Optical structure, Anti-reflective device and method for manufacturing such optical structure

Abstract

The present invention relates to an optical structure comprising a linearly polarizing layer (6), a twisted liquid crystal retarder layer (4) being arranged at a polarizer interface (8). A first orientation direction of liquid crystals just within the twisted liquid crystal retarder layer (4) at the polarizer interface (8) is set by the polarizing direction of the polarizing layer (6). Furthermore, the optical structure comprises an alignment layer (3) defining an alignment direction and an alignment interface (7) between alignment layer (3) and either the polarizing layer (6) or the twisted liquid crystal retarder layer (4). Furthermore, the present invention relates to an anti-reflective device comprising such optical structure and a method for manufacturing such optical structure.

Description

Optical structure, anti-reflective device and method for manufacturing such optical structureTECHNICAL FIELD

[0001] The present invention relates to an optical structure comprising an alignment layer, a linearly polarizing layer, and a retarder layer. In particular, the optical structure the present invention may provide a specific polarizer in the sense of polarizance (ellipticity, spectral dependency), a specific diattenuator and a combination of both, independent from each other. Furthermore, the present invention relates to an anti- reflective device comprising such optical structure. Moreover, the present invention relates to a method for manufacturing such optical structure. Accordingly, an anti- reflective device, a specific polarizer, a specific diattenuator and a combination of both may be manufactured by the method of present invention.BACKGROUND OF THE INVENTION

[0002] Anti-reflective devices are widely used, for example for OLED (organic lightemitting diode) displays. OLED displays are brighter than LCDs. Unfortunately, reflection of ambient light at the metallic anode layer of an OLED display reduces contrast and hence readability. To reduce the light reflection, OLED displays are equipped with a circular polarizer, which converts incident ambient light into circularly polarized light, which upon reflection at the metallic anode layer is then absorbed by the circular polarizer. Typically, the circular polarizer comprises a linear polarizer and a quarter wave plate (QWP) as retarder, wherein the slow axis of the quarter wave plate is at ±45 degrees with respect to the absorption axis of the linear polarizer. Unpolarized ambient light is polarized by the linear polarizer; the linearly polarized light is converted to circularly polarized light upon passing through the QWP. The handedness of the circularly polarized light is switched upon reflection from the metallic layer, so most of the reflected light does not pass through the linear polarizer anymore. Therefore, circular polarizers applied on top of OLED devices are used to address the problem of contrast degradation due to ambient light reflection.

[0003] However, a quarter wave plate converts linearly polarized light into circularly polarized light only for a certain wavelength. For longer or shorter wavelengths, this relationship no longer applies exactly, so that part of the light of different wavelengths is partially transmitted and not blocked.

[0004] The objective is therefore to reduce the wavelength dependence of the quarter wave plate. A quarter wave plate that exhibits no or reduced wavelength dependence is called "achromatic wave plate" or "achromatic retarder".

[0005] In order to reduce the wavelength dependence of the anti-reflective optical structure, several layers may be placed on top of each other. The layers are then aligned by a separate alignment step or by separate alignment steps in such a way that the wavelength effects cancel each other out or compensate for each other in order to reduce the wavelength dependence. In this case, however, a large number of alignment layers are required, each of which has to be aligned in separate alignment steps.

[0006] WO 2016 / 016156 A1 describes an encapsulation structure for an OLED display incorporating antireflection properties.

[0007] US 10 962 696 B2 describes an OLED display panel coated with layers providing anti-reflective properties.

[0008] US 2004 / 0109114 A1 describes a combination of half-wave plate and quarterwave plate to make an achromatic retarder device used for anti-reflective films.[oooo] US 6 717644 B2 describes stacks of LOP layers which have individual optical axis directions. Each of the LOP layers is aligned by an alignment layer, such as a photo-alignment layer. Because of this, the total number of layers in a stack of LCP layers is at least twice the number of LCP layers.

[0010] It is desirable to have methods and structures which allow reducing the number of layers required to stack LCP layers such that the director in one LCP layer does not influence the director in an adjacent LCP layer. The director is defined as a direction parallel to the average direction of the liquid crystal molecules. The solution to such a task would decrease the complexity of the production of stacked LCP layers and at the same time would reduce production costs.

[0011] WO 2018 / 019691 A1 describes a method in which at least one LCP containing layer is formed from a composition comprising polymerizable liquid crystals and one or more photo-orientable substances. Any kind of alignment treatment may be used to orient the polymerizable liquid crystal material in a desired direction and / orconfiguration. Usually, the orientation of the polymerizable liquid crystal in the composition is changed by exposing the alignment layer to linearly polarized light. Contrary to these methods, modifying the already established orientation of the liquid crystal material when exposed to polarized light is avoided. Hence, alignment is generated in the surface of the above layer by exposure to polarized light such that the alignment direction is different from the orientation of the liquid crystal director of the liquid crystals just beneath the upper surface of the layer.

[0012] The structure that is manufactured by this method may comprise an alignment layer above the substrate that orients the liquid crystal materials in a PLCPO layer, that is a layer made of a PLCPO (polymerizable liquid crystals and a photo-orientable) material. The PLCPO layer may act as linearly polarizing layer. The PLCPO layer may then provide alignment for a slave material such as a retarder layer wherein the alignment direction provided by PLCPO layer is different from the liquid crystal orientation in the PLCPO layer just beneath the upper surface. The polarizing direction of the PLCPO layer may therefore differ from an orientation direction of liquid crystals of the retarder layer. Although in this case a separate alignment layer for the retarder is not necessary it is still necessary to photo-align the alignment layer as well as the PLCPO layer.

[0013] Therefore, it is still desirable to provide optical structures that may be manufactured by a reduced number of manufacturing steps and methods for manufacturing such optical structures having a reduced number of manufacturing steps, in particular alignment steps.

[0014] Moreover, US 9298 041 B2 describes so called Multi-Twist Retarders (MTRs) that include an arrangement of at least two general nematic liquid crystal layers on a single substantially uniform alignment surface, where at least one of the layers has a nematic director, i.e. local optical axis, that is twisted over the thickness of the layer, and where subsequent layers are aligned directly by the prior layer's exposed surface.SUMMARY OF THE INVENTION

[0015] An object of the present invention is to provide an optical structure that defines at least two optical alignment directions, one for a retarder and one for a polarizer, and which production is simplified. In particular, a further alignment step besides the alignment of the alignment layer shall be avoided. Another object of the invention is toprovide methods for manufacturing such optical structure and an anti-reflective device comprising such optical structure.

[0016] This object is solved by an optical structure as defined in claim 1, an anti- reflective device as defined in claim 8 and methods as defined in claims 9 and 10.

[0017] The optical structure according to the invention comprises a linearly polarizing layer defining a polarizing direction, a twisted liquid crystal retarder layer or a stack of twisted liquid crystal retarder layers having a first outer layer and a second outer layer being opposite to the first outer layer, the twisted liquid crystal retarder layer or the first outer layer of the stack of twisted liquid crystal retarder layers being arranged at a polarizer interface between the polarizing layer and the twisted liquid crystal retarder layer or the stack of twisted liquid crystal retarder layers, wherein a first orientation direction of liquid crystals just within the twisted liquid crystal retarder layer or the stack of twisted liquid crystal retarder layers at the polarizer interface is set by the polarizing direction of the polarizing layer. Furthermore, the optical structure comprises an alignment layer defining an alignment direction and an alignment interface that is the interface between the alignment layer and a layer adjacent to the alignment layer, said adjacent layer being selected from the polarizing layer, the twisted liquid crystal retarder layer and the second outer layer of the stack of twisted liquid crystal retarder layers. If the alignment interface is an interface between the alignment layer and the polarizing layer, the alignment direction of the alignment layer is setting the polarizing direction of the polarizing layer. If the alignment interface is an interface between the alignment layer and the twisted liquid crystal retarder layer or the second outer layer of the stack of twisted liquid crystal retarder layers, the alignment direction of the alignment layer is setting a second orientation direction of liquid crystals just within the twisted liquid crystal retarder layer or the second outer layer of the stack of twisted liquid crystal retarder layers at the alignment interface.

[0018] The phrase that "the direction of one layer is set by the direction of another layer" or "the direction of one layer is setting the direction of another layer", namely that a first orientation direction of liquid crystals just within the twisted liquid crystal retarder layer or the stack of twisted liquid crystal retarder layers at the polarizer interface is set by the polarizing direction of the polarizing layer, the alignment direction of the alignment layer is setting the polarizing direction of the polarizing layer and that the alignment direction of the alignment layer is setting a second orientation direction of liquid crystals just within the twisted liquid crystal retarder layer or the second outerlayer of the stack of twisted liquid crystal retarder layers, means the direction of the one layer influences or is influenced by the direction of the other layer. In general, there is a particular angle between the direction of the one layer and the direction of the other layer. This angle may for example be 90° so that the direction of the one layer is perpendicular to the direction of the other layer. Furthermore, this angle may for example be 0° so that the direction of the one layer is parallel to the direction of the other layer. In the latter case, the direction of one layer is aligned with the direction of another layer, namely for example the alignment direction of the alignment layer is aligned with the polarizing direction of the polarizing layer and the alignment direction of the alignment layer is aligned with a second orientation direction of liquid crystals just within the twisted liquid crystal retarder layer or the second outer layer of the stack of twisted liquid crystal retarder layers.

[0019] It will be understood that, although the terms first, second, third etc. may be used herein to describe various layers, orientations and / or elements such as a monomer, these layers, orientations and / or elements should not be limited by these terms. These terms are only used to distinguish one layer, orientation and / or element from another layer, orientation and / or element. Thus, a first layer, orientation and / or element discussed herein could be termed a second layer, orientation and / or element without departing from the teachings of the present invention. If an embodiment specifies a second layer, orientation and / or element, a first layer, orientation and / or element may or may not be present.

[0020] In case of linearly polarized light, the electromagnetic fields oscillate in a plane, and this plane defines the polarization plane. A linearly polarizing layer shall refer to a layer that polarizes incident light at least in the visible wavelength range so that the light leaving the layer is linearly polarized. The polarizing direction is the direction in which the light leaving the polarizing layer is linearly polarized, according to the polarization plane.

[0021] The polarizer interface shall refer to the interface between the polarizing layer and another layer forming an interface with the polarizing layer, in particular the twisted liquid crystal retarder layer or the stack of twisted liquid crystal retarder layers. The alignment interface shall refer to the interface between the alignment layer and another layer forming an interface with the alignment layer. In the optical structure of the invention, the other layer is either the polarizing layer or the twisted liquid crystal retarder layer or an outer layer of the stack of twisted liquid crystal retarder layers.

[0022] Alignment of the liquid crystals in the polarizing layer or the twisted liquid crystal retarder layer or the outer layer of the stack of twisted liquid crystal retarder layers can be achieved by the alignment layer by any known means for aligning liquid crystals. For example, the alignment layer that may be formed on a substrate forms an aligning surface, which shall mean that the surface has the capability to align liquid crystals. The alignment layer may already provide the alignment without further treatment. For example, if a plastic substrate is used as a support, it may provide alignment on the surface due to the manufacturing method, for example extrusion or stretching of the substrate. It is also possible to brush the support or imprint a directional microstructure to generate alignment capability. For example, the alignment layer may be a rubbed polyimide.

[0023] Furthermore, a thin alignment layer of a material may be coated on the support which is especially designed regarding alignment performance. The layer may be further brushed or treated to have a directional microstructure on the surface, for example by imprinting. Alternatively, the thin alignment layer may comprise a photo- orientable substance. In this case, alignment can be generated by exposure to polarized light.

[0024] Compared to conventional alignment of liquid crystals by brushed surfaces, the photo-alignment technique has many advantages, such as high reproducibility, alignment patterning and suitability for roll to roll manufacturing. In addition, photoalignment can be applied on curved surfaces since the light which generates the alignment in the photo-alignment layers can follow the surface modulation. In the state of the art, the photo-alignment technique of photo-alignment materials is applied to thin layers on a substrate, such as a glass plate or a plastic foil, so that an alignment layer is formed.

[0025] Preferably, the alignment layer of the optical structure of the present invention is a photo-alignment layer. In particular the initial direction is determined by a linearly polarized ultraviolet (LPLIV) exposed alignment layer. Alternatively, the initial direction is determined by a rubbed polyimide. However, any other means of alignment is possible.

[0026] In the context of the present invention, a “photo-alignment layer” is made of a material in which anisotropic properties can be induced upon exposure to aligning light as, for example described in US 11 181 674 B2 (WO 2018 / 019691 A1). To this end, aphoto-alignment layer comprises a photo-orientable substance. In addition the term “photo-alignment layer” refers to a layer that has been aligned by exposure to aligning light. For the present invention the induced anisotropy must be as such that it provides alignment capability for the adjacent layer comprising e.g. the anisotropic LCP compounds. The term “alignment direction” shall refer to the preferred direction that is induced in the adjacent layer, for example the alignment direction is the direction in which the LCP compounds would be aligned.

[0027] Photo-orientable substances incorporate photo-orientable moieties, which are capable of developing a preferred direction upon exposure to aligning light and thus creating anisotropic properties. Such photo-orientable moieties preferably have anisotropic absorption properties.

[0028] For example, photo-orientable moieties are substituted or un-substituted azo dyes, anthraquinone, coumarin, mericyanine, 2-phenylazothiazole, 2-phenylazobenzthiazole, stilbene, cyanostilbene, fluorostilbene, cinnamonitrile, chaicone, cinnamate, cyanocinnamate, stilbazolium, 1 ,4-bis(2- phenylethylenyljbenzene, 4,4’-bis(arylazo)stilbenes, perylene, 4,8- diamino-1,5- naphthoquinone dyes, aryloxycarboxylic derivatives, arylester, N-arylamide, polyimide, diaryl ketones, having a ketone moiety or ketone derivative in conjugation with two aromatic rings, such as for example substituted benzophenones, benzophenone imines, phenylhydrazones, and semicarbazones.

[0029] Preparation of the anisotropically absorbing materials listed above are well known as shown, e.g. by Hoffman et al., U.S. Patent No. 4,565,424, Jones et al., in U.S. Patent No. 4,401 , 369, Cole, Jr.et al., in U.S. Patent. No. 4,122,027, Etzbach et al., in U.S. Patent No. 4,667,020, and Shannon et al., in U.S. Patent No. 5,389,285.

[0030] Preferably, the photo-orientable moieties comprise arylazo, poly(arylazo), stilbene, cyanostilbene, cinnamate or chaicone.

[0031] A photo-orientable substance may in particular be a monomer, a oligomer or a polymer. The photo-orientable moieties can, for example, be covalently bonded within the main chain or within a side chain of a polymer or oligomer or they may be part of a monomer or other compounds which are not polymerizable. A photo-orientable substance may further be a copolymer comprising different types of photo-orientablemoieties or it may be a copolymer comprising side chains with and without photo- orientable moieties.

[0032] Polymers denotes for example to polyacrylate, polymethacrylate, polyimide, polyurethane, polyamic acids, polymaleinimide, poly-2-chloroacrylate, poly-2- phenylacrylate; unsubstituted or with Ci-Cealkyl substituted poylacrylamide, polymethacyrlamide, poly-2-chloroacrylamide, poly-2-phenylacrylamide, polyether, polyvinylether, polyester, polyvinylester, polystyrene-derivatives, polysiloxane, straightchain or branched alkyl esters of polyacrylic or polymethacrylic acids; polyphenoxyalkylacrylates, polyphenoxyalkylmethacrylates, polyphenylalkylmethacrylates with alkyl residues of 1-20 carbon atoms; polyacrylnitril, polymethacrylnitril, cycloolephinic polymers, polystyrene, poly-4- methylstyrene or mixtures thereof.

[0033] Preferably, the twisted liquid crystal retarder layer or a twisted liquid crystal retarder layer of the stack of twisted liquid crystal retarder layers of the optical structure of the present invention is made of an oriented LCP layer, which has been made from an LCP material comprising chiral dopants that adopt a twisted configuration so that the twisted liquid crystal retarder layer is formed.

[0034] Regarding polymerizable chiral dopants and liquid crystal cholesteric mixtures containing them reference is made to US patent No. 6 120 859, in particular to example 4. Accordingly, in the liquid crystal layer the dopants induce a helix structure. In the homogeneously orientated state of such a layer (Grandjean texture), light is split into its left- and right-circular polarized components within a specific wavelength range (selective reflection range). Where one of the circularly polarized components is fully reflected and the other is transmitted unattenuated, depending on the direction of rotation of the cholesteric helix structure. Light outside the selective reflection range is transmitted uninfluenced. The position of the reflection band in the spectrum is determined by the pitch of the cholesteric helix, the width of the band being correlated with the double refraction (birefringence) of the material. Layers with these optical properties are ideal media for a wide range of applications including colour filters, optical pass band filters and polarizers.

[0035] In particular, the chiral dopant of the twisted liquid crystal retarder layer comprises one or more chiral additives. Chiral additives cause the liquid crystals to twist. In an oriented layer comprising a chiral additive a left or right handed twist deformation may be induced, wherein the twist angle depends on type andconcentration of the chiral additive and on the layer thickness. For example, if a twist of 90° develops in such a layer, the liquid crystals just beneath the upper surface of the layer are oriented at 90° with regard to the liquid crystals at the bottom of the layer.

[0036] For a non-twisted liquid crystal retarder layer, the orientation direction of the liquid crystal molecules does not change along the thickness direction of the layer. However, for a twisted liquid crystal retarder layer, such as a cholesteric liquid crystal retarder layer, the orientation direction varies along the thickness direction. The phrase "just within the layer at an interface" with regard to the position of liquid crystals in a layer shall refer to those liquid crystal molecules which are closest to the interface with another layer. Accordingly, "the orientation direction of the liquid crystals just within the layer" at a certain position shall refer to the average orientation of the liquid crystal molecules, which are closest to the interface at said position. For a layer comprising only liquid crystal molecules, the liquid crystal molecules of the layer that are closest to the interface are directly at the interface.

[0037] In a preferred embodiment of the invention, the polarizing layer is a coatable layer. In particular, the polarizing layer may comprise liquid crystal polymer material and at least a dichroic dye and / or at least a fluorescent dye. Preferably, a coatable polarizer is used for the polarizing layer. The coatable polarizing layer typically comprises anisotropically absorbing molecules, such as carbon nanotubes or dichroic dyes, which are dispersed or dissolved in a host material. For example, polarizers that may be used as polarizing layer are described in US patent No. 10 385215.

[0038] In a preferred embodiment of the invention, the twisted liquid crystal retarder layer or an outer layer of the stack of twisted liquid crystal retarder layers is a coatable layer. It is a formulation containing a solvent mixture and a solid content which features at least one LCP material, a photoinitiator and possibly other additives

[0039] In a preferred embodiment of the invention, the twisted liquid crystal retarder layer or the stack of twisted liquid crystal retarder layers is made of a twisted reversedispersion material. A reverse wavelength dispersion means a smaller retardation value with shorter wavelengths. This type of material typically exhibits a lower dependence of the retardance with the wavelength thus being closer to achromatic. In terms of anti-reflection performance, one such layer has similar result compared with two layers of conventional material.

[0040] In a preferred embodiment of the invention, the optical structure comprises the stack of twisted liquid crystal retarder layers, wherein at least one retarder interface is formed between two adjacent twisted liquid crystal retarder layers of the stack of twisted liquid crystal retarder layers. In general, a retarder interface shall refer to an interface between two different twisted liquid crystal retarder layers.

[0041] To achieve better compensation of the wavelength dependence of the retarder, a retarder is used whose liquid crystal molecules are continuously twisted around the vertical axis ("twisted retarders"). According to an embodiment of the invention two or even more twisted liquid crystal retarder layers forming a stack are used to compensate the wavelength dependencies even better. Such structures may also be called “achromatic multi-twist retarders”.

[0042] In a preferred embodiment of the invention, at the retarder interface between one of the twisted liquid crystal retarder layers and another of the twisted liquid crystal retarder layers an orientation direction of the liquid crystals just within the one of the twisted liquid crystal retarder layers at the retarder interface is aligned with an orientation direction of the liquid crystals just within the other of the twisted liquid crystal retarder layers at the retarder interface. Accordingly, the optical structure of this embodiment includes at least two birefringent layers, i.e. twisted liquid crystal retarder layers, in particular a stack of twisted liquid crystal retarder layers. The structure may include an arrangement of at least two general nematic liquid crystal layers, where one of the layers has a nematic director (i.e., local optical axis) that is twisted over the thickness of the layer, and where the subsequent layer is aligned directly by the prior layer's exposed surface, i.e. at the retarder interface. Therefore, an achromatic multitwist retarder (MTR) may be provided. Advantageously a further alignment step for manufacturing such optical structure is not necessary as at the retarder interface the liquid crystal director of one retarder layer aligns the liquid crystal director of another retarder layer.

[0043] In a preferred embodiment of the invention, a third orientation direction of the liquid crystals just within the one of the two adjacent twisted liquid crystal retarder layers at the retarder interface is aligned with a fourth orientation direction of the liquid crystals just within the other of the two adjacent twisted liquid crystal retarder layers at the retarder interface.

[0044] In a preferred embodiment of the invention, the helical pitch of one twisted liquid crystal retarder layer of the stack of twisted liquid crystal retarder layers is different from the helical pitch of another twisted liquid crystal retarder layer of the stack of the twisted liquid crystal retarder layers. Advantageously, the achromatic properties can be further improved thereby. The term “helical pitch” (helix pitch, spiral pitch) as used herein shall mean the distance for a full 360 degrees turn of the liquid crystal orientation direction.

[0045] In a preferred embodiment of the invention, the twisted liquid crystal retarder layer or at least one twisted liquid crystal retarder layer of the stack of the twisted liquid crystal retarder layers is a high twist retarder. Accordingly, the twisted liquid crystal retarder layer or at least one twisted liquid crystal retarder layer of the stack of the twisted liquid crystal retarder layers does not behave as a waveguide as the twist angle does not fulfill the Maugin condition (waveguide mode limitation):wherein (p is the twist angle, A is the wavelength, An is the birefringence and d is the thickness. If this equation is reversed it can be stated that the pitch of a high twist retarder must not exceed a critical value:

[0046] In particular, the helical pitch of this twisted liquid crystal retarder layer is less than 11 pm, in particular less than 8 pm and preferably less than 5 pm, for example to cover the whole visible range. For anti-reflective coating the angle between the director of the retarder and the director of the polarizer is important. If these directors are parallel, there will be no effect for the anti-reflective coating. At the interface between retarder and polarizer the directors are set by the layers, e. g. the directors are parallel if the retarder is aligned with the polarizer. The director of the twisted retarder then changes in the direction perpendicular to the interface. Therefore, the pitch and thickness of the twisted retarder or the pitches and thicknesses of the stack of twisted retarders or the twist angle between the opposite surfaces of the retarder or retarders of the stack of twisted retarders are of importance. A so-called low twist retarder shows effects similar to a waveguide. According to the invention, at least one high twist retarder is preferably used providing better effects in particular for anti-reflective coatings.

[0047] In a preferred embodiment of the invention, the pitch of the twisted liquid crystal retarder layer or the pitches of the twisted liquid crystal retarder layers of the stack is / are set to provide an optical structure having achromatic anti-reflective properties. In particular, the pitch(es) of the twisted liquid crystal retarder layer(s) may be set to provide an achromatic quarter-wave retarder.

[0048] In a preferred embodiment of the invention, the alignment interface is an interface between the alignment layer and the twisted liquid crystal retarder layer or the second outer layer of the stack of twisted liquid crystal retarder layers. Furthermore, the polarizer interface is arranged between the polarizing layer and the twisted liquid crystal retarder layer or the stack of twisted liquid crystal retarder layers. The optical structure of this further embodiment comprises a further twisted liquid crystal retarder layer or a further stack of twisted liquid crystal retarder layers having a further first outer layer and a further second outer layer being opposite to the further first outer layer, the further twisted liquid crystal retarder layer or the further first outer layer of the stack of twisted liquid crystal retarder layers being arranged at a further polarizer interface between the polarizing layer and the further twisted liquid crystal retarder layer or the further stack of twisted liquid crystal retarder layers. The first orientation direction of liquid crystals just within the further twisted liquid crystal retarder layer or the further stack of twisted liquid crystal retarder layers at the further polarizer interface is set by the polarizing direction of the polarizing layer. Preferably, the optical structure of this embodiment may provide a specific diattenuator.

[0049] According to the invention, the optical structure may comprise a retarder layer or a stack of retarder layers on top of polarizer layer(s) and, upside-down, a polarizer layer or layers on top of retarder layer or a stack of retarder layers. According to the invention, this structure may be extended to a structure made of a polarizer layer or polarizer layers in-between retarder layers or in between stacks of retarder layers to make it not only a specific polarizer but also a specific diattenuator. The manufacturing may start by coating an alignment layer, then a retarder layer or a stack of retarder layers, then a polarizer layer or polarizer layers and then again a further retarder layer or a further stack of retarder layers. Accordingly, not only a circular polarizer may be provided but also any specific polarizer in the sense of polarizance (ellipticity, spectral dependency etc.), any specific diattenuator and a combination of both, independent from each other.

[0050] The present invention relies on the idea that it is possible to align a coatable polarizing layer with an LCP retarder layer, e. g. to align a coatable polarizing layer on top of an LCP retarder layer, in which case the absorption direction will be determined by the slow axis of the said retarder. Alternatively, an upside down arrangement is possible, i. e. to align an LCP retarder layer with a coatable polarizing layer. However, with a simple straight retarder configuration a circular polarization effect could not be reached since the absorption axis of the polarizer and the slow axis of the retarder will coincide. According to the invention at least one twisted liquid crystal retarder layer is used. Several twisted liquid crystal retarder layers in a stack are also possible, such that the liquid crystal director vector on the outer layer of the stack aligns with that on the adjacent layer at the interface. The initial direction of the optical structure is determined by the alignment layer. In general, the optical structure of the present invention provides a specific polarizer, a specific diattenuator, or a combination of both.

[0051] The advantage of this construction of the optical structure of the present invention is to reduce the number of layers as only one alignment layer or surface is necessary in total. Moreover, the number of alignment steps, e. g. UV exposures and / or stretching steps, to provide alignment is reduced. The performance of the device is maintained, if not improved, versus the current standard optical stack structures.

[0052] According to the invention no alignment layer is used at the interface between the linearly polarizing layer and the twisted liquid crystal retarder layer or the outer layer of the stack of twisted liquid crystal retarder layers. Likewise, no alignment layer may be used between different twisted liquid crystal retarded layers of a stack. Omitting such alignment layers that are usually used to have different directors between adjacent layers leads to a continuous transition of the liquid crystal director at the respective interface. The thickness and the pitch of the twisted liquid crystal retarder or the twisted liquid crystal retarders are usually optimized to achieve a certain optical effect; for example, the conversion of linearly polarized light into elliptical or circular polarized light. By omitting a further alignment layer at the polarizer or retarder interface, another constraint is introduced, which has to be taken into account during the optimization. For example, at the polarizer interface, the linearly polarizing layer is setting the director of the twisted liquid crystal retarder layer. Therefore, at this interface, the director of the polarizer or the director of the twisted liquid crystal retarder cannot be freely chosen. According to the invention, optimizing differently with this constraint would achieve essentially the same optical effects compared to an opticalstructure in which separate alignment layers are used to choose the director of the polarizing layer or the retarder layer at the interface. However, the manufacturing process is simplified because it is not necessary to insert a further alignment layer at the polarizer interface or the retarder interface. In addition, no further alignment step, for example exposure to ultraviolet light, is necessary.

[0053] The optical structure of the present invention may, therefore, be adapted to provide a large variety of optical effects for the light passing the linearly polarizing layer and the twisted liquid crystal retarder layer or the stack of twisted liquid crystal retarder layers. For example, an elliptical or circular polarizer or a specific diattenuator may be provided, wherein the generated polarized light may be customized to the particular application for which the optical structure shall be used.

[0054] The anti-reflective device of the present invention comprises the optical structure as described above, wherein the pitch of the twisted liquid crystal retarder layer or the pitches of the twisted liquid crystal retarder layers of the stack are set to form an achromatic quarter-wave retarder.

[0055] As the anti-reflective device comprises the above described optical structure, the anti-reflective device has the same advantages as the above described optical structure. In particular, the method for manufacturing such anti-reflective device is simplified. Furthermore, advantageously, the device may be designed to provide improved achromatic properties by choosing a particular pitch and / or thickness of the twisted liquid crystal retarder layer or the pitches and / or thicknesses of the stack of twisted liquid crystal retarder layers.

[0056] The present invention is also directed to a diattenuator that comprises the optical structure as described above. The optical properties of such diattenuator may be set by the pitch of the twisted liquid crystal retarder layer or the pitches of the twisted liquid crystal retarder layers of the stack.

[0057] The Method for manufacturing an optical structure according to a first aspect of the present invention, comprises the steps of:(a) forming an alignment layer defining an alignment direction on a substrate, (b1) forming a linearly polarizing layer defining a polarizing direction on the alignment layer by setting the polarizing direction of the polarizing layer by the alignment direction of the alignment layer thereby forming an alignment interface,(c1) forming a twisted liquid crystal retarder layer or a stack of twisted liquid crystal retarder layers having a first outer layer and a second outer layer being opposite to the first outer layer, the twisted liquid crystal retarder layer or the first outer layer of the stack of twisted liquid crystal retarder layers being arranged on the polarizing layer thereby forming a polarizer interface, wherein a first orientation direction of liquid crystals just within the twisted liquid crystal retarder layer or the first outer layer of the stack of twisted liquid crystal retarder layers at the polarizer interface is set by the polarizing direction of the polarizing layer.

[0058] This method may be used to manufacture the above-mentioned optical structure. Advantageously, only a single alignment layer needs to be formed. The alignment of the orientation direction of liquid crystals of the twisted liquid crystal retarder layer at the polarizer interface with the polarizing direction of the polarizing layer is carried out without a further alignment layer. Therefore, an alignment step for aligning the liquid crystals of the alignment layer or the first outer layer of the stack of twisted liquid crystal retarder layers may be omitted.

[0059] The Method for manufacturing an optical structure according to a second aspect of the present invention, comprises the steps of:(a) forming an alignment layer defining an alignment direction,(b2) forming a twisted liquid crystal retarder layer or a stack of twisted liquid crystal retarder layers having a first outer layer and a second outer layer being opposite to the first outer layer, the twisted liquid crystal retarder layer or the second outer layer of the stack of twisted liquid crystal retarder layers being arranged on the alignment layer thereby forming an alignment interface, wherein a second orientation direction of liquid crystals just within the twisted liquid crystal retarder layer or the second outer layer of the stack of twisted liquid crystal retarder layers at the alignment interface is set by the alignment direction of the alignment layer,(c2) forming a linearly polarizing layer defining a polarizing direction on the twisted liquid crystal retarder layer or the first outer layer of the stack of twisted liquid crystal retarder layers, thereby forming a polarizer interface, wherein the polarizing direction of the polarizing layer is set by a first orientation direction of liquid crystals just within the twisted liquid crystal retarder layer or the first outer layer of the stack of twisted liquid crystal retarder layers at the polarizer interface.

[0060] This method may also be used to manufacture the above-described optical structure. The method of the second aspect of the present invention differs from themethod according to the first aspect in that the alignment layer sets, in particular aligns, the orientation direction of liquid crystals just within the twisted liquid crystal retarder layer or an outer layer of the stack of twisted liquid crystal retarder layers at the alignment interface and not the alignment of the polarizing direction of the polarizing layer. However, the method of the second aspect of the invention provides the same advantages as the method of the first aspect.

[0061] The alignment layer defining the alignment direction may be formed on a substrate.

[0062] In a preferred embodiment of the invention, in the method according to the first or second aspect of the present invention step (a) further comprises: providing a solution of a photopolymer, coating said photopolymer on a substrate, and exposing said photopolymer to polarized light for aligning said photopolymer.

[0063] In a preferred embodiment of the invention, in the method according to the second aspect of the present invention step (b2) further comprises: coating a second composition comprising a second liquid crystal monomer and a second chiral dopant on said alignment layer, thereby forming the alignment interface, said second composition containing polymerizable liquid crystals; annealing the second composition, whereby the second orientation direction of liquid crystals just within the second composition coating at the alignment interface is set by the alignment direction of the alignment layer; initiate polymerization of the polymerizable liquid crystals in the second composition by exposure to UV light, thereby forming the twisted liquid crystal retarder layer or the second outer layer of the stack of twisted liquid crystal retarder layers at the alignment interface.

[0064] In a preferred embodiment of the invention, the method according to the second aspect of the present invention further comprises: coating a first composition comprising a first liquid crystal monomer and a first chiral dopant on said polymerized second composition, said first composition containing polymerizable liquid crystals; annealing the first composition, whereby a third orientation direction of liquid crystals just within the first composition coating at the interface between the polymerized second composition and the first composition coating is aligned with a fourth orientationdirection of liquid crystals just within the polymerized second composition at the interface between the polymerized second composition and the first composition coating; initiate polymerization of the polymerizable liquid crystals in the first composition by exposure to UV light, thereby forming a further layer of the stack of twisted liquid crystal retarder layers.

[0065] Representative second and / or first compositions comprising a second and / or first liquid crystal monomer and a second and / or first chiral dopant are described in US patent No. 6 120 859, especially in example 4.

[0066] In particular, the further layer of the stack of twisted liquid crystal retarder layers is the first outer layer of the stack of twisted liquid crystal retarder layers.

[0067] In a preferred embodiment of the invention, the method according to the second aspect of the present invention further comprises: coating a polarizer solution on the twisted liquid crystal retarder layer or the stack of twisted liquid crystal retarder layers, thereby forming the polarizer interface, said polarizer solution containing polymerizable liquid crystals; annealing the polarizer solution, whereby the polarizing direction of the polarizer solution is set by the first orientation direction of liquid crystals just within the twisted liquid crystal retarder layer or the first outer layer of the stack of twisted liquid crystal retarder layers at the polarizer interface; initiate polymerization of the polymerizable liquid crystals in the polarizer solution by exposure to UV light, thereby forming the polarizer layer.

[0068] In an embodiment of the invention, the method according to the second aspect of the present invention further comprises: coating a further polarizer solution on the polymerized polarizer solution, said further polarizer solution containing polymerizable liquid crystals; annealing the further polarizer solution, whereby the polarizing direction of the further polarizer solution is aligned with the polarizing direction of the polymerized polarizer solution at the interface between the polymerized polarizer solution and the further polarizer solution; initiate polymerization of the polymerizable liquid crystals in the further polarizer solution by exposure to UV light, thereby forming a stack of polarizer layers having parallel polarizing directions. In particular, the additional layer of the coatable polarizermay be provided if thickness requirements need to be met that might not be reached by a single coating step.

[0069] In a preferred embodiment of the invention, the method according to the second aspect of the present invention further comprises: forming a further twisted liquid crystal retarder layer or a further stack of twisted liquid crystal retarder layers having a further first outer layer and a further second outer layer being opposite to the further first outer layer, the further twisted liquid crystal retarder layer or the further second outer layer of the further stack of twisted liquid crystal retarder layers being arranged on the of the polarizing layer thereby forming a further polarizer interface, wherein the first orientation direction of liquid crystals just within the further twisted liquid crystal retarder layer or the further second outer layer of the further stack of twisted liquid crystal retarder layers at the further polarizer interface is set by the polarizing direction of the polarizing layer. Preferably, the method of this embodiment may provide a specific diattenuator.

[0070] Embodiments of the present invention will now be described with reference to the attached drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 shows a first embodiment of the optical structure of the present invention,Figure 2 shows the second embodiment of an optical structure of the present invention, Figure 3 shows the third embodiment of an optical structure of the present invention, Figure 4 shows the optical structure of the third embodiment in another representation, Figure 5 shows an experimental conoscopy figure of the optical structure of the third embodiment from 30° to 50° incidence with an air gap between the mirror and the device,Figure 6 shows the fourth embodiment of an optical structure of the present invention, Figure 7 shows another representation of the optical structure of the fourth embodiment,Figure 8 shows a graph showing the retardance and the optical axes depending on the wavelength of the fourth embodiment of the optical retarder,Figure 9A shows a reflection conoscopy figure from simulation of the intensity of a known optical structure using a polarizer with a straight reverse-dispersion retarder at 45° angle,Figure 9B shows a reflection conoscopy figure from simulation of the colour AE of a known optical structure using a polarizer with a straight reverse-dispersion retarder at 45° angle,Figure 10A shows a reflection conoscopy figure from simulation of the intensity of an optical structure of the present invention using a combination of two twisted, standarddispersion retarders with the same polarizer aligned with a top director layer,Figure 10B shows a reflection conoscopy figure from simulation of the colour AE of an optical structure of the present invention using combination of two twisted, standarddispersion retarders with the same polarizer aligned with a top director layer,Figure 11A shows a reflection conoscopy figure from simulation of the intensity of an optical structure of the present invention using a single twisted, reverse-dispersion retarder with a polarizer aligned with the top angle of the retarder,Figure 11B shows a reflection conoscopy figure from simulation of the colour AE of an optical structure of the present invention using a single twisted, reverse-dispersion retarder with a polarizer aligned with the top angle of the retarder, andFigure 12 shows the fifth embodiment of an optical structure of the present invention.DETAILED DESCRIPTION OF THE INVENTION

[0072] A first embodiment of the optical structure 1 of the present invention is described with reference to Figure 1 :

[0073] The optical structure 1 comprises a glass substrate 2 that is 1.1 mm thick and known as such.

[0074] The glass substrate 2 is coated with an alignment layer 3 having a thickness of about 100 nm. The alignment layer 3 is exposed to linearly polarized ultraviolet (LPLIV) light and provides alignment direction for subsequent layers.

[0075] The alignment layer 3 defines an alignment interface 7 for a further layer to be aligned. Furthermore, the alignment layer 3 defines an alignment direction for the alignment of liquid crystals of a layer next to the alignment layer 3. The alignment layer 3 is known per se. Instead of a photo-alignment layer, a rubbed polyimide may also be used as alignment layer.

[0076] In the optical structure 1 of the first embodiment, a twisted liquid crystal retarder layer 4 is arranged at the alignment interface 7. Therefore, in the first embodiment ofthe optical structure 1 , the alignment interface 7 is the interface between the alignment layer 3 on the one hand and the twisted liquid crystal retarder layer 4 on the other hand. In this case, the alignment direction of the alignment layer 3 is setting an orientation direction of liquid crystals just within the liquid crystal retarder layer 4 at the alignment interface 7. In case of the present embodiment, the alignment direction of the alignment layer 3 is aligned with the orientation direction of liquid crystals just within the liquid crystal retarder layer 4 at the alignment interface 7 as the alignment direction is parallel to this the orientation direction. This orientation direction is also designated as second orientation direction.

[0077] The twisted liquid crystal retarder layer 4 is a coatable layer and is made of a twisted reverse-dispersion material. The thickness of the twisted liquid crystal retarder layer 4 is 3.50 pm. The helical pitch is 20.0 pm.

[0078] On the opposite interface of the twisted liquid crystal retarder layer 4, a linearly polarizing layer 6 is arranged. Therefore, the interface between the twisted liquid crystal retarder layer 4 and the polarizing layer 6 is called a polarizer interface 8. The orientation direction of liquid crystals just within the twisted liquid crystal retarder layer 4 at the polarizer interface 8 is setting the polarizing direction of the polarizing layer 6. In case of the present embodiment, the orientation direction of liquid crystals just within the twisted liquid crystal retarder layer 4 at the polarizer interface 8 is aligned with the polarizing direction of the polarizing layer 6. This orientation direction of liquid crystals of the twisted liquid crystal retarder layer 4 is also called a first orientation direction.

[0079] The polarizing layer 6 is a coatable layer. The polarizing layer 6 comprises dichroic dyes which are dispersed in a host material.

[0080] The parameters, in particular the thickness and the helical pitch of the twisted liquid crystal retarder layer 4 is set to provide an achromatic quarter wave plate. Together with the linearly polarizing layer 6, the optical structure 1 of the first embodiment provides an anti-reflective device. Incident light that passes the optical structure 1 from the linearly polarizing layer 6 to the substrate 2 that is then reflected on a material having a higher reflective index is absorbed and will essentially not pass the linearly polarizing layer 6 in an opposite direction over a wide wavelength range.

[0081] A second embodiment of the optical structure 1 of the present invention is described with reference to Figure 2:

[0082] In the optical structure 1 of the second embodiment, the substrate 2, the alignment layer 3 and the alignment interface 7 are identical to the corresponding optical structure 1 of the first embodiment. In the optical structure 1 of the second embodiment, the linearly polarizing layer 6 is arranged at the alignment interface 7. Therefore, the alignment interface 7 is the interface between the alignment layer 3 on the one hand and the polarizing layer 6 on the other hand. Therefore, the alignment direction of the alignment layer 3 is setting the polarizing direction of the polarizing layer 6. In case of the present embodiment, the alignment direction of the alignment layer 3 is aligned with the polarizing direction of the polarizing layer 6.

[0083] On the opposite side of the polarizing layer 6, i.e. at the polarizing interface 8, the twisted liquid crystal retarder layer 4 is arranged. Accordingly, a first orientation direction of liquid crystals just within the twisted liquid crystal retarder layer 4 at the polarizer interface 8 is set by the polarizing direction of the polarizing layer 6. In case of the present embodiment, a first orientation direction of liquid crystals just within the twisted liquid crystal retarder layer 4 at the polarizer interface 8 is aligned with the polarizing direction of the polarizing layer 6. The orientation direction of liquid crystals of the twisted liquid crystal retarder layer 4 then rotates in the direction of the thickness of the twisted liquid crystal retarder layer 4. The composition, the helical pitch and the thickness of the twisted liquid crystal retarder layer 4 of the optical structure of the second embodiment are identical to the twisted liquid crystal retarder layer 4 of the optical structure 1 of the first embodiment described above.

[0084] The optical structure 1 of the second embodiment therefore essentially corresponds to the optical structure 1 of the first embodiment. However, in case of the optical structure 1 of the second embodiment, the alignment layer 3 aligns the linearly polarizing layer 6 and not the twisted liquid crystal retarder layer 4.

[0085] A third embodiment of the optical structure 1 of the present invention is described with reference to Figures 3 to 5:

[0086] As in the first and second embodiment the optical structure 1 of the third embodiment comprises a glass substrate 2 that is 1.1 mm thick and known as such, said glass substrate 2 being coated with an alignment layer 3 having a thickness of about 100 nm. The alignment layer 3 corresponds to the alignment layer 3 of the first and second embodiment.

[0087] In the optical structure 1 of the third embodiment, a stack 10 of twisted liquid crystal retarder layers is arranged at the alignment interface 7. The stack 10 comprises a plurality of twisted liquid crystal retarder layers that form at least a first outer layer 10- 1 and a second outer layer 10-2 being opposite to the first outer layer 10-1. In the third embodiment, stack 10 comprises two twisted liquid crystal retarder layers, i. e. the first outer layer 10-1 and the second outer layer 10-2 as shown in Figure 3. In the third embodiment of the optical structure 1 , the alignment interface 7 is the interface between the alignment layer 3 on the one hand and the second outer layer 10-2 of the stack 10 on the other hand. In this case, the alignment direction of the alignment layer 3 is setting a second orientation direction of liquid crystals just within the second outer layer 10-2 of the stack 10 at the alignment interface 7. In case of the present embodiment, the alignment direction of the alignment layer 3 is aligned with a second orientation direction of liquid crystals just within the second outer layer 10-2 of the stack 10 at the alignment interface 7.

[0088] The second outer layer 10-2 of the stack 10 of twisted liquid crystal retarder layers is a coatable layer and is made of a twisted reverse-dispersion material. The thickness of this outer layer 10-2 of the stack 10 is 1.05 pm. The helical pitch is 8 pm. The twist angle in the direction of the thickness di of the second outer layer 10-2 is 78.2°. The twist angle is the difference of the orientation direction <Di of liquid crystals at the alignment interface 7 and the orientation direction <Di of liquid crystals at the retarder interface 9. Therefore, the second outer layer 10-2 of the stack 10 is a high- twist retarder. As it can be seen in Figure 4, the angle E> 1 of the director of the second outer layer 10-2 of the stack 10 changes from the alignment direction that is given by the alignment layer 3 in a twisted manner to another angle E> 1 at the opposite interface provided by the second outer layer 10-2 of the stack 10, that is a retarder interface 9 as described below and as shown in Figure 1 .

[0089] At the retarder interface 9, the first outer layer 10-1 of the stack 10 is arranged next to the second outer layer 10-2 of the stack 10. Therefore, the retarder interface 9 is an interface between two different twisted liquid crystal retarder layers.

[0090] As it can be seen in Figure 4, a fourth orientation direction of the liquid crystals just within the second outer layer 10-2 of the stack 10 at the retarder interface 9 is aligned with a third orientation direction of the liquid crystals just within the first outer layer 10-1 of the stack 10 at the retarder interface 9. Therefore, at the retarder interface9, the second outer layer 10-2 of the stack 10 directly aligns the first outer layer 10-1 of the stack 10 at the retarder interface 9.

[0091] As it can be seen in Figure 4, the third orientation direction of the first outer layer 10-1 of the stack 10 rotates with the thickness of the first outer layer 10-1 of the stack 10 so that depending on the thickness d2 of the first outer layer 10-1 of the stack 10 the orientation direction <t>2at the retarder interface 9 is different from the orientation direction <t>2on the opposite interface of the first outer layer 10-1 of the stack 10.

[0092] The thickness of d2 of the first outer layer 10-1 of the stack 10 is 2.12 pm. The helical pitch of the first outer layer 10-1 of the stack 10 is 38 pm. The twist angle in the direction of the thickness of the first outer layer 10-1 is 20.1 °. The twist angle is the difference of the orientation direction <t>2of liquid crystals at the retarder interface 9 and the orientation direction <t>2of liquid crystals at the polarizer interface 8.

[0093] On the opposite interface of the first outer layer 10-1 of the stack 10, a linearly polarizing layer 6 is arranged. Therefore, the interface between the first outer layer 10- 1 of the stack 10 and the polarizing layer 6 is a polarizer interface 8. The first orientation direction of liquid crystals just within the first outer layer 10-1 of the stack 10 at the polarizer interface 8 is setting the polarizing direction of the polarizing layer 6. In case of the present embodiment, the first orientation direction of liquid crystals just within the first outer layer 10-1 of the stack 10 at the polarizer interface 8 is aligned with the polarizing direction of the polarizing layer 6 as it can be seen in Figure 4.

[0094] The polarizing layer 6 corresponds to the polarizing layer 6 of the first embodiment shown in Figure 1.

[0095] The parameters, in particular the thicknesses di , d2 and the helical pitches of the first and second outer layers 10-1 , 10-2 of the stack 10 are set to provide an achromatic quarter wave plate. Together with the linearly polarizing layer 6, the optical structure 1 of the third embodiment provides an anti-reflective device. Incident light that passes the optical structure 1 from the linearly polarizing layer 6 to the substrate 2 that is then reflected on a material having a higher reflective index is absorbed and will essentially not pass the linearly polarizing layer 6 in an opposite direction over a wide wavelength range.

[0096] Figure 5 shows the results of a conoscopy from 30° to 50° incidence with an air gap between the mirror and the device. The color rendering is close to neutral grey and the anti-reflection effect is confirmed. The air gap, however, reduces this effect.

[0097] With reference to Figures 6 to 8, the fourth embodiment of the optical structure 1 is described:

[0098] In the optical structure 1 of the fourth embodiment, the substrate 2, the alignment layer 3 and the alignment interface 7 are identical to the corresponding optical structure 1 of the third embodiment. In the optical structure 1 of the fourth embodiment, the linearly polarizing layer 6 is arranged at the alignment interface 7. Therefore, the alignment interface 7 is the interface between the alignment layer 3 on the one hand and the polarizing layer 6 on the other hand. Therefore, the alignment direction of the alignment layer 3 is setting the polarizing direction of the polarizing layer 6. In case of the present embodiment, the alignment direction of the alignment layer 3 is aligned with the polarizing direction of the polarizing layer 6.

[0099] On the opposite side of the polarizing layer 6, i.e. at the polarizing interface 8, the stack 10 of twisted liquid crystal retarder layers is arranged. Accordingly, a first orientation direction of liquid crystals just within the first outer layer 10-1 of the stack 10 at the polarizer interface 8 is set by, in particular is aligned with, the polarizing direction of the polarizing layer 6. The orientation direction of liquid crystals of the first outer layer 10-1 of the stack 10 then rotates in the direction of the thickness of the first outer layer 10-1 of the stack 10 as shown in Figure 7. The composition, the helical pitch and the thickness of the first outer layer 10-1 of the stack 10 of the optical structure 1 of the fourth embodiment is identical to the first outer layer 10-1 of the stack 10 of the optical structure 1 of the third embodiment described above.

[0100] Similar to the optical structure 1 of the third embodiment, the optical structure 1 of the fourth embodiment also comprises a second outer layer 10-2 of the stack 10 at the retarder interface 9 between the first and second outer layers 10-1, 10-2 of the stack 10. The composition, the helical pitch and the thickness of the second outer layer 10-2 of the stack 10 correspond to the second outer layer 10-2 of the stack 10 of the optical structure 1 of the third embodiment, as described above.

[0101] The optical structure 1 of the fourth embodiment therefore essentially corresponds to the optical structure 1 of the third embodiment. However, in case of theoptical structure 1 of the fourth embodiment, the alignment layer 3 aligns the linearly polarizing layer 6 and not an outer layer of the stack 10 of twisted liquid crystal retarder layers.

[0102] In all embodiments, only a single alignment layer 3 is necessary to provide the optical structure 1. Furthermore, only a single alignment step for the alignment layer needs to be carried out. Further alignments are directly provided by the layer that is provided at the interface between the layers.

[0103] Figure 8 depicts a graph showing the retardance expressed in waves as well as the optical axis of the optical structure 1 of the fourth embodiment depending on the wavelength. It can be seen that the retardance is essentially independent from the wavelength within the range between 400 nm and 700 nm so that an achromatic retarder is provided by the optical structure 1.

[0104] Figures 9A and 9B depict reflection conoscopy figures from simulation for the intensity (Figure 9A) and for the colour AE (Figure 9B), up to 50° oblique incidence, for a known optical structure. In this structure, the same linearly polarizing layer 6 was used as in the third and fourth embodiment of the optical structure 1. Furthermore, a straight reverse-dispersion retarder having an orientation direction of liquid crystals having a 45° angle relative to the polarizing direction of the polarizing layer had been used.

[0105] For comparison, Figures 10A and 10B show corresponding reflection conoscopy figures from simulation for a combination of two twisted, standard dispersion retarders with the same polarizer aligned with the top director layer that is an alignment layer corresponding to the alignment layer 3 of the optical structure 1 of the third embodiment.

[0106] Furthermore, Figures 11 A and 11 B show corresponding reflection conoscopy figures from simulation with a reduced number of layers. A single twisted reversedispersion retarder with a polarizer aligned with its top angle had been used. Again, the single twisted reverse-dispersion retarder had been aligned by an alignment layer that corresponds to the alignment layer 3 of the optical structure 1 of the first or third embodiment. It can be seen that the performance is very close between the different designs.

[0107] With reference to Figures 12, the fifth embodiment of the optical structure 1 is described:

[0108] In the optical structure 1 of the fifth embodiment, the substrate 2, the alignment layer 3, the alignment interface 7, the twisted liquid crystal retarder layer 4, the polarizer interface 8 and the linearly polarizing layer 6 are identical to the corresponding optical structure 1 of the first embodiment.

[0109] In addition, the optical structure 1 of the fifth embodiment comprises a further twisted liquid crystal retarder layer 11. The further twisted liquid crystal retarder layer 11 is arranged at a further polarizer interface 12 between the polarizing layer 6 and the further twisted liquid crystal retarder layer 11. Again, the first orientation direction of liquid crystals just within the further twisted liquid crystal retarder layer 11 at the further polarizer interface 12 is set by, e. g. aligned with, the polarizing direction of the polarizing layer 6.

[0110] The optical structure 1 of the fifth embodiment provides a specific diattenuator.[oom] In the following, a first embodiment of the method of the present invention is described. The optical structure 1 of the third embodiment shown in Figure 3 may be manufactured by the first embodiment of this method.

[0112] A solution of photopolymer is spin-coated onto a glass substrate 2, then baked at 180°C for 10 minutes on a hotplate. It is subsequently exposed for a total of 330 mJ / cm2to polarized UV-light. An alignment layer 3 is thereby formed on a substrate 2, the alignment layer 3 defining an alignment direction. Forming an alignment layer from a solution of photopolymer is known as such. For example, US 9298 041 B2 describes alignment layers.

[0113] A solution of a composition comprising a liquid crystal monomer, also designated herein as second liquid crystal monomer, and a chiral dopant, such as described in example 4 of US patent No. 6 120 859 or in US 9298 041 B2, is spin- coated onto the sample, followed by an annealing step of 1 minute at 100°C on a hotplate. It is then exposed to UV light for a total of 3000 mJ / cm2under a saturated nitrogen gas atmosphere. The thickness of the layer thus obtained is 1.05 pm. Moreover, the helical pitch of the layer induced by the chiral dopant is 8 pm and the twist angle in the direction of the thickness of the layer is 78.2°. Therefore, a twistedliquid crystal retarder layer, i. e. the second outer layer 10-2 of the stack 10 of twisted liquid crystal retarder layers, is formed on the alignment layer 3 thereby forming an alignment interface 7. Furthermore, an orientation direction of liquid crystals just within the second outer layer 10-2 of the stack 10 of twisted liquid crystal retarder layers at the alignment interface 7 is aligned with the alignment direction of the alignment layer 3.

[0114] Subsequently, a solution of a composition comprising a liquid crystal monomer, also designated herein as first liquid crystal monomer, and a chiral dopant, such as described in example 4 of US patent No. 6 120 859 or in US 9298 041 B2, is applied following this process. The thickness of the layer thus obtained is 2.12 pm. Moreover, the helical pitch of the layer induced by the chiral dopant is 38 pm and the twist angle in the direction of the thickness of the layer is 20.1°.

[0115] By annealing this composition comprising the first liquid crystal monomer, an orientation direction of liquid crystals just within the composition coating comprising the first liquid crystal monomer at the interface between the polymerized composition comprising the second liquid crystal monomer and the composition coating comprising the first liquid crystal monomer is aligned with an orientation direction of liquid crystal molecules just within the polymerized composition comprising the first liquid crystal monomer at the interface between the polymerized composition comprising the second liquid crystal monomer and the composition coating comprising the first liquid crystal monomer. The polymerization of liquid crystals in the composition comprising the first liquid crystal monomer by exposure to UV light forms the first outer layer 10-1 of the stack 10.

[0116] Therefore, the stack 10 forms a multi-twist retarder (MTR) that may be manufactured as described in US 9298 041 B2.

[0117] Thereafter, a coatable polarizer solution is spin-coated onto the sample and annealed at 100 °C for 3 minutes on a hotplate. It is subsequently exposed to UV light for a total of 1500 mJ / cm2under a saturated nitrogen gas atmosphere. The polarizer solution is described in US patent No. 10 385215. Therefore, a linearly polarizing layer 6 defining a polarizing direction on the first outer layer 10-1 of the stack 10 is formed. In addition, a polarizer interface 8 is formed. Furthermore, the polarizing direction of the polarizing layer 6 is aligned with the orientation direction of liquid crystals just within the first outer layer 10-1 of the stack 10 at the polarizer interface 8.

[0118] Optionally, a second layer of coatable polarizer may be added following the same process as the above-mentioned coatable polarizer solution if thickness requirements need to be met. The embodiments of the optical structure 1 as shown in Figures 1 , 2, 4, 6 and 12 may be manufactured by methods similar to the method of the first embodiment.LIST OF REFERENCE SIGNS1 optical structure2 substrate3 alignment layer 4 twisted liquid crystal retarder layer6 linearly polarizing layer7 alignment interface8 polarizer interface9 retarder interface 10 stack of twisted liquid crystal retarder layers10-1 first outer layer10-2 second outer layer11 further twisted liquid crystal retarder layer12 further polarizer interface

Claims

CLAIMS1. Optical structure (1), comprising: a linearly polarizing layer (6) defining a polarizing direction; a twisted liquid crystal retarder layer (4) or a stack (10) of twisted liquid crystal retarder layers having a first outer layer (10-1) and a second outer layer (10-2) being opposite to the first outer layer (10-1), the twisted liquid crystal retarder layer (4) or the first outer layer (10-1) of the stack (10) of twisted liquid crystal retarder layers being arranged at a polarizer interface (8) between the polarizing layer (6) and the twisted liquid crystal retarder layer (4) or the stack (10) of twisted liquid crystal retarder layers, wherein a first orientation direction of liquid crystals just within the twisted liquid crystal retarder layer (4) or the stack (10) of twisted liquid crystal retarder layers at the polarizer interface (8) is set by the polarizing direction of the polarizing layer (6), and an alignment layer (3) defining an alignment direction and an alignment interface (7) that is the interface between the alignment layer (3) and a layer adjacent to the alignment layer (3), said adjacent layer being selected from the polarizing layer (6), the twisted liquid crystal retarder layer (4) and the second outer layer (10-2) of the stack (10) of twisted liquid crystal retarder layers, wherein the alignment direction of the alignment layer (3) is setting the polarizing direction of the polarizing layer (6) if the alignment interface (7) is an interface between the alignment layer (3) and the polarizing layer (6), or wherein the alignment direction of the alignment layer (3) is setting a second orientation direction of liquid crystals just within the twisted liquid crystal retarder layer (4) or the second outer layer (10-2) of the stack (10) of twisted liquid crystal retarder layers at the alignment interface (7) if the alignment interface (7) is an interface between the alignment layer (3) and the twisted liquid crystal retarder layer (4) or the second outer layer (10-2) of the stack (10) of twisted liquid crystal retarder layers.

2. Optical structure (1) of claim 1 , wherein the polarizing layer (6) is a coatable layer.

3. Optical structure (1) of claim 1 or 2, wherein the polarizing layer (6) comprises liquid crystal polymer material and at least a dichroic dye and / or at least a fluorescent dye.

4. Optical structure (1) of any of the preceding claims, comprising the stack (10) of twisted liquid crystal retarder layers, wherein at least one retarder interface (9) is formed between two adjacent twisted liquid crystal retarder layers (10-1 , 10-2) of the stack (10) of twisted liquid crystal retarder layers.

5. Optical structure (1) of any of the preceding claims, wherein the helical pitch of one twisted liquid crystal retarder layer (10-1) of the stack (10) of the twisted liquid crystal retarder layers is different from the helical pitch of another twisted liquid crystal retarder layer (10-2) of the stack (10) of the twisted liquid crystal retarder layers.

6. Optical structure (1) of any of the preceding claims , wherein the twisted liquid crystal retarder layer (4) or at least one twisted liquid crystal retarder layer (10-2) of the stack (10) of the twisted liquid crystal retarder layers is a high twist retarder.

7. Optical structure (1) of any of the preceding claims , wherein the alignment interface (7) is an interface between the alignment layer (3) and the twisted liquid crystal retarder layer (4) or the second outer layer (10-2) of the stack (10) of twisted liquid crystal retarder layers, the polarizer interface (8) is arranged between the polarizing layer (6) and the twisted liquid crystal retarder layer (4) or the stack (10) of twisted liquid crystal retarder layers, and the optical structure (1) comprises a further twisted liquid crystal retarder layer (11) or a further stack of twisted liquid crystal retarder layers having a further first outer layer and a further second outer layer being opposite to the further first outer layer, the further twisted liquid crystal retarder layer (11) or the further first outer layer of the stack of twisted liquid crystal retarder layers being arranged at a further polarizer interface (12) between the polarizing layer (6) and the further twisted liquid crystal retarder layer (11) or the further stack of twisted liquid crystal retarder layers, wherein the first orientation direction of liquid crystals just within the further twisted liquid crystal retarder layer (11) or the further stack of twisted liquid crystal retarder layers at the further polarizer interface (12) is set by the polarizing direction of the polarizing layer (6).

8. Anti-reflective device comprising the optical structure (1) of any of the preceding claims, wherein the pitch of the twisted liquid crystal retarder layer (4) or the pitches of the twisted liquid crystal retarder layers (10-1, 10-2) of the stack (10) are set to form an achromatic quarter-wave retarder.

9. Method for manufacturing an optical structure (1), comprising the steps of:(a) forming an alignment layer (3) defining an alignment direction,(b1) forming a linearly polarizing layer (6) defining a polarizing direction on the alignment layer (3) by setting the polarizing direction of the polarizing layer (6) by the alignment direction of the alignment layer (3) thereby forming an alignment interface (7),(c1) forming a twisted liquid crystal retarder layer (4) or a stack (10) of twisted liquid crystal retarder layers having a first outer layer (10-1) and a second outer layer (10-2) being opposite to the first outer layer (10-1), the twisted liquid crystal retarder layer (4) or the first outer layer (10-1) of the stack (10) of twisted liquid crystal retarder layers being arranged on the polarizing layer (6) thereby forming a polarizer interface (8), wherein a first orientation direction of liquid crystals just within the twisted liquid crystal retarder layer (4) or the first outer layer (10-1) of the stack (10) of twisted liquid crystal retarder layers at the polarizer interface (8) is set by the polarizing direction of the polarizing layer (6).

10. Method for manufacturing an optical structure (1) having a layer structure, comprising the steps of:(a) forming an alignment layer (3) defining an alignment direction,(b2) forming a twisted liquid crystal retarder layer (4) or a stack (10) of twisted liquid crystal retarder layers having a first outer layer (10-1) and a second outer layer (10-2) being opposite to the first outer layer (10-1), the twisted liquid crystal retarder layer (4) or the second outer layer (10-2) of the stack (10) of twisted liquid crystal retarder layers being arranged on the alignment layer (3) thereby forming an alignment interface (7), wherein a second orientation direction of liquid crystals just within the twisted liquid crystal retarder layer (4) or the second outer layer (10-2) of the stack (10) of twisted liquid crystal retarder layers at the alignment interface (7) is set by the alignment direction of the alignment layer (3),(c2) forming a linearly polarizing layer (6) defining a polarizing direction on the twisted liquid crystal retarder layer (4) or the first outer layer (10-1) of thestack (10) of twisted liquid crystal retarder layers, thereby forming a polarizer interface (8), wherein the polarizing direction of the polarizing layer (6) is set by a first orientation direction of liquid crystals just within the twisted liquid crystal retarder layer (4) or the first outer layer (10-1) of the stack (10) of twisted liquid crystal retarder layers at the polarizer interface (8).

11. Method of claim 10, wherein step (b2) comprises: coating a second composition comprising a second liquid crystal monomer and a second chiral dopant on said alignment layer (3), thereby forming the alignment interface (7), said second composition containing polymerizable liquid crystals; annealing the second composition, whereby the second orientation direction of liquid crystals just within the second composition coating at the alignment interface (7) is set by the alignment direction of the alignment layer (3); initiate polymerization of the polymerizable liquid crystals in the second composition by exposure to UV light, thereby forming the twisted liquid crystal retarder layer (4) or the second outer layer (10-2) of the stack (10) of twisted liquid crystal retarder layers at the alignment interface (7).

12. Method of claim 11 , further comprising: coating a first composition comprising a first liquid crystal monomer and a first chiral dopant on said polymerized second composition, said first composition containing polymerizable liquid crystals, annealing the first composition, whereby a third orientation direction of liquid crystals just within the first composition coating at the interface between the polymerized second composition and the first composition coating is aligned with a fourth orientation direction of liquid crystals just within the polymerized second composition at the interface between the polymerized second composition and the first composition coating; initiate polymerization of the polymerizable liquid crystals in the first composition by exposure to UV light, thereby forming a further layer of the stack (10) of twisted liquid crystal retarder layers.

13. Method of any one of claims 10 to 12, further comprising: coating a polarizer solution on the twisted liquid crystal retarder layer (4) or the stack (10) of twisted liquid crystal retarder layers, thereby forming thepolarizer interface (8), said polarizer solution containing polymerizable liquid crystals; annealing the polarizer solution, whereby the polarizing direction of the polarizer solution is set by the first orientation direction of liquid crystals just within the twisted liquid crystal retarder layer (4) or the first outer layer (10-1) of the stack (10) of twisted liquid crystal retarder layers at the polarizer interface (8); initiate polymerization of the polymerizable liquid crystals in the polarizer solution by exposure to UV light, thereby forming the polarizer layer (6).

14. Method of claim 13, further comprising: coating a further polarizer solution on the polymerized polarizer solution, said further polarizer solution containing polymerizable liquid crystals; annealing the further polarizer solution, whereby the polarizing direction of the further polarizer solution is aligned with the polarizing direction of the polymerized polarizer solution at the interface between the polymerized polarizer solution and the further polarizer solution; initiate polymerization of the polymerizable liquid crystals in the further polarizer solution by exposure to UV light, thereby forming a stack of parallel polarizer layers.

15. Method of any one of claims 10 to 14, further comprising: forming a further twisted liquid crystal retarder layer (11) or a further stack of twisted liquid crystal retarder layers having a further first outer layer and a further second outer layer being opposite to the further first outer layer, the further twisted liquid crystal retarder layer or the further second outer layer of the further stack of twisted liquid crystal retarder layers being arranged on the polarizing layer (6) thereby forming a further polarizer interface (12), wherein the first orientation direction of liquid crystals just within the further twisted liquid crystal retarder layer (11) or the further second outer layer of the further stack of twisted liquid crystal retarder layers at the further polarizer interface (12) is set by the polarizing direction of the polarizing layer (6).

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