Electrically controllable optical element made of an optically isotropic liquid and method for its production based on liquid composites

Abstract

Electrically controllable optical element with a Kerr cell filled with Kerr fluid, comprising two substrates (1) and a conductive layer (2) applied to the inner surface of the substrate (1), characterized in that the Kerr fluid (K) has a mixture of rod-shaped (5) and non-rod-shaped (4) molecules as active composites and that the Kerr fluid (K) forms a thin layer with a preformable, open-mesh, anisotropic network (9) between the structured and / or planar conductive layers (2) applied to the substrates (1), such that the state of the active composites (4, ) is determined according to the electro-optical Kerr effect.5) the Kerr fluid (K) is isotropic without an electric field in the operating temperature range (RT) and a change in the voltage-induced phase shift or refractive index shift of the optical element is produced by electrically continuous adjustment of the voltage (U) or by switching the voltage (U) on / off.

Description

[0001] The invention relates, according to claim 1, to an electrically controllable optical element, in particular a lens made of an optically isotropic liquid, and according to claim 7 to a method for its production based on liquid composites.

[0002] Adjustable optical elements are currently primarily mechanical systems. These are heavy and bulky, and due to the use of mechanical drives, they are prone to malfunctions and often too slow. Non-mechanical optical elements are therefore advantageous and the subject of current development. While solutions have already been commercialized for such optical elements with smaller usable diameters, this is not yet the case for larger apertures.

[0003] The electro-optic Kerr effect (J. Kerr 1875), also known as the quadratic electro-optic effect or electric birefringence, describes the occurrence of optical birefringence—or a corresponding change in an existing birefringence—in a medium of arbitrary symmetry (e.g., nitrobenzene, carbon disulfide), which increases quadratically with the electric field strength of an applied field. In this effect, molecules with a permanent dipole moment in an isotropic liquid align themselves in the electric field. This alignment renders the material optically anisotropic in the field, resulting in a higher refractive index in the field direction and a lower refractive index perpendicular to it, compared to the isotropic liquid in its stress-free state.In a Kerr cell, the refractive and polarization properties of a material are altered by an externally applied electric field, allowing electrical signals to be converted into optical signals. An electric field is applied across electrode plates in the Kerr fluid, perpendicular to the direction of the transmitted light. Pure nitrobenzene, which has a Kerr constant K of 2.44 x 10⁻⁶, is most commonly used as the Kerr fluid. -12 m / V 2 Nitrotoluene is liquid at room temperature and mixes well with alcohols, ethers, and benzene. For comparison, nitrotoluene has a Kerr constant K of 1.37 x 10⁻⁶. -12 m / V 2 and water has a Kerr constant K of 5.1 x 10 -14 m / V 2With the aforementioned liquid and typical cell sizes in the centimeter range, electrical voltages in the range of a few kilovolts are required. The front and back of the Kerr cell are made of transparent glass, while the metal side walls serve as electrode plates.

[0004] The technical application of the Kerr cell is thus a capacitor with the Kerr liquid – usually nitrobenzene – as its dielectric. It is placed between crossed polarizers whose principal optical axes are each inclined at 45° to the direction of the electric field. If no voltage is applied to the cell, light cannot penetrate this arrangement.

[0005] In the electric field, the Kerr fluid becomes birefringent, meaning that the component of light oscillating in the direction of the field has a different propagation speed than the component oscillating perpendicular to the field. Therefore, a phase shift δ exists between the two components as they exit the Kerr cell. The optical arrangement now produces the amount of light L, which is related to δ and the amount of light L0 that occurs under optimal conditions in the following way: L=L0⋅sin2δ / 2

[0006] The phase shift δ depends on the field strength E, the length l of the light path between the capacitor plates and the Kerr constant of the dielectric B according to the relationship δ=2π⋅B⋅1⋅E2

[0007] This will L=L0⋅sin2(π⋅B⋅1⋅E2)

[0008] To significantly increase the sensitivity of the Kerr cell as a light relay, it is known from DE 555 249 A to impart an additional, unchanging phase shift to the partial beams of the polarized light used, in addition to the voltage-dependent phase shift generated in the Kerr cell. A birefringent crystal plate is inserted into the light path to generate this additional phase shift.

[0009] For liquids—such as nitrobenzene—the observed refractive index difference is very low, even when voltages in the kilovolt range are applied. Due to the small magnitude of the long-known Kerr effect, the fabrication of active optical elements based on it initially appears impossible. For example, the Kerr constant, which quantifies the strength of the Kerr effect, is only about 2.54 x 10⁻⁶ for nitrobenzene (Zinth, W., Optics, Oldenbourg-Verlag, Munich, 2011). -12 mV -2 .

[0010] For many decades, attempts have been made to find liquids with a higher Kerr constant than the previously used nitrobenzene, thereby achieving the same effect at a lower control voltage. For example, DE 622 368 A discloses a Kerr cell liquid in which double or triple substitutes of the benzene ring, which are solid at room temperature, are dissolved in a solvent whose Kerr constant is on the order of that of nitrobenzene. The NO₂ radical is advantageously preferred. Examples of solvents used include: Nitrobenzene ........ (Kerr constant 11 000) Nitrotoluene meta (Kerr constant approximately 5,500) Nitrotoluene ortho (Kerr constant approximately 5900).

[0011] The following substances are proposed for dissolution: Dinitrobenzene ortho, Nitraniline (preferably para), Nitrotoluene para, Chloronitrobenzene ortho, Chlorodinitrobenzene 1:2:3, Dichloronitrobenzene 1:2:3, Nitronaphthalene alpha, Dinitronaphthalene 1:8.

[0012] Currently, elements based on macroscopically oriented liquid-crystalline materials are still in use. However, these systems have the disadvantage that they only affect one polarization direction of the light, leaving light with the perpendicular polarization direction almost unaffected. Because of this, either absorptive polarizers are combined with such elements, but this necessarily leads to a reduction of the light intensity to less than 50%. To overcome this disadvantage, it has been proposed to combine two (or more) identical elements with orthogonal orientations. This results in significantly higher complexity and introduces additional sources of error due to spatial crosstalk, optical losses, and further problems regarding the precise alignment of the elements relative to each other.

[0013] Other isotropic LC states include scattering PDLCs, low-scattering nano-PDLC systems, and isotropic polymer-reinforced isotropic blue LC phases. These are characterized by high switching voltages caused by the polymer matrix and domain boundaries. In contrast to reflective elements, such as LC phase modulators, the residual scattering that occurs in transmissive elements is a problem.

[0014] In isotropic liquids with high anisotropy of the dipole moment and a rod-shaped molecular form, significantly larger refractive index differences can be achieved, resulting in Kerr constants that are orders of magnitude higher. This has already been demonstrated, for example, for isotropic phases of liquid crystals at higher temperatures.

[0015] Significantly higher values ​​can be found in isotropic melts of liquid crystals (J. Chem. Soc, Faraday Trans. 2, 1976, 72, 1447-1458 / DOI: 10.1039 / F29767201447) just above the clarification temperature or in polymer-stabilized isotropic liquid crystals (Appl. Phys. Lett. 98, 023502 (2011) / DOI: 10.1063 / 1.3533396) and polymer-stabilized blue phases of liquid crystals. Here, Kerr constants of up to 300 x 10⁻⁶ were measured. -12 mV -2 measured.

[0016] A major disadvantage of the Kerr effect in such systems, however, is its very strong temperature dependence, as described, for example, for the isotropic melt of the liquid crystal 5CB (Dunmur DA and Tomes A. E., 1981, Mol. Cryst. Liq. Cryst. 76, 231). In polymer-stabilized isotropic liquid crystals, the temperature dependence could be reduced, but only for a very limited temperature range (J. Phys. D: Appl. Phys. 42 (2009) 112002 / DOI: 10.1088 / 0022-3727 / 42 / 11 / 112002). For most of these systems, the required high voltage and long switching times prove to be disadvantageous.

[0017] To create an improved optical waveguide device in the field of optical communication, suitable for applications requiring modulation, attenuation, polarization control, and switching of optical signals, a waveguide device is known from WO 2004 / 046796-A1. This device comprises an optical waveguide core and a plating optically coupled to the core. The plating includes an optically functional zone with a Kerr-effect medium having a refractive index that varies in response to a control signal applied to the optically functional zone. The refractive index of the optically functional zone is lower than that of the optical waveguide core at the optical wavelength and temperature during device operation.The plating comprises an optically functional zone defined by an unpolished, substantially isotropic or substantially anisotropic polymeric plating medium. In particular, the plating medium is a polymeric / chromophoric lining characterized by chromophoric mobility sufficient to enable the optically functional zone to undergo a passive transition from a substantially oriented state to a substantially isotropic state in less than about one second. Specifically, the polymeric / chromophoric lining contains at least about 5 wt% and about 20 wt% of chromophore and is plasticized, and the Kerr effect medium comprises a polymer selected from polycarbonate, terpolymer, PMMA, and polycyclohexane. Preferably, the chromophore includes a donor component, a bridging component with a conjugated and / or aromatic component, and an acceptor component.Specifically, 12 different structural formulas of the Kerr-effect medium are given. Furthermore, a controller is provided that is configured to control or regulate the operating temperature of the waveguide device. The polymeric plating or lining medium is characterized by an effective glass transition temperature that is lower than the operating temperature of the device. It should be noted that the effective glass transition temperature of a material is the temperature at which the reorientation mobility of the chromophore shows a relatively large increase as a function of the material's temperature. The effective glass transition temperature of an electro-optical material can be determined by measuring the electro-optical response of the material as a function of its temperature.The plating medium exhibits an acceptable degree of chromophore mobility and physical stability, either by incorporating a plasticizing agent into the plating medium or by ensuring that the effective glass transition temperature of the plating medium is lower than the operating temperature of the device. Specifically, the plating media of WO 2004 / 046796 A1 are characterized by effective glass transition temperatures below approximately 120°C, down to 20°C. To achieve sufficient chromophore mobility, a solvent is provided in the plating medium. In the case of a polymeric plating medium containing a chromophore and a base polymer, suitable solvents dissolve both the chromophore and the polymer. In many cases, the use of such solvents results in suitable operating temperatures of the devices at or near room temperature.An electric field E is generated in the optically functional zone of the plating using control electrodes. Alternatively, the control signal can induce a thermal signal, with the optically functional zone of the plating responding to the magnitude of the thermal signal. In either case, the waveguide device includes a suitable controller configured to independently modify the optical properties of optically functional sections of the optically functional zone. In particular, applying a control voltage to an electro-optic polymer plating or coating also induces successive phase shifts ΔΦ in the optical signal; however, successive phase shifts of the same value are induced with progressively smaller increases in the control voltage V (E approximately sin ΔΦ). 2 Φ, where Φ = BV 2Therefore, in the case of successive phase shifts ΔΦ of 180°, the magnitude of the successive control voltage increments Vπ required to index successive 180° phase shifts decreases with increasing magnitude of the control voltage V. To operate a Mach-Zehnder interferometer (i.e., a beam splitter with two arms for measuring phase shifts or for modulating light by selective phase modulation in one arm of the interferometer, or for wavelength-dependent demultiplexing) over a 180° phase shift, approximately 340 volts are required for the waveguide device of WO 2004 / 046796 A1. However, the next 180° phase shift at approximately 520 volts is achieved simply by increasing the drive voltage by approximately 180 volts (the difference between 520 volts and 340 volts). A third 180° phase shift occurs at approximately 610 volts, an increase of only about 90 volts.A simple extrapolation suggests that with a bias voltage of approximately 3000 volts, a Vπ drive voltage of approximately 4 volts can be achieved. By improving the polymer plating or coating medium and refining the electrode configuration used as control electrodes, a 180° phase shift can be achieved with drive voltages of less than 5 volts and a bias voltage of approximately 1000 volts.

[0018] Another technical application of the electro-optical Kerr or Pockels effect in the field of optical transmission or communication is the use of gratings with electrically adjustable refractive index and electrically adjustable spatial periodicity as input / output couplers, waveguide coupling elements (interfaces), mode / polarization converters, mode / polarization filters, deflectors, and reflectors. For this purpose, a diffraction grating with an electrically adjustable refractive index and an electrically adjustable spatial frequency is known from EP 1 155 355 B1, wherein the grating comprises: • a substrate; • an electro-optical structure extending over the substrate, wherein the electro-optical structure includes a waveguide with a propagation axis; • a first and a second electrode structure for generating an electric field between them, wherein the electric field induces the diffraction grating in the waveguide, wherein the first and second electrode structures are arranged on opposite sides of the electro-optic structure and each extends in a plane parallel to the propagation axis of the waveguide, wherein the first electrode structure comprises a first and a second set of fingered fingers, wherein the first set of fingers is under a potential V0 and the second set of fingers is under a potential V0 + ¢ V, wherein V0 is variable for adjusting the refractive index of the diffraction grating and is variable for switching the spatial periodicity of the grating between discrete values.

[0019] Alternatively, the diffraction grating can be used to modify externally incident light: • a substrate; • an electro-optical structure that extends over the substrate; • a first and a second electrode structure for generating an electric field between them, wherein the electric field induces the diffraction grating in the electro-optic structure, wherein the first and second electrode structures extend along superimposed planes parallel to each other and to the electro-optic structure and are arranged on opposite sides of the electro-optic structure, wherein the first electrode structure comprises a first and a second set of fingered fingers, wherein the first set of fingers is under a potential V0 and the second set of fingers is under a potential V0 + ¢ V, wherein V0 is variable for adjusting the refractive index of the diffraction grating and is variable for switching the spatial periodicity of the grating between discrete values.

[0020] In specific cases, the diffraction grating of EP 1 155 355 B1 can be configured to act as a Bragg filter, or the grating can be used for collinear reverse coupling for a reflector function, serving as an active optical filter for distributed feedback (DFB) or distributed Bragg reflection (DBR) lasers. Other embodiments relate to applications in wavelength division multiplexing (WDM) systems for fiber optic communication. The grating can be used alone or in combination with other electro-optical components to form integrated structures.The lattice comprises an electro-optical structure, for example, an electro-optical rod made of a material such as LiNbO3 or an electro-optical polymer, preferably with a thickness of approximately 0.5 to 2 µm and a width of about 5 µm, extending over a substrate. First and second electrode structures are positioned on opposite sides of the electro-optical structure, parallel to the direction of propagation. The first and second electrode structures, which are particularly comb-like in shape, are subjected to different potentials to generate an electric field between them and thus the periodicity resulting from the electric field. Modern nanofabrication technologies enable fingered electrode structures with submicron finger spacing.The fingered electrodes are made of a transparent conductive material and indium tin oxide (ITO), for example, with a width a = 105 µm and an ITO thickness of 0.1 µm. The gaps between the waveguide and the electrode fingers are filled with buffer layers of a dielectric material, such as SiO₂, which has a lower refractive index than that of the waveguide. These buffer layers form the cladding of a waveguide within the electro-optic rod and protect the guided wave from lossy interactions with the electrodes.

[0021] Finally, in the technical field of stereo television, the utilization of the electro-optical Kerr effect in conjunction with prismatic lenses of polarizing glasses is known from DE 28 28 910 A1. For this purpose, a polarizing device is arranged and dimensioned such that it extends over the entire surface of the screen of a television set, its polarization plane being rotatable for selective alignment with the polarization plane of the left or right lens of the viewer's polarizing glasses. The rotation of the polarization plane of the polarizing device is preferably achieved by a Kerr effect cell consisting of an optically active material that rotates the polarization plane of the transmitted polarized light depending on a voltage applied to opposite electrodes of the cell by a control voltage source.It is possible to reverse the role of the left or right lens, whereby in one state of the polarizing device the left lens transmits the light passing through the polarizing device, while the right lens acts as a dark filter; and in the other state of the device, the lenses block or transmit the light passing through the polarizing device. Furthermore, at the viewer's discretion, either the left or the right lens can be selected to act as a dark filter, thus making the image appear closer or farther away from the viewer as desired. The control voltage source can supply the aforementioned voltage depending on a command signal, which can be provided along with a television signal (in the case of television signal playback) or depending on a signal recorded on a film (in the case of film playback).The Kerr-effect cell can be made from PLZT (polycrystalline lanthanum-modified lead zirconate tanate) or other known ferroelectric ceramic materials.

[0022] Furthermore, liquid-crystalline compounds with an O-heterocyclic ring, three partially fluorinated benzene rings, and a CF₂O bridge between the rings are known from DE 10 2011 122 559 A1. DE 10 2011 122 559 A1 also relates to liquid-crystalline media produced using these compounds and liquid crystal display devices (LC displays) containing these media. In detail, DE 10 2011 122 559 A1 teaches the application of a polymer-stabilized blue phase in the sense of a Kerr cell. The blue phase is a highly ordered liquid-crystalline phase characterized by a cubic order of nanostructured, doubly twisted cylinders (see also: Yuan Chen, ST Wu, J. Appl. Polym. Sci. 2014, 40556). This state is optically isotropic and has been investigated using various optical methods. In contrast to the composites described, the blue phases are ordered, i.e.They exhibit a well-defined structure that can be detected by X-ray and calorimetric methods. The internal structure of this highly ordered state is discussed, for example, in the review article on PSBP (Yuan Chen, ST Wu, J. Appl. Polym. Sci. 2014, 40556). Accordingly, the subject matter of DE 102011 122 559 A1 is optically isotropic liquid crystalline systems, such as polymer-stabilized blue phases.

[0023] Finally, the Kerr effect in liquid-crystalline media is described in the publication Yang, Young-Cheol; Yang, Denk-Ke: Electrooptic Kerr effect in polymer-stabilized isotoropic liquid crystals. In: Applied Physics Letters, Vol. 98, 2011, No. 2, Art.-No. 023502, 3 pp. - ISSN 0003-6951. According to this publication, the Kerr media are in the ordered state of the blue LC phase (polymer-stabilized blue phase). The layers are optically isotropic due to the existence of stochastically oriented, very small domains of ordered liquid crystals in which the specific order of blue phases is formed. The polymer network in the solution described in the publication is built up in the isotropic phase and is not anisotropic. This network therefore does not support the strain-induced alignment of the low-molecular-weight, rod-shaped components of the composite.On the contrary, in the solutions according to the printed document, it stabilizes the isotropic, disordered state.

[0024] Both approaches according to DE 10 2011 122 559 A1 and the printed publication have in common that the effect of the polymer network fixes a state that would otherwise not exist within the operating temperature range. The range of existence of a desired state is thus extended – therefore, these states are also called “polymer-stabilized”.

[0025] As the preceding review of the prior art demonstrates, various devices utilizing the electro-optical Kerr effect, particularly for light modulation, are known. However, the effect's strong temperature dependence, combined with high operating voltages, is a disadvantage for widespread industrial application.

[0026] The invention is based on the objective of designing or manufacturing an electrically controllable optical element in such a way that, based on the electro-optic Kerr effect, a low threshold and operating voltage, a minimized temperature dependence of the effect and a low response time are achieved.

[0027] This problem is solved with an electrically controllable optical element having the features of claim 1.

[0028] Furthermore, this problem is solved by a method for manufacturing an electrically controllable optical element according to the features of claim 7.

[0029] The present invention utilizes the electro-optical Kerr effect (see also the online encyclopedia Wikipedia, https: / / de.wikipedia.org / wiki / Kerr-Effekt) in isotropic liquids. Therefore, these are not liquid-crystalline states. In particular, the optical element according to the invention enhances the optical Kerr effect through self-organization based on a preformable, open-mesh, loosely anisotropic network. Thus, the use of the composites and the manufacturing technology according to the invention, which leads to specific cell configurations (similar to a mosaic of small Kerr cells within the Kerr cell), can surprisingly overcome the effects of the inventive process. The state of the active composites according to the invention is isotropic in the operating temperature range without an electric field. No domains or microdomains of ordered molecules exist that could lead to scattering.In particular, fixed mesogen clusters are generated, which are also part of a precursor mixture (i.e., as starting materials for a further synthesis step). They enhance the Kerr effect and minimize the temperature dependence of the effect. The entire composite remains in a liquid, isotropic state.

[0030] The optical thin-film elements based on the invention are electrically continuously adjustable phase modulators, lenses, and visual aids, or alternatively, phase modulators that can be switched between defined states. They are characterized by being optically isotropic, i.e., polarization-free and non-scattering. They are realized with electro-optically active composites that can be applied without polarization in the transmission direction and are characterized by a high electrically inducible phase shift and / or refractive index shift. Particularly for the application of visual aids, the operating voltage range of the DC voltage U is between 15 V and 40 V, preferably between 25 V and 30 V.

[0031] In a further development of the invention, according to claim 2, the Kerr cell has two glass or polymer substrates, the inner surface of which is each provided with a conductive ITO electrode as a conductive layer and on this with an orientation layer, and that the pre-formation for a network designed as a polymer network is carried out by means of photoinitiators, aliphatic monomers and reactive mesogens added to the cooled Kerr liquid, alignment of the rod-shaped molecules by applying a voltage U and UV irradiation of the Kerr liquid.

[0032] This further development of the invention has the advantage that, in a surprising way, approximately one half of the molecular framework can be designed to be mesogenic and is characterized by pronounced interactions with the mesogens of the composites, and approximately the other half of the molecular framework is non-mesogenic, whereby the deviation from the rod-shaped structure does not reduce the strength of the dielectric anisotropy.

[0033] In a preferred embodiment of the invention, according to claim 3, the orientation layer leading to a homeotropic orientation contains photocrosslinkable or photopolymerizable groups and the anisotropic polymer network is covalently fixed to the two conductive layers (2), whereby the polymer network retains its anisotropic form even when the Kerr liquid is isotropized by raising the temperature to approximately room temperature.

[0034] This embodiment of the invention has the advantage that, in a surprising way, the effect of the aligning layers enhances a homeotropic orientation (i.e., the molecules align themselves perpendicular to the substrates) and the network is fixed to the two substrate interfaces as a mosaic of small Kerr cells within the Kerr cell.

[0035] Further advantages and details can be found in the following description of a preferred embodiment of the invention with reference to the drawing. The drawing shows: Fig. 1 A preferred embodiment of the electrically controllable optical element, left in the initial state, right after pre-marking and Fig. 2 the optical element after Fig. 1 at approximately room temperature RT, left without application of voltage U and right after application of voltage U.

[0036] The Fig. 1 to Fig. Figure 2 shows a preferred embodiment of the electrically controllable optical element, in particular to illustrate the Kerr effect according to the invention of isotropic composites of a Kerr liquid K based on mixtures of rod-shaped molecules 5 and non-rod-shaped molecules 4 in anisotropic networks 9.

[0037] The composites 4, 5 of the Kerr liquid K according to the invention are characterized by a suitable combination of rod-shaped molecules 5 with a large dipole moment, as well as non-rod-shaped, dipolar molecules 4, which prevent the formation of ordered states such as liquid crystal phases, photocrosslinkable mono-, bi-, and trifunctional molecules, in particular reactive mesogens 8, and photoinitiators 6. Photoinitiators are chemical compounds that decompose in a photolysis reaction after absorption of (UV) light, thus forming reactive species that initiate a reaction; in the context of the Kerr liquid K according to the invention, a polymerization. The reactive species are radicals or cations. Further components, such as molecules with non-covalent interactions and fixed mesogen clusters, significantly improve the desired properties.

[0038] The non-rod-shaped dipolar molecules (semimesogens) 4 suppress the formation of a liquid-crystalline phase at room temperature or in the operating temperature range RT. Their function is to induce a clearing point depression of the liquid-crystalline phases of the rod-shaped molecules 5. Such a balanced clearing point depression can be achieved, for example, by bulky wing groups, lateral substituents, a smaller length-to-width ratio, deviations from the rod-shaped structure, etc.

[0039] On the other hand, the molecular shape of the semimesogens 4, their molecular interactions and their concentration are designed such that they function at low temperatures below the operating temperature range (T <RT) die Ausbildung einer homöotropen Orientierung der Mischung erlauben. Eine Hälfte des Molekülgerüstes ist also mesogen und durch ausgeprägte Wechselwirkungen mit den Mesogenen der Komposite gekennzeichnet und die andere ist nicht-mesogen, wobei die Abweichung von der stäbchenförmigen Struktur nicht die Stärke der dielektrischen Anisotropie herabsetzt. Diese Moleküle sind aber auch durch ein möglichst hohes Dipolmoment gekennzeichnet, wodurch sie zur Stärke des Kerr-Effektes des Komposits beitragen.

[0040] UV irradiation of the homeotropically oriented layers generates radicals that cause polymerization of the oriented reactive mesogens 8. This results in a loosely dispersed, anisotropic network 9. At room temperature or within the operating temperature range RT, the composite of rod-shaped molecules with a large dipole moment and non-rod-shaped, dipolar molecules becomes isotropic again. When a voltage U is applied, the molecules of the composite 4, 5 orient themselves in the direction of the electric field lines (homeotropically, i.e., perpendicular to the substrate surfaces) (see Fig. 1 left).

[0041] In one embodiment, an alignment layer 3, leading to homeotropic orientation, contains photocrosslinkable or photopolymerizable groups. In this case, the anisotropic network 9 is covalently fixed to the two substrate interfaces, whereby the network retains its anisotropic form to a particularly high degree even when the composite 4, 5 is isotropized by increasing the temperature to the working range (approximately room temperature) RT.

[0042] The polymer-based anisotropic matrix or anisotropic network 9 serves to support the electrically induced switching state and reduces the switching voltage. It is generated in the precursor mixture 4, 5 of the active material by a special process. The generation of the assisting network 9 does not cause any macroscopically perceptible phase separation, as is the case with polymer-stabilized blue phases, polymer-stabilized isotropic phases, and isotropic polymer-dispersed phases.

[0043] The wide-mesh, anisotropic polymer network 9 with its orientation function (bulk alignment, orientation layer 3) results in better temperature stability, low switching voltages and an enhancement of the optical Kerr effect through self-organization.

[0044] In extreme cases, when a sufficiently high voltage U is applied and suitable intermolecular interactions occur with the precisely matched composition of the composite 4, 5, and only through this process, a liquid crystalline phase is formed.

[0045] To achieve a lower temperature dependence through stabilization resulting from the fixation of order in the Kerr fluid K, the Kerr effect of isotropic composites can be achieved with nanoscale clusters and nanoparticles of shape-anisotropic molecules with a large dipole moment in the open-mesh anisotropic polymer network 9. For this purpose, nanoscale clusters and nanoparticles of shape-anisotropic molecules 4, which are fixed in their ordered arrangement by covalent bonds or intermolecular interactions, are used as components of the precursor mixture. Such nanoscale anisotropic clusters and nanoparticles extend the aforementioned approach of the Kerr effect of isotropic composites based on mixtures of rod-shaped and non-rod-shaped molecules 4, 5 in anisotropic networks 9.The difference lies in the fact that the rod-shaped molecules with a large dipole moment 5 are fixed in the form of nanoscale clusters or nanoparticles in an ordered arrangement (analogous to the pre-ordering effect of swarms just above the clearing point of the nematic phase). Such clusters and nanoparticles are nanoscopically small (1 nm to 200 nm, preferably 5 to 20 nm). Due to their nanoscopic size, they do not cause scattering of the composite. Rod-shaped molecules with reactive groups (reactive mesogens) 5 can be fixed by photopolymerization in formed droplets, nanoparticles, or nematic pre-order swarms just above the clearing point. Instead of fixation through covalent bonds, this can also occur through non-covalent intermolecular interactions (hydrogen bonds, ionic interactions, and ππ interactions), which also leads to more stable, ordered nanoscopic molecular arrangements. Such molecular clusters, orSpherical and aspherical nanoparticles are dissolved or dispersed in semi-mesogenic solvents. The resulting composites are isotropic, non-scattering liquids.

[0046] Orderly fixed clusters and nanoparticles based on shape-anisotropic molecules, on the one hand, increase the Kerr effect through their high stable orientation order and, in particular, minimize the pronounced temperature dependence of the Kerr effect in solutions or LC materials above the clearing point.

[0047] To achieve a lower temperature dependence by exploiting the different temperature dependencies of intermolecular complex formation and the pre-orientation effect in the Kerr liquid K, the Kerr effect of isotropic composites based on the complex formation of shape-anisotropic molecules with large dipole moments 5 can be achieved through intermolecular interactions in the open-mesh anisotropic polymer network 9. For this purpose, the rod-shaped molecules 5 with large dipole moments are initially formed through intermolecular interactions such as hydrogen bonds, ionic interactions, and ππ interactions (e.g., pyridine / acid, acid / acid, etc.). The different temperature dependencies of intermolecular interactions and the pre-orientation effect are used to stabilize the effect.The ordering tendency of the rod-shaped molecules 5 is increased – in accordance with the above approach of the Kerr effect of isotropic composites based on mixtures of rod-shaped and non-rod-shaped molecules 4, 5 – by non-covalent intermolecular interactions.

[0048] Accordingly, a tension-induced ordering or amplification effect is caused by self-organization, leading to high Kerr constants.

[0049] To achieve a lower temperature dependence due to the differing temperature dependence of intermolecular complex formation and the pre-ordering effect in the Kerr liquid K, the thermal stabilization of the Kerr effect of isotropic composites with non-form-anisotropic molecules formed by intermolecular interactions can be achieved. The non-rod-shaped semimesogens 5 are formed only through intermolecular interactions, such as hydrogen bonds or ionic or ππ interactions. The formation of the semimesogens 5 generates an isotropic phase at room temperature or within the operating temperature range RT. An increase in temperature weakens the non-covalent interactions, and the complexes are (partially) degraded. This reduces their concentration, thus enabling control of the ordering tendency. The thermally induced degradation of the complexes, i.e.,The reduced formation of, for example, hydrogen bonds counteracts the charge blurring caused by complex formation, and the fragments acquire a higher dipole moment. In this way, the composite gains higher dielectric anisotropy with increasing temperature, and the Kerr effect is enhanced.

[0050] By utilizing the active composites 4, 5 according to the invention and applying them in thin layers between structured and / or planar electrodes 2, a variety of applications can be realized: • Refractive and diffractive optical elements, • Electrically continuously adjustable lens or lens switchable between two states, • Electrically continuously adjustable and locally modifiable lens (corrective lens, aspheric lenses), • Locally and continuously adjustable visual aid or visual aid switchable between two states (switchable near vision section), • Electrically controllable diffraction gratings, • Polarization-independent phase modulators.

[0051] The structure of the electrically controllable optical element according to the invention is described in detail below with reference to Fig. 1 and Fig. 2 described. Cell substrates 1:

[0052] The cell substrates can be made of glass or plastic. The substrates can be planar, concave, or convex, or they can also contain microlenses. The substrates 1 are held at a uniform distance of a few micrometers by spacers (pieces of glass or plastic fibers, beads, or lithographically produced polymer structures). Optical adhesive is used to fix the two substrates 1 to each other at their end faces. Electrodes 2:

[0053] The glass or plastic substrates 1 are provided with transparent electrodes 2. These are preferably electrodes made of ITO, metals, or conductive polymers, which are applied to the substrates 1 by sputtering, vapor deposition, printing, etc. The electrodes 2 can be large-area or structured, with the structuring being achieved by printing or by using masks during electrode application. Alternatively, large-area electrodes can be structured within the scope of the invention. Alignment layer 3:

[0054] Thin layers of aligning layers 3 (20 nm to 1 µm) are applied to the ITO electrodes 2, which ensure the homeotropic orientation of the composite at low temperatures or below the operating temperature range (T <RT). Dafür werden dafür im Rahmen der Erfindung Polymere wie Polyimide, Polyvinylalkohol, photosensitive Polymere, Lecithin u.a. eingesetzt.

[0055] Optionally, the aligning layers contain 3 (photo-)crosslinkable groups that allow the covalent connection of the reactive mesogens or networks formed therefrom 9. Composite 4, 5:

[0056] The composite is isotropic in the working range of approximately room temperature (RT). An anisotropic state is achieved at low temperatures through interface orientation using orientation layers. This anisotropic intermediate state is required exclusively for the synthesis of the anisotropic network 9 by photopolymerization of the reactive mesogens 5. In the working temperature range (RT), the isotropic state of the polar, rod-shaped, and non-rod-shaped molecules 4, 5 is then restored.

[0057] The conditions during the manufacture and operation of the electrically controllable optical element according to the invention are: 1. Isotropic initial state of the composite mixture 4, 5 between the substrates 1 at room temperature RT. 2. Homeotropic state at low temperatures prior to photopolymerization (see Fig. 1 left). 3. Homeotropic state with an anisotropic network generated by photopolymerization at low temperatures (see Fig. 1 right). 4. Isotropic state with anisotropic network at room temperature RT by switching off the voltage U (see Fig. 2 links). 5. Orientation of the composite by applying an electrical voltage U. The orientation is supported by the anisotropic network 9 (see Fig. 2 right).

[0058] The invention is not limited to the illustrated and described embodiments, but also includes all embodiments of the electrically controllable optical element according to the invention or of the method for its manufacture that have the same effect in the sense of the invention, i.e. the invention is only limited by the patent claims.

Claims

[1] Electrically controllable optical element with a Kerr cell filled with Kerr fluid, comprising two substrates (1) and a conductive layer (2) applied to the inner surface of the substrate (1), characterized by, that the Kerr liquid (K) has a mixture of rod-shaped (5) and non-rod-shaped (4) molecules as active composites and that the Kerr liquid (K) forms a thin layer with a preformable, wide-meshed, anisotropic network (9) between the structured and / or planar conductive layers (2) applied to the substrates (1), such that, according to the electro-optic Kerr effect, the state of the active composites (4, 5) of the Kerr liquid (K) is isotropic without an electric field in the operating temperature range (RT) and that a change in the voltage-induced phase shift or refractive index shift of the optical element is generated by electrically continuous adjustment of the voltage (U) or by switching the voltage (U) on / off. [2] Electrically controllable optical element according to claim 1, characterized by, that the Kerr cell has two glass or polymer substrates (1) whose inner surface is each provided with a conductive indium tin oxide electrode (2) as a conductive layer and on this with an orientation layer (3) and that the pre-formation for a network designed as a polymer network (9) is carried out by means of photoinitiators (6), aliphatic monomers (7) and reactive mesogens (8) added to the cooled Kerr liquid (K), alignment of the rod-shaped molecules (5) by applying a voltage (U) and UV irradiation of the Kerr liquid (K). [3] Electrically controllable optical element according to claim 2, characterized by, that the orientation layer (3) leading to a homeotropic orientation contains photocrosslinkable or photopolymerizable groups and that the anisotropic polymer network (9) is covalently fixed to the two conductive layers (2), whereby the polymer network (9) retains its anisotropic form even when the Kerr liquid (K) is isotropized by increasing the temperature to the working range of approximately room temperature (RT). [4] Electrically controllable optical element according to claim 1, characterized by, that the rod-shaped molecules (5) with large dipole moment are present as active composites of the Kerr liquid (K) in the form of nanoscale clusters or in the form of nanoparticles fixed in the network (9), wherein the fixation of the rod-shaped molecules (5) in the network (9) is effected by non-covalent intermolecular interactions, and wherein the clusters or nanoparticles are dissolved or dispersed in semimesogenic solvents, thereby achieving on the one hand an increase in the Kerr effect through high stable orientational order of the orderedly fixed clusters or nanoparticles and on the other hand minimizing the temperature dependence of the Kerr effect in solutions or liquid crystal materials above the clearing point. [5] Electrically controllable optical element according to claim 4, characterized by, that the intermolecular interactions of the shape-anisotropic molecules (5) are based on hydrogen bonds, ionic interactions and ππ interactions of pyridine / acid or acid / acid and that the different temperature dependence of the intermolecular interactions and the preorientation effect is used to stabilize the Kerr effect. [6] Electrically controllable optical element according to claim 1, characterized by , that the non-rod-shaped molecules (4) are formed as active composites of the Kerr fluid (K) by intermolecular interactions and that an increase in temperature leads to a weakening of the non-covalent interactions, whereby complexes of molecules (4, 5) are partially degraded. [7] Method for producing an electrically controllable optical element with a Kerr cell filled with Kerr fluid, having two substrates (1) and a conductive layer (2) applied to the inner surface of the substrate (1), characterized by , that a) the Kerr fluid (K) has a mixture of rod-shaped (5) and non-rod-shaped (4) molecules as active composites, reactive mesogens (8), photoinitiators (6) and aliphatic monomers (7), b) the Kerr fluid (K) is filled into the Kerr cell, c) the Kerr liquid (K) is cooled to a lower temperature (T) than room temperature (RT) at which a liquid crystal phase with a homeotropic orientation forms, d) by UV irradiation of the homeotropically oriented molecules, radicals are generated which cause polymerization of the oriented reactive mesogens (8) such that a wide-meshed, loosely anisotropic network (9) of mesogens (10) cross-linked with the aliphatic monomers (7) is formed in the Kerr liquid (K), so that without voltage (U) in the operating temperature range (RT) the composite of rod-shaped molecules (5) with large dipole moment and non-rod-shaped molecules (4) is again isotropic and when the voltage (U) is applied the molecules of the composite (4, 5) orient themselves in the direction of the E-field lines, namely perpendicular to the inner surface of the substrates (1). [8] Method according to claim 7, characterized by , that an orientation layer (3) leading to a homeotropic orientation contains photocrosslinkable or photopolymerizable groups.

Images