

[0011] a first liquid crystal layer disposed between the first front substrate and the first rear substrate, the first liquid crystal layer comprising:

[0012] a non-polymerised liquid crystal portion; and a polymerised liquid crystal portion forming a first three-dimensional polymer structure; and

[0013] a first pair of electrodes configured to apply an electric field to the first liquid crystal layer; and

[0014] a second liquid crystal device comprising:

[0015] a second optical cell comprising a second front substrate and a second rear substrate;

[0016] a second liquid crystal layer disposed between the second front substrate and the second rear substrate, the second liquid crystal layer comprising:

[0017] a non-polymerised liquid crystal portion; and a polymerised liquid crystal portion forming a second three-dimensional polymer structure; and

[0018] a second pair of electrodes configured to apply an electric field to the second liquid crystal layer;

[0019] wherein the first liquid crystal device is coupled to the second liquid crystal device at an interface comprising the first rear substrate and the second front substrate.

[0020] In essence, the LC-based optical device comprises two LC devices that are stacked on top of one another. The LC devices may have the same, complementary or different three-dimensional polymer structures. As discussed further below, the three-dimensional polymer structures are integrated into the respective liquid crystal layer by selective polymerisation of the desired portions thereof. The non-polymerised liquid crystal portion and the polymerised liquid crystal portion may comprise the same composition (e.g., the same monomer-LC mixture), except that in the polymerised liquid crystal portion the monomers are polymerised to form a polymer network, I.e., part of the liquid crystal layer is polymerised to form the three-dimensional structure, whilepart of the liquid crystal layer remains unpolymerised. This may advantageously enable the LC layers to be manufactured from a single material (e.g., a polymer-LC mixture). This may enable easier manufacturing, compared to having to provide additional, separate structures / components within the LC layer.

[0021] The first liquid crystal layer may comprise:

[0022] a non-polymerised liquid crystal portion of the first liquid crystal layer; and a polymerised liquid crystal portion of the first liquid crystal layer, the polymerised liquid crystal portion forming a first three-dimensional polymer structure within the first liquid crystal layer.

[0023] The second liquid crystal layer may comprise:

[0024] a non-polymerised liquid crystal portion of the second liquid crystal layer; and a polymerised liquid crystal portion of the second liquid crystal layer, the polymerised liquid crystal portion forming a second three-dimensional polymer structure within the second liquid crystal layer.

[0025] The first three-dimensional polymer structure and / or the second three-dimensional polymer structure may be configured to selectively modify a characteristic of incident light used to illuminate the liquid crystal-based optical element. As discussed further herein, the LC-based optical device of the present invention may be used to modify incident light. The three-dimensional polymer structures with the LC layers may function as optical elements which can be configured to provide several desirable optical effects dependent on the shape / structure of the three-dimensional polymer structures. The three-dimensional polymer structures may generally form a grating, a vortex generator or a waveplate / phase plate, for example.

[0026] The first three-dimensional polymer structure and / or the second three-dimensional polymer structure may be configured to selectively modify a phase profile of incident light used to illuminate the liquid crystal-based optical element. The three-dimensional polymer structures may be shaped / structured so as to provide a phase shift to at least part of the incident light. The three-dimensional polymer structures may be configured to shift the phase of the entire incident wavefront (e.g., by introducing a phase delay) or may be configured to modulate the phase of the incident wavefront (e.g., to alter a Fresnel zone or Zernike mode), for example.Depending upon said structures, each LC device may thus be used to modify incident light in some way. For example, an LC device may be provided with a diffraction grating-like polymer structure or a blazed grating structure. Because the first and second LC devices are each provided with a pair of electrodes, they may each be operated individually. By providing a drive voltage (sometimes referred to as a read voltage) across each pair of electrodes, each LC device can be configured to exhibit a desired optical property based upon the three-dimensional polymer structure therein. In various examples discussed herein, either LC device may be switched on or off or may be continuously tuned, thereby adjusting the optical properties of each LC layer and, therefore, the total modification imparted by the LC-based optical device upon the incident light. Depending upon the configuration and operation of each LC device in the stack, the present invention may provide for: polarisation sensitivity / insensitivity while retaining the same optical performance; additional or multi-functional operation; and / or an increased operational range, when compared to a single-layer LC device. These advantages may be realised in LC-based optical elements that function as blazed gratings, vortex and structured beam generators, Fresnel zone plates or Zernike mode aberration correctors, for example. Thus, the present invention may have applications in a wide range of optical systems / technologies.

[0027] The first pair of electrodes may be located at the first front substrate and first rear substrate. Likewise, the second pair of electrodes may be located at the second front substrate and the second rear substrate. Accordingly, the electrode pairs may be configured to apply an electric field across each LC layer between the respective first and rear substrates. By providing a first pair of electrodes either side of the first LC layer and a second pair of electrodes either side of the second LC layer, the first and second LC layers may be operated independently. I.e., different voltages may be provided across the pairs of electrodes, resulting in different electric fields being applied to each LC layer. This may provide for a greater degree of freedom in controlling the LC-based optical device of the present invention. The first and second LC layers may be tuned independently so as to provide desired optical effects. However, other electrode and electric field configurations may be provided. For example, electrodes may be located at the sides of the optical cells. The electrode pairs may be configured to provide an electric field in the plane of the LC layer. The electric field may not be solely across the LC layer or solely in the plane of the LC layer; a non-orthogonal or non-uniform electric field may be applied. Each LC device may comprise more than two electrodes / more than one pair of electrodes.

[0028] The first three-dimensional polymer structure may comprise a spatially varying height profile across at least part of a surface area of the first liquid crystal layer. The second three-dimensional polymer structure comprises a spatially varying height profile across at least part of a surface area of the first liquid crystal layer.

[0029] The height profile (i.e., topology) of the three-dimensional polymer structures may vary across the LC layer. Where the LC layer is in the x-y plane (e.g., sandwiched between substrates and / or electrodes in the x-y plane), the height of the three-dimensional polymer structures may vary in the z-axis. As discussed further below, the three-dimensional polymer structures may have a variety of different structures. The three-dimensional polymer structures may have a discrete height variance, such as posts or grating lines. The posts / grating lines may have a height (e.g., equal to the thickness of the LC layer or a fraction there of) and may be separated by gaps (e.g., with no polymerised portion / zero height or a shallower polymerised portions). The three-dimensional polymer structures may have a discrete height variance, such as a sawtooth pattern in the x-axis or y-axis, or an undulating surface. By configuring the respective heights of the three-dimensional polymer structures within the LC layer, the optical properties of the LC layer (e.g., a phase shift imparted on the incident light) may be configured appropriately. The three-dimensional polymer structures may span the entire surface area of the LC layer, or part thereof.

[0030] The first three-dimensional polymer structure may comprise a plurality of liquid crystal molecules with a constrained (predetermined) orientation. The second three-dimensional polymer structure may comprise a plurality of liquid crystal molecules with a constrained (predetermined) orientation.

[0031] The liquid crystal layers comprise liquid crystal material comprising a plurality of liquid crystal (LC) molecules (e.g., nematic liquid crystals). It will be understood that the orientation of LC molecules may be controlled by varying an electric field across the LC layer. However, in the polymerised liquid crystal portions, developed polymer networks may constrain the orientation of the LC molecules. The polymerised liquid crystal portions may comprise LC molecules whose orientation is fixed. It will beunderstood that this constraint / fixing of the LC molecules may be effective across an expected or nominal working range of electric field strengths that the LC layer(s) is to be subject to. When a voltage is applied to the pair of electrodes either side of the LC layer, the resulting electric field is not strong to overcome the links of the polymer network that constrains the LC molecules. By contrast, the LC molecules in the nonpolymerised liquid crystal portion(s) may be free to move. The LC molecules in the non-polymerised liquid crystal portion(s) may be reorientated by the application of an electric field across the LC layer (by applying a voltage to the pair of electrodes either side of the LC layer).

[0032] The predetermined orientation of the LC molecules in the three-dimensional polymer structure / polymerised portion may differ to the orientation of the LC molecules in the non-polymerised portion when no voltage is applied to the relevant pair of electrodes (e.g., when the LC molecules in the non-polymerised portion are in a relaxed state). The orientation of the LC molecules within the polymerised liquid crystal portions may be set using a ‘writing’ process, as discussed further below. The LC layer may be subject to an electric field when the three-dimensional polymer structure is formed via polymerisation of the relevant portion of the LC layer. This may mean that the LC molecules in the three-dimensional polymer structure are fixed in an excited state.

[0033] For example, the relaxed state of the LC layer may be homogenous alignment (with the LC molecules aligned parallel to the substrate). However, by forming the three-dimensional polymer structure when an electric field is applied across the LC layer, the LC molecules of the three-dimensional polymer structure may be fixed in a homeotropic alignment (with the LC molecules aligned perpendicular to the substrate). The structures may be vice versa in some examples - i.e. the relaxed state of the LC layer may be homeotropic and the LC molecules of the three-dimensional polymer structure may be fixed in a homogenous alignment.

[0034] As discussed further below, by applying an electric filed across the LC layers when in use (by applying a voltage across the pair of electrodes), the LC molecules within the non-polymerised portions can generally be reoriented to match (where the applied voltage was that used during writing of the three-dimensional polymer structures) or not match (e.g., with zero voltage) the orientation of the fixed LC molecules within the three-dimensional polymer structure. This operation may be binary (where theorientation match or do not match) or continuously tuneable (where the LC molecules can be reorientated between the two states). Similarly, by using suitable alignment and a writing process, the orientations of the LC molecules within the polymerised and nonpolymerised portions may match when zero voltage is applied (e.g., when the LC molecules of the non-polymerised portion are in the relaxed state) and not match when a non-zero voltage is applied (e.g., when the LC molecules of the non-polymerised portion are in the relaxed state). Therefore, by configuring the LC layers appropriately and selectively adjusting the voltages that are applied, the optical properties of the device can be varied with a high degree of freedom and repeatability.

[0035] The first LC device and the second LC device may be arranged such that a rubbing direction of the first LC device is non-parallel to a rubbing direction of the second LC device.

[0036] The rubbing directions of the first LC device and the second LC device may be perpendicular to one another, for example. The rubbing direction generally describes the direction in which the LC molecules lie at the substrate for the optical cell, resulting from grooves or a ‘grain’ of the substrate. Due to the birefringence of LCs, each LC layer may be polarisation sensitive. Thus, by arranging the first LC device and second LC device such that their rubbing directions are not aligned, the polarisation sensitivity / selectively of the LC-based optical device may be adjusted or defined.

[0037] Alternatively, the rubbing directions of the first LC device and the second LC device may be parallel to one another, so that both LC devices exhibit the same polarisation sensitivity.

[0038] In some embodiments the first LC device and / or the second LC device may have a homeotropic alignment. An appropriate surface treatment may be used to promote a homeotropic alignment.

[0039] The first three-dimensional polymer structure and the second three-dimensional polymer structure may be the same. When used with perpendicular rubbing directions, as discussed above, this may allow both of the LC devices to modulate light in the same way but for different polarisations of light. This may be used to provide a polarisation insensitive diffraction grating, for example. The first three-dimensional polymerstructure and the second three-dimensional polymer structure may be the same but orientated differently, e.g., horizontal and vertical diffraction gratings.

[0040] The first three-dimensional polymer structure and the second three-dimensional polymer structure may be different. For example, both of the three-dimensional polymer structures may be lens-like structures but having different focal lengths, or may be vortex beam generating structures of different orders. The combination of two different three-dimensional polymer structures may increase the functionality or operational range of the LC-based optical device.

[0041] The first three-dimensional polymer structure and / or the second three-dimensional polymer structure may comprise:

[0042] (i) a plurality of discrete ridges arranged in parallel to one another;

[0043] (ii) a plurality of discrete concentric rings;

[0044] (iii) a continuous structure with a spatially varying height profile, the height profile varying along one or more directions;

[0045] (iv) a continuous structure with a periodic spatially varying height profile; (v) one or two holographic structures; and / or

[0046] (vi) a planar structure with an aperture through the centre of the planar structure.

[0047] The three-dimensional polymer structures may be configured in a variety of different ways, thereby enabling each LC device to have a wide range of optical properties. A plurality of discrete ridges arranged in parallel may provide a diffraction grating, for example. A plurality of discrete concentric rings may provide a lens-like device, such as a zone plate, for example. A continuous structure with a spatially varying height profile, the height profile varying along one or more directions may provide a prism like structure (where the height profile is an angled plane) or a vortex beam generator (where the height profile varies azimuthally), for example. A continuous structure with a periodic spatially varying height profile may provide a spatial modulator or a beam steering device, for example. Holographic structures may be operated in a binary on / off state depending upon the operation of the LC device, for example. A planar structure with an aperture through its centre (i.e., a ring or an annulus) may function as a phase shifting ring. The height profile of the planar structure may be flat or may be ramped. Further examples of each of these configurations are discussed below.‘Discrete’ is used to refer to structures that have step-change boundaries defining the polymerised LC portion. For example, there may be a first polymer ridge of height X, a gap (either with height zero or height < X) and then a second polymer ridge. The ridges can be regarded as being discrete structures. Meanwhile, ‘continuous’ is used to refer to structures that have a gradient-like spatial variation in height. For example, there may be a ramp profile with height increasing across the width of the LC layer. Evidently there may be some overlap between these categories - a sawtooth profile has regions of continuous, ramped height increase but also step-changes at the end of each tooth.

[0048] The first three -dimensional polymer structure and / or the second three-dimensional polymer structures may each comprise an azimuthally varying height profile.

[0049] An azimuthally varying height profile may comprise a ramp that increases in height around the circumference of a circle but is a constant height across the radius of the circle (similar to a spiral staircase). Across 360 degrees of azimuth, the height may continuously increase from zero (i.e., no polymer structure) to the full thickness of the LC layer (i.e. the entire height of the LC layer is polymerised). An azimuthally varying height profile may be used to provide an LC-based optical device that functions as a vortex beam generator.

[0050] The first three-dimensional polymer structure may comprise one azimuthally varying height profile, the variance in height being from 0 to a thickness of the first LC layer across 360 degrees of azimuth. Such a height profile may provide for a continuously tuneable phase depth of 0 to 4TI radians. In other words, the first three-dimensional polymer structure of the first LC device can provide all vortex states, including fractional vortex states, from 0thorder to 2ndorder. The order can be continuously tuned by varying the drive voltage provided to the first pair of electrodes.

[0051] The second three-dimensional polymer structure may comprise n azimuthally varying height profiles, the variance in height being from 0 to the thickness of the second LC layer across 360 / w degrees of azimuth, wherein n is > 2. The second three-dimensional polymer structure may comprise two ramps over 180 degrees of azimuth each, or three ramps over 120 degrees of azimuth each. Providing different azimuthal structures, rather than repeating the structure of the first LC device, can enable a greater range of vortex orders to be achieved. The vortex state order of the second LC device can be adjusted(either continuously or discretely, dependent upon the structure) by varying the drive voltage provided to the second pair of electrodes. In combination with adjusting the vortex state order of the first LC device, the whole LC-based optical device can provide for a greater operational range than a single LC device.

[0052] The periodic spatially varying height profile may comprise a sawtooth profile, a ramp profile or a sinusoidal profile.

[0053] Any suitable height profile may be provided in the first or second three-dimensional polymer structure. The skilled person will understand that polymer structures may be configured to replicate conventional optical elements, depending upon a desired application. For example, a sawtooth profile may be provided such that the LC device functions as a blazed grating. A linear ramp profile may be provided such that the LC device functions as a prismatic reflector / deflector.

[0054] The height profile of the first three-dimensional polymer structure and / or the second three-dimensional polymer structure may be configured to reduce an aberration present in incident light used to illuminate the liquid crystal-based optical element. The aberration may correspond with a Zernike mode. The height profile of the first three-dimensional polymer structure and / or the second three-dimensional polymer structure may correspond with a Zernike mode. The first liquid crystal device may be configured to correct a first Zernike mode, and the second liquid crystal device may be configured to correct a second, different, Zernike mode.

[0055] As discussed herein, the LC-based optical device may be used in a range of optical applications and systems. Such systems may be susceptible to aberrations in their optics. Thus, the optical properties of the first and / or second LC device may be configured (via the configuration of the three-dimensional polymer structures and the drive voltage applied to each pair of electrodes) to correct for said aberration. For example, the first three-dimensional polymer structure may be configured to provide an elongation in a first axis and the second three-dimensional polymer structure may be configured to provide an elongation in a second, orthogonal axis. Thus, by tuning the first and second LC devices (or switching them on / off), the LC-based optical device may be used to correct for an elongation or astigmatism in the incident beam. The aberrations in the wider optical system may correspond to a Zernike mode. The first and / or second LCdevice may be configured to correct for such an aberration. For example, the first and / or second three-dimensional polymer structures may comprise a central structure configured to provide a phase shift to light at the centre of the incident beam (thus compensating for the phase shift caused by an annular aberration).

[0056] The planar structure and aperture may be configured to generate at least one Zernike mode when illuminated with incident light.

[0057] By providing a three-dimensional polymer structure comprising a ring / annulus, the LC device may be operated as a variable Zernike phase plate. A Zernike phase plate may comprise a polymer structure corresponding with a Zernike mode shape. By varying the drive voltage applied to the electrode pair, a phase contrast between the polymer structure and the non-polymerised liquid portion can be adjusted, and an effective amplitude of phase shift corresponding with the Zernike mode of the polymer structure may be continuously varied. Other annular structures may be written into the three-dimensional polymer structure, such as Boersch phase plates.

[0058] The first LC layer may comprise an additional polymerised LC portion forming an additional three-dimensional polymer structure. The first three-dimensional polymer structure may be located adjacent (or anchored to) to the first front substrate and the additional three-dimensional polymer structure may be located adjacent (or anchored to) to the first rear substrate. Similarly, an additional three-dimensional polymer structure may also be provided in the second LC layer.

[0059] Providing additional polymerised LC portions can enable the LC devices to have a plurality of binary states. For example, a first diffraction grating-like polymer structure may be formed using a first write voltage at the bottom of the LC layer, and a second diffraction-grating like polymer structure may be formed using a second write voltage at the top of the LC layer. Thus, by setting the drive voltage to the first or second write voltage respectively, the first or second polymer structures may be switched off respectively. Thus, the LC device can be operated in one of two binary states. Where both the first and second LC devices are provided with such additional polymer structures, they may each be operated independently, so that the LC-based optical device has four total states (or more generally for n stacked LC devices, 2" states).The additional polymerised LC portion may have a different three-dimensional polymer structure to the first and / or second three-dimensional polymer structure. I.e., the additional three-dimensional polymer structure may not simply be a ‘mirror image’ / replica of the first / second three-dimensional polymer structure. The additional three-dimensional polymer structure may comprise a different size, shape or spatial structure and / or frequency compared to the first / second three-dimensional polymer structure. By providing a different polymerised LC portion / three-dimensional polymer structure, the LC device may be able to provide a greater range of optical effects - e.g., by adjusting the drive / read voltage to write voltage of the first / second three-dimensional polymer structure or the additional three-dimensional polymer structure as desired.

[0060] The first front substrate and / or the second rear substrate may have a thickness of at least 1 mm, at least 0.8 mm, at least 1.2 mm or at least 1.5 mm. Having thicker outer substrates (i.e., the front-most and rear-most substrates) may provide the LC-based optical device with sufficient mechanical strength.

[0061] The first rear substrate and / or the second front substrate may have a thickness of no more than 0.5 mm, no more than 0.35 mm or no more than 0.2 mm. Having thinner inner substrates (i.e., substrates that form the interface between adjacent LC devices) may increase the transmittance at the interface and / or reduce aberrations. This may improve the optical performance of the LC-based device. Increased transmittance may also enable both the first and second LC device to be written from the same direction (e.g., writing the second LC device through the first LC device, as discussed further below).

[0062] The first front substrate and the first rear substrate may comprise an electrically conductive material, the electrically conductive material forming the first pair of electrodes. The first front substrate and the second rear substrate may each comprise a glass sheet with a layer of indium tin oxide (ITO) deposited on a surface thereof, for example. Any other suitable material may be used. Plastic, PET, silicon, sapphire or quartz may be used instead of glass, for example. Copper, silver or gold may be used instead of ITO, for example.

[0063] The second front substrate and the second rear substrate may comprise an electrically conductive material, the electrically conductive material forming the second pair ofelectrodes. The substrates and electrodes of the second LC device may have the same structure as the first LC device.

[0064] The first front substrate, the first rear substrate, the second front substrate and / or the second rear substrate may consist of an electrically conductive layer. Rather than having an electrode layered onto a substrate, as discussed above, the electrode and substrate may be the same component. The substrates may each be a sheet of ITO, for example. A single substrate / electrode layer may be thinner than a dual-layer configuration. This may be particularly advantageous at the first rear substrate and the second front substrate, as the thickness of the interface between the first and second LC devices may be reduced. An insulating layer or spacer may be provided between the first rear substrate and the second front substrate so that they are electrically insulated.

[0065] The first rear substrate and the second front substrate may together be formed by a single (or shared) substrate layer. Rather than the first LC device and the second LC device each having a substrate layer at the interface therebetween (i.e., two separate optical cells), the first and second LC devices may share a substrate at the interface. The single substrate layer may be a glass sheet, for example. A first layer of conductive material may be deposited on a first side of the glass sheet (forming one electrode of the first pair of electrodes) and second layer of conductive material may be deposited on a second side of the glass sheet (forming one electrode of the second electrode pair).

[0066] All of the substrates of the LC devices may be transparent. The substrates may be considered ‘transparent’ insofar as they are transparent to a wavelength of interest (e.g., the wavelength of a laser source used to illuminate the LC-based optical device during manufacturer or use). This may mean that light can be transmitted through both the first and second LC devices. This may enable the LC-based optical device to be used as a spatial light modulator (SLM).

[0067] Alternatively, the first front substrate or the second rear substrate may be reflective. The substrate may be considered ‘reflective’ insofar as it reflects a wavelength of interest (e.g., the wavelength of a laser source used to illuminate the LC-based optical device during manufacturer or use). This may enable the LC-based optical device to be used as a deformable mirror (DM). The LC-based optical device may be illuminatedfrom the direction of the first front substrate, with the second rear substrate being reflective so that the light makes two passes through the first and second optical cells.

[0068] The liquid crystal -based optical device may further comprise:

[0069] a third liquid crystal device comprising:

[0070] a third optical cell comprising a third front substrate and a third rear substrate;

[0071] a third liquid crystal layer disposed between the third front substrate and the third rear substrate, the third liquid crystal layer comprising:

[0072] a non-polymerised liquid crystal portion; and a polymerised liquid crystal portion forming a third three-dimensional polymer structure; and

[0073] a third pair of electrodes configured to apply an electric field to the third liquid crystal layer;

[0074] wherein the second liquid crystal device is coupled to the third liquid crystal device at a second interface comprising the second rear substrate and the first front substrate.

[0075] The third liquid crystal layer may comprise:

[0076] a non-polymerised liquid crystal portion of the third liquid crystal layer; and a polymerised liquid crystal portion of the third liquid crystal layer, the polymerised liquid crystal portion forming a third three-dimensional polymer structure within the first liquid crystal layer.

[0077] The third three-dimensional polymer structure may be configured to selectively modify a characteristic of incident light used to illuminate the liquid crystal-based optical element.

[0078] The LC-based optical device may comprise a stack of three, four, or more LC devices. The third (and fourth, etc.) LC device may comprise any or all of the features discussed above with regard to the first and second liquid crystal devices. The third LC device may be arranged relative to the first or second LC device as discussed above. For example, the LC devices may be arranged such that the rubbing direction of the second LC device is at 45 degrees relative to the rubbing direction of the first LC device, and the rubbing direction of the third LC device is at 90 degrees relative to the rubbing direction of the first LC device. Providing more LC devices may enable furtherexpansion or improvement in the functionality discussed herein compared to two stacked LC devices. The third pair of electrodes may be arranged and configured similarly to the first and second pair of electrodes (e.g., located at the third front substrate and third rear substrate and configured to apply an electric field across the LC layer).

[0079] The second front substrate and the second rear substrate may consist of an electrically conductive layer. As discussed above, the substrates may be a sheet of ITO or other electrically conductive material, so that the substrate also functions as the respective electrode. This may be particularly advantageous for the second LC device where three LC devices are stacked together (the second LC device being sandwiched between the first and third LC devices) because both interfaces between LC device may be made thinner.

[0080] According to a second aspect of the invention, there is provided a system comprising:

[0081] a liquid crystal -based optical device comprising:

[0082] a first liquid crystal device comprising:

[0083] a first optical cell comprising a first front substrate and a first rear substrate;

[0084] a first liquid crystal layer disposed between the first front substrate and the first rear substrate, the first liquid crystal layer comprising:

[0085] a non-polymerised liquid crystal portion; and a polymerised liquid crystal portion forming a first three-dimensional polymer structure; and

[0086] a first pair of electrodes configured to apply an electric field to the first liquid crystal layer; and

[0087] a second liquid crystal device comprising:

[0088] a second optical cell comprising a second front substrate and a second rear substrate;

[0089] a second liquid crystal layer disposed between the second front substrate and the second rear substrate, the second liquid crystal layer comprising:

[0090] a non-polymerised liquid crystal portion; and a polymerised liquid crystal portion forming a second three-dimensional polymer structure; anda second pair of electrodes configured to apply an electric field to the second liquid crystal layer;

[0091] wherein the first liquid crystal device is coupled to the second liquid crystal device at an interface comprising the first rear substrate and the second front substrate; and

[0092] a controller configured to independently provide a first drive voltage to the first pair of electrodes and a second drive voltage to the second pair of electrodes.

[0093] The LC-based optical device may be according to the first aspect of the invention.

[0094] The controller, which may comprise a power supply and a manual and / or automated control means can provide a separate voltage to the pair of electrodes of each device. This can allow for each LC device to be operated independently from one another. Therefore, each LC device may be turned on / off (by setting the drive voltage equal to a write voltage) or may be continuously tuned by varying the drive voltage. Thus, the optical properties of the overall LC-based optical device can be adjusted as desired, providing greater control than using a single LC device.

[0095] The system may further comprise a light source configured to illuminate the LCl-based optical device. The light source may be a laser or an LED, for example, with a wavelength configured to match the transmitted wavelength of the LC layers / substrates. Together, the controllable LC-based optical device and the light source may be used for applications including microscopy, microfabrication, steerable / shapable optical system, lidar, or optical communications, for example.

[0096] According to a third aspect of the invention, there is provided a method of manufacturing a liquid crystal-based optical device, the liquid crystal optical device comprising:

[0097] a first liquid crystal device comprising:

[0098] a first optical cell comprising a first front substrate and a first rear substrate;

[0099] a first liquid crystal layer disposed between the first front substrate and the first rear substrate, the first liquid crystal layer comprising:

[0100] a non-polymerised liquid crystal portion; anda polymerised liquid crystal portion forming a first three-dimensional polymer structure; and

[0101] a first pair of electrodes configured to apply an electric field to the first liquid crystal layer; and

[0102] a second liquid crystal device comprising:

[0103] a second optical cell comprising a second front substrate and a second rear substrate;

[0104] a second liquid crystal layer disposed between the second front substrate and the second rear substrate, the second liquid crystal layer comprising:

[0105] a non-polymerised liquid crystal portion; and a polymerised liquid crystal portion forming a second three-dimensional polymer structure; and

[0106] a second pair of electrodes configured to apply an electric field to the second liquid crystal layer;

[0107] wherein the first liquid crystal device is coupled to the second liquid crystal device at an interface comprising the first rear substrate and the second front substrate, and wherein the method comprises:

[0108] applying a first write voltage to the first pair of electrodes;

[0109] while the first write voltage is applied, selectively applying light to liquid crystal material contained within the first optical cell so as to polymerise the liquid crystal material in regions where the light is applied, thereby forming the first three-dimensional polymer structure;

[0110] applying a second write voltage to the second pair of electrodes;

[0111] while the second write voltage is applied, selectively applying light to liquid crystal material contained within the second optical cell so as to polymerise the liquid crystal material in regions where the light is applied, thereby forming the second three-dimensional polymer structure.

[0112] The method may be used to manufacture an LC-based optical device according to the first aspect of the invention.

[0113] Polymerising portions of the LC material to form three-dimensional polymer structures, while a write voltage is applied across the LC material, may be referred to as direct laser writing (DLW). The applied light may be generated by a laser, for example, the wavelength of which may be selected so as to polymerise the LC material in question.When a write voltage is applied to the LC material, the orientation of the liquid crystal will change to minimise an energy state with respect to the electric field produced by the write voltage. For a liquid crystal composition with positive dielectric anisotropy, the molecules will tend to align themselves parallel with the electric field. For a liquid crystal composition with negative dielectric anisotropy, the molecules will tend to align themselves perpendicular to the electric field. The orientation of the liquid crystal at the write voltage is then ‘locked’ in the portions of the LC material that are polymerised while the write voltage is applied. The light may be moved relative to the surface of the optical cell (i.e ., in the x-axis and y-axis) to control which regions of the LC material are polymerised, thereby allowing 3D printing of polymer structures within the LC layer contained within the optical cell. DLW can be used to create different three-dimensional structures in each LC device. The DLW process may comprise lithography-based manufacturing techniques, such as two-photon polymerisation (2PP).

[0114] The LC material may have a nematic phase. However, other LC phases may be used; the present invention is equally applicable to smectic phase LCs or chiral nematic phase LCs, for example. When an electric field is applied across the LC layer (either as a result of the write voltage during manufacture or a read voltage during operation), the LC molecules will align themselves relative to the electric field to minimise an energy state .

[0115] The first LC device and the second LC device may be separate from one another when the first three-dimensional polymer structure and the second three-dimensional polymer structure are formed. The method may further comprise subsequently affixing the first rear substrate to the second front substrate. In all the methods discussed herein, the LC devices may be affixed / coupled to one another using any suitable adhesive, for example. This process may be referred to as a “write -then-stack” method; both the LC devices are written before they are stacked together.

[0116] Alternatively, prior to forming the first or second three-dimensional polymer structures, the first LC device and the second LC device may be coupled to one another. Light may be applied to the first optical cell from the direction of the first front substrate and to the second optical cell from the direction of the second rear substrate. This process may be referred to as a “write-flip-write again” method; the first and second LC devices are coupled to one another (stacked) before any writing occurs. Between writing the firstand second LC devices, the stacked LC-based optical device may be flipped over, so that writing for both LC devices is done ‘top down’. Alternatively, the DLW apparatus may be capable of writing from the top and bottom, meaning that the LC-based optical device does not need to be flipped between writing the first and second LC devices.

[0117] Both the write-then-stack or write-flip-write again methods can enable commercially available LC devices / optical cells to be used in the stacked LC-based optical device. Writing is only done through the first front substrate / second rear substrate, meaning that the interface between the LC devices may not impact the writing.

[0118] Alternatively, prior to forming the first or second three-dimensional polymer structures, the first liquid crystal device and the second liquid crystal device may be coupled to one another. Light may be applied to both the first optical cell and the second optical cell from the direction of the first front substrate or the second rear substrate. This process may be referred to as a “single side writing” method; both the LC devices are written from the same direction while they are stacked, e.g., top down. The single side writing method may provide for easier alignment of the first and second three-dimensional polymer structures since the written layers are not moved relative to one another or flipped. The same structure could be polymerised in the both LC devices in the same x-y position, just at a different focal length / height, for example.

[0119] Alternatively, the second three-dimensional polymer layer may be formed first and, subsequent to this, the first optical cell may be deposited on top of the second liquid crystal device. Light may be applied to both the first optical cell and the second optical cell from the direction of the first front substrate. This process may be referred to as “write a layer, build a layer”; LC are layers written and subsequently built / stacked on top of one another. Depositing the first optical cell on top of the second optical cell may comprise placing a complete, pre-filled LC device on top of the written second LC device. This may allow commercially available LC devices / optical cells to be used. Alternatively, the first LC device may be ‘built’ on top of the written second LC device. Spacers / side walls may be deposited on the second front substrate, the first front substrate deposited on top of said spacers to form the first optical cell, and the first optical cell may then be filled with LC material, for example.Similar to the single side writing method above, the write a layer, build a layer method may provide for easier alignment of the first and second three-dimensional polymer structures. The write a layer, build a layer method may enable many LC devices (including commercially available LC devices / optical cells) to be stacked, since writing is only ever done in the top-most LC layer.

[0120] Between the formation of the first three-dimensional polymer structure and the second three-dimensional polymer structure, the position of a focal point / region of the light may be adjusted so that the focal point is moved from within the first optical cell to within the second optical cell, or vice versa.

[0121] The DLW apparatus may be calibrated so that the focal point / region of the light is within the second optical cell. The light may then be moved in plane / in the x and y-axes to write the second three-dimensional polymer structure. The DLW apparatus may then be adjusted to move the focal point / region of the light up into the first optical cell, so that the first three-dimensional polymer structure can be written. The focal point / region may be moved up or down mechanically (by physically moving the DLW apparatus relative to the LC based optical device) or optically (by adjusting optical elements to alter the focal length). This movement of the focal point / region in the z-axis may enable the “single side writing” and “write a layer, build a layer” methods discussed above. The DLW apparatus may comprise adaptive optics configured to adjust the focal point / region position as it is moved between or around the optical cells.

[0122] During formation of the first three-dimensional polymer structure, the position of a focal point of the light may be moved in a direction normal to the first front substrate and the first rear substrate, such that the first three-dimensional polymer structure varies in height across a planar surface of the first optical cell. During formation of the second three-dimensional polymer structure, the position of a focal point of the light may be moved in a direction normal to the second front substrate and the second rear substrate, such that the second three-dimensional polymer structure varies in height across a planar surface of the second optical cell.

[0123] In addition to moving the focal point / region in the x-axis and y-axis, so as to polymerise different planar regions of the LC material, the focal point / region may also be moved in the z-axis so that the polymerisation depth within the LC layer can be varied. Thiscan enable any of the three-dimensional polymer structures discussed above to be produced. This may also be used to produce the additional three-dimensional polymer structures in the first and / or second LC layer discussed above. The first or second three-dimensional polymer structure may be formed at a rear substrate. The focal point of the light may then be brought upwards to enable an additional three-dimensional polymer structure to be formed at a front substrate, for example.

[0124] The first write voltage and / or the second write voltage may be at least 10 V. The first write voltage and / or the second write voltage may be at least 25 V, at least 50 V, at least 75 V, at least 100V, at least 150 V or at least 200 V. The voltage may be a peak-to-peak voltage. The voltage amplitude may have a square wave profile.

[0125] Using high write voltages may improve the polymerisation of the LC material. In the absence of a high electrical field, the resulting polymer structures can appear scrambled. This may be due to realignment of the LC molecules induced by the electric field of the writing light. In addition to reducing the onset of polymer structure scrambling, using higher write voltages / electric fields may allow for a greater tuning range of the LC devices’ optical properties. 10 V peak-to-peak may be sufficient to prevent scrambling, but a 100 V writing voltage may be used to provide greater tuneability of the device, for example.

[0126] The method may further comprise polymerising a portion of the liquid crystal material in the first optical cell and / or the second optical cell so as to produce an alignment feature, the alignment feature subsequently being used to position the first three-dimensional polymer structure relative to the second three-dimensional polymer structure .

[0127] Alignment features may improve the accuracy with which the first and second three-dimensional polymer structures can be aligned, which may improve the optical properties of the LC-based optical device. This may be particularly important where layers are moved / flipped relative to each other or the DLW apparatus during manufacture.

[0128] The alignment features may include:(i) staircases, which may be used to determine a z-axis position of a substrate in a given optical cell. The first step may indicate starting height for DLW;

[0129] (ii) helicoidal (spiral) staircase (i.e., optical vortex generator), where the angular position of first step may indicate starting height for DLW.

[0130] (iii) crosshairs, which may be used to determine x, y and rotational alignment for each LC layer;

[0131] (iv) a grating, whereby phase imaging of grating lines can be used to determine optical cell tilt from a single fabrication location. The grating lines may also be used for x,y and rotational alignment for each layer;

[0132] (v) square patch array, which may be used to determine non linearities in height step / phase step relationship; and / or

[0133] (vi) hatching, a combination of staircase and crosshairs which may be used to determine tilt, x and y alignment.

[0134] The alignment features may themselves be part of the first or second three-dimensional polymer structures. For example, lines of a diffraction grating may be used for alignment.

[0135] DESCRIPTION OF FIGURES

[0136] Embodiments of the invention will be described, purely by way of example, with reference to the accompanying drawings, in which:

[0137] Figure 1 shows a schematic diagram of a single liquid crystal device that may be used to form the stacked liquid crystal-based optical devices of the present invention;

[0138] Figure 2 shows an illustration of direct laser writing being used to form a three-dimensional polymer structure in a liquid crystal layer;

[0139] Figure 3 shows illustrations of examples of three-dimensional polymer structures in a liquid crystal layer;

[0140] Figure 4 shows experimental and simulated results of a liquid crystal device comprising a liquid crystal layer with a blazed grating liquid crystal polymer structure;Figure 5 shows transmission results for a beam steering experiment using a liquid crystal device like that of Figure 4;

[0141] Figure 6 shows a schematic diagram of an example of a liquid crystal-based optical element according to the present invention;

[0142] Figure 7 shows schematic diagrams of example structures that may be used for forming liquid crystal-based optical element according to the present invention;

[0143] Figure 8 shows a schematic diagram of an example liquid crystal-based optical element with orthogonal rubbing directions;

[0144] Figure 9 shows an image of an example of a liquid crystal-based optical device according to the present invention configured as a polarisation insensitive diffraction grating;

[0145] Figure 10 shows experimental and simulated diffraction results for the liquid crystal-based optical device of Figure 9;

[0146] Figure 11 shows an image of an example of a liquid crystal-based optical device according to the present invention configured as a polarisation sensitive diffraction grating;

[0147] Figure 12 shows experimental diffraction results for the liquid crystal-based optical device of Figure 11;

[0148] Figure 13 shows a schematic diagram and images of an example of a liquid crystal -based optical device according to the present invention configured as a polarisation insensitive Fresnel zone plate;

[0149] Figure 14 shows experimental diffraction results for the liquid crystal-based optical device of Figure 13;

[0150] Figure 15 shows experimental results of a bifocal optical system comprising a liquid crystal-based optical device according to the present invention;Figure 16 shows experimental results for a liquid crystal -based optical device according to the present invention configured as a polarisation selective hologram;

[0151] Figure 17 shows experimental results for a liquid crystal -based optical device according to the present invention configured as a selective astigmatism device;

[0152] Figure 18 shows an image and a phase illustration of an LC device configured as a vortex beam generator;

[0153] Figure 19 shows a phase illustration of a liquid crystal -based optical device according to the present invention configured as a vortex beam generator;

[0154] Figure 20 shows an image of the liquid crystal-based optical device according to the present invention configured as a vortex beam generator as in Figure 19;

[0155] Figure 21 shows a phase illustration of a liquid crystal -based optical device according to the present invention configured as a vortex beam generator; and

[0156] Figure 22 shows illustrations for various method of manufacturing a liquid crystal-based optical device according to the present invention.

[0157] DETAILED DESCRIPTION

[0158] Referring to Figure 1, an example of a single liquid crystal (LC) device 1 is shown. The LC device 1 comprises an optical cell. The optical cell comprises a front substrate 12 and a rear substrate 14. The front and rear substrates are paired together with a small airgap (e.g., 5 to 30 microns) therebetween. The front and rear substrates 12, 14 may be made of glass, for example. The optical cell further comprises side wall elements 11 such the optical cell encloses a volume. The side wall elements may comprise spacer beads, a gasket, or a vertical wall.

[0159] The volume within the optical cell, once enclosed, is filled with a liquid crystal material, thereby providing a liquid crystal layer 20. The LC material comprises liquid crystal molecules 21, which arrange / align themselves in response to an applied electric field. Thus, the LC layer 20 can demonstrate controllable electro-optic properties, asdiscussed further below. Any suitable LC material may be used in the devices of the present invention. For example, the LC material may comprise 69%wt Host nematic liquid crystal BL006, 20%wt reactive mesogen, RM257, and l%wt photoinitiator, IR819. Photoinitiator IR651 may be used instead of IR819. Similarly, nematic liquid crystal E7 may be used instead of BL006.

[0160] The pair of electrodes 16a, 16b are provided on the front substrate 12 and the rear substrate 14 respectively. A voltage may be applied across the pair of electrodes 16a, 16b, thereby generating an electric field (sometimes referred to a drive or read field) across the LC layer 20. The electrodes 16a, 16b may be made from a transparent material (e.g., ITO), such that the LC device 1 is transparent to illuminating incident light passing from the front substrate 12 to the rear substrate 14.

[0161] The LC device 1 may also comprise a front and / or rear alignment layer 17a, 17b. The alignment layers 17a, 17b may cause the LC molecules at the top and bottom of the LC layer 20 (i.e., LC molecules in contact with the alignment layers 17a, 17b) to align with a directional feature on the alignment layers 17a, 17b. These directional features may be grooves or ridges on the surface of the alignment layers 17a, 17b and define a “rubbing direction”. The alignment layers 17a, 17b may be rubbed polyimide layers, for example, but other alignment mechanisms may be used (e.g., photoalignment, homeotropic alignment, etc.). Alternatively, the substrates 12, 14 or the electrodes 16a, 16b may function as an alignment layer.

[0162] Referring to Figure 2, an example of a direct laser writing (DLW) process is shown being used to create a three-dimensional polymer structure in an LC layer. This DLW process may be used to manufacture any or all of the LC devices that constitute the stacked, LC-based optical devices of the present invention.

[0163] During the DLW process, an electric field is applied across the LC layer 20. For example, an optical cell having a pair of electrodes 16a, 16b may be filled with LC material, like in the example LC device of Figure 1. A voltage, typically referred to as a write voltage V„rite, is then applied across the pair of electrodes, thus generating the electric field across the LC layer 20. The electric field causes the LC molecules 21 within the LC layer 20 to reorient themselves - e.g., align themselves along the electric field gradient. The left-hand image of Figure 2 shows a simplified illustration of this;the LC molecules are shown arranged in a homeotropic structure between the pair of electrodes 16a, 16b.

[0164] While the write voltage V„rite is applied to LC layer 20, a light source 310 is used to illuminate the LC material with a writing or polymerising light beam 312. The light source 310 may comprise a laser, for example, configured to emit a writing light beam 312 at a wavelength that can polymerise the LC material. The wavelength of the light source 310 may be LC material-specific and / or selected based upon the transmission spectrum of the optical cell in which the LC layer 20 is contained.

[0165] The writing light beam 312 has a focal volume / region 314. In this focal region 314, the intensity of the light is sufficient to cause polymerisation of the LC material, at least within a pre-determined dwell time. For example, where the polymerising light beam 312 is applied to a region of the LC layer for a dwell time td, the LC material within the volume of the focal region 314 shall be polymerised and the LC material within the remainder of the polymerising light beam 312 will remain non-polymerised.

[0166] The light source 310 can be moved while the writing light beam 312 is applied to the LC layer 20. As shown in the left-hand image of Figure 2, the light source 310 is moved along a path D from the left-hand side of the LC layer 20 to the right-hand side of the LC layer 20. The writing light beam 312 may be applied continuously during the movement of the light source 310, or the writing light beam 312 may be applied intermittently between movements of the light source 310. The writing light beam 312 can be moved in the plane of the LC layer 20 (i.e., along the x and y axes) so that different two-dimensional, planar regions of the LC layer 20 are polymerised. Additionally, the writing light beam 312 may be moved perpendicularly to the LC layer 20 (i.e., along the z axis) so that the height of the material within the LC layer 20 that is being polymerised can be varied. The writing light beam 312 may be moved (both in the x-y plane and the z direction) mechanically - e.g., by physically repositioning the light source 310 - and / or optically - e.g., by adjusting lenses and mirrors to redirect the writing light beam 312 as desired. For example, the position of the focal region 314 may be varied in the x-y plane by moving the light source 310, and in the z axis by adjusting the focal length of lenses / optical elements arranged in the optical path of the writing light beam 312.The right-hand image of Figure 2 shows the same LC layer 20 after completion of the DLW process; the write voltage V„rite applied to the pair of electrodes 16a, 16b is removed and the light source 310 is switched off. Following the polymerisation via DLW, the LC layer 20 comprises a non-polymerised portion 22 and a polymerised portion 24.

[0167] The polymerised portion 24 constitutes a three-dimensional polymer structure 25 within the LC layer 20. The polymerised portion 24 occupies some planar area of the LC layer 20, dependent upon the movement of the focal region 314 in the x-y plane during the DLW. The polymerised portion 24 has some height profile h across its structure, dependent upon the movement of the focal region 314 in the z axis during the DLW. Within the polymer structure 25, the LC molecules 21 are frozen in the alignment in which they were in when the write voltage V„rite was applied. That is, once the write voltage V„rite is removed, the LC molecules 21 in the polymerised portion 24 retain the alignment provided by the electric field across the LC layer. On the other hand, the LC molecules 21 in the non-polymerised portion 22 relax and return to the state that they were in prior to DLW and the application of the write voltage V„rite. The LC molecules 21 in the non-polymerised portion 22 may be aligned randomly or in parallel to the rubbing direction of the optical cell, for example.

[0168] The three-dimensional polymer structures that are produced using DLW may be regarded as ‘discrete’ or ‘continuous’. Discrete polymer structures may comprise a plurality of polymerised portions 24 distributed across the x-y plane of the LC layer 20 and separated by non-polymerised portions 22. The polymerised portion(s) may all have the same height. Discrete polymer structures may be configured to form ridges, pillars or other shapes, for example. Discrete polymer structures may be utilised as diffraction gratings or holograms, for example. Continuous polymer structures may comprise a single polymerised portion 24 having a non-uniform height profile. Each continuous polymer structure may be configured to have a ramped height profile, a spatially periodically varying height profile (e.g., a sawtooth pattern or a sinusoidal profile) in the x and / or the y axis, for example. Continuous polymer structures may be utilised as a vortex beam generator, an astigmatism or aberration generator / corrector or a prismatic / beam steering device, for example.Although the general operating principles of DLW are discussed above, it will be understood that any suitable DLW process may be used to form the three-dimensional polymer structures discussed herein. For example, specific DLW methods used to form polymer structures similar to those discussed herein are described in Xu et al, “Laser-Written Tunable Liquid Crystal Aberration Correctors” ACS Photonics 2023 10 (9), 3401-3408, and in Nourshargh et al, "Continuously tuneable single electrode pair liquid crystal optical vortex generators" Nanophotonics, 2024, the subject matter of which is incorporated herein by reference.

[0169] Referring to Figure 3, some example polymer structures that may be formed by DLW are shown. In the top images a), the polymer structure comprises a plurality of ridges or pillars distributed across the x-y plane of the LC layer. The polymerised portions 24 extend across the full height of the LC layer between the pair of electrodes 16a, 16b. The polymerised portions 24 are separated by non-polymerised portions 22. Such a polymer structure may be referred to as a two-dimensional or planar structure due to the fact that the structure varies in the plane of the LC layer (the x-y plane) but has a constant height (in the z axis). Nonetheless, a polymer structure as shown in a) can still be regarded as a three-dimensional polymer structure; such structures may be utilised in LC devices of the present invention.

[0170] As discussed above, the polymerised portions 24 are formed via DLW while a write voltage V„rite is applied to the LC layer. In operation, a read voltage Vread can be applied across the pair of electrodes 16a, 16b. The read voltage Vread similarly results in an electric field being generated across the LC layer. This results in the LC molecules in the non-polymerised portion 22 reorienting themselves, while the LC molecules in the polymerised portion 24 remain in their frozen alignment from the DLW process (at least until a high breakdown or scrambling voltage is reached, where the LC molecules overcome the polymerisation). By varying the read voltage Vread, the optical properties of an LC device containing such a polymer structure like this can varied or tuned.

[0171] In the left-hand image of Figure 3 a), a read voltage Vread is applied to the pair of electrodes 16a, 16b, wherein the read voltage Vread is not the same as the write voltage V„rite used during DLW. Vread may be zero, for example. In this image, the LC molecules of the non-polymerised portion 22 are shown having the same alignment as that of the right-hand image of Figure 2 (where no voltage is applied). Under such a condition, theLC in the non-polymerised 22 and polymerised portions 24 are different; the LC molecules in the two portions are aligned differently. Thus, if light is passed through said LC layer, it may be spatially modulated by the polymer structure. In this example, the periodic ridges of the polymerised portion 24 may act as a diffraction grating to light passing through the LC layer.

[0172] By contrast, in the right-hand image of Figure 3 a), the read voltage applied to the electrodes 16a, 16b is the same as the write voltage V„rite. As such, all of the LC molecules - in both the non-polymerised 22 and polymerised 24 portions - have the same alignment. Thus, there is no spatial variation in the LC material and therefore there is no spatial modulation of incident light. In alternative examples, using a suitable writing procedure and V„rite, there may be no spatial modulation when zero voltage is applied to the electrodes 16a, 16b. For example, the write voltage V„rite may be zero such that when Vread = 0V, the orientation of the LC molecules within the non-polymerised 22 and polymerised 24 portions is the same. When a non-zero Vread is then applied, the LC molecules within the non-polymerised portion may then be reoriented so as to induce a spatial modulation within the LC layer. It will be understood that the configuration of the device may be adjusted based upon factors including the LC material used, the write voltage and the ‘relaxed’ alignment of the LC molecules (e.g., the configuration of alignment layers).

[0173] A polymer structure like that shown in Figure 3 a) may therefore be operated as a binary device. By varying the read voltage that is applied to the LC device, the LC layer may be switched ‘on’ or ‘off’. In the on state, the read voltage may be configured to cause an alignment in the LC molecules of the non-polymerised portion 22 so as to induce half of retardance difference between the polymerised 22 and non-polymerised 24 LC portions. In the off state, the read voltage is set to the write voltage used in DLW such that the LC molecules are uniformly aligned.

[0174] In the bottom images b) of Figure 3, the polymerised portion 24 does not extended across the full height of the LC layer between the electrodes 16a. 16b - the polymerised portion 24 forms a series of ridges at the lower electrode 16b. Instead, an additional polymerised portion 26 is provided, forming a separate, second polymer structure - the polymerised portion 26 forms a series of ridges at the upper electrode 16a. This is just one example of a dual-structure LC layer; each polymerised portion 24, 26 may haveany of the structures discussed herein. The polymerised portions 24, 26 may have a different three-dimensional structure. For example, the polymerised portions 24, 26 may comprises ridges that have different spacings or are offset from one another, which may impact the phase modulation / diffraction of light enabled by the LC layer. The polymerised portions 24, 26 may comprises three-dimensional structures with different sizes or shapes.

[0175] Both the polymerised portions 24, 26 are formed using the same DLW process discussed above. However, the polymerised portion 24 and the additional polymerised portion 26 are formed using two different write voltages. In the example shown in Figure 3 b), the polymerised portion 24 may be formed while a write voltage V2 is applied and the additional polymerised portion 26 may be formed while a write voltage Vi is applied. One of Vi or V2 may be zero voltage. The height of the focal region of the DLW apparatus writing light beam may be adjusted between the writing of the polymerised portion 24 and the additional polymerised portion 26, such that the two polymer structures are formed at different heights within the LC layer (as shown).

[0176] By applying a suitable read voltage to the pair of electrodes 16a, 16b, the spatial modulation resulting from both the polymerised portion 24 and the additional polymerised portion 26 can be controlled. In the left-hand image, a read voltage of Vi is applied. Accordingly, the LC molecules of the non-polymerised portion 22 and the additional polymerised portion 26 are uniformly aligned, while the polymerised portion 24 retains its alternative alignment (resulting from writing at a write voltage V2). Thus, the polymer structure 24 is the only structure that causes spatial modulation of light passing through the LC layer. By contrast, in the right-hand image, a read voltage of V2 is applied. Accordingly, the LC molecules of the non-polymerised portion 22 and the polymerised portion 24 are uniformly aligned, while the additional polymerised portion 26 retains its alternative alignment (resulting from writing at a write voltage Vi). Thus, the additional polymer structure 26 is the only structure that causes spatial modulation of light passing through the LC layer. Thus, this device can be operated in two discrete ‘on’ states. The two polymer structures may be configured to produce diffraction patterns or different holograms, for example. Additionally, a read voltage that is neither Vi or V2 may be used (not shown), in which case both the polymerised structure 24 and additional polymerised structure 26 will at least partially modulate light (because theLC molecules of the non-polymerised portion 22 will be aligned with the LC molecules of neither polymer structure).

[0177] The polymer structures in Figure 3 are discussed in terms of being binary structures; the one or more polymer structures can either be turned on or off. However, DLW may be used to produce polymer structures that are continuously tuneable. Particularly where the polymer structures have a varying height profile, the amount of modulation generated by the LC may be varied proportionally to the applied read voltage depending upon the degree of uniformity between the non-polymerised and polymerised portion(s). Tuneable devices like this are discussed in further examples below.

[0178] In certain applications, binary devices like those above may have limited capabilities. For example, a lens-like device may be approximated using a polymer structure comprising a plurality of concentric binary rings. However, such a binary phase lens structure is limited in efficiency (i.e. in the fraction of incident light that is directed into the desired diffraction order). By contrast, a lens-like device may be fabricated using a polymer structure with a continuous height profile variance. This can allow DLW LC layers to be operated as lenses, optical vortex generators, aberration correctors, and blazed gratings, for example, with high efficiency and continuously tuneable phase depth. Nonetheless, both binary and continuously tuneable structures may be used in the stacked LC-based optical devices of the present invention.

[0179] Referring to Figures 4 and 5, an example of a single LC device is shown in order to illustrate the functionality of LC devices comprising three-dimensional polymer structures. This LC device may form one layer of a stacked LC-based optical device discussed further below.

[0180] In this example, a commercially available LC optical cell was used, having an LC layer thickness of 20 microns. DLW, as discussed above, was used to fabricate a blazed grating structure within the LC material of the LC layer. As shown in the bottom row of images, the blazed grating three-dimensional polymer structure comprises a plurality of sawtooth like features; the boundary between the non-polymerised portion 122 and the polymerised-portion 124 of the LC layer forms a series of linear ramps with a periodically varying height in the z axis across the width (x axis) of the device. Such a LC device may be used as a discrete beam steering element.The top row of Figure 4 shows experimental polarised optical microscopy (POM) images for different applied drive voltage image. The polariser (P), analyser (A) and rubbing direction (R) positions are indicated with arrows in the top left corner. The inset portion of each image shows a zoomed-in section of the device showing two discontinuities around one full pitch of the LC layer. For different drive voltages, reading from right to left: at 8.0 V, there is a uniform intensity profile across the pitch and the whole device, indicating plane wave transmission. At 3.0 V, there is one light fringe and one dark fringe per pitch, indicating a 2TI phase ramp, as necessary for steering into the first order. At 1.0 V, there are two light fringes and two dark fringes per pitch, indicating a 4TI phase ramp, as necessary for steering into the second order. The middle row of images shows simulated POM images illustrating the same blazed grating profile at the different drive voltages. The bottom row of images shows the profile of the non-polymerised portion 122 and polymerised portion 124, including simulated LC molecule director profiles (shown as a vector arrow for each molecule).

[0181] In the optimal condition, a blazed grating will yield 100% diffraction efficiency into a given order, thereby providing perfect, discrete beam steering. The optimal condition is achieved when the phase discontinuity between each pitch is 2nm radians, whereby the grating will steer the beam into the mth order. Figure 5 shows transmission measurements illustrating beam steering between discrete positions at varying drive voltages, with maximum steering efficiency achieved at 1.0 V, 3.0 V and 8.0 V as identified in Figure 4.

[0182] Referring to Figure 6, an example of a system 200 is shown. The system 200 comprises an example of a liquid crystal -based optical device 100. The LC-based optical device 100 comprises two LC devices, each of which may have any of the features of the LC device / LC layer discussed above in relation to Figures 1 to 5.

[0183] The first LC device (the top device of Figure 6) comprises a first optical cell 110, the first optical cell 110 comprising a first front substrate 112 and a first rear substrate 114. The first optical cell 110 contains a first LC layer 120 made of LC material. The LC material has been subjected to the polymerisation discussed above, such that the first LC layer 120 comprise a first non-polymerised portion 122 and a first polymerised portion 124. In this example, the first polymerised portion 124 forms athree-dimensional polymer structure comprising a plurality of ridges / pillars, which may be used as a diffraction grating. The first LC device further comprises a first pair of electrodes 116a, 116b, comprising an electrode layer disposed on each of the first front substrate 112 and the first rear substrate 114 respectively.

[0184] The first LC device (the bottom device of Figure 6) comprises a second optical cell 130, the second optical cell 130 comprising a second front substrate 132 and a second rear substrate 134. The second optical cell 130 contains a second LC layer 140 made of LC material. The LC material has been subjected to the polymerisation discussed above, such that the second LC layer 140 comprise a second non-polymerised portion 142 and a second polymerised portion 144. In this example, the second polymerised portion 144 forms a three-dimensional polymer structure comprising a plurality of ridges / pillars, which may be used as a diffraction grating. The second LC device further comprises a second pair of electrodes 136a, 136b, comprising an electrode layer disposed on each of the second front substrate 132 and the second rear substrate 134 respectively.

[0185] In this example, the first and second LC devices are essentially the same (e.g., having the same three-dimensional polymer structure in the LC layer 120, 140). However, in other examples, the two devices may be different.

[0186] The first and second LC devices are coupled together; the first rear substrate 114 is coupled to the second front substrate 132. The LC devices may be coupled together using any suitable adhesive, for example a UV curing glue. The first rear substrate 114 and the second front substrate 132 may be regarded as forming an interface between the two LC devices / optical cells 110, 130.

[0187] The system 200 further comprises a controller 210. The controller 210 (e.g., a variable / controllable power supply) is electrically coupled to the first pair of electrodes 116a, 116b and the second pair of electrodes 136a, 136b. The controller 210 is configured to independently provide a voltage to each of electrode pairs, thus producing a first electric field Ei across the first LC layer 120 and a second electric field E2 across the second LC layer 140. As discussed herein, the electric fields Ei, E2 can cause LC molecules in the non-polymerised portions 122, 142 of the LC layers 120, 140 to realign themselves. The realignment of the LC molecules, in combination with the fixed alignment of LC molecules in the polymerised portions 124, 144, can result in a changein the optical properties of each LC device - e.g., the phase-shift applied to illuminating light passing through each LC layer 120, 140. By independently controlling the electric fields Ei, E2 applied to the LC layers 120, 140 (by varying the voltage provided by the controller 210), the optical properties of each LC device can be independently controlled.

[0188] The system 200 further comprises a light source 220, for example a laser. The light source 200 is configured to provide an illuminating beam 222a that is incident on the LC-based optical device 100. As the illuminating / incident beam 222a transits the LC layers 120, 140 of the LC-based optical based device 100, it is modified based on the properties of the LC material(s) and their electric field-dependent configuration (i.e, the electric field Ei, E2 applied to each LC layer 120, 140. The resultant beam 222b that exits the LC-based optical device 100 may be deflected, re-shaped, phase-shifted or retarded, for example.

[0189] In the example of Eigure 6, the LC-based optical device is configured as a transmissive device; the illuminating beam 222a enters the first front substrate 112 and exits the second rear substrate 134. I.e., all of the layers (particularly the substrates) are at least partially transparent to the light provided by the light source 220. This transmissive device may be used as a spatial light modulator, for example. In an alternative example (not shown), the second rear substrate 134 may comprise a reflective material, for example a metallic layer. Thus, the illuminating beam 222a may enter the first front substrate 112 and be reflected by the second rear substrate 134, thereby passing through the LC layers 120, 140 twice. A reflective device like this may be used a deformable mirror, for example.

[0190] Referring to Figure 7, two alternative examples of stacked LC devices that may be used in optical devices 100a, 100b of the present invention are shown. In both cases, the optical devices 100a, 100b each comprise a first LC layer 120 and a second LC 140 contained within two optical cells, similar to as discussed above. For brevity, the side walls of the optical cells are not shown.

[0191] In the first alternative LC-based optical device 100a (shown on the left-hand side of Figure 7), a single substrate layer 118, or shared substrate, is provided at the interface between the first optical cell and the second optical cell, rather that the optical cellscomprising a separate first rear substrate and second front substrate respectively. In other words, the first LC layer 120 is contained in a first optical cell comprising a first front substrate 112 and the shared substrate 118, and the second LC layer 140 is contained in a second optical cell comprising the shared substrate 118 and a second rear substrate 134. Similar to the optical device 100 shown in Figure 6, a first pair of electrodes 116a, 116b are provided on either side of the first LC layer 120 (disposed on the first front substrate 112 and a front side of the shared substrate 118) and a second pair of electrodes 136a, 136b are provided on either side of the second LC layer 140 (disposed on the second rear substrate 134 and a rear side of the shared substrate 118).

[0192] In the second alternative LC-based optical device 100b (shown on the right-hand side of Figure 7), the first LC layer 120 is contained in a first optical cell comprising a first conductive front substrate 113 and a first conductive rear substrate 115, and the second LC layer 140 is contained in a second optical cell comprising a second conductive front substrate 133 and a second conductive rear substrate 135. Thus, the two optical cells are separate from one another, similar to the optical device 100 shown in Figure 6.

[0193] However, in this instance, the aforementioned substrates comprise an electrically conductive material such that they also perform the function of the pairs of electrodes discussed above. A first electric field can be generated across the first LC layer 120 by applying a first voltage across the first conductive front substrate 113 and the first conductive rear substrate 115. A second electric filed can be generated across the second LC layer 140 by applying a second voltage across the second conductive front substrate 133 and the second conductive rear substrate 135. The substrates may each consist of an ITO sheet, for example. An electrically insulating layer 119 is provided between the first conductive rear substrate 115 and the second conductive front substrate 133, so that they are electrically insulated (and each pair of electrodes is independent from one another). The insulating layer 119 may comprise a sheet of insulating material or may comprise one or more spacers, such that an airgap is formed between the substrates.

[0194] Both of the alternative structures shown in Figure 7 may enable an LC-based optical device 110a, 100b that is thinner (between the first front substrate and the second rear substrate) than the optical device 100 of Figure 6. In particular, the interface between the two optical cells may be thinner, due to providing a shared substrate 118 or combination electrode-substrates. This may reduce the amount of aberration orreflectance / absorbance occurring at the interface. This may improve the optical characteristics of the LC-based optical device when in use and / or enable different manufacturing techniques (e.g., writing the second LC layer through the first LC layer / optical cell interface).

[0195] Referring to Figure 8, an arrangement of two stacked optical cells 110, 130 is shown. The optical cells 110, 130 are parts of an LC-based optical device as discussed above, but for simplicity, some components are not shown (e.g., pairs of electrodes).

[0196] The first optical cell 110 contains a first LC layer 120 and is configured with a first rubbing direction Ri lying in the x axis. The second optical cell 130 contains a second LC layer 140 and is configured with a second rubbing direction R2 lying in the y axis. The rubbing direction of each optical cell can be configured using any suitable means. For example, each optical cell may comprise an alignment layer comprising a rubbed polymer sheet. In this instance, the first and second rubbing directions Ri, R2 are arranged perpendicularly to each other. But, in other examples, the first and second rubbing directions Ri, R2 may be parallel or some other non-zero angle. This can be achieved by rotating one of the optical cells 110, 130 relative to the other in the x-y plane prior to stacking them.

[0197] Due to the birefringence of LCs, optical devices that comprise a single LC layer are generally polarisation sensitive; they modulate the phase for light polarised parallel to the rubbing direction and have no effect on the orthogonal polarisation. By controlling the alignment of rubbing directions in the stacked optical devices discussed herein, a range of polarisation sensitive or insensitive devices may be obtained.

[0198] Referring to Figures 9 to 16, experimental results are shown for a variety of LC-based optical device according to the present invention. These optical devices comprise a pair of commercially available optical cells, each containing a 5 -micron thick LC layer. These LC-based optical devices were manufactured using the write-flip-write method discussed further below.

[0199] Figure 9 shows a POM image of an example LC-based optical device. The polariser (P), analyser (A) and rubbing direction (R) positions are indicated with arrows in the top left corner. The optical device comprises two optical cells (Cell 1 and Cell 2) eachcontaining an LC layer. Both LC layers comprise identical three-dimensional polymer structures; a plurality of discrete, parallel ridges that form a diffraction grating. The diffraction grating of Cell 2 is hidden under that of Cell 1 in Figure 9. However, the optical cells are arranged such that their rubbing directions Ri, R2 are perpendicular to one another. Thus, this LC-based optical device functions as a polarisation insensitive diffraction grating. In the on state, the optical device produces a fixed diffraction pattern, irrespective of the polarisation of the illuminating beam.

[0200] Figure 10 shows simulations (left-hand images) and transmission measurements (righthand images) for a polarisation insensitive diffraction grating like that of Figure 9. The images of row (a) show that in the off state, where a drive voltage of 10 V was applied to both LC layers, there is no diffraction irrespective of the illuminating beam polarisation. The images of rows (b) to (d) show diffraction results in the on state, where a drive voltage of 3.5 V was applied to both LC layers. For illuminating beam polarisations of 0, 45 and 90 degrees, the diffraction pattern of the resultant beam is constant, demonstrating polarisation insensitivity.

[0201] Alternatively, the two optical cells / LC layers can be fabricated with orthogonal, rather than identical, diffraction gratings. As above, the optical cells are arranged such that their rubbing directions Ri, R2 are perpendicular to one another. Thus, the resultant LC-based optical device can function as a polarisation sensitive diffraction grating. The two LC layers may be turned on / off (by varying the drive voltage provided to each layer) so that orthogonal illuminating beam polarisations are diffracted in orthogonal directions. By varying the drive voltage applied to each LC cell, the optical device may be used to select which input polarisation state diffracts and which is unperturbed.

[0202] Figure 11 shows a POM image of the polarisation selective diffraction grating. The polariser (P), analyser (A) and rubbing direction (R) positions are indicated with arrows in the top left corner. The bottom Cell 2 appears out of focus due to the limited depth of field. Figure 12 shows transmission measurements for the polarisation selective diffraction grating. Image (a) shows the device in the off state, where a drive voltage of 10 V was applied to both LC layers. Images (b) show (d) show diffraction results in the on state, where a drive voltage of 3.5 V was applied to both LC layers. For illuminating beam polarisations of 0, 45 and 90 degrees, the amount of light diffracted horizontally and vertically vary.In another example, the LC layers may be written to function as a Fresnel zone plate (FZP); a binary representation of a Fresnel lens, comprising a plurality of concentric rings of alternating phase (0, a). Figure 13 shows a schematic diagram of an example LC-based optical device and POM images of two constituent optical cells. The polariser (P), analyser (A) and rubbing direction (R) positions are indicated with arrows in the top left corner of the images. Both optical cells contain an LC layer comprising identical three-dimensional polymer structures; a plurality of discrete concentric rings that form an FZP. The optical cells are arranged such that their rubbing directions Ri, R2 are perpendicular to one another. Thus, this LC-based optical device can function as a polarisation insensitive FZP.

[0203] Figure 14 shows transmission measurements of the polarisation insensitive FZP, obtained with a collimated illuminating beam and a camera at the desired focal plane of the device. The images of column (a) show that with both optical cells in the off state, there is no focusing observed. For the images of columns (b) to (d), in the top row, cell 1 is in the on state and cell 2 is in the off state. When the polarisation of the illuminating beam is 0 degrees the light is focused, at 45 degrees the light is partially focused (indicating the horizontal polarisation has focused and the vertical polarisation has not), and at 90 degrees there is no focusing. In the middle row, cell 2 is in the on state and cell 1 is in the off state . It can be seen that the light is fully focused when the polarisation of the illuminating beam is 90 degrees, unfocused at 0 degrees, and partially focused at 45 degrees. In the bottom row, both cells are in the on state. Light is fully focused irrespective of the input polarisation, demonstrating polarisation insensitive operation.

[0204] In a switchable LC-based optical device like that above, switchable focusing of the illuminating beam can be achieved by varying the drive voltage applied to each LC layer. The polarisation insensitive FZP may be placed immediately after a fixed lens, for example. The LC-based optical device may then be switched on / off to enable bifocal operation.

[0205] An example optical system may, for example, comprise a polarisation insensitive FZP with a focal length in the on state = 50 mm and a fixed optical lens of focal length = 250 mm. The polarisation insensitive FZP can be switched off / on to enable switching between focal planes at 250 mm and 47 mm respectively. Figure 15 shows experimentalresults obtained from such an optical system. The illuminating beam has a polarisation of 45 degrees in all instances. Each row shows the intensity of the resultant beam at different camera planes located at 47 mm, 120 mm and 250 mm from the LC-based optical device / fixed lens. The images of column (a) show that with both cells off, the light is clearly focused in the 250mm plane. The images of columns (b) and (c) show that with only cell 1 or cell 2 on respectively, the light partially focuses in the 47mm plane and partially focuses in the 250mm plane. This is due to orthogonal polarisation states experiencing different focal lengths, thus demonstrates polarisation selective focusing. The image of column (d) shows that with both cells on, the light clearly focuses on the 47mm plane.

[0206] In another example, an LC-based optical device may be configured to provide polarisation insensitive or sensitive holograms. One or two binary holograms can be written into a single LC layer, like the single three-dimensional polymer structure and the dual three-dimensional polymer structure shown in Figure 3 a) and b) respectively. By varying the drive voltage provided to a single layer LC device, the hologram can be switched on / off in the single case, and between the two hologram states in the dual structure case. Similarly to the polarisation sensitive diffraction grating above, two holographic LC devices can be stacked with perpendicular rubbing directions to provide an LC-based optical device that produces polarisation sensitive / selective holograms.

[0207] Figure 16 shows transmission measurements of an LC-based optical device configured as a polarisation selective hologram. The LC-based optical device comprises two optical cells, each containing an LC layer comprising a different phase hologram (a single three-dimensional polymer structure), the optical cells being arranged such that their rubbing directions are perpendicular to one another. Cell 1 is written to produce a horizontal arrow, while cell 2 is written to produce a vertical arrow. In the top row, cell 1 is in the on state and cell 2 is in the off state. When the polarisation of the illuminating beam is 0 degrees, the horizontal arrow of cell 1 can be seen clearly. At 45 degrees, it is dimmer due to the vertically polarised light not being diffracted, and it disappears at 90 degrees. In the middle row, cell 2 is in the on state and cell 1 is in the off state. When the polarisation of the illuminating beam is 90 degrees, the vertical arrow of cell 2 can be seen clearly. At 45 degrees, it is dimmer due to the horizontally polarised light not being diffracted, and it disappears at 0 degrees. In the bottom row, both cells are in the on state. As can be seen, as the polarisation of the illuminating beam varies, the amountof light forming the horizontal vs. vertical arrow is changed. Thus, the holograms produced by this LC-based optical device can be selected based upon the polarisation of the illuminating beam.

[0208] Referring to Figure 17, stacking of LC devices in an LC-based optical device can provide for additional or multifunctionality, in addition to or instead of polarisation sensitivity / insensitivity. An example optical device comprises a pair of commercially available optical cells, each containing a 20-micron thick LC layer. The optical cells are arranged with the rubbing directions arranged parallel to one another. In this example, the LC-based optical device was manufactured using the write-then-stack method discussed further below. The optical cells were aligned by fabricating both of them with the same offset from a reference point in the cell (e.g., an alignment feature written into the LC material within the cell). A 3D-printed chuck was then used to position the two optical cells on top of one another.

[0209] Each of the LC devices were written to have different three-dimensional polymer structures within the LC layers. The polymer structures were astigmatic structures; structures that result in an elongation of the focal spot along a particular axis. In this example, the two LC devices were written to provide vertical or horizontal elongation; i.e., astigmatism of opposite magnitude. By fabricating and stacking the devices in such a way, the drive voltage / electric field applied to each of the LC devices can be varied so that the operation of each LC device can be switched or combined. Figure 17 shows the transmission results for such an LC-based optical device. By switching either of the LC devices on or off (by applying an appropriate drive voltage to each LC device), the overall optical device can provide either vertical elongation (left most image) or horizontal elongation (right most image) of an incident beam with a circular focal spot (centre image).

[0210] Referring to Figures 18 to 21, LC-based optical device may also be configured to provide an extended operational range compared to a single LC device. The optical devices of these figures were manufactured using custom-made optical cells. Each cell was made from substrates of ITO-coated glass, with one substrate having a thickness of 1.1 mm and the other a thickness of 0.2 mm. The substrates were coated with a polyvinyl acetate (PVA) solution and rubbed to provide an alignment direction. The pair of substrates were assembled as a cell using double sided spacer tape. The spacer tape wasconfigured to provide a cell gap between the two substrates of 20 microns. Two optical cells were manufactured in this way, and they were then coupled together using UV curing adhesive. The two optical cells were arranged so that their rubbing directions were parallel and the thinner substrate of each cell were on the inside of the device (similar to as shown in Figure 6). By reducing the inter-cell thickness (i.e., the thickness at the interface of the first rear substrate and the second front substrate), the three-dimensional polymer structures in both optical cells could be fabricated using the single side writing method, discussed further below. The polymer structure of the lower LC device could be written from the top, through the upper LC device, without needing to flip the optical device over or subsequently stack the two LC devices. This made aligning the polymer structures in each optical cell easier.

[0211] By stacking multiple similar devices, the switching / operational range of the devices can be extended compared to a single LC device while still providing continuous tuning (i.e., not a binary on / off state). In Figures 18 to 21, for example, the LC layers are written to function as vortex beam generators. The three-dimensional polymer structures constitute a linear ramp around the centre of a circle; the height profile continuously varies dependent upon azimuthal angle, except where a step change occurs at the 360-to-0-degree point.

[0212] Figure 18 shows a POM image (left-hand image) for a single vortex beam generating LC device with a phase that varies with azimuth (right-hand image). The tuning range of the phase depth of the device is 0-4TI radians. The phase depth is determined by the azimuthal height profile of the three-dimensional polymer structure. Where the polymer structure is written across the full height of the LC layer, the LC molecules will be fixed in the homeotropic state. But as the height of the polymer structure decreases, a greater thickness of the LC molecules are non-polymerised and so free to move. By varying the drive voltage applied across the LC layer, the degree of phase shift applied to the incident beam can therefore be tuned continuously. A phase depth tuning range of 0-4TI radians enables continuous tuning of the vortex order, through fractional vortex states, from 0 to 2.

[0213] This tuning range is limited by the thickness of the cell and the birefringence of the LC material, so could be increased without stacking. Alternatively, the tuning range can be increased by adding a second vortex beam generating LC device. As the phase profilesof subsequent devices can be considered to simply add together, the extended range would be equivalent to a similar, thicker single device. Two devices like that shown in Figure 18 could be stacked together to provide continuous O-871 tuneable phase depth.

[0214] However, stacking like devices in this way would yield inefficiencies as there is a redundancy in the tuneability. A second order vortex could be achieved by configuring the two LC devices with either: a 0thorder + a 2ndorder, two 1storders, or a 2ndorder and a 0thorder. Using two like LC devices as above, the maximum vortex order achievable with n cells (having n drive voltages) would be In.

[0215] The operational range can be extended by writing the two LC devices to have different polymer structures. Figure 19 shows phase depth profiles for an example first LC device / optical cell and a second LC device / optical cell, which may be stacked together. The first LC device has a phase depth profile like that discussed above in relation to Figure 18; a continuous linear, azimuthal ramp is written into the LC layer to provide continuous switching from a 0thto 2ndorder vortex (top image of Figure 19). To make full use of this continuous range, the second LC device may be configured to switch discretely between the 0-2-4 vortex orders. The second LC device was written so that the polymer structure comprised two linear azimuthal ramps, each covering 180 degrees of azimuth (top image of Figure 19).

[0216] Figure 20 shows POM images obtained of this LC-based optical device comprising two different LC devices. As above, continuous tuning is enabled, but these POM images show each device being switched fully on or off. Using this configuration, the second LC device enables coarse tuning of the vortex order, while the first LC device enables fine, continuous tuning. In this way, an w-layer device with n independently addressable electrode pairs will be continuously tuneable over 2 / 3"1+ 1 vortex orders.

[0217] If continuous tuning between the orders is not a requirement, then the operational range can be further increased. Figure 21 shows phase depth profiles for the LC devices of an alternative LC-based optical device. As above, the first LC device is tuneable from 0-4TI radians (top image of Figure 21). However, the second LC devices is written to have three linear, azimuthal ramps across 120 degrees, so that the second device can switch between the 0-3-6 vortex states (bottom image of Figure 21). Thus, each integer state can be reached, and the final redundancy is removed.In these examples, vortex beam generators are used to show how a stacked LC-based optical device of the present invention be used to extend operational range. However, this approach is universally applicable for any optical device having a continuously varying phase profile, such as tilt devices (e.g., blazed gratings), lenses (e.g., Fresnel lenses), etc. For example, two differing blazed grating polymer structures could be coupled together to provide a greater range of diffraction angles. By stacking multiple LC devices and independently switching their drive voltages, accurate and precise beam steering may be achieved.

[0218] Referring to Figure 22, schematic diagrams are shown illustrating different methods by which the stacked LC-based devices of the present invention may be manufactured. For clarity, each individual LC device is shown as a simple block A or B pre-filled with LC material, but these LC devices may comprise any of the features discussed herein.

[0219] A first manufacturing method 300a may be referred to as “write then stack”. Firstly, in step i), the first LC device A is written using direct laser writing (DLW) to form a first three-dimensional polymer structure within the first LC layer. Secondly, in step ii), the second LC device B is written using DLW to form a second three-dimensional polymer structure within the second LC layer. Finally, in step iii), the two LC devices are stacked on top of one another. The LC devices may be coupled to one another using an adhesive. During the DLW of the first and second polymer structures, alignment features may also be written. For example, a pair of crosshairs could be written into part of the LC layer in both devices. These crosshairs may then be used to align the two LC devices appropriately during stacking. Further LC devices may be stacked using this method.

[0220] A second manufacturing method 300b may be referred to as “write-flip-write again”. Two LC devices A and B are first stacked (e.g., using an adhesive). Then, in step i), the first LC device A is written using DLW to form a first three-dimensional polymer structure within the first LC layer. Secondly, in step ii), the whole device is flipped over. As shown in this illustration, the DLW is done from the top down, with the first LC device A on top first. The device is then flipped 180 degrees so that the second LC device B is on top. Finally, in step iii) the second LC device B is written using DLW to form a second three-dimensional polymer structure within the second LC layer. The write-flip-write again method may reduce alignment issues compared to the write thenstack method, since the LC devices are already coupled together. Nonetheless, alignment features may still be written into the first LC device, which may be used as a reference for writing the second LC device.

[0221] A third manufacturing method 300c may be referred to as a “single side writing” method Two LC devices A and B are first stacked (e.g., using an adhesive). Then, in step i), the second LC device B is written using DLW to form a second three-dimensional polymer structure within the second LC layer. DLW of the second LC device B is done through the first LC device. The two LC devices may comprise thinner substrates than other devices (e.g., those that use commercially available optical cells) to enable this. The position of the focal region of the DLW writing apparatus (the region in which the intensity of the light is sufficient to cause polymerisation of the LC material) is then adjusted to move within the second LC device up to the within the first LC device. This may be done mechanically (e.g., by moving the DLW laser upwards) or optically (e.g., by adjusting optical elements to alter the focal length of the DLW apparatus). Finally, in step ii) the first LC device A is written using DLW to form a first three-dimensional polymer structure within the first LC layer. The single side writing method may greatly improve the ease with which the polymer structures are aligned since the LC devices are not moved at all during the manufacturing. Where the two LC devices have the same three-dimensional polymer structure, for example, this can simply be repeated in the same x-y position in each LC device. The DLW apparatus may comprise automated adaptive optics which may be configured to correct the focussing of the light as it is moved from the second to the first LC layer. For example, the interface between the two optical cells may cause some diffraction of the light. This DLW writing apparatus may be configured to automatically account for this so that the focal region is positioned at the correct height.

[0222] A fourth manufacturing method 300d may be referred to as “write a layer, build a layer”. Firstly, in step i), the second LC device B is written using DLW to form a second three-dimensional polymer structure within the second LC layer. Secondly, the first LC device A is built upon the top of the second LC device B. Where commercially available optical cells are used, this building may comprise simply stacking another optical cell on top of the bottom one (using an adhesive, for example). Alternatively, the first LC device A may custom built on top of the second LC device B; substrate and electrodes layers may be deposited to form the optical cell, which is then filled with LC material. Finally,in step iii), the first LC device A is written using DLW to form a first three-dimensional polymer structure within the first LC layer. The focal region of the DLW apparatus may be moved mechanically or optically to write the upper, first LC device A. The write a layer, build a layer method may provide for easier alignment as above (the bottom LC device does not move and may be provided with alignment features for aligning the upper LC device). However, this method may enable LC-based optical devices with many stacked layers to be manufactured, since only the topmost LC device is written at any given point.

[0223] A hybrid of the single side writing method 300c and the write a layer, build a layer 300d may also be used to manufacture LC-based optical devices. LC devices may first be stacked, but with all but the bottom LC device empty. After writing the bottom LC device the upper LC device may then be filled with LC material and written. This may be regarded as either the single side writing method with an intermediate filling step, or the write a layer, build a layer, where the building comprises simply filling a pre-stacked empty optical cell. Writing through an empty LC device may be easier than the method 300c shown in Figure 22. Likewise, having the empty optical devices pre-stacked may allow for easier alignment than individually stacking devices like in the method 300d shown in Figure 22.

[0224] Although specific examples have been described, the skilled person will appreciate that variations are possible, within the scope of the invention, which should be determined with reference to the accompanying claims.

Claims

CLAIMS1. A liquid crystal-based optical device comprising:a first liquid crystal device comprising:a first optical cell comprising a first front substrate and a first rear substrate;a first liquid crystal layer disposed between the first front substrate and the first rear substrate, the first liquid crystal layer comprising:a non-polymerised liquid crystal portion of the first liquid crystal layer; anda polymerised liquid crystal portion of the first liquid crystal layer, the polymerised liquid crystal portion forming a first three-dimensional polymer structure within the first liquid crystal layer; anda first pair of electrodes configured to apply an electric field to the first liquid crystal layer; anda second liquid crystal device comprising:a second optical cell comprising a second front substrate and a second rear substrate;a second liquid crystal layer disposed between the second front substrate and the second rear substrate, the second liquid crystal layer comprising:a non-polymerised liquid crystal portion of the second liquid crystal layer; anda polymerised liquid crystal portion of the second liquid crystal layer, the polymerised liquid crystal portion forming a second three-dimensional polymer structure within the second liquid crystal layer; anda second pair of electrodes configured to apply an electric field to the second liquid crystal layer;wherein the first liquid crystal device is coupled to the second liquid crystal device at an interface comprising the first rear substrate and the second front substrate; andwherein the first three-dimensional polymer structure and / or the second three-dimensional polymer structure are configured to selectively modify a characteristic of incident light used to illuminate the liquid crystal-based optical element.

2. The liquid crystal-based optical device of claim 1, wherein:the first three-dimensional polymer structure comprises a spatially varying height profile across at least part of a surface area of the first liquid crystal layer; and / or the second three-dimensional polymer structure comprises a spatially varying height profile across at least part of a surface area of the first liquid crystal layer.

3. The liquid crystal -based optical device of claim 1 or claim 2, wherein:the first three-dimensional polymer structure comprises a plurality of liquid crystal molecules with a constrained orientation; and / orthe second three-dimensional polymer structure comprises a plurality of liquid crystal molecules with a constrained orientation.

4. The liquid crystal -based optical device of any preceding claim, wherein the first liquid crystal device and the second liquid crystal device are arranged such that a rubbing direction of the first liquid crystal device is non-parallel to a rubbing direction of the second liquid crystal device.

5. The liquid crystal -based optical device of any preceding claim, wherein:the first three-dimensional polymer structure and the second three-dimensional polymer structure are the same; orthe first three-dimensional polymer structure and the second three-dimensional polymer structure are different.

6. The liquid crystal -based optical device of any preceding claim, wherein the first three-dimensional polymer structure and / or the second three-dimensional polymer structure comprises:(i) a plurality of discrete ridges arranged in parallel to one another;(ii) a plurality of discrete concentric rings;(iii) a continuous structure with a spatially varying height profile, the height profile varying along one or more directions;(iv) a continuous structure with a periodic spatially varying height profile; (v) one or two holographic structures; and / or(vi) a planar structure with an aperture through the centre of the planar structure.

7. The liquid crystal-based optical device of claim 5(iii), wherein:the first three-dimensional polymer structure and the second three-dimensional polymer structures each comprise an azimuthally varying height profile.the first three-dimensional polymer structure comprises one azimuthally varying height profile, the variance in height being from 0 to a thickness of the first liquid crystal layer across 360 degrees of azimuth; andthe second three-dimensional polymer structure comprising n azimuthally varying height profiles, the variance in height being from 0 to the thickness of the second liquid crystal layer across 360 / w degrees of azimuth, wherein n is > 2.

8. The liquid crystal -based optical device of claim 6, wherein the height profile of the first three-dimensional polymer structure and / or the second three-dimensional polymer structure is configured to reduce an aberration present in incident light used to illuminate the liquid crystal-based optical element, and optionally wherein the aberration corresponds to a Zernike mode.

9. The liquid crystal -based optical device of claim 6(vi), wherein the planar structure and aperture are configured to generate at least one Zernike mode when illuminated with incident light.

10. The liquid crystal -based optical device of any preceding claim, wherein the first liquid crystal layer comprises an additional polymerised liquid crystal portion forming an additional three-dimensional polymer structure, wherein the first three-dimensional polymer structure is located adjacent to the first front substrate and the additional three-dimensional polymer structure is located adjacent to the first rear substrate, wherein optionally the additional polymerised liquid crystal portion comprises a different three-dimensional polymer structure to the first three-dimensional polymer structure .

11. The liquid crystal-based optical device of any preceding claim, wherein:the first front substrate and / or the second rear substrate have a thickness of at least 1 mm; and / orthe first rear substrate and / or the second front substrate have a thickness of no more than 0.5 mm.

12. The liquid crystal-based optical device of any preceding claim, wherein:the first front substrate and the first rear substrate comprise an electrically conductive material, the electrically conductive material forming the first pair of electrodes; andthe second front substrate and the second rear substrate comprise an electrically conductive material, the electrically conductive material forming the second pair of electrodes.

13. The liquid crystal -based optical device of any preceding claim, wherein:the first front substrate, the first rear substrate, the second front substrate and / or the second rear substrate consist of an electrically conductive layer; and / orthe first rear substrate and the second front substrate are together formed by a single substrate layer.

14. The liquid crystal-based optical device, wherein:all of the substrates of the liquid crystal devices are transparent to a wavelength of interest; orthe first front substrate or the second rear substrate is reflective to a wavelength of interest.

15. The liquid crystal -based optical device of any preceding claim, further comprising:a third liquid crystal device comprising:a third optical cell comprising a third front substrate and a third rear substrate;a third liquid crystal layer disposed between the third front substrate and the third rear substrate, the third liquid crystal layer comprising:a non-polymerised liquid crystal portion of the first liquid crystal layer; anda polymerised liquid crystal portion of the first liquid crystal layer, the polymerised liquid crystal portion forming a third three-dimensional polymer structure within the first liquid crystal layer; anda third pair of electrodes configured to apply an electric field to the third liquid crystal layer;wherein the second liquid crystal device is coupled to the third liquid crystal device at a second interface comprising the second rear substrate and the first front substrate;andwherein the third three-dimensional polymer structure is configured to selectively modify a characteristic of incident light used to illuminate the liquid crystalbased optical element; and optionallywherein the second front substrate and the second rear substrate consist of an electrically conductive layer.

16. A system comprising:the liquid crystal-based optical device of any preceding claim;a controller configured to independently provide a first drive voltage to the first pair of electrodes and a second drive voltage to the second pair of electrodes; and optionallya light source configured to illuminate the liquid crystal -based optical device.

17. A method of manufacturing a liquid crystal-based optical device, the liquid crystal optical device comprising:a first liquid crystal device comprising:a first optical cell comprising a first front substrate and a first rear substrate;a first liquid crystal layer disposed between the first front substrate and the first rear substrate, the first liquid crystal layer comprising:a non-polymerised liquid crystal portion; and a polymerised liquid crystal portion forming a first three-dimensional polymer structure; anda first pair of electrodes configured to apply an electric field to the first liquid crystal layer; anda second liquid crystal device comprising:a second optical cell comprising a second front substrate and a second rear substrate;a second liquid crystal layer disposed between the second front substrate and the second rear substrate, the second liquid crystal layer comprising:a non-polymerised liquid crystal portion; anda polymerised liquid crystal portion forming a second three-dimensional polymer structure; anda second pair of electrodes configured to apply an electric field to the second liquid crystal layer;wherein the first liquid crystal device is coupled to the second liquid crystal device at an interface comprising the first rear substrate and the second front substrate, and wherein the method comprises:applying a first write voltage to the first pair of electrodes;while the first write voltage is applied, selectively applying light to liquid crystal material contained within the first optical cell so as to polymerise the liquid crystal material in regions where the light is applied, thereby forming the first three-dimensional polymer structure;applying a second write voltage to the second pair of electrodes;while the second write voltage is applied, selectively applying light to liquid crystal material contained within the second optical cell so as to polymerise the liquid crystal material in regions where the light is applied, thereby forming the second three-dimensional polymer structure.

18. The method of claim 17, wherein the first liquid crystal device and the second liquid crystal device are separate from one another when the first three-dimensional polymer structure and the second three-dimensional polymer structure are formed, the method further comprising subsequently affixing the first rear substrate to the second front substrate.

19. The method of claim 17, wherein:prior to forming the first or second three-dimensional polymer structures, the first liquid crystal device and the second liquid crystal device are coupled to one another; andlight is applied to the first optical cell from the direction of the first front substrate and to the second optical cell from the direction of the second rear substrate.

20. The method of claim 17, wherein:prior to forming the first or second three-dimensional polymer structures, the first liquid crystal device and the second liquid crystal device are coupled to one another; andlight is applied to both the first optical cell and the second optical cell from the direction of the first front substrate or the second rear substrate.

21. The method of claim 17, wherein:the second three-dimensional polymer layer is formed first and, subsequent to this, the first optical cell is deposited on top of the second liquid crystal device; and light is applied to both the first optical cell and the second optical cell from the direction of the first front substrate.

22. The method of claim 20 or 21, wherein between the formation of the first three-dimensional polymer structure and the second three-dimensional polymer structure, the position of a focal point of the light is adjusted so that the focal point is moved from within the first optical cell to within the second optical cell, or vice versa.

23. The method of any claim 17 to 22, wherein:during formation of the first three-dimensional polymer structure, the position of a focal point of the light is moved in a direction normal to the first front substrate and the first rear substrate, such that the first three-dimensional polymer structure varies in height across a planar surface of the first optical cell; and / orduring formation of the second three-dimensional polymer structure, the position of a focal point of the light is moved in a direction normal to the second front substrate and the second rear substrate, such that the second three-dimensional polymer structure varies in height across a planar surface of the second optical cell.

24. The method of any of claims 18 to 23, wherein the first write voltage and / or the second write voltage is at least 10 V.

25. The method of any of claims 18 to 24, further comprising polymerising a portion of the liquid crystal material in the first optical cell and / or the second optical cell so as to produce an alignment feature, the alignment feature subsequently being used to position the first three-dimensional polymer structure relative to the second three-dimensional polymer structure.

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