Improvements in and relating to optical components

Abstract

An electrically switchable optical modulator for modulating an optical wavefront transmitted therethrough, the optical modulator comprising a birefringent first optical element and a birefringent second optical element each having a corresponding ordinary and extraordinary refractive index. A birefringent liquid crystal material is sandwiched between the first and second optical elements. The extraordinary refractive index of the liquid crystal material is electrically switchable between: a first state in which it has a first value; and, a second state in which it has a second value different from the first value. One or both of the first value and the second value do not match the extraordinary refractive index of the first optical element for light polarized in a first direction of linear polarization, and do not match the extraordinary refractive index of the second optical element for light polarized in a second direction of linear polarization orthogonal to the first direction. This switchably produces a relative contrast in the extraordinary refractive index between the liquid crystal material and the first and second optical elements to modulate the wavefront.

Description

Technical Field

[0001] The present invention relates to optical elements or assemblies, such as diffractive and / or refractive optical elements or assemblies. The present invention relates to optical elements or assemblies, whose diffractive and / or refractive optical effects are electrically switchable. The present invention relates to optical elements or assemblies comprising an electrically switchable liquid crystal material. Background Art

[0002] There is a need to provide optical modulator devices, such as lenses, beam steering devices, and diffraction gratings or holograms, that can be switched independently of polarization. These will be very desirable components in smartphones, medical imaging, and photovoltaic applications. The desired performance is fast switching, polarization independence, low power consumption, and low-cost mass production potential. Polarization independence allows the optical modulator to apply the desired optical modulation to unpolarized light that is insensitive to the two orthogonal polarization components inherently present in all unpolarized light. That is, any unpolarized light can be decomposed into two equal components of linearly polarized light, one component polarized in a direction orthogonal to the other, or with opposite chirality (if circularly or elliptically polarized).

[0003] Existing approaches to attempting to provide polarization independence in optical modulators involve physically separating unpolarized light into its two polarization components, then processing each component separately, and then bringing the two processed components back together again. Such approaches often employ a system that uses two optical processing trains / devices arranged in parallel - one optical train / device for each corresponding one of the two orthogonal polarization states of the unpolarized light. This arrangement is often seen in 3D displays and projectors, where the two orthogonal polarization components in a light beam are separated using a polarization beam splitter (PBS) and directed to independent optical processing devices arranged alongside each other for parallel optical processing appropriate for the corresponding polarization states, and then brought back together (e.g., on a screen).

[0004] Alternatively, two independent optical components, each operating on a respective one of the orthogonal polarizations, may be arranged in series. These may be mounted together and coupled using an index matching fluid or adhesive. However, arrangements such as this present problems associated with parallax, such as between the two independent optical components, which are particularly important to avoid in lens systems. Such an arrangement is not suitable for some applications, such as beam redirectors and the like. This can quickly become an impractical arrangement if other optical elements are required in series to mitigate these parallax problems.

[0005] These arrangements, whether parallel or serial arrangements of optical elements, are expensive, complex and relatively bulky. They require replication of components to replicate the optical process in each of the two orthogonal polarization states of unpolarized light. The present invention provides an alternative. SUMMARY OF THE INVENTION

[0007] In a first aspect, the present invention may provide an electrically switchable optical modulator for modulating an optical wavefront transmitted therethrough, comprising: a birefringent first optical element and a birefringent second optical element each having a corresponding ordinary and extraordinary refractive index; a birefringent liquid crystal material sandwiched between the first and second optical elements; wherein the extraordinary refractive index of the liquid crystal material is electrically switchable between: a first state, in which it has a first value; and a second state, in which it has a second value different from the first value. One or both of the first and second values ​​do not match the extraordinary refractive index of the first optical element for light polarized in a first direction of linear polarization, and do not match the extraordinary refractive index of the second optical element for light polarized in a second direction of linear polarization orthogonal to the first direction, thereby switchably rendering a relative contrast in the extraordinary refractive index between the liquid crystal material and the first and second optical elements for modulating the wavefront. The first and second values ​​of the extraordinary refractive index of the liquid crystal material may be respective average values, or bulk values ​​of the liquid crystal material as a whole.

[0008] In this way, by controlling the component of birefringence of the liquid crystal sandwiched between the optical elements in the direction of light propagation, it is possible to enable the first of the two optical elements to obtain an optical modulation function with respect to one polarization component / state of light, and simultaneously enable the second of the two optical elements to obtain an optical modulation function with respect to another (transverse) polarization component / state of light. Optical modulation functions include, but are not limited to: lensing; focusing; defocusing; refraction, diffraction; beam steering. Preferably, the optical modulation function, effect or role of the first optical element is substantially the same as the modulation function, effect or role of the second optical element (for example, both act as lenses, prisms or diffraction gratings, which modulate the corresponding wavefronts to focus the corresponding polarization components of the light or diffract the light in substantially the same way and with substantially the same results). As a result, the optical modulator can provide a switchable optical modulator whose net action or effect is substantially independent of the polarization state of the light incident thereon. The present invention substantially avoids or mitigates the parallax problem caused by the prior art tandem optical arrangement by enabling the two polarizations to be operated by a single device.

[0009] Like all liquid crystal optical devices, the optical modulator of the present invention comprises two surfaces and is arranged so that one surface acts on one optical polarization state and the other on an orthogonal optical polarization state.

[0010] Desirably, a polychromatic dye may be included in the liquid crystal material. The liquid crystal material may contain an element that absorbs one polarization more than the other for a particular wavelength range. In such a case, the optical modulation may be intentionally made wavelength-dependent, so that one wavelength band is modulated as desired, while a different wavelength band is not optically modulated or is optically modulated differently.

[0011] Desirably, the first value matches the corresponding extraordinary refractive index of both the first and second optical elements. Thus, the optical modulator is switchable between states in which it has substantially no effective modulating effect on incident light (e.g., the first state) because there is substantially no effective refractive index contrast between the liquid crystal material and the optical elements surrounding it, and a state in which such contrast exists (e.g., the second state). If neither the first value nor the second value matches the corresponding extraordinary refractive index of either of the first and second optical elements, the optical modulator is operatively switchable between states in which different corresponding contrasts are induced for the extraordinary refractive index between the liquid crystal and the two optical elements. Desirably, the ordinary refractive index of the liquid crystal material matches the corresponding ordinary refractive index of the first and second optical elements. It will be appreciated, and will be apparent to one skilled in the art, that the refractive index values ​​of materials are typically wavelength dependent (i.e., optical materials exhibit dispersion). To this extent, references herein to refractive index "matching" or "mismatching" may refer to situations in which such a contrast occurs for at least wavelengths or electromagnetic radiation that fall within the operating spectrum / band of the optical modulator. Preferably, one refractive index value may be considered to "match" another refractive index value if the two values ​​under investigation agree with each other to the two decimal places (i.e., do not differ). For example, a first refractive index value may be a decimal value: a.bcd, where a, b, c, and d are all integer values, and a second refractive index value may be a decimal value: α.βγδ, where α, β, γ, and δ are each integers. The two refractive index values ​​may be considered to "match" to the two decimal places when: a=α, b=β, c=γ, and d does not necessarily (but may) equal δ. For birefringent materials, this may apply to materials with an extraordinary refractive index n. e and / or ordinary refractive index n o More preferably, one refractive index value may be considered to "match" another refractive index value if the two values ​​under investigation agree within + / - 0.005 (i.e., differ by no more than ). Even more preferably, one refractive index value may be considered to "match" another refractive index value if the two values ​​under investigation agree within three decimal places. However, the inventors have found that satisfactory operation may be achieved when the refractive indices are less closely matched.

[0012] In the second state, the extraordinary refractive index of the liquid crystal material may be different from the extraordinary refractive index of the first optical element for light polarized in a first direction of linear polarization. In the second state, the extraordinary refractive index of the liquid crystal material may be different from the extraordinary refractive index of the second optical element for light polarized in a second direction of linear polarization orthogonal to the first direction.

[0013] In the first state, the extraordinary refractive index of the liquid crystal material may be different from or may be substantially the same as (e.g., 'matching') the extraordinary refractive index of the first optical element for light polarized in a first direction of linear polarization. In the first state, the extraordinary refractive index of the liquid crystal material may be different from or may be substantially the same as (e.g., 'matching') the extraordinary refractive index of the second optical element for light polarized in a second direction of linear polarization orthogonal to the first direction.

[0014] Thus, the optical modulator is capable of switchably generating a relative contrast (or changing from one contrast to another different contrast) in the extraordinary refractive index between the liquid crystal material and the first and second optical elements to modulate the wavefront.

[0015] The extraordinary refractive index of the liquid crystal material is electrically switchable between a first state and a second state, in which it matches the corresponding extraordinary refractive index of both the first and second optical elements. In the second state, the extraordinary refractive index of the liquid crystal material may not match the extraordinary refractive index of the first optical element for light polarized in a first direction of linear polarization, and may also not match the extraordinary refractive index of the second optical element for light polarized in a second direction of linear polarization orthogonal to the first direction.

[0016] Desirably, the ordinary refractive index of the liquid crystal material matches the corresponding ordinary refractive index of the first and second optical elements.

[0017] Desirably, the first and second optical elements are formed of a birefringent polymer material (eg, formed using a reactive mesogenic material) and may each define a respective director, wherein the director of the first optical element is transverse to the director of the second optical element.

[0018] Desirably, the liquid crystal material is a nematic liquid crystal material, wherein, when in said first state, an optical axis of the liquid crystal material is: aligned substantially parallel to the director of the first optical element; and, aligned substantially parallel to the director of the second optical element. Desirably, the first and second optical elements are arranged relative to each other to define a 90° twisted configuration within the nematic liquid crystal between the optical elements.

[0019] Long pitch cholesteric can join the nematics so that the pitch is many times larger than the bulk thickness of the liquid crystal (ie the spacing between optical elements, the cell gap), but the chirality of tilt and twist is substantially preserved for both states without unwanted scattering artifacts.

[0020] Desirably, the first optical element and the second optical element each comprise a birefringent polymer material (eg formed using a reactive mesogen material).The first and second optical elements may be formed from a reactive mesogen that is cured from a reactive state to form a birefringent polymer.

[0021] Twisted nematic (TN) liquid crystal material is beneficial because it works well for a wide range of incident light angles (relative to the optical axis of the modulator) and over a wide spectrum of wavelengths of electromagnetic radiation. It is also relatively insensitive to changes / variations that may occur in the thickness of the liquid crystal cell due to structured surface relief patterns on the first and second optical elements that sandwich it (e.g. when no counter surface structure is used).

[0022] The optical modulator may be configured to provide a half-wave plate. The liquid crystal material may be arranged so that the directors are preferably parallel to each other across the cell, but the cell gap and birefringence are arranged to produce a half-wave delay of the incident light. This causes the two polarizations to be reoriented by 90°, thereby achieving the desired result. The alignment of the liquid crystal material at one or two optical element surfaces (e.g., RM surfaces) may be achieved by providing the surface (e.g., the top layer of the RM surface) with an independent alignment layer having a strong anchoring energy. Utilizing a TN liquid crystal arrangement, together with an optical element presenting an RM surface, requires that the directors of the liquid crystal comply with those of the RM, thereby automatically supporting the correct alignment.

[0023] Desirably, one or each of the first and second optical elements presents a curved surface to which the liquid crystal material conforms.

[0024] Desirably, one or each of the first and second optical elements provides a surface relief pattern to which the bulk shape of the liquid crystal material conforms.

[0025] Desirably, the first and second optical elements each provide a surface relief pattern to which the bulk shape of the liquid crystal material conforms. Conforming to the surface relief pattern means that the fluidity of the liquid crystal material is able to closely conform to the surface relief shape, achieving substantially continuous contact with the surface of the relief pattern on the surface and leaving substantially no gap between the liquid of the liquid crystal material and the surface of the surface relief pattern. The material fluidity properties may be temperature dependent. This may apply, for example, when the optical modulator is designed to operate with a ferroelectric smectic liquid crystal material.

[0026] In these cases, the liquid crystal may be caused to conform to the surface relief pattern or surface shape of the first / second optical element at an elevated temperature, which is a temperature above the temperature at which the liquid crystal is designed to operate normally or optimally. For example, the liquid crystal may be brought into a state of conforming to the surface relief pattern or surface shape of the first / second optical element at a temperature at which the liquid crystal is an isotropic liquid or a nematic liquid crystal during the manufacturing steps, and cooled to a lower temperature liquid crystal phase for operation.

[0027] Desirably, the first and second optical elements each present a surface relief pattern to which the bulk shape of the liquid crystal material conforms, wherein the pattern presented by the first optical element is a reciprocal (eg, opposite or complementary shape) of the pattern presented by the second optical element.

[0028] Desirably, each of the first and second optical elements presents a surface relief pattern to which the liquid crystal material conforms, wherein the pattern presented by the first optical element is substantially the same as the pattern presented by the second optical element.

[0029] Desirably, the first optical element is opposite to the second optical element. One or both of the optical elements may include a substantially planar substrate on the surface on which the corresponding surface relief pattern is formed. The substrate may be non-birefringent, and the surface relief pattern may be formed by a birefringent material deposited on the surface of either substrate. Alternatively, the planar substrate may be birefringent as a whole (e.g., formed by a material having birefringence).

[0030] Desirably, the first optical element defines a first optical axis and the second optical element defines a second optical element substantially coaxial with the first optical axis.

[0031] Desirably, one or both of the optical elements are shaped to define an axis of cylindrical symmetry that is substantially perpendicular to a planar surface provided on one side of the respective optical element. Preferably, both optical elements are shaped to define the respective axes of cylindrical symmetry and the axes of cylindrical symmetry of the first and second optical elements are coaxial.

[0032] Desirably, the liquid crystal material is disposed between two independent electrodes, the electrodes being configured to support an electric field therebetween, the electric field being operable to switch the extraordinary refractive index of the liquid crystal material from the first state to the second state. The liquid crystal material may be electrically switchable between states in which the local directors of the liquid crystal material are substantially homeotropically aligned, and states in which successive local directors of the liquid crystal material are progressively twisted aligned to define a twisted nematic state. The thickness of the liquid crystal material sandwiched between the first and second optical elements may be substantially equal to one quarter of the length of a pitch of a 360 degree twist in the local directors of the liquid crystal material.

[0033] The first state of the extraordinary refractive index of the liquid crystal material may correspond to a twisted nematic state of the liquid crystal material, wherein the local directors of the liquid crystal material are substantially transverse (e.g., perpendicular) to the optical axes of the first and second optical elements. The second state of the extraordinary refractive index of the liquid crystal material may correspond to a homeotropically aligned state of the liquid crystal material, wherein the local directors of the liquid crystal material are substantially aligned (e.g., parallel) along the optical axes of the first and second optical elements.

[0034] Desirably, one or each of the first optical element and the second optical element is disposed between the two independent electrodes.

[0035] The optical modulator may include or may be configured to include a voltage unit, which is arranged to be electrically connected to both the first and second electrodes and is configured to switchably apply a predetermined voltage (V) between the two electrodes to generate an electric field directed across the liquid crystal material of the optical modulator. The two electrodes may be arranged so that the electric field is directed in a direction substantially parallel to a shared optical axis of one or both of the first and second optical elements.

[0036] Desirably, one of two independent electrodes is disposed between the first optical element and the liquid crystal material.

[0037] Desirably, one of two independent electrodes is disposed between the second optical element and the liquid crystal material.

[0038] One or both of the optical elements may include a substrate, on the surface of which a corresponding surface relief pattern is formed. The substrate may be non-birefringent, and the surface relief pattern may be formed by a birefringent material deposited on the surface of the substrate. The aforementioned electrode may be arranged on the surface of the substrate, and the surface relief pattern may be arranged at / on the surface of the electrode. The aforementioned electrode may be arranged on the surface of the surface relief pattern, or on the exposed surface of the two surfaces of the relief pattern of the substrate, if this is present between the elements of the surface relief pattern.

[0039] Stick-shaped reactive mesogens (RM) can be used to form birefringent optical elements. Stick-shaped nematic liquid crystals can be used as liquid crystal materials sandwiched between them (in optical contact with them). The range of suitable values ​​of the birefringence (Δn) of the RM is preferably: Δn=0.05 to 0.35; more preferably, Δn=0.07 to 0.3; even more preferably Δn=0.17 to 0.27. In essence, the birefringence can be as high as possible without compromising the photostability. Therefore, Δn=0.2 to 0.27 is also a suitable range. In each of these cases, the birefringence is quoted as an operating wavelength range, and the values ​​used are typical for stable high birefringence liquid crystals in optical modulators.

[0040] However, the present invention can be used to enable optical modulators to operate outside the optical wavelength range, such as in the infrared (IR), mm-wave or microwave spectral wavelength ranges.

[0041] The spacing (d) between the first and second optical elements, for accommodating the liquid crystal and thus the thickness of the liquid crystal material, may desirably be selected according to the Gooch Tarry conditions given below:

[0042]

[0043] Where Δn is the birefringence of the liquid crystal and d is the spacing between two opposing optical elements (eg, the average spacing when including spacing variations due to structured optical elements).

[0044] Typically, the spacing d may be at least about 5 μm, preferably larger. Preferably, the cell gap d is much higher than the height / amplitude of the surface / pattern features in the birefringent optical element under investigation. For example: d>3h may be a desired criterion. Preferably: d>10h or d>15h or d>20h or d>25h to ensure a substantially complete 90° rotation of the polarization direction of light when it is transmitted through the liquid crystal in the twisted nematic state.

[0045] For optical quality and achromatism, it is preferred that: m > 4. If lens speed is more important or the lens is required for a narrow wavelength range, a lower value of m is chosen.

[0046] The liquid crystal material may be any suitable nematic liquid crystal. An example is MLC-6204-000. Alternatively, other positive Δε nematic materials may be used, such as E7, MDA 1551, MLC 6437, TL 213, MLC 14200, and especially high birefringence nematic mixtures such as BLO36 and BLO37. The reactive mesogen material may be any suitable RM. Examples include RMM1850 (Merck Chemicals Ltd.) or reactive mesogen RM257. Typically, the reactive mesogen is formed from a bifunctional acrylate system to help ensure that a polymer with positive birefringence is formed. It may include other photo-crosslinkable and non-crosslinkable components to suitably adapt the properties of the birefringent polymer, including strongly crosslinking compounds (e.g. TMPTMA), viscosity modifiers, components for improving the average refractive index, components for increasing birefringence, components for reducing the temperature dependence of the refractive index, components for changing the wavelength dependence of the polymer (e.g. dyes, dispersing components, fluorophores), and photoinitiators and sensitizers (e.g. ITX). In embodiments where the birefringent surface relief structure is placed on a substrate, the material of the substrate may be ITO. The material of the first and second electrodes may be ITO.

[0047] Candidate electrode materials include evaporated gold, silver or aluminum, conductive organic layers (such as PEDOT), sintered conductive nanoparticles made of silver or gold, carbon nanotubes and graphene. The interaction between liquid crystals and electric fields (such as the electric field of electromagnetic waves) depends on the direction of the liquid crystal director (ε 平行 ) and vertical (ε 垂直 ) The difference between these two values ​​is called dielectric anisotropy (△ε)

[0048] △ε=ε 平行 -ε 垂直

[0049] The optical modulator may employ a liquid crystal material having a positive dielectric anisotropy at a selected operating frequency, or may employ a liquid crystal material having a negative dielectric anisotropy.

[0050] Optical elements forming a first surface of the device may be configured to provide a different optical modulation (eg, in relation to light of a first polarization) than the optical modulation that the second surface is configured to apply (eg, to light of a second (orthogonal) polarization).

[0051] The first optical element may define a lens. The second optical element may define a lens. The focal length of the first optical element (e.g., for light of a first polarization state) may be different than the focal length of the second optical element (e.g., for light of a second polarization state). The focal length of the first optical element may be longer than the focal length of the second optical element.

[0052] The focal length of the first optical element (e.g. acting on the first polarization state of light) can be sufficiently greater / higher than the focal length of the second optical element (e.g. acting on the second polarization state) such that the difference in focal lengths is substantially equal to the optical thickness of the liquid crystal medium between the two optical elements. In this way, slight optical differences between the first and second surfaces can be compensated, thereby achieving a desired (e.g. lowest possible) depth of focus.

[0053] In a second aspect, the present invention may provide a method for manufacturing an optical modulator comprising one or more birefringent optical elements shaped into a desired shape for modulating an optical wavefront transmitted therein, wherein each birefringent optical element is provided according to a method comprising: providing a flexible sheet having an embossed surface, the sheet being embossed with a shape that is inverse to the desired shape (e.g., an opposite or complementary shape); providing a quantity of photoreactive material comprising reactive mesogens in an uncured state on a surface of the substrate; bending the embossed surface to press against the quantity of photoreactive material, gradually across the surface of the quantity of photoreactive material, thereby forming a shaped birefringent portion having the desired shape; and curing the shaped birefringent portion to form the birefringent optical element.

[0054] The curvature of the embossed surface can be caused by bending the flexible sheet. The bending of the flexible sheet can be caused by pressing against the side of the localized surface area of ​​the flexible sheet (the side opposite to the side with / presenting the embossed surface) so that the flexible sheet reacts by bending to form a depression in the localized surface area and a reverse convexity on the opposite side of the sheet, the opposite side being the side presenting the localized embossed surface. Such a local convexity of the embossed surface can be pressed against the amount of photoreactive material on the substrate and can present an embossed surface that is partially parallel to the surface of the substrate, where it is caused to be in close contact with it and partially inclined to it, where the local convexity causes the embossed surface to gradually separate from the surface of the substrate. By gradually moving the curved position of the embossed surface in the direction along the surface of the substrate, the inclined portion of the embossed surface sweeps over the surface of the substrate and pushes a certain amount of reactive mesogen fluid material in front of it as it advances. The movement of the positional curvature can be achieved by pressing a roller against the aforementioned localized surface area and rolling the roller over the flexible sheet to gradually advance to a new adjacent localized surface area.

[0055] The optical modulator may be electrically switchable.The optical modulator may comprise an electrically switchable liquid crystal material sandwiched between two such birefringent optical elements.

[0056] The method may include providing the embossed surface with a preferential surface alignment and thereby imparting the preferential surface alignment to the embossed surface of the shaped birefringent portion, the surface alignment being configured to align the liquid crystal material. Such preferential surface alignment may be achieved by, but is not limited to, scratching the shaped birefringent portion, or applying an overcoat polymer thereto, or applying a photoreactive overcoat thereto and photoalignment of the overcoat by applying polarized light thereto, or by applying a suitable coating comprising a surfactant. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1A and Figure 1B Two states of an electrically switchable optical modulator are schematically shown;

[0059] Figure 2 Demonstrated schematically Figure 1B a function of the interfaces between components of an electrically switchable optical modulator;

[0060] Figure 3A and Figure 3B schematically showing a respective one of two states of an electrically switchable optical modulator, and Figure 3C , Figure 3D and Figure 3E each schematically showing two states of an electrically switchable optical modulator;

[0061] Figure 4 Demonstrated schematically Figure 3A An exploded view of a portion of an electrically switchable optical modulator;

[0062] Figure 5 Demonstrated schematically Figure 3B An exploded view of a portion of an electrically switchable optical modulator;

[0063] Fig. 6A and 6B Schematic diagram showing the manufacturing process that can be used Figure 1A , 1B , 3A or 3B;

[0064] Figure 7 (a) to 7(h) present views of optical elements embossed with materials comprising birefringent polymers formed using reactive mesogens;

[0065] Figure 8 (a) to 8(c) show the lateral beam profile of a laser beam transmitted through a switchable optical modulator and the associated diffraction efficiency as a function of the applied switching voltage;

[0066] Fig. 9 Displaying the transverse beam properties of a laser beam transmitted through a switchable optical modulator and the associated input beam polarization state;

[0067] Fig.10 (a) to 10(c) and 10(d) to 10(f) respectively show the peak optical power values ​​of the light beam and its beam diameter with respect to the light beam transmitted by the switchable optical modulator;

[0068] Fig.11 (a) and 11(b) to 11(e) show simulation results and measurement results, respectively, illustrating the point spread function of a switchable optical modulator;

[0069] Fig.12 (a) to 12(i) respectively show: (a) a master mold for manufacturing an optical element; (b, c) images of light passing through an optical element (Fresnel lens) using Fig.12 (a) an image of an optical element (diffraction grating) embossed with the master, and observed according to two different optical polarization states; (d, e, f) an image of an optical element (diffraction grating) embossed with the master, and an optical diffraction pattern therefrom; (g, h, i) an image of an optical element (microlens array) embossed with the master, a view of the beam profile of a laser beam passing therethrough, and an image of a photograph imaged through the microlens array;

[0070] Fig.13 (a) and 13(b) show the cross-sectional surface profiles of two Fresnel zone plates;

[0071] Fig.14 (a) and 14(b) show the cross-sectional surface profiles of two Fresnel zone plates;

[0072] Fig.15 (a) and 15(b) show the cross-sectional surface profiles of two Fresnel zone plates;

[0073] Fig.16 (a) to (f) show images of an electrically switchable optical modulator subjected to different applied switching voltages;

[0074] Fig.17 Schematic representation of the setup for measuring the diffraction efficiency of an optical modulator (Fresnel lens);

[0075] Fig.18A and 18B Schematically showing the arrangement of ordinary and extraordinary beams relative to the optical axis (director) of a material comprising a liquid crystal arrangement;

[0076] Fig.19A and Fig.19B schematically showing a respective one of two states of an electrically switchable optical modulator;

[0077] Fig. 20A and Fig. 20B Schematically shows a switchable polarization-independent diffraction grating according to an embodiment of the present invention. Fig. 20A In , the diffraction grating is in the "field off" state, where no diffraction occurs and the two interfaces (reactive mesogen / liquid crystal) are index matched, while in Fig. 20B , the diffraction grating is in the "field-on" state, where diffraction occurs and the two interfaces (reactive mesogen / liquid crystal) are not index matched, so that different polarizations are diffracted at the top / bottom interface;

[0078] Fig.21 A refractive lens using an embodiment of the present invention in an imaging application is schematically illustrated.

[0079] Description of the implementation plan

[0080] Figure 1A and Figure 1B Schematically illustrated are cross-sectional views of an electrically switchable optical modulator 1 in each of two switching states which will be described in detail below.

[0081] In such an embodiment, the optical modulator is a compound Fresnel lens. The compound lens comprises a pair of optically transparent and optically birefringent optical elements (2, 4) comprising a first Fresnel lens sheet 2 and an opposing second Fresnel lens sheet 4 spaced a distance (d) from the first Fresnel lens sheet, and filled with a twisted nematic liquid crystal material 6. Each of the first and second Fresnel lens sheets defines a respective optical axis, and the two opposing lens sheets are arranged in parallel opposition, and their respective optical axes 9 are arranged in a collinear manner.

[0082] The opposing surfaces of the two Fresnel lens sheets each present a surface relief pattern (7, 8) to each other, the pattern consisting of annular concentric annular concave portions or segments, each of which is functionally equivalent to an annular portion of a corresponding conventional concave lens surface. The height (h) or amplitude of the surface relief features is substantially the same on the patterned surface of each lens sheet. In other words, the depth of the depression formed by any one of the annular concave portions is substantially the same as the depth of the depression formed by any other of those annular concave portions. The focal length of the first Fresnel lens sheet is substantially equal to the focal length of the second Fresnel lens sheet.

[0083] Each of the two Fresnel lens sheets is formed of a birefringent polymer and is made using a birefringent polymerizable liquid crystal material called a reactive mesogen (RM) in the form of a shaped bulk of a resin cured into a solid form. In birefringence, each Fresnel lens sheet has an ordinary refractive index n o and the extraordinary refractive index n e Birefringent polymer materials are substantially homogeneous and transparent over at least the wavelength range over which transmission occurs. The speed of light transmitted through the material is determined by the direction of oscillation of the light vibrations. When a single beam of light enters a birefringent material, two beams may emerge that deviate from each other. There are directions of linear polarization of light for which there is a definite refractive index; the refractive index associated with each of the two orthogonal directions of linear polarization is different.

[0084] The orientation of the 'optical axis' of a birefringent material relative to the propagation direction of the oscillations of the electric field (E) of the light determines the refractive properties of the material. If there is propagation of light along the direction of the optical axis, then all orientations of the electric field components of the light oscillate perpendicular to the optical axis and there is no birefringence. This is schematically illustrated in Fig.18A , where the perpendicular orientations of the polarizations of light (a) and (b) are also each perpendicular to the optical axis. Here, the optical axis is defined by the 'director' of the liquid crystal material. However, if there is propagation of light transverse to the optical axis, birefringence occurs, provided that, in the case of a birefringent material, the electric field component of the light wave oscillates in a direction along the optical axis (or at least can be resolved along this axis). Otherwise, no optical delay occurs. This is schematically shown in Fig.18B, which shows the perpendicular orientation of the polarization of light (a) and (b). The polarization of light beam (b) is completely perpendicular to the optical axis and experiences the ordinary refractive index of the material. However, light ray (a) has its electric field component oriented to oscillate in-plane with the optical axis, then it experiences the extraordinary refractive index of the material. This is true even if the light ray is tilted at an angle (θ) relative to the optical axis, as shown by light ray (c).

[0085] The value of the extraordinary refractive index varies depending on the angle of propagation of such a ray in the following manner:

[0086]

[0087] Therefore, when such a light ray is tilted at an angle (θ = 0), as shown in Fig.18A , then the extraordinary and ordinary refractive indices are equal and no birefringence occurs. Conversely, when such a light ray is tilted at an angle (θ = 90 degrees), as shown in Fig.18B (a), then it experiences maximum birefringence and experiences maximum optical retardation.

[0088] Birefringence is quantified by the difference between the values ​​of the extraordinary and ordinary refractive indices, while retardation is defined as the total phase difference experienced by orthogonal polarizations after passing a birefringent material over a certain distance. This distance can be taken as the thickness of the optical medium under study, such as a reactive mesogen or liquid crystal material, such as can be applied to and between optical elements of an optical modulator.

[0089] Mesogen materials can exhibit both solid-like and liquid-like properties. This liquid crystal state (LC) is called an mesophase and occurs between a crystalline solid state and an isotropic liquid state over different temperature ranges. Liquid crystals can include stick-like molecules with the long axes of adjacent molecules roughly aligned with each other. A vector, called a "director," is used to represent the preferred orientation direction of the molecules in the material at any given point. The birefringent polymer material of the first and second Fresnel plates is in the form of a cured resin in which the reactive mesogen is no longer reactive, but its director is retained.

[0090] In the first Fresnel lens sheet 2, the director 3 of the reactive mesogen material is aligned in a direction perpendicular to the optical axis 9 of the lens sheet. Similarly, in the second Fresnel lens sheet 4, the director 5 of the reactive mesogen material is aligned in a direction perpendicular to the optical axis 9 of the lens sheet and perpendicular to the direction of the director 3 of the material of the first lens sheet 2.

[0091] The twisted nematic liquid crystal material 6 sandwiched between the first and second Fresnel lens plates is a birefringent liquid crystal material. The director of the liquid crystal material is locally aligned with the director of the reactive mesogen material on each of the first and second lens plates, respectively, where the liquid crystal material and the lens plates are joined. In a first static state of the liquid crystal 6, the progressive twisting of the director of this twisted nematic material ensures that the liquid crystal director is aligned with two mutually orthogonal directors of the opposing first and second Fresnel lens plates. In this static state, both the ordinary and extraordinary refractive indices of the liquid crystal material match the ordinary and extraordinary refractive indices of each of the two Fresnel lens plates. Therefore, at the boundary between the liquid crystal material and the opposing lens plates, there is no refractive index contrast in the ordinary and extraordinary refractive indices of these three components of the optical modulator. Therefore, non-polarized light (or arbitrarily polarized light, elliptically or circularly polarized light, etc.) 10 can pass through the optical modulator without substantially experiencing any refraction when passing from the first Fresnel lens plate to the liquid crystal material and then from the liquid crystal material to the second Fresnel lens plate. Such a situation is schematically illustrated in Figure 1A .

[0092] On the contrary, as schematically shown in Figure 1B In a second state, in which the optical modulator obtains a lensing function, the liquid crystal material is acted upon by an applied electric field which changes the direction of the liquid crystal director 17 so as to be substantially parallel to the shared optical axis of the first and second Fresnel lens sheets. The optical modulator comprises a pair of transparent conductive electrodes (not shown) Figure 1A ), which comprises a first electrode 12 adjacent to the first Fresnel lens sheet, and a second electrode 13 adjacent to the second Fresnel lens sheet. A voltage unit (not shown) is provided in electrical connection with both the first and second electrodes, and is configured to switchably apply a predetermined voltage (V) between the two electrodes, thereby generating an electric field 14, which is directed across the liquid crystal material of the optical modulator in a direction substantially parallel to the shared optical axis 9 of the first and second Fresnel lens sheets.

[0093] On display at Figure 1B In an embodiment of the invention, the first and second Fresnel lens plates are arranged together with the liquid crystal material between the first and second electrodes. However, in an alternative embodiment (see Figure 3C ), one of the two electrodes may be disposed between the first Fresnel lens plate and the liquid crystal material or between the second Fresnel lens plate and the liquid crystal material. In a further embodiment, (see Figure 3D ) One of the two electrodes may be disposed between the first Fresnel lens plate and the liquid crystal material, and the other of the two electrodes may be disposed between the second Fresnel lens plate and the liquid crystal material.

[0094] Due to the application of the electric field 14 to the liquid crystal material and the resulting change in the orientation of the liquid crystal director, the extraordinary refractive index of the liquid crystal material is made to mismatch with the extraordinary refractive index of the first Fresnel lens plate 3 with respect to the polarization of light 15 in a first direction of linear polarization. At the same time, the extraordinary refractive index of the liquid crystal material is also made to mismatch with the second Fresnel lens plate 4 with respect to the polarization of light 16 in a second direction of linear polarization orthogonal to the first direction of linear polarization. Of course, the incident light 10 directed onto the optical modulator 1 is unpolarized and can therefore be considered to be composed of or decomposable into equal components / ratios of light polarized in the first and second directions of linear polarization. The lenses under the action of the optical modulator act on the two orthogonal components / ratios of linearly polarized light at two separate continuous interfaces between the liquid crystal material and the first and second Fresnel lens plates, respectively. This is schematically shown in Figure 1B .

[0095] In order to more clearly demonstrate the performance of the optical modulator and the functional steps implemented by the independent optical components of the modulator, reference is now made to Figure 2 , in which the four optical interfaces of the optical modulator are schematically presented and their corresponding optical functions are shown.

[0096] In particular, the optical modulator may be considered to include a first optical interface 20, which is the interface between the upper surfaces of the first Fresnel lens sheet 2, where unpolarized light or light 10 of any polarization enters the optical modulator at this interface.

[0097] Importantly, the optical modulator according to the invention in its first aspect and the embodiments described herein is operable not only with respect to completely unpolarized incident light, but also equally operable with respect to incident light having any degree (%) of polarization. Thus, if there is some degree of polarization in the incident light (e.g. reflected sunlight) or if circularly or elliptically polarized light is incident, the optical modulator modulates said light in the same manner.

[0098] The birefringence of the material of the first lens plate means that the first component of light 24 polarized in a first direction of linear polarization experiences a different refractive index than the second component of light 25 polarized in a second direction of linear polarization. Therefore, birefringence occurs, which is schematically demonstrated by the spacing of these two orthogonal polarization components (24, 25) of the light. Figure 2The forward propagation of these two light components brings them to a second interface 21 within the optical modulator, which is the interface between the first lens plate 2 and the liquid crystal material 17 in the second state. At this interface, only with respect to the second component of light, the extraordinary refractive index of the first lens plate does not match the extraordinary refractive index of the liquid crystal material, so that the second component of light experiences a refractive index contrast across the interface, thereby causing the first lens plate to perform its intended lensing function 26. However, with respect to the first component of light, which does not experience any refractive index contrast at the interface, the ordinary refractive index of the first lens plate remains matched to the ordinary refractive index of the liquid crystal material, thereby preventing the first lens plate from performing any lensing function with respect to that component of light.

[0099] The forward propagation of the two components of light brings them to a third interface 22 within the optical modulator, which is the interface between the second lens plate 4 and the liquid crystal material 17 in the second state. At this interface, only with respect to the first component of light, the extraordinary refractive index of the second lens plate does not match the extraordinary refractive index of the liquid crystal material, so that the first component of light experiences a refractive index contrast at the interface, causing the second lens plate to perform its intended lensing function 27. However, with respect to the second component of light, the ordinary refractive index of the second lens plate remains matched to the ordinary refractive index of the liquid crystal material, which does not experience any refractive index contrast at the interface, thereby preventing the second lens plate from performing any lensing function with respect to that component of light.

[0100] Finally, the subsequent propagation of the two components of light brings them both to the fourth interface 23, which is the interface between the outer edge of the second lens plate and the surroundings beyond the optical modulator. The output light 28 comprises two combined components of mutually orthogonal linear polarizations, each of which is modulated (i.e. focused in this case) separately within the optical modulator, but output as a combined modulated light output.

[0101] Figure 3A and 3B Another embodiment of the present invention is schematically shown, wherein the optical modulator 30 employs two opposing, optically transparent and parallel substrates separated by a spacing "d". Each substrate carries a birefringent Fresnel zone plate pattern on a respective substrate surface. The first optical element is provided by a first optical substrate 32 having a patterned first Fresnel zone plate pattern 34 thereon, the pattern comprising a birefringent polymer (cured reactive mesogen) material having a first director 33 parallel to the planar surface of the first optical substrate. The second optical element is provided by a second optical substrate 38 having a patterned second Fresnel zone plate pattern 37 thereon, the pattern comprising a birefringent polymer (cured reactive mesogen) material having a second director 39 parallel to the planar surface of the second optical substrate, but perpendicular to the direction of the first director 33.

[0102] The first and second zone plate patterns are complementary to each other with respect to their spatial shapes. That is, where the first zone plate pattern presents a body of birefringent polymer (cured reactive mesogen) on the first substrate, the directly opposite area of ​​the second zone plate presents an open area of ​​the second substrate between adjacent bodies of birefringent polymer (cured reactive mesogen) on the substrate. In this way, the location on the first substrate 32 showing the presence of birefringent polymer (cured reactive mesogen) 34 is inverse to the opposite portion of the second substrate 38 due to the absence of birefringent polymer (cured reactive mesogen) 37, and vice versa.

[0103] The reactive mesogenic directors (33, 39) of the first and second optical elements define respective optical axes that are perpendicular to each other. This results in a twisted configuration of the liquid crystal material 35 sandwiched between the two optical elements. When the optical modulator 30 is in the closed state, as schematically shown in FIG. Figure 3A , each of the two perpendicular polarization components of the unpolarized light 31 propagating through the structure undergoes a corresponding 90° polarization rotation, but passes without a corresponding wavefront 40 (phase front) distortion.

[0104] When the optical modulator 30 is in the on state, as schematically shown in FIG. Figure 3B By applying a voltage 41 between electrodes (not shown) provided on the first and second optical elements, the liquid crystal 43 is reoriented in a direction perpendicular to the plane of each of the first and second substrates along an electric field generated by the voltage.

[0105] Such reorientation prevents the corresponding 90° polarization rotation that would otherwise be provided in the off state, and both of the two perpendicular components of polarization (44, 45) present in the incident light 42 undergo their respective wavefront (phase fronts 46 and 47) distortion at the respective one of the first and second optical elements. This results in one of the two components with respect to polarization 44 being lensed from the first optical element, and the other of the two components with respect to polarization 45 being lensed independently from the second optical element.

[0106] The complementary design of the Fresnel zone plate patterns (34, 37) on the first and second optical elements achieves a more uniform electric field distribution throughout the box compared to alternative designs in which the Fresnel zone plate patterns are not complementary. Thus, voltage independent lensing can be achieved once a critical voltage is exceeded, wherein the lensing effect remains stable to changes in the applied voltage (V). This arrangement is less sensitive to imperfect refractive index matching between the refractive indices of the liquid crystal material and the birefringent Fresnel zone plate, because the distortion imposed on the initially undistorted incident wavefront 31 (caused by imperfections in the first optical element (32, 43), as present, (resulting in a slightly distorted phase front 36)) is substantially compensated by the complementary distortion imposed on the slightly distorted wavefront 36, producing a substantially undistorted output wavefront 40 when passing through the second optical element (37, 38), as shown in FIG. Figure 3A This makes the design resilient to index matching imperfections that may occur due to index variations within the device.

[0107] On display at Figure 3A and 3B In an embodiment, the first and second Fresnel zone plates are arranged together with the liquid crystal material on the first and second electrodes (12, 13: see for example Figure 1B However, in an alternative embodiment (see Figure 3C ), one of the two electrodes 12 is disposed between the first Fresnel zone plate and the liquid crystal material or between the second Fresnel zone plate and the liquid crystal material. In a further embodiment, (see Figure 3D ) One of the two electrodes 12 is disposed between the first Fresnel zone plate and the liquid crystal material, and the other of the two electrodes 13 is disposed between the second Fresnel zone plate and the liquid crystal material. In yet another embodiment, (see Figure 3C and Figure 3E ), one of the two electrodes (12, 13) (12, Figure 3C ) or each (12, 13; Figure 3E ) are arranged with their corresponding Fresnel zone plates between the RM surface relief pattern (Fresnel zone pattern) and the transparent planar plate / substrate on which the RM surface relief pattern is formed. In such an arrangement, the optical effect of the conductive electrodes is preferably minimized. However, the use of these arrangements removes the field losses that occur on the non-electroactive reactive mesogenic optical elements, thereby reducing the operating voltage and electrical power.

[0108] Fig.19A and 19BAn alternative embodiment according to the present invention is shown. In this alternative, applying an electric field to the liquid crystal sandwiched between the first and second optical elements can cause the optical modulator to cease its modulation function, while removing the electric field allows the modulator to resume modulation. This is the above reference Figure 3A and 3B The reverse of the arrangement described. In such an alternative arrangement, a vertically aligned twisted nematic liquid crystal 310 is employed between the first and second optical elements (32, 38).

[0109] Fig.19A The vertical state of the liquid crystal 310 is shown when the optical modulator is in the "field off" state. In this state, the voltage unit 41 does not apply a voltage (V) between the electrodes of the system (12, 13), and there is a pre-tilt in the liquid crystal (inset: Fig.19A ). The pretilt is in the direction of the optical axis 33 of the birefringent polymer forming the adjacent Fresnel zone plate 34.

[0110] In this embodiment, the arrangement of the optical modulator is based on the above reference Figures 3A to 3E Embossed anisotropic Fresnel zone plates (34, 37). In addition, however, in this embodiment, the Fresnel zone plates are each overcoated with a vertical alignment material 300 (eg, polyimide, silane, lecithin, photo-alignment material).

[0111] The liquid crystal has a pre-tilt 310 in the direction of the optical axis of the underlying Fresnel zone plate, which can be induced by scratching or photo-alignment. The modulator comprises a first and a second optical element (32, 33; 37, 38), and each of them has an optical axis perpendicular to each other (i.e. Fresnel lens: directors 33, 39 of birefringent material).

[0112] The liquid crystal adopts a vertical configuration 310 in the "field off" state [Figure 19(A)], and this state allows polarization-independent lensing by an optical modulator, as described above with reference to Figure 3A described.

[0113] Fig.19B The "field on" state is shown, wherein a voltage (V) is applied by a voltage unit 41 to the electrodes (12, 13) of the system. Here, an electric field induced twisted nematic configuration 311 is provided in the liquid crystal. The liquid crystal may have a negative dielectric anisotropy. In the "field on" state, the liquid crystal 311 is oriented perpendicular to the electric field due to the negative dielectric anisotropy. Here, the liquid crystal 311 is index matched to the ordinary and extraordinary refractive indices of the birefringent material (e.g. formed from a cured reactive mesogen) forming the Fresnel zone plate, as described above with reference to Figure 3AAs described. In this state essentially no lensing occurs. The pre-tilt of the liquid crystal 310 in the direction of the optical axis of the underlying birefringent Fresnel lens (directors: 33, 39) results in a twisted nematic configuration [FIG. 19(B)]. In such a case, the optical modulator does not focus light because the isotropic Fresnel zone plate is index matched to the liquid crystal.

[0114] In an alternative embodiment, only one of the two optical elements (32 or 38) exhibits / provides pretilt 310, while the other is perpendicular to the substrate.Twist under applied voltage can be induced using chiral nematics with d / p≈0.25, where d is the cell gap and p is the pitch of the chiral nematics.

[0115] The rod-shaped reactive mesogens (RMs) can be used to form birefringent optical elements / polymers, and the rod-shaped nematic liquid crystals can be used as the liquid crystal material sandwiched between them (in optical contact with them). The nematic liquid crystals can be arranged in a substantially planar manner, such as in Figure 3A and 3B and can be arranged to be uniformly aligned to prevent scattering.

[0116] The RM preferably has a high birefringence. The range of suitable values ​​of the birefringence (Δn) of the RM is preferably: Δn=0.05 to 0.35; more preferably Δn=0.07 to 0.3; and even more preferably Δn=0.17 to 0.27, where the values ​​are given for the visible wavelength range. Of course, the device can be designed to operate in other wavelength ranges, including IR, mm-wave, microwave, and THz. In essence, the birefringence can be as high as possible without compromising light stability. Therefore, Δn=0.2-0.27 is also a suitable range.

[0117] RMs may be effectively bifunctional. This may be the case, for example, when stick-like (rod-like) polymerizable groups (e.g., acrylate groups) are attached to each end of the RM. This ensures that the RMs induce a plane alignment in the liquid crystal. The desired alignment of the RM material is most preferably maintained throughout the optical element formed by the RM. This may be a mixed alignment or a twisted alignment, but in most cases, in some implementations, a uniform alignment of the plane of the RM director in the entire optical element is preferred. However, the twisted nematic configuration of the liquid crystal within the optical modulator may bring about some beneficial properties. In particular, improved wavelength dependence and less dispersion may be achieved. The examples of optical modulators provided herein employ a substantially planar configuration / structure because such a structure is simple to manufacture and design. However, the present invention is not limited to such a substantially planar structure or design.

[0118] The optical element may be structured (e.g., shaped or patterned) to provide a refractive optical effect and / or a diffractive optical effect on the transmitted light wavefront through a refractive index difference between the contacting liquid crystal and the RM optical element (which is shaped to provide the desired effect). For example, the optical element may be shaped in the form of a refractive lens (e.g., a simple lens or a Fresnel lens) or a refractive prism, or in the form of a diffractive structure, such as a diffraction grating or a Fresnel diffractive optical element. The liquid crystal material is selected to be compliant so that the bulk liquid can adopt a shape or structure that complies with (i.e., forms the opposite shape / structure) of the optical element that interfaces with it. This provides a refractive or diffractive interface / surface for modulating the optical wavefront.

[0119] Preferably, each optical element is deposited on an alignment layer less than about 100 nm thick, which in turn is preferably deposited on a transparent electrode. The optical element is preferably designed and fabricated so that the maximum electric field can be applied to the liquid crystal material between two opposing electrodes. That is, the minimum distance between the alignment layer and the surface of the optical element adjacent to the liquid crystal material is preferably as low as possible. For example, the optical structure formed by the RM material of the optical element preferably displays a pattern amplitude (such as a grating structure or a groove in a Fresnel lens / plate) that is as close as possible to the alignment and electrode layers. This minimizes the value of the minimum distance between the electrode and the adjacent active liquid crystal layer and ensures that the switching voltage remains as low as possible.

[0120] The liquid crystal material and the optical element (diffractive or refractive) may preferably be formed of a rod-shaped liquid crystal and a birefringent polymer (cured reactive mesogen), respectively, and further, a discotic liquid crystal material may be used for the former. If a positive birefringent rod-shaped type is used for both the birefringent optical element and the liquid crystal therebetween, a component of light polarized parallel to the optical axis (director) of the birefringent optical element is refracted / diffracted, while a component of light polarized perpendicular to the optical axis (director) of the birefringent optical element is transmitted without significant refraction or diffraction, as described above.

[0121] The two optical elements of the optical modulator are ideally separated by a contacting liquid crystal medium that is substantially transmissive in the operating wavelength range of the optical modulator. The liquid crystal between the two optical elements of the optical modulator is preferably arranged to be refractive index matched at least locally near the interface surface of the two optical elements. This means that the director profile of the liquid crystal is desirably arranged to be 90° twisted to form a twisted nematic.

[0122] The pitch of the optical element and hence the thickness of the twisted nematic liquid crystal material may desirably be selected in accordance with the Gooch Tarry condition given below:

[0123]

[0124] Where Δn is the birefringence of the liquid crystal, and d is the spacing between two opposing optical elements (eg, the average spacing when including spacing variations due to structured optical elements).

[0125] For optical quality and achromatism, it is preferred that: m>4. If speed is more important, or the lens is required for a narrow wavelength range, a lower value of m may be chosen. Typically, the spacing d may be at least about 5 μm, preferably larger.

[0126] It is important that the cell gap d is much higher than the height / amplitude of the surface / pattern features in the birefringent optical element being investigated. For example: d>3h may be a desirable criterion to be established, as the desired director profile becomes substantially uniform in polarization direction for the polarization component of the light to be rotated 90°. Preferably: d>>3h, such as d>10h or d>15h or d>20h or d>25h, to ensure a substantially complete 90° rotation of the polarization direction of the light when it passes through the liquid crystal in the twisted nematic state. Such a condition would desirably be achieved at a distance of approximately 1h at the top / outer portion of the surface pattern features of the optical element, and thus the 3h condition occurs when considering two such opposing surfaces and a uniform central region (where the twisted nematic liquid crystal is substantially uniform). Such a central uniformly twisted region preferably has a thickness greater than h.

[0127] Fabrication of embossed and oriented birefringent polymers

[0128] The present invention may provide a method of manufacturing an optical element for an optical modulator. For example, a method of manufacturing the above-mentioned electrically switchable optical modulator is provided, the optical modulator comprising an electrically switchable liquid crystal material sandwiched between two birefringent optical elements shaped into a desired shape for modulating an optical wavefront transmitted therein. Any or each birefringent optical element of the modulator may be provided by a method comprising: providing a flexible sheet presenting an embossed surface, the embossed surface being embossed with a shape inverse to the desired shape; providing a certain amount of reactive mesogen material in an uncured state on the surface of the substrate; pressing the embossed surface against the amount of reactive mesogen material and gradually crossing the surface of the amount of reactive mesogen material to form a shaped birefringent portion having the desired shape; curing the shaped birefringent portion to form the birefringent optical element. The method may include providing the embossed surface with a preferential surface alignment and thereby imparting the preferential surface alignment to the embossed surface of the shaped birefringent portion, the surface alignment being configured to align the liquid crystal material.

[0129] The RM in liquid form can be formed into a desired shape (e.g., a lens shape or other pattern) between a substrate comprising a transparent electrode and an embossed alignment layer of the RM in contact with the RM in liquid form, thereby embossing the desired shape therein and simultaneously imparting the desired alignment to the surface of the RM. The following process can be used:

[0130] In a first step: an original master of or for a birefringent optical element is defined, for example using diamond cutting or lithographic methods (photolithography, electron beam, etc.). Procedures such as those readily available to a person skilled in the art, such as those applied to the production of gratings, lenses or prismatic elements, can be used for this step.

[0131] The original master mold can then be replicated into a metal pad (e.g., nickel) using, for example, electroforming. Alternatively, the original master mold can be used to make a replica.

[0132] In the second step: the inverse of the desired pattern or shape is embossed into a resin material provided on a flexible backing substrate such as a PET film (eg using hot foil or photo embossing). This forms an embossed surface for embossing the RM material.

[0133] In the third step: a preferential alignment is imparted to the upper surface of the embossed surface. This can be induced by scratching the upper surface of the resin (polymer), or coating the embossed resin structure with an alignment layer (preferably thin), such as a liquid crystal polymer, a surfactant or a photoalignment agent, and imparting the correct orientation to the surface of the opposite optical element.

[0134] In the fourth step: an optical substrate (e.g. glass or transparent non-birefringent plastic) is provided on which the optical element (RM material) is to be deposited, the substrate having an electrode structure deposited thereon and an alignment layer (usually a scratched polyimide or photo-alignment layer) of a suitably aligned plane. A certain amount of uncured / liquid RM material is then deposited on one surface of the substrate in the form of lines or strips, and a flexible embossed surface is placed in contact with the liquid RM material. A roller is applied to the opposite surface of the flexible backing substrate to bend the substrate and the embossed surface thereon while pushing the embossed surface into the liquid RM material. Under appropriate pressure and speed, while the RM material is in contact with the embossed surface, the roller is pushed along the opposite surface of the flexible backing substrate to gradually bend the embossing along the surface of the optical substrate into the liquid RM material, thereby spreading the RM material thereon while embossing it. That is, embossing is performed so that the RM is pressed into the shape of the original optical template structure, and is performed with minimal deviation between the base and the electrode of the feature in the element.

[0135] The resin on the film is usually arranged so that the orientation direction of its surface coating is equal to (parallel or antiparallel) to the orientation direction of the surface. The effect of the alignment layer on either side of the RM causes it to form a uniform birefringent state with (or close to) the complete birefringence possible for the RM. The RM is then cured to form a polymer, and when the film and resin mold are removed, the aligned polymer RM remains on the glass substrate.

[0136] The embossing process can be completed at an elevated temperature to obtain an optimum speed for practical embossing speed and pressure. The process can be completed at a temperature where the RM is an isotropic liquid. In this case, the cooling of the RM in contact with the film can be carried out in a controlled manner. Preferably, the thermal gradient during cooling is in the direction of the optical axis of the RM.

[0137] Curing should be accomplished at cold temperatures, but in the nematic phase of the uncured RM so that the order parameter and hence the birefringence is at a maximum.

[0138] Preferably, solidification will be carried out at the lowest temperature possible in the nematic phase of the RM to maximize the order parameter of the layer and the birefringence therefore. In such a case, the liquid crystal is basically matched with the RM in the operating temperature range of the equipment, rather than at the temperature of curing the RM. The RM of the alignment will usually give the nematic liquid crystal in contact with a sufficiently strong alignment. Such an alignment is spontaneous in the desired direction for correct operation. However, the operation of the device can be improved by strengthening such an alignment through extra scraping, light alignment or a surfactant layer.

[0139] Agents may be added to the RM to improve the adhesion between the glass and the cured RM. Alternatively, different agents may be added that selectively reduce the adhesion between the cured resin and the resin. In each case, the concentration of the additive should be small enough to ensure that the resin properties are not affected. Preferably, these additives are surface specific for improving efficiency.

[0140] Similarly, UVO3 or plasma treatment of the resin can be used to help reduce the adhesion between the cured RM and the resin. The RM structure can be patterned by printing appropriate areas of adhesion promoter for the RM so that the RM selectively adheres to the glass substrate in areas where the promoter was deposited but removed from the glass, where adhesion to the film resin is stronger.

[0141] The optical element may be a simple or Fresnel-type refractive lens, or it may be a diffractive lens using a Fresnel zone. Alternatively, it may form other refractive structures, such as prisms, or diffractive structures, such as gratings or holographic elements.

[0142] An alternative design would have a conductive electrode deposited on top of the optical element. Such a design may be chosen where it is desirable to minimize the lowest switching voltage. For example, in a large refractive lens, even a Fresnel type, the height of the structure will cause a high voltage drop across the dielectric material. This will be minimized if the electrode (or a second electrode connected to the electrode below) is deposited onto the upper surface of the RM optical structure. The electrode may be formed from a sputtered layer such as ITO, or by a thin coating of conductive (e.g. Ag) nanoparticles which is then annealed at high temperature to form a uniform conductive coating, or by using a thin layer of a conductive polymer such as PEDOT. In each case, an additional planar uniform alignment layer on the conductor is required. The preferred alignment direction should be parallel to the direction of the aligned RM.

[0143] Figure 4 Demonstrated schematically Figure 3A A portion of an optical modulator comprises an upper optical element including a transparent substrate 32 carrying a RM surface pattern 34, the RM having a director 33.

[0144] Liquid crystal material 35 is only present in the region adjacent the upper optical element. In the 'off' state, the directors 51 of the liquid crystal material are oriented on average perpendicular to the lens optical axis 48, which is the axis of cylindrical symmetry of the Fresnel lens, perpendicular to the plane of the transparent substrate 32.

[0145] An exploded view of a portion of the liquid crystal material 35 is shown in Figure 4 , which is in close proximity to a portion of the RM surface pattern of the Fresnel zone plate forming the upper optical element. This illustrates that, while the local liquid crystal directors 51 are oriented mostly perpendicular to the lens axis 48, and at the same time substantially parallel to the RM optical axis (director) 33 within the material of the Fresnel zone plate 34, the immediate interface between the liquid crystal material 35 and the vertical surface of the RM pattern forms a surface-anchored boundary layer 52, wherein the local liquid crystal directors 51 are substantially parallel to the lens axis 48 and substantially perpendicular to the RM optical axis (director) 33. The exploded view also shows the location of the electrodes 50 on the upper surface of the transparent substrate 32, and that an alignment layer 49 is provided on the lower surface of the substrate, which contacts the liquid crystal material 35 and the RM material 34 within the optical modulator.

[0146] Figure 5 Shown on Figure 44, wherein the optical modulator is switched to an 'on' state. Here, the electric field generated between opposing electrodes of the optical modulator is relatively high in those areas of the modulator where no RM material is present and where liquid crystal material 43 is present (shown in a field aligned state). However, areas of liquid crystal material in the optical modulator that are immediately adjacent to areas of RM material experience lower electric field strengths due to the shielding effect of the dielectric RM substance and also due to the surface anchored boundary layer 52 formed on the RM surface. This causes the directors of the liquid crystal material that are immediately adjacent to the RM material to adopt an orientation that is perpendicular to the lens axis 48. This has the net effect of raising the effective height of the Fresnel zone plate elements 34 to slightly above their actual height.

[0147] At low voltages, the director in the on state will not approach the surface vertically, requiring a field significantly above the Freidricksz transition. The Freidricksz transition is a phase transition in liquid crystals that occurs when a sufficiently strong electric or magnetic field is applied to the liquid crystal in an undistorted state. Below a certain field threshold, the director remains undistorted. As the field value is gradually raised from this threshold, the director starts to splay until it is aligned with the field.

[0148] This results in a first inner surface (ie on the incident side) giving an imperfect refractive effect at lower fields, where the director is not vertical at the surface (see Figure 5 connected state).

[0149] The material for the lens can be chosen so that the ordinary index of refraction of the RM is slightly higher than that of the LC, helping to optimize the contrast enhancement between the on and off states. If the electrodes are below the lens, for example if a diffractive Fresnel lens is used, the depth of the lens structure can be reduced compared to the original mold to achieve the same efficiency, because the dielectric effect of the lens will reduce the field in those areas to near the surface (e.g. Figure 5 That is, the effective height of the diffractive structure will rise above h for the switched state, more than for the off state, as shown in Figure 4 .

[0150] If perfectly vertical, one polarization refracted or diffracted by the first surface will have an error due to the angular dependence of the beam. The dielectric constant of the material is preferably arranged to favor refraction of the applied field having similar properties to the optical effect. If the electrode is deposited onto a grating or lens element, the optical and electro-refractive effects should automatically be similar.

[0151] The present invention realizes a new method for polarization-independent adaptive optics that can be applied to a wide variety of passive optical components so that they can be electrically switched. A method is provided for embossing a desired shape into an optically anisotropic liquid crystal material. Polarization independence can be achieved by combining two of those substrates, for example, with orthogonal optical axes, into one cell. The cell can be filled with nematic liquid crystals having a twisted nematic configuration. The refractive index of the liquid crystal can be matched to the refractive index of the substrate.

[0152] We now describe another example of the first application on a Fresnel zone lens, which can switch from a non-focusing state in the absence of an electric field to a focusing state in the field-on state. This can be extended to multi-stage Fresnel lenses to overcome the 41% efficiency limit of a binary structure. We further show examples of polarization-independent gratings and microlens arrays to demonstrate that the technology is applicable to a wide variety of optical elements.

[0153] A Fresnel zone plate (FZP) relies on diffraction effects to focus light at a focal point M. Light transmitted from a collimated source and through the FZP can be characterized by constructive or destructive interference at point M, depending on the path length from the point in the plane of the FZP to the focal point. Focusing at M is achieved by phase shifting light in the destructive interference region by π relative to other regions. This results in a partitioning of the plate in the name-giving region.

[0154] According to simple geometric arguments and interference conditions, the area radius r m Given by:

[0155]

[0156] Where m is the number of rings, f is the nominal focal length, and λ is the wavelength of the light being studied.

[0157] In the following example, embossing technique is used to imprint Fresnel zone plates on reactive mesogens. Embossing is the method of choice here because it allows replication of the structure with essentially no deviation. Zero deviation conditions are very desirable in electro-optical devices to avoid unnecessary voltage drops and large electrical variations on the device. Other imprinting techniques may be prone to some deviation, no matter how high the pressure is, because the liquid photopolymer is prevented from flowing around the rigid structured element. In this example, by embossing, the structured element is formed on a flexible film that allows the liquid photopolymer to be pushed from one side of the substrate to the other, filling in the gap formed between the electrode and the film.

[0158] A schematic diagram of the embossing process is shown in Fig. 6A and 6BThe flexible structured film 60 with the reverse side 61 of the desired structure is pressed onto the substrate 63 by a roller 62 with a pressure p so that a certain amount of liquid reactive mesogen 64 is gradually distributed over the entire substrate surface. In particular, referring to Fig. 6A , a rubber-coated roller 62 presses a pre-structured flexible film 60 onto the surface of the substrate. The substrate 63 may be rigid, such as glass, ITO or silicon, or it may be flexible, such as PET (polyethylene terephthalate). Between the substrate and the structured film, the reactive mesogens are spread by pushing down the roller with a specific pressure and speed. These can be appropriately adjusted to produce a zero deviation range. The temperature of the system can be changed to control the speed of the material that can be fixed, so that the pressure and speed required for the process are within the desired range. For example, a pressure of about 4-7 bar can be used for RM materials. The speed of RM materials at room temperature can be about 500-800cP. The roller and the base plate can be heated to 50°C to 70°C to achieve a speed of 100-200cP. The embossing speed can be about 0.1 to 2cm / min. After the embossing step, the reactive mesogens are UV cured (i.e., irradiated with ultraviolet light) and the structured film is removed, leaving the desired polymerized structure. The temperature at which curing is carried out may be different from the temperature at which embossing is carried out, or indeed ambient temperature. The inventors have found that by cooling the film to below ambient temperature (but above the freezing temperature and glass transition temperature) during curing, higher birefringent optical structures can be formed from RM. The speed values ​​described herein may be measured according to the International Organization for Standardization (ISO) standard "ISO 3219:1993".

[0159] The process of producing birefringent polymer (cured reactive mesogen) embossed Fresnel structures is shown in Figure 6B . A master mold is used for the first example of a Fresnel zone lens and this master mold is made by direct laser writing. However, in general, existing structures can be used for this process. The master mold is coated with a release layer and molded using a UV-curable molding material. This material is deposited on a flexible backing, which is a prerequisite for the present embossing process. The negative mold is then coated with an alignment layer and scratched. Reactive mesogens are deposited on an ITO-covered glass that has been treated with an alignment layer. The scratched mold is placed on a liquid RM and embossed using a rubber roller. After UV curing of the RM, the mold is removed and a master copy of the RM is obtained. If the embossing process is optimized, deviations due to the imprinting technique can essentially disappear.

[0160] Embodiments of the present invention provide a polarization-independent device based on a birefringent polymer (cured reactive mesogen) embossed Fresnel zone plate. The device consists of two substrates each carrying a Fresnel zone plate. As described above, the lensing of the Fresnel zone lens relies on alternating π / 2 phase shifts, so it is unimportant which zone (even or odd) exhibits such a phase change. For better performance of the device, it is advantageous to use two substrates ( Figure 3A ). The birefringent polymer (cured reactive mesogen) layer is uniformly aligned, with the optical axes of the RM substrates perpendicular to each other. Filled with liquid crystals, the cell becomes a twisted nematic cell due to this perpendicular alignment. The liquid crystals used are index matched to the RM for both ordinary and extraordinary refractive indices.

[0161] The operating principle is summarized in Figure 3A and Figure 3B In the field-off state, collimated incident light does not experience pre-phase distortion as it would with a birefringent polymer (cured reactive mesogen), and the liquid crystal has the same (or very similar) ordinary and extraordinary refractive indices. Therefore, collimated light will remain collimated without focusing. However, due to the twisted geometry, each input polarization is rotated by 90°. The use of complementary structures on the top and bottom structures is intended to compensate for the imperfect refractive index match. Due to the refractive index mismatch, the optical phase space change that occurs at the top will occur at a complementary position at the bottom, so both will be balanced.

[0162] In the field-on state at sufficiently high voltage (above the critical voltage), the polarization switching function of the liquid crystal stops and the output polarization is the same as the input polarization. The reorientation of the liquid crystal directors to the vertical state further stops the index matching achieved previously and lensing of the top and bottom substrates occurs. Thus, lensing of both polarizations is achieved by such a Fresnel lens cell. The complementary design of the Fresnel zone cell helps to achieve a more uniform electric field distribution across the device, which should reduce the voltage dependence.

[0163] Therefore, the phase shift adopted due to the liquid crystal should remain constant throughout the lens, so that the lateral phase modulation is determined only by the birefringent polymer (cured reactive mesogen). It should be mentioned that the technique to achieve the presentation of polarization independence is not limited to Fresnel zone plates, but can be extended to a wide range of optical components, as will be shown later.

[0164] Figure 7 a to 7d show polarized optical micrographs of normal (a, b) and complementary (c, d) Fresnel zone plates (FZP) embossed with RM. Birefringent polymer (cured reactive mesogen) embossed substrate. (a, b) with normal structure between crossed polarizers, with the director (a) at 45° and (b) 0° relative to the polarizer. Figure 7 (c, d) show complementary structures. Images were recorded with a constant exposure time. Figure 7 (eh) show the assembled cell with the directors of the two substrates perpendicular to each other. The unfilled cell between crossed polarizers is shown at (e) 45° and (f) 0° relative to the polarizers. The filled (TN-like) cell between Figure 7 (g) (for crossed polarizers), and Figure 7(h) (for parallel polarizers). Both structures are exact replicas of the original mold and additionally show very good alignment of the cured RM (for profilometry, see Fig.13 and Fig.14 ). The dark areas between the RM rings confirm that the embossing process produces little or zero structural deviation, since there is no visible birefringence. Figure 7 The dark state in (d) reveals some minor defects, which are most likely caused by the scratching process of the textured surface.

[0165] The normal and complementary FZPs are assembled into the box with the directors of the substrates perpendicular to each other. The unfilled state of the box is Figure 7 (e, f). Due to the complementary design, the cell exhibits nearly uniform brightness between crossed polarizers if the director is at a 45° angle to the polarizer. Figure 7 (e)]. Placed parallel to the polarizer, the sample exhibits a well-defined dark state. Figure 7 (f)]. Filling the cell with liquid crystals results in a twisted nematic cell that exhibits bright light between crossed polarizers. Figure 7 (g)] and the parallel polarizers show darkness [ Figure 7 (h)]. This confirms that the alignment of RM is well transferred to the liquid crystal.

[0166] Figure 8 (a, b) show a collimated Gaussian beam with diagonal polarization transmitted through the Fresnel zone plate cell. In the off state, the beam remains collimated, however, small distortions are introduced due to errors in the filled FZL cell. In the on state, the light is focused on a central spot.

[0167] Specifically, Figure 8 Shown is the measured transmission of a laser beam (wavelength = 594 nm) through a Fresnel zone cell on a CCD camera at a distance of 20 cm: Figure 8 (a) Display closed state, Figure 8 (b) shows the on state (10 V voltage applied). Figure 8 (c) shows the efficiency of the power focused onto a central spot for different input polarizations and transmission properties of a twisted nematic (TN) cell modeled according to the lens specifications (dashed black line).

[0168] Figure 8 (c) shows the measured values ​​for different input polarizations (see Fig.17). Independent of polarization, the efficiency saturates to a similar value at roughly 33%. However, while the horizontal polarization saturates rapidly, the vertical polarization starts similarly at low voltages but then exhibits a slower saturation. The diagonally polarized input gives an intermediate behavior as it is a mixture of vertical and horizontal polarizations. The overall switching behavior is very similar to that of the twisted nematic cell, which has been plotted as a reference. The details of the simulation are as follows. The director profiles were calculated with COMSOL Multiphysics 5.3. The free energy is defined as follows:

[0169] and is implemented as a weak formation with a strong anchoring condition. Here, n i is the director n in the Cartesian coordinate system (i = x, y, z) = [n x , n y , n z ] T The weight, K jj E is the elastic constant (j=1, 2, 3). z represents the electric field in the z-direction, ∈0 is the vacuum dielectric constant, Δ∈ is the dielectric anisotropy, and ∈ ⊥ is the perpendicular dielectric constant of the liquid crystal. The director field is then used to calculate the transmission of linearly polarized light by applying the Jones operation. The physical properties of the liquid crystal (MLC6204-000) used for the simulation are as follows:

[0170] performance MLC-6204-000 RMM1850 <![CDATA[Elastic constant K 11 > 6.0 - <![CDATA[Elastic constant K 22 > 4.3 (estimated) - <![CDATA[Elastic constant K 33 > 15.2 - <![CDATA[Dielectric constant, parallel ∈ || > 44.8 - <![CDATA[Dielectric constant, vertical ∈ ⊥ > 9.5 3.68 (cured) <![CDATA[ordinary refractive index n o > 1.504 1.510(uncured) <![CDATA[Unusual refractive index n e > 1.652 1.654 (uncured)

[0171] Theoretically, a diffraction efficiency of about 41% is expected. There are three factors that negatively affect the diffraction efficiency. They are: the profile of the embossed structure, the effect of the sharp edges of the liquid crystal, and the alignment quality of the birefringent polymer (cured reactive mesogen). From the profile of the embossed RM lens ( Fig.13 ), we see the embossed structure compared to the original mold.

[0172] A more detailed study of the beam profile is shown in Fig. 9 This shows a series of measured transmitted laser beam profiles at different applied switching voltages for different input polarizations (red arrows): vertical, horizontal, diagonal.

[0173] For different voltages applied between the two electrodes of the optical modulator, a central point of increased intensity appears. For different input polarizations (horizontal, vertical, diagonal), there is little change in intensity or shape. However, a notable difference is that for horizontal and vertical polarizations, the profiles are extended along their respective axes. This does not happen for diagonal inputs designed with the device, where the horizontal and vertical axes are the principal axes of the lens. The diagonally polarized input is therefore divided into equal parts horizontally and vertically, which compensates for the ellipticity observed for purely horizontal and vertical inputs.

[0174] Fig.10 The voltage dependence of the peak intensity of the beam for x and y cuts is shown for different input polarizations ( Fig.10 (ac)) and beam width (full width at half maximum, FWHM). Specifically, Fig.10 (ac) show the peak intensity of the central peak of the transmitted laser beam as a function of the applied switching voltage for different input and output polarizations. Fig.10 (df) shows the voltage-dependent FWHM cut by peak position x and y. The input polarization is: Fig.10 (a, d) vertical; Fig.10 (b, e) horizontal; Fig.10 (c, f) diagonal.

[0175] The peak power density without output polarizer shows a very similar behavior to the efficiency measurements after a steep increase and a fast saturation afterwards. Polarization-dependent output measurements show that the polarization transition stops quickly after exceeding a critical voltage due to the TN effect. Therefore, the output polarization corresponds to the input polarization. The amplitude drop is caused by the polarizer, which transmits about 80% of the incident light. For a diagonal input, the polarized output shows equal parts of horizontally and vertically polarized light. The horizontal component saturates faster than the vertical component, which has been observed in the efficiency measurements [ Fig.10 (c)]

[0176] Beam diameter (full width at half maximum, FWHM)[ Fig.10 (df)] decreases with increasing voltage due to improved focusing. The ellipticity of the horizontally and vertically polarized inputs is clearly visible. Furthermore, compensation for the diagonal input can be expected from the 2D beam profile ( Fig.10 (d)). This is further supported by looking at the absolute value of the FWHM. For the diagonal, the beam width of 35 μm is very close to the larger diameters found for horizontal and vertical polarizations, 36 μm and 37 μm. For comparison, the smaller diameter is about 30 μm.

[0177] The ellipticity is assumed to be due to imperfect alignment near the edges of the RM rings, which is transferred to the alignment of the liquid crystal. In a perfectly aligned birefringent polymer (cured reactive mesogen) Fresnel zone lens, the RM director is uniformly aligned within the ring including the region where the director is perpendicular on the ring edge. In practice, however, distortion is more likely to occur at these edges, since PVA treatment of the mold will promote anchoring of the plane at this location.

[0178] For horizontally polarized light this means that, along the horizontal axis, there is less focusing due to the decreased refractive index contrast, which results in an increased spot size along this axis.

[0179] Fig.11 Displays a z-scan of a focused laser beam around the focal point. Specifically, Figure 8 The z-scan of the point spread function of a Fresnel lens is shown below: Fig.11 (a) Simulation; Fig.11 (be) Experimental measurement at an applied switching voltage of 10 V; the input light polarization is: Fig.11 (b, c) horizontally; Fig.11 (d, e) vertically. Theoretical results [ Fig.11 (a)] based on a perfect binary phase distribution and a beam diameter (FWHM) of 24 μm. The experimental minimum values ​​obtained are 30 μm and 32 μm [ Fig.11 (b) and (d)], both beam diameters are measured perpendicular to the input polarization. As expected, the diameter parallel to the polarization is significantly larger [ Fig.11 (c) and (e)].

[0180] As mentioned before, the presented technique is not limited to simple Fresnel zone plates but can be applied to a wide range of optical components. First, to overcome the efficiency limitations of binary Fresnel lenses, we apply the patterning process together with a polarization-independent design to multi-level Fresnel lenses [ Fig.12 (ac)]. The measured efficiency of the three-stage design was 53%, demonstrating that such an approach can exceed 50%. This is still short of the theoretically predicted 68%, but we believe that by optimizing the technique, the device can approach such efficiencies (see Fig.15 surface profile). Fig.15 Surface profiles of two three-order Fresnel lenses are shown: (a) master mold made of SU-8 on silicon, (b) replica of reactive mesogens (RM).

[0181] Fig.12 The display is as follows: Fig.12 (ac) are optical microscopic images of the following three-stage Fresnel lens: Fig.12 (a) Master mold on silicon, Fig.12(b, c) Patterned lens substrate between crossed polarizers. Fig.12 (d) shows an embossed diffraction grating (2 μm pitch, 125 nm amplitude) between crossed polarizers. Fig.12 (e, f): Electrically switchable diffraction for different input polarizations. Fig.12 (g) shows a patterned microlens array (MLA) between crossed polarizers. Fig.12 (h) Camera image of the Gaussian laser beam focused by the MLA cell when a 10 V switching voltage is applied to the optical modulator. Fig.12 (i) shows a photograph of the embossing seen between crossed polarizers.

[0182] Fig.12 Two examples of replicating existing optical components are given in (dh). Fig.12 In (d), a 1D grating has been embossed into the RM using a commercially available grating film. In this case, the process of molding the master can be skipped. Using two such substrates - one with a director in the direction of the grooves and the other with a director perpendicular to the grooves - a switchable polarization-independent grating is produced [ Fig.12 (e, f)].

[0183] Fig.12 (g) shows a RM replication of a microlens array (MLA), which is overall a more expensive component. To avoid damage to the MLA, it has been molded in PDMS and then press molded in a hard resin to obtain a master mold for further processing.

[0184] Two replicas of the birefringent polymer (cured reactive mesogen) were fabricated and then assembled into a liquid crystal cell. The results of the switched-on MLA are shown in Fig.12 (h) shows the focusing of a Gaussian laser beam with a 45 degree input polarization relative to the RM director.

[0185] Fig.13 and Fig.14 Display of two-stage Fresnel lens ( Fig.13 ) and a complementary two-stage Fresnel lens ( Fig.14 ) For example, the surface profile for the example of FIG. 3: Fig.14 (a) Master mold made of SU-8 on silicon. Fig.14 (b) Replication of birefringent polymer (cured reactive mesogen) (RM). Fig.16 A liquid crystal filled two-stage Fresnel lens between crossed polarizers at different applied switching voltages (0 V, 1 V, 2 V, 5 V, 10 V and 50 V) is shown according to Figure 3. The director n (optical axis) of the reactive mesogen is aligned parallel to the polarizer (P) / analyzer (A). Fig.17The diffraction efficiency of a Fresnel lens used to measure an embodiment of the present invention is shown schematically (the figure is labeled as follows - POL: polarizer, QWP: quarter wave plate, FZL: Fresnel zone lens, PD: photodiode).

[0186] Original model making

[0187] In one example, the Fresnel lens has a focal length of 200 mm at 594 nm, an outer diameter of 5 mm and a nominal feature height of 2.1 μm. The latter is derived from the birefringence of the birefringent polymer (cured reactive mesogen) and the desired phase shift π. Two versions of the zone plate were manufactured: a normal structure in which all odd zones are made of RMs and a complementary structure with RMs in the even zones. The Fresnel zone plate master mold was manufactured using a direct write laser system. SU-8-2025 (38% in cyclopentanone) was spin-plated on silicon at 500 rpm (100 rpm / s) for 10 seconds and at 1000 rpm (300 rpm / s) for 40 seconds. Dilutions of neat SU-8 were used to adjust the film thickness. The samples were soft-baked at 30°C for 30 minutes. At a nominal dose of 3000 mJ / cm 2 The writing process was performed with a 375nm (wavelength) laser. Post-exposure baking was performed at 50°C for 30 minutes. The master mold was developed by spin-plating SU-8 developer and then rinsed with IPA. Finally, a hard bake was performed at 180°C for 1 hour. The feature height of 2.1μm was confirmed by measuring the surface profile (Dektak XT).

[0188] For better demoulding, the master mold was spin coated with a release layer consisting of Teflon AF1600 (1.7%) in FC40 (both from SigmaAldrich) at 500 rpm (100 rpm / s) for 10 seconds and at 3000 rpm (300 rpm / s) for 40 seconds, and dried at 180°C for 15 minutes.

[0189] Mould making

[0190] The isotropic resin used to mold the master mold consisted of 45% HDDA (Sigma Aldrich), 15% TMPTA (Sigma Aldrich) and 40% Actilane 420. The resin was deposited on a master mold with a flexible 125 μm thick PET film (Melinex 506, HIFI film) and then heated at 10 mW / cm 2 UVA cure for 10 minutes.

[0191] After curing, the film was lifted off and the cured resin adhered to the plastic backing. The mold was UV-ozone treated for 5 minutes (T10X10 / OES, UVOCS), and then the PVA solution (1% in H2O) was spin coated at 500 rpm (100 rpm / s) for 10 seconds and at 1500 rpm (300 rpm / s) for 30 seconds.

[0192] The PVA layer was dried for 10 minutes at 90° C. and for 30 minutes at 130° C. In a final step, the mold was scratched using a scratching machine equipped with a velvet cloth.

[0193] Substrate preparation and embossing

[0194] The ITO substrates were cleaned using different solvents and treated with UV / ozone (T10X10 / OES, UVOCS) for 30 minutes. The samples were covered with a solution consisting of polyimide SE 3510 (66 wt%, Nissan Chemicals), dimethylformamide (33 wt%, SigmaAldrich) and reactive mesogen RM257 (1 wt%, Merck) for alignment and adhesion. The solution was spin coated at 300 rpm (100 rpm / s) for 10 seconds and 3000 rpm (300 rpm / s) for 60 seconds and cured at 180°C for 1 hour. Finally, the samples were scratched for planar alignment.

[0195] For embossing, a drop of reactive mesogen mixture RMM1850 (Merck Chemicals Ltd.) was deposited on the ITO substrate and the scratched mold was placed on it. The embossing process was carried out at a substrate temperature of 85°C, a roller pressure of 4.5 bar and a base plate speed of 6.5 mm / s. After embossing, the sample was cooled to room temperature within a period of 5 minutes. The sample was irradiated with a UV-A intensity of 10 mW / cm 2 UV-curing for 10 minutes (measured with a radiometer RM-12, Opsytec Dr Groebel GmbH). Finally, the film was removed, leaving the replicated structure.

[0196] Lens assembly and measurement

[0197] The normal and complementary Fresnel zone plate substrates were assembled into a box so that the directors formed a 90° angle. Alignment was performed manually under a microscope to ensure good positioning of the structure. Mylar spacers were used to control the box gap. The box was sealed on three of the four sides to enable vacuum filling. Such a filling method is necessary because capillary filling usually results in bubbles being trapped in the box. The box was filled with nematic liquid crystal MLC-6204-000.

[0198] After filling and electrical contacting, the cell was measured using a HeNe laser (λ=594 nm) which had been expanded to a beam of approximately 300 μm diameter (FWHM). The output state was measured using a beam profiler (Thor labs BC106N-VIS).

[0199] As discussed above, the optical modulation functions provided by the present invention include, but are not limited to: lensing; focusing; defocusing; refraction, diffraction; beam steering. Preferably, the optical modulation function, effect or role of the first optical element of the present invention is substantially the same as the modulation function, effect or role of the second optical element of the present invention.

[0200] For example, they can all be used as lenses, prisms or diffraction gratings to modulate the corresponding wavefronts to focus light or diffract polarization components of the corresponding light in substantially the same way with substantially the same results. Examples of uses of the present invention are provided below.

[0201] Fig. 20A and Fig. 20B Schematically shown is a switchable polarization-independent diffraction grating according to an embodiment of the present invention, comprising a birefringent first diffractive optical element separated from a birefringent second diffractive optical element by a liquid crystal. Each optical element is referenced according to the method described herein Figure 6B To make. Fig. 20A In , the diffraction grating is in the "field off" state, where no diffraction occurs and the two interfaces (reactive mesogen / liquid crystal) are index matched. Fig. 20B In , the diffraction grating is in the "field-on" state, where the two interfaces (reactive mesogen / liquid crystal) are not index matched, so that different polarizations are diffracted at the top / bottom interface.

[0202] Fig.21 A refractive lens in an imaging application using an embodiment of the present invention is schematically illustrated. When the Fresnel lens (1) is in the "field off" state ("off"), where the two interfaces (reactive mesogen / liquid crystal) are index matched, the focal length of the lens (100) dominates the image of the imaged object on the CMOS camera (concentric rings and linear scale, printed on the sheet). However, when the Fresnel lens (1) is in the "field on" state ("on"), where the two interfaces (reactive mesogen / liquid crystal) are not index matched, so that refraction occurs in the Fresnel lens (1), the focal length of the combination of the lens (100) and the Fresnel lens (1) dominates the image of the imaged object on the CMOS camera (number '1' printed on the sheet). The image formed by the CMOS camera is shown in FIG. Fig.21 , in each case, adjacent to the imaged object under investigation.

[0203] Although some preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the appended claims. The present invention is not limited to the details of the foregoing embodiments. The present invention extends to any new one or any new combination of the features disclosed in this specification (including any appended claims, abstracts and drawings), or to any new one or any new combination of the steps of any method or process so disclosed.

Claims

1. An electrically switchable optical modulator for modulating an optical wavefront transmitted therethrough, the electrically switchable optical modulator comprising: a birefringent first optical element and a birefringent second optical element each having respective ordinary and extraordinary refractive indices, wherein the first and second optical elements are formed from a reactive mesogen that cures from a reactive state to form a birefringent polymer, and the reactive mesogen is formed from a bifunctional acrylate system to help ensure formation of a polymer having positive birefringence; a birefringent liquid crystal material sandwiched between the first and second optical elements; The extraordinary refractive index of the liquid crystal material can be electrically switched between: a first state, wherein it has a first value; and, a second state in which it has a second value different from the first value; wherein one or both of the first value and the second value do not match the extraordinary refractive index of the first optical element for light polarized in a first direction of linear polarization, and do not match the extraordinary refractive index of the second optical element for light polarized in a second direction of linear polarization orthogonal to the first direction, thereby switchably generating a relative contrast for modulating the wavefront in the extraordinary refractive index between the liquid crystal material and the first and second optical elements.

2. An electrically switchable optical modulator according to claim 1, wherein the first value matches the corresponding extraordinary refractive index of both the first and second optical elements.

3. An electrically switchable optical modulator according to any one of claims 1 to 2, wherein the ordinary refractive index of the liquid crystal material matches the corresponding ordinary refractive index of the first and second optical elements.

4. An electrically switchable optical modulator according to any one of claims 1 to 2, wherein the first and second optical elements are formed of a birefringent polymer and define a director, wherein the director of the first optical element is transverse to the director of the second optical element.

5. An electrically switchable optical modulator according to claim 2, wherein the liquid crystal material has a twisted nematic liquid crystal arrangement, wherein when in the first state, the optical axis of the liquid crystal material: substantially parallel to the director alignment of the first optical element; and, The director is aligned substantially parallel to the second optical element.

6. An electrically switchable optical modulator according to any one of claims 1 to 2, wherein one or each of the first and second optical elements presents a curved surface to which the liquid crystal material conforms.

7. An electrically switchable optical modulator according to any one of claims 1 to 2, wherein one or each of the first and second optical elements presents a surface relief pattern to which the liquid crystal material conforms.

8. An electrically switchable optical modulator according to claim 7, wherein each of the first and second optical elements presents a surface relief pattern to which the liquid crystal material conforms, wherein the pattern presented by the first optical element is the inverse of the pattern presented by the second optical element.

9. An electrically switchable optical modulator according to claim 6, wherein each of the first and second optical elements presents a surface relief pattern to which the liquid crystal material conforms, wherein the pattern presented by the first optical element is substantially the same as the pattern presented by the second optical element.

10. An electrically switchable optical modulator according to any one of claims 1 to 2, wherein the first optical element is opposite to the second optical element.

11. An electrically switchable optical modulator according to any one of claims 1 to 2, wherein the first optical element defines a first optical axis and the second optical element defines a second optical axis, the second optical axis being substantially coaxial with the first optical axis.

12. An electrically switchable optical modulator according to any one of claims 1 to 2, wherein the liquid crystal material is arranged between two independent electrodes, and the electrodes are configured to support an electric field between them, and the electric field is used to switch the extraordinary refractive index of the liquid crystal material from the first state to the second state.

13. An electrically switchable optical modulator according to claim 12, wherein one or each of the first optical element and the second optical element is arranged between the two independent electrodes.

14. An electrically switchable optical modulator according to claim 12, wherein one of the two independent electrodes is arranged between the first optical element and the liquid crystal material.

15. An electrically switchable optical modulator according to claim 13, wherein one of the two independent electrodes is arranged between the first optical element and the liquid crystal material.

16. An electrically switchable optical modulator according to claim 12, wherein one of the two independent electrodes is disposed between the second optical element and the liquid crystal material.

17. An electrically switchable optical modulator according to claim 13, wherein one of the two independent electrodes is disposed between the second optical element and the liquid crystal material.

18. An electrically switchable optical modulator according to claim 14, wherein one of the two independent electrodes is disposed between the second optical element and the liquid crystal material.

19. An electrically switchable optical modulator according to claim 15, wherein one of the two independent electrodes is disposed between the second optical element and the liquid crystal material.

20. An electrically switchable optical modulator according to any one of claims 1 to 2, wherein one or both of the optical elements are shaped to define an axis of cylindrical symmetry that is substantially perpendicular to a planar surface disposed on a side of the respective optical element.

21. An electrically switchable optical modulator according to claim 20, wherein both optical elements are shaped to define respective said axes of cylindrical symmetry, and the axes of cylindrical symmetry of the first and second optical elements are coaxial.

22. A method of manufacturing an optical modulator comprising one or more birefringent optical elements shaped to a desired shape for modulating an optical wavefront transmitted therein, wherein each birefringent optical element is provided according to a method comprising: providing a flexible sheet material exhibiting an embossed surface, the embossed surface being embossed with a shape that is the inverse of the desired shape; providing an amount of reactive mesogen material in an uncured state on a surface of a substrate; bending the embossed surface to press against and gradually across the surface of the amount of reactive mesogen material to form a shaped birefringent portion having the desired shape; curing the shaped birefringent portion to form the birefringent optical element, The reactive mesogen is formed from a difunctional acrylate system to help ensure the formation of a polymer having positive birefringence.

23. A method of manufacturing an optical modulator according to claim 22, wherein the optical modulator is electrically switchable.

24. A method of manufacturing an optical modulator according to claim 23, the optical modulator comprising an electrically switchable liquid crystal material sandwiched between two of said birefringent optical elements.

25. A method according to claim 24, comprising providing the embossed surface with a preferential surface alignment and thereby imparting the preferential surface alignment to the embossed surface of the shaped birefringent portion, the surface alignment being configured to align the liquid crystal material.

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