Polarization gratings in mesogenic films   

Abstract: A polarizer including a polarization grating comprising a polarization sensitive photo-alignment layer and a liquid crystal composition arranged on said photo-alignment layer. An alignment pattern, corresponding to the polarization pattern of a hologram, is recorded in the photo-alignment layer, and the liquid crystal composition is aligned on the photo-alignment layer.

TECHNICAL FIELD

The present invention relates to novel polarization gratings, as well as areas of application for such novel polarization gratings.

Conventional diffraction gratings operate by periodically modulating the phase or amplitude of light propagating through them, potentially splitting incident light into multiple diffraction orders.

Polarization gratings, which periodically modulate the polarization state of light traveling through them, have been known since the 1970s, when initial publications about the more general case of polarization holograms appeared in Soviet journals.

It was soon recognized that the most compelling advantage of polarization gratings over conventional diffraction gratings was the possibility to control the polarization state of the diffracted orders while at the same time making the efficiency in each order dependent on the polarization of the incident light. Initial success at reducing the theory of polarization gratings to practice came in photochromic silver-chloride (AgCl) glass using holography. In this approach, two nearly orthogonally polarized coherent laser beams were superimposed with nearly parallel propagation, creating a standing optical wave with a periodic modulation of the polarization state while maintaining a constant intensity. Since linearly polarized light induced considerable optical anisotropy (linear birefringence) in the materials through the Weigert-effect, the periodic patterns, where polarization is changing from linear to circular and back, were captured as a polarization grating.

This holographic method eventually made a substantial advance when organic materials containing azobenzene moieties were shown to be able to record these polarization holograms as a relatively strong birefringence. In these materials, the azobenzene groups undergo a reversible trans=>cis=>trans isomerization process and an associated orientational redistribution of the chromophores. Research has shown that a variety of azo-containing polymers and also dispersions may also be used.

In many of these polymers, a surface, relief grating is also formed during irradiation. Although the primary reason of the surface generation process is not well understood, several theories have tried to explain the existing phenomenology, and it is agreed that the surface relief appears a result of a mass diffusion mechanism. While it can be useful, this surface relief structure diffracts as a phase grating, and does not lead to a modulation of the polarization state of light propagating through it. In fact, this surface relief grating often degrades the unique diffraction properties of polarization gratings since the properties of both are superimposed. Azo-containing materials are colored in the visible so the range of wavelength applicability is limited. In addition, the long-term stability is usually limited, especially when the grating is exposed to light in the absorption band of the material or subject to high temperature thermal treatments common in applications such as LCD manufacturing.

Other materials have also been studied as alternative polarization hologram materials, including bacteriorhodopsin, holographic polymer dispersed liquid crystals, and a porous glass system imbibed with an azobenzene liquid crystal molecule. Lithographic processing of sub-wavelength metal-stripe structures has also been shown to successfully form a polarization grating by inducing a spatially periodic anisotropic absorption. In this approach, a conductive layer on a substrate is patterned into parallel lines with a sub-wavelength pitch (creating a linear polarizer), where the orientation of these lines determines the transmission/absorption axis of the polarizer. This orientation is varied periodically by the lithography at a pitch greater than the wavelength, forming the polarization grating.

This type of grating operates at infrared wavelengths, but the principle also valid at visible wavelengths (but the fabrication is more difficult since the dimensions are substantially smaller). While good optical quality can be achieved, it is an absorbing optical element (typically 50% of incident light is absorbed) and the fabrication process requires substantial photolithographic processing such as is used for semiconductor wafers (clean room environment, expensive shadow masks, photoresist development, wet chemical etching of inorganic conductive layers, etc.).

One recent method for the production of polarization gratings based on liquid crystals is described by Eakins et al, “Zero voltage Freedericksz transition in periodically aligned liquid crystals”, Applied Physics Letters 85, no 10, pp 1671-1673, 2004, using a holographic exposure to photo-polymerize a polarization sensitive alignment layer, and aligning a liquid crystal composition there on.

However, there still remains a need for new high quality polarization gratings that are easy to produce, temperature stable and useful in practical applications.

SUMMARY

One object of the present invention is to overcome the above-mentioned problems of the prior art and to provide polarization gratings which are easy to produce and which exhibits high diffraction efficiency, transparency in visible/IR wavelengths, moderate to large useable areas, stability when exposed to moderate temperatures and visible light, and flexible design features.

The inventors have surprisingly found that a polarization grating fulfilling this object could be produced by using a polarization hologram, recording its polarization pattern in a photo-alignment film, and aligning a liquid crystal composition on the photo-alignment film.

Thus, in a first aspect, the present invention provides a polarization grating comprising a polarization sensitive photo-alignment layer, for example arranged on a substrate, and an integral liquid crystal composition arranged on said photo-alignment layer. The pattern corresponding to the polarization pattern of a hologram is arranged (recorded) in the photo-alignment layer, such as in form of an anisotropic pattern of chemical bonds. A liquid crystal composition is arranged on the alignment layer, with the result that the local mesogens director of the liquid crystal composition adjacent to the alignment layer follows the anisotropic pattern, i.e. the directors of the mesogens, and thus the local optical axis, will follow the polarization pattern of the hologram. Due to the nature of liquid crystal materials, the orientation of the mesogens adjacent to the surface of the alignment layer will propagate through the thickness of the liquid crystal composition to produce a transparent film with patterned high value anisotropy and birefringence.

As the origin for the patterned birefringence is a polarization hologram recorded in a photo-alignment layer, an essentially defect-free pattern can be obtained with this approach. The patterned orientation of the birefringence results in a very strong diffraction as a polarizing grating.

In embodiments of the present invention, the alignment direction of the anisotropic alignment pattern is periodic along at least one line in the plane of the alignment layer.

In embodiments of the present invention, the alignment direction exhibits a periodical variation which over one period corresponds to the polarization direction variation along a circle on the Poincaré sphere.

As the anisotropic pattern corresponds to a hologram with constant intensity and a periodically changing polarization profile, this pattern is most conveniently mapped onto the Poincaré sphere. One of the surprising discoveries during the pursuit of this invention is that an alignment pattern corresponding to any circle on the Poincaré sphere may be created by holographic methods, allowing polarization gratings to be created which diffracts light into any desired set of orthogonal polarizations, including linear, circular and/or elliptical polarizations.

In embodiments of the present invention, the liquid crystal composition may comprise a polymerizable compound, such as for example polymerizable mesogens or polymerizable non-mesogenic compounds.

In embodiments of the present invention, a polymerizable liquid crystal composition may be at least partly polymerized to form a solid film from the liquid crystal composition. In embodiments of the present invention, a polarization grating may comprise a liquid crystal composition as described above, sandwiched between and aligned by two alignment layers. In other embodiments, the polarization grating may comprise of a first liquid crystal composition arranged on an alignment layer and a second liquid crystal composition arranged on aligned by the first liquid crystal composition. In addition, a third liquid crystal composition may be arranged on the second liquid crystal composition, and so on.

In embodiments of the present invention, the liquid crystal composition may comprise additional functional compounds, such as different type of dyes and particles having anisotropic shape and/or spectral properties. Such compounds may be oriented in the composition by the mesogens, and thus confer additional, for example optical, properties to the polarization grating.

In embodiments of the present invention, a polarization grating may comprise means for establishing an electric and/or magnetic field in said liquid crystal composition, such as for example electrodes. As the orientation of mesogens may be affected by such a field, the optical properties may also be affected, providing a switchable grating.

The present invention also relates to broadband polarization gratings having a broader wavelength interval with high efficiency transmission. Such broadband polarization gratings may be an attractive alternative to conventional polarization gratings.

The present invention also relates to display devices comprising polarization gratings of the present invention as optical switches. Switches of this type may operate on unpolarized light, obviating the use of polarizers and thus, enabling the potential for a very high overall optical efficiency.

The present invention also relates to beam-splitters based on polarization gratings, where a high diffraction angle may be obtained without the need for a small grating pitch.

The present invention also relates to new polarizers comprising polarization gratings of the present invention. Such polarizers have the ability to convert unpolarized light into polarized light at efficiencies markedly higher than 50%.

The present invention also relates to security arrangements comprising polarization gratings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described further by the following preferred embodiments with reference to the accompanying drawings, in which:

FIG. 1 illustrates two different setups of orthogonally polarized superimposed beams, and the resulting polarization pattern. FIG. 1a: A left and a right hand circularly polarized beam. FIG. 1b: A vertical and a horizontal linearly polarized beam.

FIG. 2 illustrates an anisotropic polarization pattern that is spiraling repeating in two dimensions.

FIG. 3a illustrates schematically an embodiment of a polarization grating according to the present invention. FIG. 3b is a photograph of a polarization grating as shown in FIG. 3a between crossed polarizers.

FIG. 4 illustrates schematically another embodiment of a polarization grating according to the present invention.

FIG. 5a illustrates schematically another embodiment of a polarization grating according to the present invention. FIG. 5b is a photograph of a polarization grating as shown in FIG. 5a between crossed polarizers.

FIG. 6 illustrates the transmission curve of a standard polarizing grating and a broadband polarizing grating.

FIG. 7a illustrates a first embodiment of a broadband polarizing grating.

FIG. 7b illustrates a second embodiment of a broadband polarizing grating.

FIG. 7c illustrates the anisotropic director pattern of a broadband polarizing grating.

FIG. 8 illustrates the transmission curve for a polarization grating vs. applied voltage.

FIG. 9 illustrates transmission curves of a polarization grating optimized for red (FIG. 9a), green (FIG. 9b) and blue (FIG. 9c) light, respectively.

FIG. 10 illustrates a display device comprising a polarization grating of the invention.

FIG. 11 illustrates an embodiment of a polarizer comprising a polarization grating.

FIG. 12 illustrates another embodiment of a polarizer comprising a polarization grating.

FIG. 13 illustrates a beam-splitter comprising two polarization gratings.

FIG. 14 illustrates an embodiment of a security device comprising a polarization grating.

FIG. 15 illustrates another embodiment of a security device comprising a polarization grating.

FIG. 16 shows the first order diffraction efficiency vs. voltage for a polarization grating.

DETAILED DESCRIPTION

One embodiment of a polarization grating of the present invention is shown in FIG. 3a. The polarization grating of this embodiment comprises a substrate 1 on which a polarization sensitive photo-alignment layer 2 is arranged.

In the photo-alignment layer, an anisotropic pattern corresponding to a polarization hologram is recorded as an anisotropic pattern of chemical bonds within the polymer constituting the photo-alignment layer (see for example FIGS. 1a and 1b).

On the photo-alignment layer 2, a liquid crystal composition 3 is arranged. The mesogens located adjacent to the surface of the photo-alignment layer will orient themselves along the anisotropic pattern in the photo-alignment layer.

Thus, the directors of the liquid crystal mesogens located adjacent to the surface of the photo-alignment layer 2 will be oriented in the direction of the polarization in each position of the polarization hologram.

Due to the nature of liquid crystal composition 3, the patterned director arrangement will propagate through the composition to form a transparent film with a patterned anisotropy and birefringence, giving as a result the optical properties of a polarization grating.

Herein the terms “mesogen” and “liquid crystal” are used to indicate materials or compounds comprising one or more mesogenic groups, such as (semi)rigid rod-shaped, banana-shaped, board shaped or disk-shaped mesogenic groups, i.e. groups with the ability to induce liquid crystal phase behavior. Liquid crystal compounds with rod-shaped or board-shaped groups are also known in the art as “calamitic” liquid crystals. Liquid crystal compounds with a disk-shaped group are also known in the art as “discotic” liquid crystals. Compounds or materials comprising mesogenic groups do not necessarily have to exhibit a liquid crystal phase themselves. It is also possible that they show liquid crystal phase behavior only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerized.

As used herein, the term “liquid crystal composition” refers to a composition which comprises mesogens and which exhibits liquid crystal phase behavior.

The substrate 1 on which the photo-alignment layer 2 is arranged may be any rigid or flexible substrate. Examples of suitable substrates include, but are not limited to, glass, transparent ceramic, fused silica, transparent polymers which may be thermosetting or thermoplastic and (semi)-crystalline or amorphous, such as PC (polycarbonate), PMMA (polymethylmethacrylate), PET (polyethylene terephthalate), PVC (polyvinylchloride), PS (polystyrene), (polycarbonate), COC (cyclic olefin copolymers), PET (polyethylene terephthalate), PES (polyether sulphone) and also crosslinked acrylates, epoxies, urethane and silicone rubbers.

Other suitable substrate materials include reflective substrates, such as metallic substrates, e.g. silver and aluminum, etc., and semiconducting substrates, e.g. silicon, etc. Additional substrate materials will be apparent to those skilled in the art.

The alignment layer 2 may be arranged on the substrate 1 by known methods, such as spin coating, doctor blade coating, casting, etc, to form a thin film on the substrate. Polarization sensitive photo-alignment layer materials are well known in the art and include, for example, linearly photo-polymerizable polymers, such as those commercially available from Rolic, Vantico and Huntsman, and similar materials available from JSR and LG Cable.

Alternatively, azo-containing materials, as described above, could also be used as an alignment layer, as these could both record the polarization pattern and align mesogens to the polarization pattern.

To obtain the anisotropic alignment pattern in the photo-alignment layer, two or more coherent and orthogonal polarized laser beams (linear, elliptical, or circular polarizations) may be superimposed on the alignment layer, leading to a spatially periodic variation in the polarization of the light.

The basic theory for this is known, for example, from Nikolova et al, Optica Acta 31, 579 (1984).

The case of two orthogonally circular polarized superimposed beams yields a one-dimensionally spatially extending periodic “rotating” pattern in linear polarization, where the polarization pattern in each period is represented by a great-circle (where S3=0) around the equator of the Poincaré sphere (see FIG. 1a).

The case of two orthogonally linear polarized superimposed beams yields a one-dimensional periodic spatially pattern composed of a variety of polarizations, including linear, circular, and elliptical, where the polarization pattern in each period is represented by a great-circle around the Poincaré sphere (where S1=0) which travels through the poles and the points representing the polarization of the two superimposed beams (see FIG. 1b).

The general case of two orthogonally elliptical polarized superimposed beams yields a one-dimensional spatially periodic pattern composed of a variety of polarizations, which may generally be chosen in such a way to correspond to any circle on the Poincaré sphere. When the two beams have the same intensity, then the hologram maps to a “great-circle” on the sphere. When the two beams have unequal intensities, the hologram maps to any circle on the sphere.

When a liquid crystal composition is arranged on the alignment layer, the mesogens in the composition will align to the alignment pattern, forming a birefringent grating where the optical axis close to the alignment layer, i.e. the director of the mesogens adjacent to the alignment layer, follows the alignment pattern. The director pattern propagates through the layer in a manner that is specific for the liquid crystal composition used. For example, in a cholesteric liquid crystal composition, the director pattern will twist with the distance from the alignment layer and at any distance from the alignment layer, the director pattern is still present, however twisted at an angle defined by the pitch of the liquid crystal composition.

In general, the diffractive properties of such gratings are such that the diffracted orders are polarized with the same polarizations as the orthogonal beams that formed the hologram.

The period (Λ) of this polarization pattern is determined by the wavelength of the laser beams (λ) and the angle (φ) separating the coherent superimposed beams according to the following formula:

Λ=λ/(2*sin(φ/2))   (I)

Two-dimensional (2D) polarization patterns, i.e. where the polarization pattern exhibits a periodic spatially rotating pattern both in both dimensions of the plane, may be formed in several different ways, including a “one-step”-method and a “sequential” method. In either case, this type of grating is interesting and useful since the diffracted orders are polarized according to the beams that formed the hologram (just as in the above discussed one-dimensional case).

Sequential formation will be discussed first since it is the most intuitive and known in prior art, followed by an explanation of the “one-step” formation that is not in prior art to our knowledge.

In a sequential method, a one-dimensional (1D) grating is formed in as described above (by interfering two orthogonal coherent beams), after which the substrate is rotated by a chosen angle (e.g. 90°). Subsequently, a second exposure is performed (with the same or different two orthogonal coherent beams) which results in a second 1D grating being superimposed on the first. This may be repeated as many times as desired (H. Ono et al., Opt. Exp. 11, 2379-2384 (2003)).

One advantage of this method is that the same setup which created the 1D polarization grating can be used to create a 2D polarization grating. However, many interesting 2D polarization patterns cannot be formed in this way, and in many instances, the subsequent recordings degrade the quality of the previous recordings. Also, in some cases (R. C. Gauthier and A. Ivanov, Opt. Expr. 12, 990-1003 (2004)), the rotation stage requires exceedingly stringent positioning/rotation requirements.

An alternative approach to provide 2D polarization gratings is through a single step formation. This is done by superimposing three or more coherent laser beams with propagation and polarization parameters chosen such that a constant or almost constant intensity pattern results. For example, the interference pattern from three superimposed coherent beams can be described by the following formula:

I  ( x , y ) = 1 + V   12 * cos  ( G 12  X * x + G 12  Y * y ) + V   23 * cos  ( G 23  X * x + G 23  Y * y ) + V   31 * cos  ( G 31  X * x + G 31  Y * y )

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