Patent Description:
Tunable spectrum, intensity and phase modulation devices, such as liquid crystals, have recently become of great interest due to their ability to provide spectral filtering, intensity modulation, polarisation control, tunable lensing and other many applications in privacy and smart windows, optical imaging, sensing and optical telecommunications. However, when it comes to practical implementation in various field systems, these devices show some serious drawbacks, such as strong incidence angle and polarisation dependence, slow speed and narrow tuning range. These problems of tunable and spectral modulation devices are yet to be overcome.

There are several situations where polarisation independence of incident light helps increasing the light throughput, for example in spectral imaging, privacy windows operation, vision tunable correction and wave-front sensing through the atmosphere. In privacy windows, for instance, the light polarisation dependence is a drawback because it deteriorates the light throughput. As a result, the need for polarisers makes such privacy window extremely expensive and less efficient.

The present application describes new birefringence-tunable devices, such as liquid crystals, which are built to solve the aforementioned problems. These devices can be used, for example, in privacy windows and other photonic applications.

Disclosed embodiments will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended figures. The drawings included and described herein are schematic and are not limiting the scope of the disclosure. It is also noted that in the drawings, the size of some elements may be exaggerated and, therefore, not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the disclosure.

In the following description, various aspects of the present application will be described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the present application. However, it will also be apparent to one skilled in the art that the present application may be practiced without the specific details presented herein. Furthermore, well-known features may be omitted or simplified in order not to obscure the present application.

The term "comprising", used in the claims, is "open ended" and means the elements recited, or their equivalent in structure or function, plus any other element or elements which are not recited. It should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It needs to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising x and z" should not be limited to devices consisting only of components x and z. Also, the scope of the expression "a method comprising the steps x and z" should not be limited to methods consisting only of these steps.

Unless specifically stated, as used herein, the term "about" is understood as within a range of normal tolerance in the art, for example within two standard deviations of the mean. In one embodiment, the term "about" means within <NUM>% of the reported numerical value of the number with which it is being used, preferably within <NUM>% of the reported numerical value. For example, the term "about" can be immediately understood as within <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% of the stated value. In other embodiments, the term "about" can mean a higher tolerance of variation depending on for instance the experimental technique used. Said variations of a specified value are understood by the skilled person and are within the context of the present invention. As an illustration, a numerical range of "about <NUM> to about <NUM>" should be interpreted to include not only the explicitly recited values of about <NUM> to about <NUM>, but also include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as <NUM>, <NUM>, and <NUM> and sub-ranges, for example from <NUM>-<NUM>, from <NUM>-<NUM>, and from <NUM>-<NUM>, as well as <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>, individually. This same principle applies to ranges reciting only one numerical value as a minimum or a maximum. Unless otherwise clear from context, all numerical values provided herein are modified by the term "about". Other similar terms, such as "nearly", "substantially", "generally", "up to" and the like are to be construed as modifying a term or value such that it is not an absolute. Such terms will be defined by the circumstances and the terms that they modify as those terms are understood by those of skilled in the art. This includes, at very least, the degree of expected experimental error, technical error and instrumental error for a given experiment, technique or an instrument used to measure a value.

It will be understood that when an element is referred to as being "on", "attached to", "connected to", "coupled with", "contacting", etc., another element, it can be directly on, attached to, connected to, coupled with or contacting the other element or intervening elements may also be present. In contrast, when an element is referred to as being, for example, "directly on", "directly attached to", "directly connected to", "directly coupled" with or "directly contacting" another element, there are no intervening elements present.

In one aspect of the present invention, a liquid crystal composite tunable device (hereinafter, "liquid crystal device", or "LCD" for the sake of simplicity) (<NUM>) for fast polarisation-independent modulation of a light beam comprises:.

In general, an index-matching material is a substance, such as liquid, gel or liquid crystal, which has a refraction index that closely approximates that of another substance, such as glass or polymer. When the two substances with the same index are in contact, light passes from one to the other with neither reflection, nor refraction or scattering. A mismatch between the refraction indices of the two substances in optical devices usually causes serious problems, such as spherical aberration, reduction of the effective numerical aperture by total internal reflection, and "fish tank" effect. However, the "refractive index mismatch" finds its application in the present invention as will be discussed below.

The composite structure of the present embodiment, made of liquid crystal and porous microparticles, is used in the LCD to modulate the incident light intensity with no polarisation dependence. Such device can therefore be used as a controlled transparency window. The idea behind the invention is that the refractive index mismatch between the microparticles and the liquid crystal, which surrounds and infiltrates these microparticles, is tunable upon applying external fields such as electric or magnetic. The refractive index mismatch can also be tunable under thermal or optical impact.

By a "microparticle" we mean a particle having spatial dimensions typically in the range of about <NUM>-<NUM> microns. The porous microparticles may be dielectric, magnetic or metallic, preferably with high porosity, and may have a shape of tubes, rods, hollow fibres or shells. The aforementioned size of these microparticles is approximately in the order of magnitude or larger than the incident light wavelength.

The porous microparticles are advantageous over non-porous microparticles, since the surrounding liquid crystal molecules in the composite can infiltrate the pores of the microparticles and change the refractive index to achieve optimum modulation of the scattering. Another advantage of the porous microparticles is that their density becomes close to that of the LC as they get infiltrated with the LC molecules and therefore sedimentation effects are minimized. The transparency of such device is thus polarisation-independent and can be tuned with electric, magnetic, optical or thermal fields. Infiltration of the liquid crystal molecules into the porous microparticles imparts additional functionality to the composite structure of the embodiments, depending on how the molecules are oriented inside the particle. The average refractive index of the liquid crystal infiltrating the porous microparticle is close to the refractive index of the isotropic state of the liquid crystal: <MAT> where n⊥ and n∥ are refractive indices of the liquid crystal molecules perpendicular and parallel to (along) their long axis respectively. The isotropic index is close to the refractive index n⊥. As a result, for a homogeneously aligned liquid crystal device, the refractive index mismatch between the bulk of the liquid crystal molecules and the microparticles is large, thereby resulting in large scattering. Therefore, and as mentioned above, the microparticles used in the device of the present embodiments are preferably porous. These porous microparticles may be functional and made of thermochromic material such as vanadium oxide, tungsten oxide, zinc oxide or titanium oxide which allows their transparency to be controlled thermally or electrically, or thermally and electrically. The microparticles may also be magnetic, photosensitive, fluorescent or electrochromic.

Thus, the LCD of the present embodiments allows polarisation-independent intensity modulation and is based on tunable scattering, which uses a composite of a liquid crystal mixed with transparent microparticles. Reference is now made to <FIG> showing schematically the porous microparticles embedded in the liquid crystal composite tunable device of the embodiments for polarisation-independent scattering. These microparticles have a refractive index nearly equals to one of the liquid crystal refractive indices, for example n⊥, so that at zero voltage, no index matching exists and large scattering occurs. The LCD of the present embodiments comprises the transparent panels (<NUM>) coated with a transparent conductive electrode (TCE) layer and with an exemplary alignment layer, and the composite made of porous microparticles (<NUM>) surrounded by liquid crystal molecules (<NUM>). <FIG> illustrates a scattering state of the LCD in which the microparticles infiltrated with the liquid crystal molecules act as scattering centres due to large refractive index mismatch. <FIG> illustrates a transparent state of the LCD in which the liquid crystal molecules are oriented on the average along one direction and hence, the refractive index mismatch is minimized. As the voltage increases, the liquid crystal molecules re-orient with the field direction. As a result, the index mismatch gradually decreases, and so does the scattering. The composite structure of the embodiments may also be transparent at zero voltage and scatters at high voltage, for example when the liquid crystal material has a negative dielectric anisotropy and homoetropically aligned.

The microparticles are formed by coagulation or agglomeration of nanoscale particles so that the porosity of each coagulated particle is high. The porous microparticles may be prepared lithographically or by chemical etching, templating, electrospinning, embossing, laser patterning or milling and then randomly distributed on at least one of the panels. Examples of such particles include porous silica, partially oxidised or nitrogenised porous silicon with various porosities and oxidation or nitrogenation levels. The microparticles may also be prepared at room temperature from a raw solid material, such as the surfactant dimethyloctadecyl[<NUM>-(trimethoxysilyl)-propyl]ammonium chloride (DMOAP) or N-methyl-<NUM>-aminopropyltrimethoxy-silane (MAP). Such materials can be in solid form or in solution form.

This solid material preferably has its melting point slightly lower than that of the liquid crystal used in the composite. The composite is prepared by dissolving the raw material and mixing it with the liquid crystal above the melting point, followed by cooling to room temperature, which then form microparticles embedded in the liquid crystal matrix. Alternatively both the liquid crystal and the raw material of the microparticles may be dissolved in a common solvent such as toluene and the composite is formed upon heating to the isotropic temperature, thus causing the solvent to evaporate and remaining with the LC-microparticles composite. Since usually the surfactants cause the LC molecules to align perpendicular to the substrates plane, then assuming the LC molecules have positive dielectric anisotropy, they will not switch upon applying an electric field. In this case, it is preferable to choose the liquid crystal having negative dielectric anisotropy which will then result in a composite exhibiting reversed behaviour, that is transparent at zero voltage and scattering at high enough voltage. In a specific embodiment, the microparticles concentration in said composite is less than <NUM>%, and preferably less than <NUM>%, for providing optimum contrast of the device upon switching it between different orientational states of the liquid crystal for privacy and smart window applications. This concentration is further more preferably less than <NUM>%, and for other non-scattering photonic devices applications even more preferably less than <NUM>%, for avoiding significant light scattering. The inclusion of less than <NUM>% microparticles in the LC composite device of the present invention does not provide significant scattering but can improve the switching speed between different orientational states of the liquid crystal, because of reducing the viscoelastic relaxation time. Similarly, for wavelengths much larger than the microparticles size such as in the long wave infrared or the THz ranges, the scattering becomes small and consequently, the device may then be used as a tunable birefringence or phase plate. The microparticles concentration is thus small enough to make sure that the scattering is insignificant and the device is used for phase, retardation or wavelength modulation, yet maintaining the faster operation in any one of the configurations of the present invention.

Examples of the porous microparticles include glass, ceramic, metallic, ferromagnetic, photosensitive, or organic microspheres, which are mixed in a liquid crystal with <NUM>-<NUM>% concentration. The microparticles may also be partially oxidized porous silicon microspheres or any other porous dielectric microstructures. Oxidation level of the porous material determines its adsorption degree, colour, refractive index and a scattering degree of the composite. Therefore, tunable architectural privacy windows, which are made from this material according to the present invention, exhibit different colours. Another preferable embodiment of the microparticles is to have their melting point less than that of the liquid crystal and maybe designed to fall near the surrounding ambient temperature above which, the device will stop to be scattering. When the microparticles melt and get mixed with the liquid crystal the scattering centres disappear and the liquid crystal layer becomes transparent without dependence on the voltage. Hence with this design one can get a thermochromic type device switching between scattering and non-scattering or vice versa depending on the temperature.

As schematically shown in <FIG>, liquid crystal molecules are frequently rod-shaped aligned, such that their long axes are on the average in the same direction making an angle Θ, called the tilt angle, with the normal to the transparent panels. The panels (<NUM>) are usually made of glass treated at their facing surfaces with transparent conducting oxide (TCO) as electrodes and a thin polymer layer to provide mono-domain alignment of the liquid crystal (LC) molecules. Some glass micro-spheres (not shown in the figures) are inserted to act as spacers to determine the thickness of the device.

Because of their shape, the LC molecules are anisotropic and the whole device would therefore act as a birefringent plate. As noted above, the majority of the liquid crystal molecules are uniaxial meaning that they have two refractive indices, one along the molecules axis (n∥) and one perpendicular to it (n⊥). When voltage is applied between the two electrodes connected to the transparent panels, the LC molecules orientation changes. Thus, the tilt angle is a function of the voltage. As shown in <FIG>, when a linearly polarised light impinges on a birefringent medium, two waves can propagate at different velocities or refractive indices, the ordinary and the extraordinary waves. The extraordinary wave is an electromagnetic wave which is excited with its polarisation along the projection of the molecular axis on the transparent panels, and its index is a function of the tilt angle which is a function of the applied voltage: <MAT>.

The ordinary wave index on the other hand remains fixed at no = n⊥, so that the effective birefringence defined as Δn = ne - no, varies from Δn = n∥ - n⊥ at zero voltage to near zero at high voltage when the molecules closely approach the orientation θ ≈ <NUM> provided that the molecules have positive dielectric anisotropy. Because of the difference in their velocities the two excited waves (ordinary and extraordinary) accumulate a phase difference, Γ = <NUM>πdΔn/λ which depends on the path length they travelled d and birefringence. This phase difference Γ causes a variation in the polarisation state. As shown in <FIG>, assuming that the polariser axis and the analyser axis are oriented at azimuthal angles P and A with respect to the optical axis (e-axis), the light transmission can be written as: <MAT>.

As an example, when liquid crystal is sandwiched between two crossed polarizers and P = π / <NUM>, the transmission varies according to T = sin<NUM> (πdΔn/λ) , while when the liquid crystal is between two parallel polarisers, it is T = cos<NUM> (πdΔn/ λ). This is the principle of polarisation or intensity modulation of the LCD of the embodiments. When the incident light polarisation is along the projection of the molecular axis on the transparent panel plane, only one wave is excited. This wave is the extraordinary wave with its polarisation direction remaining unaltered however the device in this case acts as a variable index device. There are liquid crystal modes which by their nature exhibite isotropic change of the refractive index such as blue phases of short pitch, and polymer dispersed liquid crystals with small LC droplets. Variable index modes are important to provide phase-only modulations which is of importance for spatial light modulators.

However, because of the variation of the extraordinary index with the voltage, its phase is varying. This is the principle of phase-only modulation action of the LCD of the embodiments. If the electrodes are patterned into small pixels, then each pixel is able to modify a small part of an optical beam (polarisation, intensity or phase modification). As a result, a pixilated LCD can be used as a spatial light modulator useful for improving image quality. Based on a spatial phase modulation, aberrations correction of wave-fronts can be achieved. Similarly based on a spatial polarisation modulation, polarimetric or ellipsometric images can be formed.

There are many LCD electro-optic modes which are based on the effective birefringence variation and the optic axis rotation, when an electric field is applied between the two electrodes. Non-limiting examples include the hybrid mode, twisted nematic mode, in-plane switching mode, pi-cell, vertically-aligned LC, smectic modes, chiral smectic mode, flexo-electric mode, smectic A, dual frequency mode, cholesteric LC with small pitch, blue phases, heliconical LC phases, distorted helix ferroelectric (DHF), and surface stabilized ferroelectric LC (SSFLC). In particular, the smectic and cholesteric modes exhibit other characteristics when microparticles are embedded in them. Above certain concentration, they break into regions forming focal conic texture, which is scattering. When a voltage is applied, they form a transparent nearly mono-domain state. Different voltage waveforms can be applied and tuning can be achieved by varying the amplitude, the frequency or the duty cycle of the voltage waveform. At sufficiently high frequency and depending on the voltage applied, the temperature of the device may increase which changes the transparency of the device.

As it is evident from the above discussion, majority of the existing LCDs are usually polarisation-dependent, and therefore, the light beam passing through a linear polariser results in a <NUM>% loss in the light throughput. This problem is thus solved in the present invention by introducing a polarisation-independent device of the embodiments. As will be discussed below, privacy and smart windows are of many possible applications of this device. However, it can also be useful in many other situations, for example when weak ambient light is used for imaging, or when two polarisers are used, for instance in interferometric imaging and sensing applications. Non-limiting examples are hyperspectral and wave-front correction devices based on liquid crystals.

Another problem solved in the present invention is the strong dependence on the angle of incidence. When a light beam impinges along the LC molecules principal axis, the retardation vanishes and phase modulation becomes asymmetric with respect to the normal to the transparent panels. Because, in imaging systems, the angular extent of light contains a wide spectrum of angles, each light beam may experience different phase retardation. The devices of the embodiments are capable of extending their angular field of view. Slow switching speed attributed to the majority of the existing LCDs, particularly when thick liquid crystal layers are used, is also overcome by the devices of the present embodiments.

In a further embodiment, the composite structure may be formed from liquid crystal by creating random liquid crystal micro-domains on at least one of the panels, so that there is a large index mismatch between neighbouring micro-domains at one orientational state of the liquid crystal. This mismatch actually decreases as the liquid crystal molecular orientation changes with external field tuning. The alignment layer deposited on the transparent panels is made of random liquid crystal micro-domains by one of the methods of nano-patterning of the transparent conductive electrode (TCE) layer or photo-alignment of the photosensitive polymer layer or thin chalcogenide glass. Thus, the liquid crystal micro-domains are created with refractive index mismatching between them at one state (for example, parallel n∥) and nearly full matching at the other state (for example, perpendicular n⊥), which is tuned by an external field. In the above discussion, the index matching or mismatching states (different orientational states of the liquid crystal) can be reversed depending on whether the micro-domains are hydrophilic or hydrophobic and whether the liquid crystal has negative or positive dielectric anisotropy.

Reference is now made to <FIG> illustrating the above embodiment describing the scattering medium which is achieved by having micro-domains at the panels randomly oriented. As noted above, the microparticles (<NUM>) may have a different shape, thereby resulting in different formed structures of the micro-domains. For example, <FIG> shows random micro-fibres (<NUM>) inside the liquid crystal (<NUM>) medium. <FIG> shows random micro-posts or micro-walls (<NUM>) oriented almost perpendicular between the two transparent panels (<NUM>), and the liquid crystal molecules (<NUM>) filling the space between them and maybe even infiltrating them at least partially. The micro-posts (<NUM>) may have various shapes, such as helices, cones or zigzag shapes, which can reduce the angular dependence of the scattering. <FIG> shows a skeleton structure of the micro-posts filled with the liquid crystal. <FIG> shows a mesoporous matrix of random micro-fibres filled with the liquid crystal. <FIG> illustrates an exemplary device of the embodiments with one of the panels having different micro-domains (R1-R5) that can induce different liquid crystal molecular orientations so that the refractive index mismatch between the neighbouring domains is large at one orientational state of the liquid crystal molecules and becomes smaller at different orientational states.

The random orientation of the micro-domains causes large scattering at zero voltage. But when the voltage increases, the index mismatch between the micro-domains starts to decrease and so does the scattering. Furthermore, the colour of the device may be tuned by selecting different sizes of the microparticles, different raw materials, different porosities or different refractive indices. As noted above, the porous silicon microparticles having different porosity and oxidation levels produce different colours of scattered light. Mesoporous silicon matrix at different oxidation levels is another possibility for a random scattering structure which becomes tunable when infiltrated with liquid crystals. In one of the embodiments, the porous microparticles may be functionalised with an organic or inorganic molecular layer or nano-layer to control the way the liquid crystal molecules are oriented on their surface and to maximise the randomness of the micro-regions comprising the microparticles and the liquid crystal surrounding. The pores size of the microparticles is not limited and can range from few nm in diameter to hundreds of nm or even more. The larger the number of pores and their diameter, the better as long as they are on the micron scale because then the effective index of the infiltrated microparticle becomes closer to one of the principal refractive indices of the liquid crystal and their density becomes closer to that of the liquid crystal thus minimising sedimentation effects over time.

Organic or non-organic tubes normally used for drug delivery can be used as the porous microparticles to produce the composite structure of the embodiments. One non-limiting example of such organic particles are cochleates which are cigar-like microstructures consisting of a series of lipid bilayers, formed as a result of the condensation of small unilamellar negatively charged liposomes. In the presence of calcium, the small phosphatidylserine (PS) liposomes fuse and form large sheets. These sheets have hydrophobic surfaces and tend to roll-up into the cigar-like cochleate. Electron microscopy images usually show a typical cochleate cylinder characterised by the elongated shape and by the tight packed bilayers.

Cochleates are usually prepared by mixing DOPS (dioleoyl phosphatidylserine) with DMPS (dimyristoyl phosphatidylserine) at the <NUM>:<NUM> molar ratio, and then freeze dried to get the powder form. The samples are dialysed before freeze drying to remove salts. The obtained dry powder can be directly used for analysis or by further preparing a suspension. The suspension samples are dried at room temperature under vacuum. The suspension of the cochleate particles should be sonicated to temporarily disrupt aggregates and avoid sedimentation of the particles.

Another example of the organic porous microstructure fiber maybe made from raw cotton, waste paper or other organic materials. Such structure when embedded between the two transparent panels of the embodiments and filled with the liquid crystal, forms the liquid crystal composite tunable device of the present invention. In a further embodiment, the micro-particles are functionalised with a nano-layer of material that can cause preferable orientation of the liquid crystal molecules in the vicinity of the micro-particles. This will result in a randomly deformed structure which reveals to higher switching contrast. An example of such nano-layer coating is a surfactant, but any other functionalisation material may be suitable.

In a particular embodiment, the microparticles or the mesoporous structure shown in <FIG> is made of thermochromic material, such as vanadium oxide, which is dielectric at room temperature, but becomes metallic above its transition temperature. As a result, the device incorporating the vanadium oxide microparticles can in part strongly scatter the infrared radiation from the sun in the summer period and in part reflect or absorb it, while the transmission in the visible range can still be made high enough with proper selection of the voltage applied to the device. Such device may act both as a privacy window and smart window at the same time. The device may also be formed on a single layer of thermochromic material, such as vanadium oxide, a multi-layered structure or periodic structure containing vanadium oxide or any electrochromic material or phase change material, such as Ge<NUM>Sb<NUM>Te<NUM> (GST) (chalcogenide glass), on one side of a window, while the composite liquid crystal layer is adjacent to it. The thermochromic material thus acts as a smart window while the scattering composite liquid crystal layer acts as a privacy window.

The existence of the liquid crystal layer on top of the thermochromic material helps in reducing reflections from the smart window structure boundary, thereby improving the transmission characteristics of the window. Birefringence of liquid crystals is reduced with the incident light wavelength. Therefore, at high temperatures, it is possible to adjust the voltage in order to maintain less reflection in the visible range of the spectrum while the reflection in the infra-red remains high. Thus, the liquid crystal layer in the device of the embodiments has two roles: first, to provide the privacy window with its functionality, and second, to actively control the Fresnel reflection from the thermochromic material interface thus improving its contrast in the infrared range and increasing the transmission in the visible range.

Thus, the randomly aligned micro-domains formed in the composite structure of the embodiments result in strong scattering at zero voltage. When voltage is applied, the liquid crystal molecules in all the micro-domains reorient themselves with the electric field direction, thereby gradually decreasing the scattering as the voltage increases. The device of this embodiment is used in the thermochromic smart window, first, to turn the window to become both smart and privacy window, and second, to improve the transmission characteristics of the smart window by modulation of the Fresnel reflection properties with the liquid crystal composite tunable device of the present invention. As mentioned above, in all described scattering-mode devices, the index matched or mismatched states can be reversed depending on whether the walls of the embedded micro-domains are hydrophilic or hydrophobic and whether the LC has negative or positive dielectric anisotropy. Bistability of the devices can also be achieved depending on the choice of the liquid crystal, for example ferroelectric LC are known to exhibit bistability as well as metal-organic liquid crystalline compounds.

The LCD of the embodiments may also be tuned optically by depositing a photosensitive layer, such as an intrinsic photo-conducting layer or photodiode structure, on at least one of the transparent panels. At least part of the solar spectrum impinging on the photo-sensitive layer side changes the voltage drop across the liquid crystal layer, thereby modulating the scattering. Some photo-conducting layers, such as chalcogenide layers, can be made thin to transmit large part of the incident light. They can also act as photo-alignment layers. Of particular interest is a photosensitive layer capable of absorbing the UV part of the solar spectrum and converting it into voltage drop across the device. Thus, in addition to the efficient energy consumption, such layer also provides protection from the UV radiation. Example of such highly efficient UV photoconductive layer is an aluminium-doped zinc-oxide nanorod array annealed in oxygen environment.

Similarly, photo-conducting polymers can be used as the photosensitive layers for visible light, while InGaAs can be used as a photoconductive layer in the short wave infrared range (SWIR). In yet further embodiment, a single layer is deposited on one of the transparent panels or two layers are deposited on the two panels on top of the transparent conductive electrode layer. Other examples of more transparent photovoltaic device include Cu based chalcogenide glasses, mixed with perovskites and using highly transparent electrodes such as metal grids. Although the voltage change produced by solar light or part of it might not be enough to switch the liquid layer completely, it is still possible to apply a bias voltage to maximise the effect of sunlight. This optically-addressed device can be used as a smart window capable of self-controlling its transparency depending on the sunlight intensity. In the summer period, the solar intensity is strong, so this window may automatically dim and keep the house cold, while in the winter period, when there is no strong sunlight, the window may brighten, thereby keeping the house warm.

Thus, the use of a photoconductive or photovoltaic layer in conjunction with the composite structure allows modulating the smart window's transparency by the photo-voltage or resistance change due to the incident sunlight shining on the window.

Reference is now made to <FIG> schematically showing a typical birefringent device, which in the present exemplary case is a nematic liquid crystal device having an antiparallel geometry. The geometry of the dielectric tensor associated with the liquid crystal molecules is shown in <FIG>. The polariser and analyser are installed to obtain intensity- or wavelength-modulation. For phase-only modulation, only polariser is installed, while for polarisation-independent operation, neither polariser, nor analyser is required.

One of the difficulties in liquid crystal devices used in photonic applications is obtaining uniform thickness. As shown in <FIG>, an improved gap uniformity can be obtained by etching one of the glass substrates with the etch depth equals to the desired cell gap. The panel surfaces are then coated with the necessary layers of transparent conducting oxide and alignment materials, and with optional layers, such as dielectric mirrors for a Fabry-Perot cavity tunable filter or photoconductive layers. The second (bottom) panel is not etched, but simply pressed and glued to the bottom of the device without spaces.

To obtain easy electrical connections the substrates are laterally shifted by an offset so that the connection area is exposed on both substrates from right and from left. Using modern etching techniques (chemical, ion-beam or laser-beam) one can achieve nanoscale uniformity over large area. Although not shown in these figures, different means for easy electrical connections are possible, for example by drilling holes in the transparent panels followed by filling them with a conducting material. This is important for pixelated devices in which the number of electrical connections increases with the number of pixels.

According to another aspect of the present invention, various configurations of the LCD of the present embodiments having a tunable birefringence are described below. All these configurations based on the LCD of the invention represent tunable, polarisation-independent optical systems effectively overcome the aforementioned problems of the existing LC devices.

In one embodiment, a polarisation liquid-crystal retarder system with improved light throughput comprising the LCD of the present invention is shown in <FIG>. It is based on the Mach-Zehnder interferometer configuration having an arbitrarily polarised beam split with a conventional polarised beam splitter (<NUM>) into two orthogonally polarised and collimated beams P and S. The conventional Mach-Zehnder interferometer determines the relative phase shift variations between these two collimated beams derived by splitting light from a single source. The generated phase shifts between the two beams P and S is caused by the sample or a change in length of one of the paths.

As shown in <FIG>, the P-beam passes through the liquid crystal device (<NUM>) of the embodiments oriented at <NUM> degrees, meaning that its optic axis is parallel to (or along) the P-polarisation direction. Hence, this beam is affected by the extraordinary refractive index that varies with the applied voltage. The S-beam passes through another section of the LCD (<NUM>) and is affected by the ordinary refractive index, which does not vary with the applied voltage.

Upon recombining the P- and S-beams, the resulting combined beam is again arbitrarily polarised, but modulated as the voltage varies due to the resulting phase retardation Γ = 2πd(ne - no) / λ. This is similar to the situation when the incident light beam is linearly polarised at <NUM> degrees to the optics axis. However, in the present system, there is no loss in the light that is usually encountered when the input linear polariser is used.

Reference is now made to <FIG> schematically showing three uses of the output modulated beam from <FIG>. In <FIG>, the modulated beam is passing through an optical etalon having an output transfer function of multiple narrow spectral bands, so that by modulating the LCD (<NUM>), the output spectrum includes multiple narrow spectral bands variable with the voltage. In <FIG>, the modulated beam is passing through an output of an illuminator connected to an optical fibre which can be used remotely as a light source with various spectroscopic or imaging systems. In <FIG>, the modulated beam being reflected or transmitted through a sample is then directed to a parallel detector or to an imaging system either for spectral measurement applications, or for hyperspectral imaging purposes, respectively. In order to obtain the polarisation-independent phase modulation in the configuration of <FIG>, a half-waveplate or polarisation rotator should be inserted in the S-beam path both before and after the cell, so that the beam passing the cell becomes P-polarised, and then converted back to S to be reflected by the polarised beam splitter (<NUM>).

Reference is now made to <FIG> showing the similar configuration as above, but with the bottom cell (<NUM>) empty (without liquid crystal inside) or with the filled cell oriented at <NUM> degree. The S-beam is now passing through either the empty bottom cell or a filled cell oriented at <NUM> degree (<NUM>). The filled cell must be oriented at zero degree so that both the P- and S-polarisation have the same phase modulation. When the bottom cell (<NUM>) is filled, this configuration can work without the output polariser and for the phase-only polarisation-independent modulation, which is useful in tunable lensing and wavefront modulation. Similarly, to the previous configuration, the polarisation-independent phase modulation can also be achieved in this configuration, provided that, for example, a half-waveplate or polarisation rotator is inserted in the path of the S-beam both before and after the cell, so that the light beam passing the cell (<NUM>) becomes P-polarised and converted back to S to be reflected by the polarised beam splitter (<NUM>).

In case of the empty cell, the phase retardation is Γ = 2πd(ne -<NUM>) / λ which is larger by ΔΓ = 2πd(no -<NUM>) / λ from the previous configuration shown in the <FIG>. Taking typical values for the refractive indices ne = <NUM> and no = <NUM>, the calculated phase retardation will increase by a factor of <NUM>-<NUM>, which allows thinning of the liquid crystal layer by the same factor. Since switching time of nematic liquid crystals is proportional to the square value of the thickness, the response time of the device may be improved by a factor of <NUM>-<NUM>. This is a significant improvement for spectral modulation devices with compressed sensing, FTIR spectroscopy, or phase modulators operated in a long wavelengths range (IR or THz). Thus, using the present configuration, the system of the present embodiments can be miniaturised, but still benefit from the short switching time reduced from seconds to milliseconds or less.

Reference is now made to <FIG> illustrating the system of the embodiments based on the Michelson interferometer that uses polarisation conversion mirrors in both arms for light throughput and speed improvement. The whole device maybe a single solid unit with the arms glued (using the refractive index matching glue) to the sides of the polarised beam splitter (<NUM>).

The conventional Michelson interferometer uses a beam splitter splitting a light beam into two arms. Each of those light beams is reflected back toward a beam splitter which then combines their amplitudes using the superposition principle. The resulting interference pattern that is not directed back toward the source is typically directed to some type of photoelectric detector or camera.

Similarly, in the present configuration, the S-polarised beam is first transmitted through either an empty cell (<NUM>) or a filled cell positioned at <NUM> degree, reflected from a first polarisation conversion mirror (<NUM>') converting the beam to P-polarisation, and then transmitted through a second polarisation conversion mirror (<NUM>"). The polarised P-beam is then directed to the filled LCD (<NUM>) oriented at <NUM> degrees, reflected from the second polarisation conversion mirror (<NUM>"), then converted to S-polarisation and again reflected from the first polarisation conversion mirror (<NUM>') to recombine with the beam from the other interferometer channel.

The polarisation conversion can be performed by several means, for example using a quarter waveplate (QWP) and regular mirror, a Faraday mirror, a metallic grating with the Gaussian profile of the gratings lines or other means. The net phase retardation in this geometry for the case of empty cell in the bottom arm is Γ = 2zd(ne + no - <NUM>) / λ. However, since it is not variable with the voltage, the same modulation characteristics can be achieved as in the previous case but with a fixed phase shift. The cell (<NUM>) in the bottom arm can be empty to enhance the retardation modulation or filled, oriented at <NUM> degree to provide polarisation-independent phase modulation. In the latter case, the output polariser is not necessary.

A Sagnac interferometer (or a ring interferometer) is based on a phenomenon of interference that is elicited by rotation. A beam of incident light is split and the two beams are made to follow the same path but in opposite directions. On return to the point of entry, the two beams are allowed to exit the ring and undergo interference. The relative phases of the two exiting beams, and thus the position of the interference fringes, are shifted according to the angular velocity of the apparatus. Thus, when the interferometer is at rest with respect to the earth, the light travels at a constant speed. However, when the interferometer system is spun, one beam of light will slow with respect to the other beam of light. Fibre-optic and ring-laser gyroscopes are based on this phenomenon.

Reference is now made to <FIG> showing a polarisation liquid-crystal retarder system with improved light throughput based on the Sagnac interferometer configuration. In this configuration, the P and S-beams pass twice through the LCD (<NUM>) with the help of a retroreflector (<NUM>) or two mirrors. The phase retardation is doubled, Γ = 4πd(ne - no) / λ , with the advantage of having the two beams passed along the same path.

In another embodiment, the configuration shown in <FIG> is similar to the previous configuration shown in <FIG>, but has an empty cell (<NUM>) (without liquid crystal). In this case, the phase retardation becomes Γ = 2πd(ne - no) / λ , which is twice less than in the previous case (with liquid crystal). As mentioned above, the empty cell (<NUM>) can be filled with the liquid crystal, but then it should be oriented at <NUM> degree, and no output polariser is needed. In this configuration, polarisation-independent phase-only modulation is achieved, which is important for many applications including polarisation-independent virtual reality applications and tunable focusing.

In still another embodiment, the Sagnac interferometer configurations, which are shown in <FIG>, may be further supplemented with a polarisation conversion mirror (<NUM>) installed instead of the retroreflector (<NUM>). The respective Sagnac interferometer configurations are shown in <FIG>. In all these configurations shown in <FIG>, the retroreflector (<NUM>) or the polarisation conversion mirror (<NUM>) may constitute one of the transparent panels (<NUM>) of the LCD (<NUM>) of the embodiments, or attached with refractive index-matching glue to one of the external sides of the transparent panels (<NUM>).

A Wollaston polariser consists of two birefringent right-angle triangle prisms cemented together, such that their optical axes are perpendicular. It separates randomly polarised or unpolarised light into two orthogonal linearly polarised outgoing beams. As light passes through such polariser, a symmetric deviation between the ordinary and extraordinary beams is created. The resulting beams are of orthogonal linear polarisation states and have equal intensity and a large angular deviation, which is determined by the prisms' wedge angle and the wavelength of the light. Commercial Wollaston polarisers are available with divergence angles from <NUM>° to about <NUM>°.

A Rochon polariser is very similar to the Wollaston polariser, but the ordinary beam passes through the prism without deviation. The Rochon polariser also consists of two birefringent material prisms in optical contact with one another. As the ordinary beam is not deviated on both sides of the prism, the ordinary and extraordinary beams remain collinear through the first prism. Upon entering the second prism, the ordinary rays do not experience a change in the refractive index and pass through the prism without deviation, while the extraordinary rays refract at the interface.

Reference is now made to <FIG> showing a polarisation liquid-crystal retarder system of the embodiments incorporating either two Wollaston polarisers or two Rochon polarisers (<NUM>' and <NUM>"). As mentioned above, these two polarisers have the property of splitting the incident beam into two orthogonally polarised beams with a small angle in between. The two P- and S-beams are then collimated using a lens, passes the liquid crystal device (<NUM>) of the present invention, collected again and recombined by a polariser at the output.

The similar configuration, but with the bottom cell (<NUM>) empty (without liquid crystal) or filled with the liquid crystal and positioned at <NUM> degree is shown in <FIG>. In this case, one beam passes through the filled LCD (<NUM>) and the other beam passes through the empty bottom cell (<NUM>) or filled and oriented at <NUM> degrees. The phase retardation in the case with the filled bottom cell (<NUM>) is Γ = 2πd(ne - no) / λ , while in the case with the empty bottom cell (<NUM>) is Γ = 2πd(ne -<NUM>) / λ, which is larger. Therefore, the latter configuration shown in <FIG> with the empty bottom cell (<NUM>) is preferable. When the bottom cell is filled, polarisation independent modulation is achieved and no need for the output polariser.

Reference is now made to <FIG> showing the equivalent configuration with only one Wollaston polariser or with one Rochon polariser (<NUM>), but operating in a reflection mode with the advantage of doubling the phase retardation to Γ = 4πd(ne -<NUM>) / λ. The polarisation-independent phase modulation can also be achieved in all the configurations shown in <FIG>, provided that, for example, a half-waveplate or polarisation rotator is inserted in the path of the S-beam both before and after the cell, so that the light beam passing the cell (<NUM>) becomes P-polarised and converted back to S to be recombined with the other beam in the Wollaston or Rochon polariser.

Reference is now made to <FIG> showing the Mach-Zehnder interferometer configuration with two channels. It is similar to the configuration shown in <FIG> except that there are two LCDs (<NUM>' and <NUM>"), and their optic axes are aligned at <NUM> degrees with respect to the P or S polarisations, both at the top and at the bottom of the system. As a result, the two beams become modulated, and when passing through the output polarised beam splitter (<NUM>), two modulated channels of the same or differently modulated beams are obtained. This configuration has no loss in light throughput and the advantage of having two modulated beams at the same time, which can be useful for measuring two different samples simultaneously.

A Fabry-Pérot interferometer (resonator or etalon) is a linear optical resonator (or cavity) which consists of two highly reflecting parallel mirrors or made of a transparent plate with two reflecting surfaces (having some small transmissivity) and is often used as a high-resolution optical spectrometer.

Reference is now made to <FIG> schematically showing a system configuration comprising the LCD of the present invention and based on a thick liquid crystal Fabry-Perot interferometer, which produces large number of narrow spectral bands in the transmission spectrum. The narrow bands shown in <FIG> can be modulated with the applied voltage. The dashed peaks in the spectrum are obtained at voltage V1, while the solid curves are obtained at voltage V2. This configuration has an advantage of achieving high resolution within shorter processing time in the compressed sensing methodology.

The system configuration shown in <FIG> may be combined with coloured parallel detectors. As an example, consider an output of three wavelengths centred at the standard colour camera band at <NUM>, <NUM> and <NUM>. Since the three bands are tuned, three spectral images are obtained at each voltage, and the final number of spectral images obtained is the number of the voltages multiplied by <NUM>. Similarly, with the relatively modern multispectral parallel detectors, it is possible to design the Fabry-Pérot resonator with the LCD of the present invention to produce peaks centred at the major wavelengths of the coloured pixels, and by tuning them the larger number of spectral images can be obtained. The thick LCD (<NUM>) of the present invention shown in <FIG> is sandwiched between either two regular dielectric mirrors or polarisation conversion mirrors (<NUM>). In the case of the polarisation conversion mirrors, the output is polarisation-independent, thereby creating larger light throughput. In case of the regular mirrors, the light has to be linearly polarised.

In yet further embodiment, the dielectric mirrors (<NUM>) in <FIG> may be made in a way that the reflection and transmission through the device exhibit interference with no need for polarisers, for example by having different reflectivity at different wavelengths. The tunable birefringent element (TBE) sandwiched between these two mirrors can then tune these colours by applying an external field. If the TBE layer is made thick enough, then the colours can disappear at the high retardation state, because the different interference orders become close to each other. As the retardation decreases, for example by applying voltage to the LCD (<NUM>), the interference orders become separated and colours start to appear depending on the applied voltage or temperature. Thus, using this configuration, an interference-based smart window can be built.

The system configurations of the above embodiments, which incorporate both the LCD (<NUM>) and the empty cell (<NUM>) can be used for measurements of a refractive index or thickness of transparent materials. These configurations acting as orthogonal polarisation interferometers have the LCD sample arm (<NUM>) with the S-polarisation and the empty cell reference arm (<NUM>) with the P-polarised beam. As described above, the reference arm may contain the cell filled with liquid crystal or tunable birefringent element instead and then should be positioned at <NUM> degree. The output phase of the light traversing the sample can be measured similar to the phase-shift interferometry by providing different known phase changes to the P-polarised beam passing the tunable birefringent element. The output analyser is a must in this case in order to combine the two beams. It is then possible to extract the phase change of the light traversing the sample from the output signal after the output analyser by providing at least three known different phase differences between the reference and sample arms. This can be used for refractive index measurement of gases and liquids flowing through the empty cell or for solid transparent materials. In the case of the solid transparent materials, it is possible to measure their thickness, assuming that the refractive index is known. Alternatively, it is also possible to extract intrinsic birefringence in the sample and calculate stresses in the sample material, for example, in glass.

In a further embodiment, the system of the present invention can be configured to convert one of the polarisations into the other. The system shown in <FIG> has a spatial separation configuration, while the system shown in <FIG> has an angular separation configuration. The polarisation conversion in <FIG> is done with the help of the polarised beam splitters (<NUM>) and polarisation conversion mirrors (<NUM>). The LCD (<NUM>) must be oriented at <NUM> degrees to the output polarisation direction, and the net phase retardation is then calculated as Γ = 2πd(ne - no) / λ with the advantage of having higher light throughput.

The configuration shown in <FIG> has a further advantage of using the flat elements, such as a wire grid polariser (<NUM>) or an achromatic quarter waveplate (not shown). The wire grid polariser (<NUM>) lets the P-polarisation to pass, but reflects the S polarisation. Consequently, the S-polarised beam upon reflection from the polarisation conversion mirror (<NUM>) becomes P-polarised, thus passing through the wire grid polariser (<NUM>) at different angle from the initial P-polarised beam. The angles of the two beams should be small in order to avoid any modulation variations over different sections of the beams. The liquid crystal device (<NUM>) positioned at <NUM> degrees, capable of minimising the angle dependence, such as pi-cell, is preferably used in this case.

Reference is now made to <FIG> showing a polarisation liquid-crystal retarder system with a polarisation conversion mirror (<NUM>) for polarisation-independent phase-only modulation. In this configuration, the polarisation conversion mirror (<NUM>) is installed behind the LCD (<NUM>) oriented at an arbitrary angle. In a specific embodiment, the polarisation conversion mirror (<NUM>) may comprise a quarter waveplate (QWP) combined with a mirror, or it can be made of a metallic grating with the grating lines having Gaussian profile. In another specific embodiment, a Faraday rotator can also operate as the polarisation conversion mirror (<NUM>) in the reflection mode.

The P polarisation is converted to S upon reflection, while S converts to P. As a result, the two polarisations accumulate the same phase modulation as shown in <FIG>. In fact, the retarder does not have to be oriented at a specific angle for the polarisation-independent phase-only modulation to occur. For the retarder with its optic axis oriented at an angle ξ, the Jones matrix maybe written as: <MAT> where Γav = 2πd(no + ne) / 2λ is the average retardation. In reflection using a polarisation reflection mirror, the Jones matrix becomes: <MAT>.

As a result, the assembly of an arbitrarily oriented retarder and polarisation conversion mirror acts as a polarisation-independent phase-only modulator with the phase modulation equal to 2Γav. The beam splitter (<NUM>') shown in <FIG> is optional and may be removed if the light incidence is at oblique angle. Alternatively, the Sagnac configurations shown in <FIG> can be used in this system. In some of the LCD modes, such as the ferroelectric LC modes, twisted modes or in-plane switching modes, the optic axis rotates in the plane of the transparent panels and is characterised by the angle ξ. Hence the presented polarisation-independent phase-modulation configuration can also work with ferroelectric liquid crystals which are faster.

In a further embodiment, the spectral- or phase-modulated LCDs of the present invention are used in imaging systems which usually contain large angular extent. It is therefore critical to minimise the angular dependence of the phase retardations. Reference is now made to <FIG> showing the LCDs of the present invention that are capable of minimising the angle dependence. For example, <FIG> shows two anti-parallel aligned devices of the present invention positioned at <NUM> degrees to each other, so that the liquid crystal molecules of the top LCD mirror the molecules of the bottom LCD. <FIG> shows the configuration with the parallel aligned LCD, i.e. pi-cell. Since the two halves of the pi-cell are mirror images of each other, this device has wider field of view than the anti-parallel aligned LCD. The exemplary liquid crystal modes in which the optic axis remains in the plane of the LCD transparent panels are the in-plane switching mode shown in <FIG> and the ferroelectric mode shown in <FIG>. Another exemplary liquid crystal mode is shown in <FIG> illustrating the LCD with compensating waveplates and other modes containing multiple domains: (a) a patterned vertically aligned mode; and (b) a multi-domain vertically aligned mode, in two voltage regimes (switched of and switched on).

The LCD of the present invention can be modified to exhibit coloured bands on its surface which can be tuned with the voltage. Reference is now made to <FIG> showing a wedge-type LCD of the present embodiments with partially reflecting panels (<NUM>). Each panel (<NUM>) is coated with a transparent electrode and alignment layer. This LCD has a thickness varying in the range between d<NUM> and d<NUM> over the length L of the retarder, so that the wedge angle is determined by tanω = (d<NUM> - d<NUM>) / L. Because of the wedge, the coloured interference bands appear on the facets of the wedge which can be selected serially using the slit aperture. These coloured bands correspond to the maxima in transmission or reflection. Similarly, as shown in <FIG>, the wedge can be spherical, thereby producing the spectral (coloured) concentric rings appearing on the surface. These coloured rings (spherical bands) can be selected using the annular aperture. In both cases, the same effect can be obtained using a flat uniform LCD with highly resistive electrodes at high-frequency operation, so the voltage across the electrode is non-uniform and the colours will therefore appear due to the voltage distribution. The voltage non-uniformity in this case is a function of the frequency and electrical resistance. The highly resistive electrodes have conducting contacts on one side for activating the linear wedge and an annular contact around the liquid crystal active area for activating the spherical wedge. In the case of the linear wedge, the beam-steering effect can be made tunable with the voltage because of the linear refractive index gradient. In case of the spherical wedge, the tunable lensing effect can be created by applying symmetric voltage profile when using the annular highly conducting electrodes around the liquid crystal active area. This tunable lensing effect makes it possible to use a polarisation-independent tunable lens in the configurations of the embodiments described above for polarisation-independent phase-only modulation.

Thus, there are two configurations which can impart colour variations across the liquid crystal composite tunable device of the present invention:.

The interference pictures shown in <FIG> suggest that it is possible to place an aperture (rectangular slit or circular aperture) centred at dc and having a width Δ, so that only one colour passes at a particular voltage. If the transmission function is T(λ,d) , the total transmission of the spectrum from λ<NUM> till λ<NUM> of the slit is given by: <MAT> where the variable d is changed to t inside the integral. Thus, the narrower the aperture is used, the narrower the spectral line is selected.

Reference is now made to <FIG> showing a tunable filtering system comprising the LCD (<NUM>) of the present invention with a passive linearly-variable bandpass filter (<NUM>), which is commercially available, for example from Delta Optical Thin Film, and prepared on a transparent substrate. The common mode of using such filters is by mechanically moving them. However, mechanical motion is slow and might add noise to the imaging or sensing system. In the system configuration of the present embodiment, the LCD (<NUM>) is combined with the linearly-variable passive filter (<NUM>), and therefore, it is capable of selecting one or more different transmitted spectral passbands at a time. A lens system (<NUM>) then directs the filtered beam and couples it into an optical fibre (<NUM>), or focuses it onto an imaging or sensing system (not shown in the figure).

As shown in <FIG>, the LCD of the present invention may be pixelated. Such pixelated LCD (<NUM>) is integrated within a polarisation-independent assembly (<NUM>) which is shown in <FIG>. The pixelated LCD (<NUM>) further comprises two polarisers (not shown here) positioned either in a parallel or crossed configuration, so that the voltage on each pixel or group of pixels facing the beam coming from one single passband is transmitted, while all other bands are blocked.

A single LCD (<NUM>) is capable of selecting a limited range of wavelengths and relatively wide band beam. However, for a wide range and narrow-band operation, three LCDs maybe installed in the system. Two of them form a two-stage Lyot filter, and the third is installed between the crossed polarisers, so it can block the side interference peaks of the Lyot filter, thereby improving the dynamic range by approximately the factor of <NUM>. Another option would be to use three LCDs, each one of them installed between the crossed polarisers and having their LC layer thicknesses as d, 3d and 5d, thereby providing a wider dynamic range and creating the narrowband beams with a smaller full width at the half maximum (FWHM), compared to a single LCD system.

<FIG> schematically shows a tunable filtering system comprising the LCD of the present invention with a dispersive element (<NUM>) having appropriate optics. The dispersive element (<NUM>) can be a prism or grating and can produce the array of filtered beams. The system of the present embodiment can operate also in a reflection mode with appropriate and commercially available mirrors, lenses and polarised beam splitters arrangements. In this configuration the linearly-variable passive filter (<NUM>) of the previous configuration may also be replaced with a passive wedged cavity similar to that shown in <FIG>, but without the voltage applied to the liquid crystal layer. Alternatively, the linearly-variable passive filter (<NUM>) may be replaced with an empty cavity or a cavity filled with any passive material. In case of the spherical cavity, the pixelated LC device of <FIG> and <FIG> should select an annular zone which is variable with the voltage applied.

Reference is now made to <FIG> schematically showing a double-focus system comprising the LCD (<NUM>) of the present invention and a birefringent plate, a lens or a lens system (<NUM>) installed in the path of a converging beam. It is known from ray optics that if a glass block of thickness dg is located in the path of a focused beam, the focal point shifts by an amount ΔFg = dg (tan αg / tan αi -<NUM>), where αi, αg are the incidence angle on the glass and the refraction angle inside the glass. These two angles are connected via Snell's law stating that sin αi = ng sin αg , assuming that the glass block is located in air. However, the glass is usually isotropic, and therefore the two polarisations P and S are focused at the same point. In the presence of the LC layer having the thickness dLC, these two orthogonal polarisations experience two different refractive indices no and ne inside the LC layer ("o" stands for "ordinary" and "e" stands for "extraordinary). As a result, the two polarisations are focused at focal planes separated by: <MAT> which for small angles can be approximated as AFLC ≈ dLC(ne - no) / neno.

Since the refractive index of the extraordinary ray in nematic LCs can be varied between n⊥ and n∥, a double focus imaging is obtained at two orthogonal polarisations with the distance between the two focal planes variable by the amount: <MAT> Taking the typical values of the LC refractive indices of n∥ = <NUM> and n⊥ =<NUM>, and thickness dLC of <NUM>, the calculated ΔFLC will be approximately <NUM>. As a result, using the thicker LC layer and a material with higher birefringence, it is possible to obtain higher tunability of the focus. Thus, using this configuration several functionalities can be achieved:.

It is possible to use a single channel (one camera or detector), but produce the phase shift between the two beams using a phase modulator located above the beam splitter as shown in <FIG>. Reference is now made to <FIG> schematically showing an orthogonally-polarised beam in-line interferometer based on the system configuration shown in <FIG>. A polarisation splitting mirror (<NUM>) reflecting only the ordinary wave is introduced at the focal plane Fo (ordinary), thereby producing a reference beam, while the object of interest is located at the focal plane Fe (extraordinary). The polarisation splitting mirror (<NUM>) may be made of a planar wire grid polariser or using the multi-layered flat polarizing beam splitter available from <NUM> industries. If the path length difference between the two beams is within the coherence region of the incident beam, the almost common-path orthogonal polarisation interferometry is obtained. In this configuration, the two orthogonally polarised beams are split into three channels, and a waveplate is inserted in each channel to produce different phase shifts. The two orthogonal polarisations are then recombined at an analyser plane located before each detector (camera or single detector). Without the objective and projection lenses, this interferometer can still operate as an in-line parallel-beam interferometer (an orthogonal-polarisation interferometer with parallel beams), and may be used for serial phase modulation. There are many applications of this interferometer in the field (where sub-nm resolution range and the axial direction matter), for example surface topography, focus tracking and vibrometry.

Reference is now made to <FIG> schematically showing an orthogonally-polarised beam in-line interferometer having the cholesteric LCD (<NUM>) of the present invention combined with a quarter waveplate (<NUM>). The quarter waveplate (<NUM>) is inserted near the object of interest within the focal region of a microscope objective (<NUM>) and within the temporal coherence region of the beam, so that both left and right circularly polarised beams are reflected from within the focal region of the objective (<NUM>). The reflected wave from the cholesteric LCD (<NUM>) is circularly polarised having the same helicity as the cholesteric LC helix, while the transmitted wave has the opposite helicity. This selective reflection phenomenon is also known as the circular Bragg reflection phenomenon. The opposite helicity wave passes through the quarter waveplate (<NUM>), get reflected from the object and passes again through the quarter waveplate (<NUM>). Upon this round trip the wave's helicity remains unchanged, and therefore, it consequently passes back through the cholesteric LCD (<NUM>). As a result, there are two beams that are orthogonally polarised (right and left circularly polarised) which give the rise to the in-line orthogonally polarised interferometer of the present embodiments.

The two beams are brought together to interfere on a detector. At least three phase shifts can be introduced between the two beams serially using a modulator, or in parallel using at least three channels having different phase shifts. Since the two interfering beams are now circularly polarised, the phase shifts can be introduced by passing the two beams through the linear polarisers positioned at different orientations without the need for waveplates. The serial phase modulation can be achieved by rotating the linear polariser. Since the LCD (<NUM>) reflects one circular polarisation at certain range of wavelengths, the operating wavelengths can be chosen to be within the reflection band of the LCD (<NUM>).

It is possible to tune the reflection band, and consequently, to tune the operation spectral band by applying voltage to the LCD (<NUM>). This is really important if the multiple-wavelengths phase-shift interferometry mode is used which allows overcoming the phase unwrapping problems without any complicated algorithms. This interferometer can still operate without the objective and projection lenses, simply as an in-line orthogonal-polarisation interferometer. Another group of helical structures exhibiting the circular Bragg phenomenon can be equally used instead of the cholesteric LCD such as the chiral sculptured thin films prepared by the glancing angle deposition technique.

The double focus for two orthogonally linearly-polarised beams can also be achieved using an LC lens or a combination of the LC phase-only spatial light modulator with a standard lens. To achieve this, a parabolic radial mask is written on the spatial light modulator plane with large number of pixels. The ordinary wave is not affected this way. It is focused in the close proximity to the original focal plane of the lens. The extraordinary wave is to the contrary modulated and shifted by the amount determined by the maximum phase written on the spatial light modulator and on the numerical aperture of the lens. A rough estimate for the case when the distance between the spatial light modulator and the lens is very small results in Δf = λφmax / πNA<NUM>. Polarisation-independent tunable focusing can then be obtained by cascading two devices oriented at <NUM> degrees to each other, or by combining the proposed device with the polarisation-independent assembly configuration described above.

In another embodiment, the spatial light modulator may be used to extend the depth of field of the imaging system by placing it in the exit pupil plane and writing the annular regions on it that provide equal phase shifts, but variable from one annulus to another. In a specific embodiment, the spatial light modulator uses several parabolic profiles of the phase, each corresponding to slightly different focal point. The combination of the several focus regions provides an extended depth of field. Alternatively, one can scan the different phase shift masks fast enough, so that an average image is obtained with the extended depth of field. Minor image processing can then bring the image back to nearly the same quality as the original focused image.

In yet further embodiment shown in <FIG>, a wide-range tunable spectral filter comprises a coupling medium (<NUM>), such as a prism, gratings, a waveguide or an optical fibre, coated with a multilayer structure comprising a thin absorbing layer (<NUM>), such as metal (for example, chromium metal of <NUM>-<NUM>-nm thickness) in contact with said coupling medium (<NUM>), a low refractive index dielectric layer (<NUM>) (for example, magnesium fluoride having few hundreds nanometre thickness) on top of said absorbing layer (<NUM>), the liquid crystal composite tunable device (LCD) (<NUM>) of the present invention superimposed on top of said dielectric layer (<NUM>), a transparent electrode layer (<NUM>) coating the top transparent panel (<NUM>) of said LCD (<NUM>), and a semi-infinite dielectric medium layer (<NUM>) on top of said electrode layer (<NUM>). This is actually a backward resonating structure with the resonance tuned by the liquid crystal composite tunable device of the present invention. The resonance maybe interpreted in several terms, such as a special type of a guided mode resonance, Fano resonance or coupled waveguides resonance. Tunability can be achieved by modulating an external magnetic field, electric field, optical field or thermal field applied to the filter. Since for this device, the modulation required is phase-only, then the LCD layer (<NUM>) maybe even replaced with other electro-optic, magneto-optic, photosensitive or thermo-optic material, which may provide easier means of preparation or operation or maybe exhibiting ultrafast tuning of the resonance wavelength. The dielectric layer (<NUM>) thickness affects strongly the full width at half maximum, and it is possible to get extremely narrow peaks by increasing the thickness of this layer. This is not possible by either using plasmonic structures because of their high absorption, or by the standard guided mode resonant structure at a wide spectral range as demonstrated here.

The incident light beam can be generated after polarisation splitting using one of the system configurations of the embodiments described above. Thus, the entire system based on this backward resonating structure further comprises polarisation conversion elements to get the two polarisation components modulated similarly by the same device. The tunable spectral filter of the present embodiment can be combined with the system configurations described above to achieve polarisation-independent operation.

The absorbing layer (<NUM>) and its combination with the dielectric layer (<NUM>) allows propagation of several types of surface electromagnetic waves, such as Zennick wave, Tamm wave, or surface plasmon resonance wave. The LCD (<NUM>) of the present invention incorporated in this tunable spectral filter is thick enough to allow guided waves. The interaction between the two different types of waves (surface electromagnetic waves and guided waves) causes a resonance in reflection due to constructive interference in the backward direction. The location of this resulting resonance is highly sensitive to the liquid crystal properties and therefore, can be used as a tunable filter or a refractive index sensor. The advantages of this tunable filter is in its wide range tuning, narrowband and fast response because the thickness of the LCD layer can be thinner than <NUM> micron for visible range operation. As mentioned above, other electro-optic, magneto-optic, photosensitive or thermo-optic materials can be used instead of the LCD layer.

As an example, <FIG> shows a simulated transverse electric reflectivity from the wide-range tunable spectral filter of the above embodiment shown in <FIG>. In this example, the coupling medium (<NUM>) is an SF11 glass prism, the angle of incidence inside the prism is <NUM> degrees, and the stack of layers is <NUM>-nm Cr (<NUM>) / <NUM>-nm MgF<NUM> (<NUM>) / <NUM>-nm liquid crystal (<NUM>) oriented so that the extraordinary mode is excited and the refractive index is variable as shown near each resonance peak/MgF<NUM> layer (<NUM>).

As mentioned above, the liquid crystal composite tunable device of the present invention can be used in privacy or smart windows. The system comprising the LCD (<NUM>) of the present invention for use in such tunable windows is illustrated in <FIG> and further comprises a polariser and an achromatic waveplate (<NUM>). The achromatic waveplate (<NUM>) is tunable and designed to manipulate the polarisation state of the incident light and its transmission through the LCD (<NUM>), which is in turn controlled via the tunable achromatic waveplate (<NUM>). Helical anisotropic LCDs, such as chiral smectic LCDs, cholesteric LCDs, heliconical LCDs, or helical photonic crystals prepared by the oblique angle deposition technique, are examples of the LCDs of the present invention used in the privacy or smart windows of the embodiments. These helical structures reflect only one circular polarisation which has the same helicity of the helical structure. The circular polarisation having the opposite helicity is transmitted through.

The reflection band centre wavelength is determined by λp = Pnav with P being the period of the helix and nav is the average refractive index. The full width at half the maximum of the reflected peak is given by FWHM = PΔn, where Δn is the local effective birefringence. Therefore, by choosing the helix pitch in the centre of the visible spectrum, i.e. P = <NUM>, with the high-birefringence material having Δn = <NUM>, the obtained reflection band covers most of the visible range from blue to red. It is also possible to have the pitch in the infrared range to manipulate the infrared part of the solar spectrum.

The polariser shown in <FIG> is preferably reflective, such as a metallic wire-grid polariser reflecting one component of the unpolarised solar spectrum, while the other component is transmitted. Nearly <NUM>% reflection from the polariser surface prevents seeing clearly from one side of the privacy window, whereas the other side of the window is completely transparent for seeing due to the light part that transmitted through. The linear polarisation is transmitted through the tunable achromatic waveplate (<NUM>) which can be at several switching states:.

<FIG> shows a prototype privacy window of the present invention with no voltage applied on the left and with applied voltage on the right. <FIG> shows this privacy window based on the LCD (<NUM>) of the present invention comprising the porous microparticles made of porous silica of about <NUM>-<NUM> in size and <NUM>% concentration. The liquid crystal used in this example is Nematic BL036 purchased from Merck, and the gap thickness was <NUM>. <FIG> further shows this window based on the LCD of the present invention comprising non-porous silica microspheres of <NUM> and <NUM>% concentration. The liquid crystal used in this example was also Nematic BL036 purchased from Merck, and the gap thickness was <NUM>. These two figures compare the use of the porous versus non-porous microparticles in the device of the present invention and clearly demonstrate the superiority of using the porous microparticles over non-porous microparticles.

The tunable window described herein may also be used to transmit <NUM>% of the visible light of the solar spectrum at all times while controlling the transmission of the infrared light. This can be accomplished by choosing the pitch of the helix to be in the microns range and the achromatic waveplate to operate in the infrared range. This way the window acts as a smart window to keep the house cool during the summer period and warm during the winter period.

As described above, two LCDs of the present invention, having their optic axis preferably oriented at <NUM> degrees with respect to each other, may be combined in one system. In that case, the incident polarisation is at <NUM> degrees with respect to the optic axis of the first LCD in the system. Such system, when driven at different voltages applied to the two different LCDs, provides excellent tunability between an achromatic half wave, a quarter waveplate and a full waveplate.

In another embodiment, an achromatic tunable lens comprises the LCD of the present invention. In yet further embodiment, an imaging system comprises an achromatic tunable lens, which is a refractive lens, and a diffractive lens, for example the Fresnel-type lens having a negative dispersion. The focal length of this diffractive lens is dependent on the wavelength of the incident light, so that the longer the wavelength is, the shorter the focal length is, which is opposite to the focal length dispersion of a refractive lens.

The action of an element as a lens requires lateral variation of the effective refractive index neff. The optical path length difference exists between the rays passing through the centre of the lens and at some distance r from the centre: <MAT>.

The parabolic profile is usually the desired one to minimise aberrations, so that the transmission function of the lens is described by: <MAT> where R is the radius of the lens, and d is its thickness. The focal length of such refractive lens is given by: <MAT> where nc is the refractive index at the centre of the lens (for a positive lens, nc - np > <NUM>).

In the LCD of the present invention, or any LC lens, the focal length can be tuned because of the control of ne(r) with an external field. <FIG> shows a parabolic profile of the optical path difference (wavelength) of a refractive lens with the M maximum. Since the dispersion of glasses and LC materials is such that the refractive index decreases as the wavelength increases, it is evident from the above equation of the focal length f that the focal length increases as the wavelength increases. In other words, the blue colour is focused first, then the green colour and then the red colour. This phenomenon is usually observed with the human eye, which is a typical refractive lens.

Reference is now made to <FIG> showing the corresponding segmented profile of the refractive index represented in a wrapped way of a diffractive Fresnel lens. In this presentation, the lens radius is divided into five regions and the phase is wrapped five times so that the maximum optical path difference at each zone is M/<NUM> waves. However, to achieve the same total optical path difference of M waves, the zones become denser as the distance from the centre increases. In the Fresnel lens configuration, the radius of zone j is given by <MAT>, where λ<NUM>, is the design wavelength. Because of this specific design for the specific wavelength, the focal length varies with the wavelength in an opposite way to the refractive lens. More specifically it varies according to <MAT>. Therefore, the red colour is now focused first, then the green and then the blue, opposite to the dispersion of the refractive lens. However, the focal length at the design wavelength has the dispersion due to the refractive index dispersion according to the above equation of the focal length f. This means that under assumption the diffractive lens zones are made from a material having the standard refractive index dispersion (index decreases with the wavelength), some compensation may occur, and the final focal length of the Fresnel lens might not have significant dispersion. However, this is not enough in many cases where the achromatic operation is required, for example in imaging systems.

According to the present embodiment, to get the achromatic operation of the imaging system, the diffractive lens is combined with the refractive lens having exactly the same focal length dispersion in magnitude, but opposite in sign. If the two lenses are attached together, then the total focal length is the sum of the two, i.e. f = f<NUM> + f<NUM>. Assuming the glass refractive index dispersion follows the Cauchy equation: <MAT> where A<NUM>, B<NUM> and C<NUM> are constants, the dispersion of the focal length of the diffractive lens behaves approximately as follows: <MAT> where A<NUM>, B<NUM> and C<NUM> are the Cauchy coefficients for the dispersion function of the refractive index of the Fresnel lens material.

Thus, the imaging system of the present embodiment is based on combining the refractive lens with the diffractive lens, so that the total focal length ftot = f<NUM>(λ) + f<NUM>(λ) is wavelength-independent simply because f<NUM>(λ) and f<NUM>(λ) have opposite trends with the wavelength. The refractive and diffractive lenses in their achromatic combination should be of the same power to minimise the requirement on the refractive index dispersion relation. It is also possible to choose them both to be made of the liquid crystal composite of the present invention and tune them so that at each external field the focal length obtained is wavelength-independent in a wide spectral range.

The diffractive element zones maybe optimised both in their width and in their optical path difference so that each zone is capable of minimising the chromatic aberration. Assuming the zones corresponding to one designed wavelength are <MAT>, then it is possible to introduce other zones between them that would correspond to a different design wavelength <MAT>. As a result, the obtained generalised Fresnel lens is suitable for wide range of wavelengths. In a particular embodiment of a manufacturing method shown in <FIG> and described below, these zones are created as annulus on the transparent electrodes having widths much less than the designed width of the Fresnel zone defined by rj+<NUM> - rj. Such Fresnel zone structure provides a further degree of freedom to adjust the phase profile, so as to minimise chromatic aberrations, extend the depth of field and avoid the polarisation dependence. Another advantage of having the fine structure of each Fresnel zone to be created of many annuli is a possibility to create different phase profiles interlaced with each other. Each profile can be designed to be appropriate for a specific wavelengths region or specific focus region, so that the extended depth of field and achromatic operations are obtained.

The optimum phase profiles can be found using large variety of optimisation algorithms known in the art such as neural nets, simulated annealing and machine learning. In yet further embodiment, part of the sub-Fresnel zones are nano-patterned with lines in one direction, but other sub-Fresnel zones are patterned with lines in the perpendicular direction. This is important because the LC molecules orientation follows the nano-grid pattern direction, and therefore, some regions of the Fresnel lens provide focusing for one polarisation (for instance, transverse-electric), and other regions with the perpendicular orientation provide tunable focusing of the orthogonal polarisation (such as transverse-magnetic). The nano-grid pattern can be created with variety of lithographic techniques, nanoimprinting or irradiation with femto-second- or ultrashort-pulsed lasers. The nano-grid pattern creates anisotropic surface tension which causes the LC molecules to be aligned in one direction. There are other techniques for generating the surface tension anisotropy, for example using mechanical rubbing in one direction, depositing material at oblique incidence, coating with black phosphorous layer or transition metal sulphides, coating the surface with photosensitive polymer layer or polyimide, or chalcogenide glass, and then irradiating it with polarised UV or blue light.

Reference is now made to <FIG> describing the method for manufacturing the liquid crystal composite tunable device of the present invention. In order to make electrical connections to the different electrodes in a way that they do not disturb the transmitted light intensity or wave-front, the manufacturing method comprises the following steps:.

In yet another embodiment, the annular regions and sub-regions are replaced with rectangular lines or sub-lines so that the tunable achromatic lens becomes a tunable achromatic cylindrical lens. Once can also simplify the driving by combining two such achromatic cylindrical lenses oriented at <NUM> degrees with respect to each other. Another advantage of using the rectangular regions instead of annular ones is the ease of manufacturing and electrical connections.

Claim 1:
A liquid crystal composite tunable device (<NUM>) for fast polarisation-independent modulation of an incident light beam comprising:
(a) two supporting and functional panels (<NUM>) made of transparent material, such as glass or transparent flexible material, at least one of them coated with a transparent conducting oxide forming a transparent conductive electrode layer patterned into pixels and with optionally at least one additional layer selected from an alignment layer, antireflective coating layer, thermochromic or electrochromic layer, photoconductive or photosensitive layer;
the liquid crystal composite tunable device (<NUM>) being characterised in comprising:
(b) a composite structure sandwiched between said two panels and made of a liquid crystal (<NUM>) and porous microparticles (<NUM>) surrounded by said liquid crystal and infiltrated with said liquid crystal;
wherein
(i) said porous microparticles have an average refractive index approximately equals to one of the liquid crystal principal refractive indices and having spatial dimensions in the range of about <NUM>-<NUM> microns;
(ii) an effective refractive index of the porous microparticles matches that of the liquid crystal at one orientational state (for example, parallel n∥) of said liquid crystal and exhibits large mismatch at another orientational state (for example, perpendicular n⊥ ), of said liquid crystal;
(iii) a refractive index mismatch between said microparticles and said liquid crystal is tuned by applying an external electric or magnetic field, thermally or optically, wherein said transparent conductive electrode layer is patterned into pixels and said external field being applied to said each pixel separately or to a group of said pixels, thereby resulting in said device being pixelated, wherein said each pixel is adjusted to modify polarisation, intensity or phase of an optical beam, thereby turning said device into a spatial or polarisation light modulator for improving image quality and creating polarimetric or ellipsometric images; and
(iv) concentration of said microparticles in said composite is in the range of <NUM>%- <NUM>% for providing optimum contrast of the device upon switching it between different orientational states of the liquid crystal, when said device is used as a scattering device, and less than <NUM>% for avoiding significant light scattering, when said device is used as a spectrum, phase or polarisation modulator.