WO2005034099A2

WO2005034099A2 - Information medium comprising two layers, method of manufacturing such an information medium, device for reading such an information medium

Info

Publication number: WO2005034099A2
Application number: PCT/IB2004/003033
Authority: WO
Inventors: Johan Lub; Dirk Jan Broer; Ralph Kurt; Robert Frans Maria Hendriks
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-09-30
Filing date: 2004-09-17
Publication date: 2005-04-14
Also published as: WO2005034099A3

Abstract

The present invention relates to an information medium comprising a first information layer (L1) and a second information layer (L2), said first and second information layers (L1, L2) comprising marks intended to store binary data, each mark being intended to be read by a light spot polarized according to a first direction (d1) or to a second direction (d2), wherein: the first information layer (L1) comprises first marks (101) sensitive to the first polarization direction, and second marks (102) not sensitive to said first polarization direction, said first and second marks being both not sensitive or equally sensitive to the second polarization direction, the second information layer (L2) comprising third marks (103) sensitive to the second polarization direction, and fourth marks (104) not sensitive to said second polarization direction, said third and fourth marks being both not sensitive or equally sensitive to the first polarization direction. A first photo-alignment layer is spin-coated on a substrate, exposed with uv-light having a particular polarization, spin-coating a mixture containing liquid crystal monomers and dichroic dyes and locally curing that mixture by uv-light.

Description

"Information medium comprising two layers, method of manufacturing such an information medium, device for reading such an information medium "

FIELD OF THE INVENTION The present invention relates to an information medium comprising marks intended to store information, said medium comprising two information layers. The present invention also relates to a method of manufacturing such an information medium. The present invention finally relates to a device for reading information on said information medium. This invention may be used in, for example, the field of optical data storage.

BACKGROUND OF THE INVENTION In the field of optical storage, increasing the capacity of the information medium is the trend. An already investigated way for increasing the data capacity consists in using a plurality of information layers in the information carrier. For example, a DND (Digital Video Disc) may comprise two information layers. Information is recorded on or read from an information layer by means of an optical beam, using local refractive index variations or the presence of surface relief structures.

This conventional optical data storage media is subject to limitation since they are rather expensive and difficult to manufacture.

SUMMARY OF THE INVENTION It is an object of the invention to propose an information medium comprising two information layers, and having an alternative structure compared to the prior art information medium.

To this end, the information medium according to the invention comprises a first information layer and a second information layer, said first and second information layers comprising marks intended to store binary data, each mark being intended to be read by a light spot polarized according to a first direction or to a second direction. The first information layer comprises first marks sensitive to the first polarization direction, and second marks not sensitive to said first polarization direction, said first and second marks being both not sensitive or equally sensitive to the second polarization direction, The second information layer comprising third marks sensitive to the second polarization direction, and fourth marks not sensitive to said second polarization direction, said third and fourth marks being both not sensitive or equally sensitive to the first polarization direction.

The information medium in accordance with the invention is such that the first information layer can be read by a light spot having a first polarization direction, and such that the second information layer can be read by a light spot having a second polarization direction. As a matter of fact, when the information medium is illuminated by a light spot polarized according to the first polarization direction, the first and second marks of the first information layer can provide a predetermined data pattern on a detector, whereas the second information layer only contributes to a constant data pattern on the detector, which can correspond in some cases to a fully transparent background as it will be detailed in the following. Compared to known multilayer information medium, the present invention has a number of advantages. First of all, the double layer information medium according to the invention is much thinner than the one of the prior art since no electrodes nor electrolytes are required. Fewer problems are also expected with the number of readout cycles because these media are static and non-switchable. The selection of an information layer can only be done thanks to the relationship between the polarization direction and the absorption or reflection properties of the marks. No additional means are required for selecting the appropriate information layer. Cross talk is reduced. Finally, the information medium is less complex and easier to replicate for content distribution and therefore much cheaper than other multilayer approaches.

The present invention also relates to a method of manufacturing such an information medium. The present invention finally relates to a device for reading information on such an information medium.

These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein: Fig.l depicts a first embodiment of an information medium according to the invention, Fig.2 depicts a second embodiment of an information medium according to the invention, Fig.3 depicts a third embodiment of an information medium according to the invention, Fig.4 depicts a fourth embodiment of an information medium according to the invention, Fig.5 depicts a fifth embodiment of an information medium according to the invention, Fig.6A to Fig.6C show different structures of reactive dichroic dyes, Fig.7A and Fig.7B illustrates reactive liquid crystals that can act as liquid crystalline hosts for the dyes, Fig.8 A and Fig.8B illustrates reactive liquid crystals that can act as liquid crystalline hosts for the dyes, Fig.9 depicts an embodiment of a device for reading an information medium according to the invention, Fig.lOA to Fig.10C illustrates examples of materials for reflecting right-handed or left-handed circularly polarized light.

DETAILED DESCRIPTION OF THE INVENTION In the following, statement "sensitive" relates to the state of a polarized material that absorbs or reflects, at least partially, a light beam. In the following, statement "not sensitive" relates to the state of a polarized material that does transmit a light beam (i.e. without absorbing nor reflecting). In the following, axes (x, y, z) form an orthogonal three-dimensional reference. The present invention relates to an information medium according to the invention comprises a first information layer and a second information layer, said first and second information layers comprising marks intended to store binary data, each mark being intended to be read by a light spot polarized according to a first direction or to a second direction. Said information medium is intended to be read in applying one light spot on a desired mark, or alternatively in applying a plurality of parallel light spots simultaneously on a set of marks. A light spot which is applied to a given mark is detected by a photo-detector facing the information medium. According to the level of the light spot which is detected, the data stored by said given mark is recovered. The first information layer comprises first marks sensitive to the first polarization direction, and second marks not sensitive to said first polarization direction, said first and second marks being both not sensitive or equally sensitive to the second polarization direction, The second information layer comprising third marks sensitive to the second polarization direction, and fourth marks not sensitive to said second polarization direction, said third and fourth marks being both not sensitive or equally sensitive to the first polarization direction. The first and second information layers are superimposed so that marks having a same spatial position are opposite to the marks of the other layer having the same said spatial position. The first and the second information layers are parallel to a plan (x,y) formed by the perpendicular directions x and y.

In the following, it is assumed that the information medium is illuminated by light spots which are first applied to the first information layer. In this arrangement, the second information layer is placed between the first information layer and the detector. However, it will be apparent for a skilled person that the invention also applies for the case where the information medium is illuminated by light spots which are first applied to the second information layer. In this arrangement, the first information layer is placed between the second information layer and the detector. Fig.l to Fig.5 show different embodiments of an information medium according to the invention. these Figures, the cylinders illustrate the polarization of the material, and correspond to anisotropically shaped molecules part of a polymer network. More specifically, the orientation of the cylinders represents the average orientation of anisotropically shaped molecules or molecular units. For sake of understanding, the size of molecules have been amplified. Each mark, represented by a dotted square area, is intended to store a binary data.

Fig.l depicts a first embodiment of an information medium according to the invention. The first information layer Ll is read when illuminated by a light spot having the first polarization direction dl = x, and the second information layer L2 is read when illuminated by a light spot having the second polarization direction d2 = y. Because the first direction and the second direction are perpendicular, only one of the two layers is visible when illuminated by the light source, the other one being as transparent. The first information layer (Ll) comprises first marks (101) sensitive to the first polarization direction, and second marks (102) not sensitive to said first polarization direction, said first and second marks being both not sensitive to the second polarization direction. The second information layer (L2) comprising third marks (103) sensitive to the second polarization direction, and fourth marks (104) not sensitive to said second polarization direction, said third and fourth marks being both not sensitive to the first polarization direction. First marks (101) are aligned according to axis x. Second marks (102) are not polarized, e.g. they are transparent. Third marks (103) are aligned according to axis y. Second marks (102) are not polarized, e.g. they are transparent.

Fig.2 depicts a second embodiment of an information medium according to the invention. The first information layer Ll is read when illuminated by a light spot having the first polarization direction dl = x, and the second information layer L2 is read when illuminated by a light spot having the second polarization direction d2 = y. Because the first direction and the second direction are perpendicular, only one of the two layers is visible when illuminated by the light source, the other one being as transparent. The first information layer (Ll) comprises first marks (201) sensitive to the first polarization direction, and second marks (202) not sensitive to said first polarization direction, said first and second marks being both not sensitive to the second polarization direction. The second information layer (L2) comprising third marks (203) sensitive to the second polarization direction, and fourth marks (204) not sensitive to said second polarization direction, said third and fourth marks being both not sensitive to the first polarization direction. First marks (201) are aligned according to axis x. Second marks (202) are aligned according to the vertical axis z. Third marks (203) are aligned according to axis y. Fourth marks (204) are aligned according to the vertical axis z.

Fig.3 depicts a third embodiment of an information medium according to the invention. Axis x', in the plan (x,y), is oriented so as to define a 45° angle with axis x. Axis y', in the plan (x,y), is oriented so as to define a 45° angle with axis y. Axes x' and y¹ are perpendicular.

The first information layer Ll is read when illuminated by a light spot having the first polarization direction dl = x', and the second information layer L2 is read when illuminated by a light spot having the second polarization direction d2 = x. The first information layer (Ll) comprises first marks (301) sensitive to the first polarization direction, and second marks (302) not sensitive to said first polarization direction, said first and second marks being equally sensitive to the second polarization direction. The second information layer (L2) comprising third marks (303) sensitive to the second polarization direction, and fourth marks (304) not sensitive to said second polarization direction, said third and fourth marks being equally sensitive to the first polarization direction. First marks (301) are aligned according to axis x'. Second marks (302) are aligned according to axis y'. Third marks (303) are aligned according to axis x. Fourth marks (304) are aligned according to axis y.

Only data of one layer are readable at a time, i.e. marks within this layer equal to 1 or 0 depending if they are sensitive or not to the polarization direction, and the other layer contributes to a constant absorption or reflection background, i.e. marks within that layer have substantially the same absorption or reflection value. For example, if the information medium is illuminated by a light having the first polarization direction, then marks 301 and 302 of layer Ll will absorb the same quantity of light (i.e. the projection of the polarization direction of the light into the direction of the polarized material contained in the mark) as the first polarization direction x makes an angle of 45° with the direction x' and y'. This results in a constant data pattern. The residual light outputted by the first layer Ll is then transmitted to the second information layer L2, wherein third marks 303 will absorb said residual light and fourth marks 404 will transmit said residual light. The same principle applies for an illumination by a light having the second polarization direction.

Fig.4 depicts a fourth embodiment of an information medium according to the invention. Axis x', in the plan (x,y), is oriented so as to define an angle θ with axis x. Axis y', in the plan (x,y), is oriented so as to define an angle θ with axis y. Axes x' and y' are perpendicular. The first information layer Ll is read when illuminated by a light spot having the first polarization direction dl = x, and the second information layer L2 is read when illuminated by a light spot having the second polarization direction d2 = x'. The first information layer (Ll) comprises first marks (401) sensitive to the first polarization direction, and second marks (402) not sensitive to said first polarization direction, said first and second marks being both not sensitive to the second polarization direction. The second information layer (L2) comprising third marks (403) sensitive to the second polarization direction, and fourth marks (404) not sensitive to said second polarization direction, said third and fourth marks being both not sensitive to the first polarization direction. First marks (401) are aligned according to axis y\ Second marks (402) are aligned according to vertical axis z. Third marks (403) are aligned according to axis y. Fourth marks (404) are aligned according to vertical axis z.

This embodiment allows to set angle θ to different values without restrictions.

Fig.5 depicts a fifth embodiment of an information medium according to the invention. Axis x', in the plan (x,y), is oriented so as to define a 45° angle with axis x. Axis y', in the plan (x,y), is oriented so as to define a 45° angle with axis y. Axes x' and y' are perpendicular. The first information layer Ll is read when illuminated by a light spot having the first polarization direction dl = x', and the second information layer L2 is read when illuminated by a light spot having the second polarization direction d2 = x. The first information layer (Ll) comprises first marks (501) sensitive to the first polarization direction, and second marks (502) not sensitive to said first polarization direction, said first and second marks being equally sensitive to the second polarization direction. The second information layer (L2) comprising third marks (503) sensitive to the second polarization direction, and fourth marks (504) not sensitive to said second polarization direction, said third and fourth marks being both not sensitive to the first polarization direction. First marks (501) are aligned according to axis x'. Second marks (502) are aligned according to axis y'. Third marks (503) are not polarized, e.g. they are transparent (or they are aligned according to vertical axis z, but not shown). Fourth marks (504) are aligned according to axis y'.

As described above, the information medium in accordance with the invention comprises marks made of a polarized material that is adapted to preferentially absorb light having a predetermined polarization direction. Said polarized material comprises a photo-alignment layer and liquid crystal monomers that are aligned by the photo-alignment layer. Said layer is coated on the substrate prior to the liquid crystal monomer. The photo-alignment layer achieves its orientation capability by exposing it, also prior to the application of the liquid crystal monomer, to polarized UN light. Depending of the type of photo-alignment layer, the liquid crystal monomer will align parallel or perpendicular the electrical field vector of the polarized light. Alternatively, polarized materials can be materials capable of aligning themselves by polarized UN light without the use of an alignment layer. Examples of these materials are for instance described in the European patent application n° 02077425.3 filed on 19 June 2002.

According to an embodiment of the invention, said polarized material comprises liquid crystal material or absorbing molecules aligned in a liquid crystal matrix.

Alternatively, the polarized material can comprise nano-crystals, such as nanotubes. Nanotubes are crystal structures formed of individual atoms and have therefore a certain long-range order. Absorbing molecules are, for example, dichroic dyes. Said dyes are molecules that absorb preferentially light polarized according to a predetermined polarization direction depending on the molecule, i.e. on the direction of the internal dipoles which is either parallel or perpendicular to the molecule axis. Fig.6A to Fig.6C show examples of such dichroic dyes. The dichroic dyes are called absorber guests and they have to be embedded in liquid crystal hosts, i.e. a matrix of liquid crystal material that is responsible for a good alignment or parallelism. The dichroic dyes of Fig.6A to Fig.6C are liquid crystal monomers that absorb light of a predetermined wavelength. Alternatives for these monomers are azo-compound with dichroic properties without reactive groups. They are added as non-reactive guests to reactive liquid-crystal hosts. The materials in Fig.7 A, Fig.7B, Fig.8 A and Fig.8B are all reactive liquid crystals or liquid crystalline monomers that can act as liquid crystalline hosts for the dyes. The monomer is the host material and the dye is the guest material. In other words, a blend of monomers is made, consisting of liquid crystalline monomers (together with an orientation layer they provide the aligned properties) and reactive dichroic dyes. The dyes can be liquid crystalline themselves or just isotropic dichroic. In the latter case, they must align themselves in the liquid crystalline host together with the other molecules. Fig.7A / Fig.7B differ from Fig.8A / Fig.8B by the way they are drawn. The aromatic rings in Fig.7A / Fig.7B comprise inner circle indicating the conjugated behavior of the double bonds, whereas they are given as single bonds in Fig.8 A / Fig.8B. Also the spacer groups in Fig.7A / Fig.7B are given as (CH₂)_X (in Fig.7A, x must be defined at some point being between 1 and 12), whereas each connection in the zigzag symbolizes a CH in Fig.8A / Fig.8B. In Fig.8A and Fig.8B, the spacer contains (CH₂)₆. The acryalte group in Fig.7A / Fig.7B is given as O-CO-GH CH₂ and only the bonds are drawn in Fig.8A / Fig.8B. The combination of the absorber guest and of the liquid crystal monomer results in a mixture. An example of a monomer corresponding to this mixture is presented in Fig.8 A. Said mixture can be used as such. However, by making a blend of a liquid-crystalline diacrylate, e.g. the one shown in Fig.8 A, and a liquid-crystalline monoacrylate, e.g. the one shown in Fig.8B, the crosslink density can be adjusted, and thus the mechanical properties of the resulting film, as said film becomes less brittle by blending in a monoacrylate. The viscosity of the uncured mixture is also reduced by adding the monomer depicted in Fig.8B, which make orientation of the liquid crystals easier and homeotropic alignment, i.e. alignment of the molecules perpendicular to the surface of the information layers, becomes easier. The alignment of the liquid crystal molecules is based on the use of photo-alignment material such as polyvinylcinnamates or polyvinylcoumarins, or of polyimides. Said material is a separate layer, which has to be deposited first. Said alignment layer is oriented according to a predetermined direction by illumination with polarized light and the liquid crystal layer deposited on top will follow that predetermined direction to align more or less parallel.

The information medium comprising the polarized material is manufactured according to the following method, described in the case of the embodiment of Fig.l. A thin film of a photo-alignment layer, such as polyvinylcinnamate, is applied by spin coating on a substrate. This material is exposed with polarized Ultra-Niolet UN light having a first polarization direction in order to induce its liquid crystal aligning capability. On this photo-alignment layer, a thin film is spin-coated from a solution, for example in xylene, containing the liquid crystal monomers, the dichroic dyes and a photoinitiator. The liquid crystal orients on the polyvinylcinnamate. The monomer mixture is cured locally by UN exposure giving the marks. This can be done by pulsed laser writing or by an integral exposure through a mask. The remaining non- exposed material is removed by dissolving it, for example in tetrahydrofuran. A second layer of polyvinylcinnamate is spin-coated and again exposed with polarized UN light having a second polarization direction, perpendicular to the one of the first polyvinylcinnamate layer in the case of the embodiment of Fig.l. On this layer, a layer reactive liquid crystalline monomers is spin-coated and locally cured. The remaining material is removed by dissolving. This stack of layers can be protected by a thin transparent coating. In addition, an intermediate layer can be deposited between the first information layer and the second one to separate them. This intermediate layer can be a mixture of isotropic, densely crosslinking acrylate monomers. Alternatively, the photo-alignment layer can be replaced by polyimides, well-known in the LCD industry for the rubbed alignment. The total surface of the substrate is then provided with a uniaxial alignment layer thanks to the rubbed polyimide, and the applied liquid crystal monomer is exposed locally with UN light where it polymerizes.

Fig.9 shows an embodiment of a device for reading information stored on an information medium in accordance with the invention. The device for reading data stored on an information medium comprises an optical 93 element for generating an array of light spots from a coherent input light beam 91 delivered by a light source, said array of light spots being intended to scan the marks of the information medium. This feature enables a parallel read out of data. Alternatively, a sequential readout is also possible. For that purpose, the optical element 93 is adapted to deliver a single light spot. The optical element 93 may correspond to a two-dimensional array of micro-lenses at the input of which the coherent input light beam 91 is applied. The array of micro-lenses is placed parallel and distant from the information medium so that light spots are focused on the marks of said information medium. The numerical aperture and quality of the micro-lenses determines the size of the light spots. For example, a two-dimensional array of micro-lenses having a numerical aperture which equals 0.3 can be used. The input light beam 91 can be realized by a waveguide (not represented) for expanding an input laser beam, or by a two- dimensional array of coupled micro lasers. Alternatively, the optical element 93 may correspond to a two-dimensional array of apertures at the input of which the coherent input light beam 91 is applied. The apertures correspond for example to circular holes having a diameter of 1 μm or much smaller. In this case, the array of light spots is generated by the array of apertures based on the Talbot effect which is a diffraction phenomenon working as follow. When a number of coherent light emitters of the same wavelength, such as the input light beam 204, are applied to an object having a periodic diffractive structure, such as the array of apertures 202, the diffracted lights recombines into identical images of the emitters at a plane located at a predictable distance zO from the diffracting structure. This distance zO is known as the Talbot distance. The Talbot distance zO is given by the relation zO = 2.n.d² / λ, where d is the periodic spacing of the light emitters, λ is the wavelength of the input light beam, and n is the refractive index of the propagation space. More generally, re-imaging takes place at other distances z(m) spaced further from the emitters and which are a multiple of the Talbot distance z such that z(m) = 2.n.m.d² / λ, where m is an integer. Such a re-imaging also takes place for m = Yz + an integer, but here the image is shifted over half a period. The re-imaging also takes place for m = ^XA + an integer, and for m = % + an integer, but the image has a doubled frequency which means that the period of the light spots is halved with respect to that of the array of apertures. Exploiting the Talbot effect allows generating an array of light spots of high quality at a relatively large distance from the array of apertures (a few hundreds of μm, expressed by z(n)), without the need of optical lenses. This allows inserting for example a cover layer between the array of apertures and the information medium for preventing the latter from contamination.

The light spots generated by the optical element 93 are then applied on sensitive or non-sensitive marks of the information medium. If a light spot is applied on a sensitive mark, no output light beam is generated in response by said mark. If a light spot is applied on a non- sensitive mark, an output light beam is generated in response by said mark, said output light beam being detected by a detector 94. The detector 94 is used for detecting the binary value of the data mark on which the light spot is applied. It is represented in Fig.9 in the case of an information medium working according to an absorption or transmission mode. Said detector is advantageously made of an array of adjacent CMOS or CCD pixels, as represented. For example, one pixel of the detector is placed opposite an elementary area containing one data (i.e. one bit) of the information carrier, hi that case, one pixel of the detector is intended to detect one data of the information carrier. The detector is equipped with an optimized threshold for bit detection. Optionally, a possibly switchable polarizer is used in the detection path. Advantageously, an array of micro-lenses (not represented) is placed between the information medium and the detector for focusing the output light beams generated by the information medium on the detector, for improving the detection of the data. Finally, the reading device comprises means 92 for controlling the polarization direction of the input light beam in such a way that the first information layer Ll is read for a first polarization direction and that the second information layer L2 is read for a second polarization direction. Said control means are, for instance, based on the use of a liquid crystal element that rotated the polarization of the light when a certain voltage is applied. It is also possible to use a retardation plate that is rotated to rotate the polarization of the light. For certain types of laser, it is also possible to switch the polarization by changing the drive current. In that case, no extra optical element is required. If the light source is not a laser, the control means further comprises a controllable polarizer for polarizing the light spot or array of light spots according to a first or a second polarization direction, depending on the information layer Ll or L2 to be read. The controllable polarizer is mechanically or electrically controlled. The drawings and their description hereinbefore illustrate rather than limit the invention. It will be evident that there are numerous alternatives that fall within the scope of the appended claims. In this respect the following closing remarks are made. It is to be noted that the present invention is not limited to two layers. It is possible to have more than two layers stacked on top of each other. However, in such a case, the layers are usually not anymore independent from each other. It means cross talk between the different layers can occur, which usually makes it more difficult to reconstruct the decoded data. Moreover, the present invention covers both linear polarized light as well as circular polarized light including corresponding beam, modulation and detection. Fig.l, for example, is then interpreted as follows. The first marks 101 correspond, for example, to material that is transparent for left-handed circular polarized light and that reflects right-handed polarized light back. The third marks 103 correspond to material that is transparent for right-handed circular polarized light and that reflects left-handed polarized light back. This result in the same effects as described for linear polarized light. hi this case, cholesteric monomers are used instead of using guest-host based monomers with different orientation directions. The information medium according to said embodiment is manufactured according to the following method. On a substrate, eventually provided with rubbed polyimide, a chiral nematic monomer (or a monomer mix in order to optimize properties such as reflection wavelength, etc) with a right-handed helix is spin-coated in a thickness of about 3.5 mm. This film is exposed to UN light by local exposure through a shadow-mask or by direct laser writing. The unexposed material is polymerized by a flood exposure at relatively high temperatures. At such temperatures, the reflection wavelength shift far away from the wavelength used in the optical system thanks to the thermo-chromic effect, or the materials become isofropic (no reflection at all). In this case the reflection wavelength is shifted by an isomerization step through a shadow mask on air followed by polymerization step using a flood exposure in nitrogen. Subsequently, a thin separating coating is applied and eventually coated with rubbed polyimide. Then, a second chiral-nematic material is spin-coated, but now with a left-handed helix. The same procedures for information recording are applied and eventually a cover coating is applied. Reading can then be done with circularly polarized light. The islands in the lower cholesteric bits will reflect right-handed circularly polarized light whereas the upper cholesteric bits will reflect left-handed circular polarized light. Examples of materials are mixtures depicted in Fig.lOA and 10B for reflecting right- handed circularly polarized light and in Fig.lOA and 10C for reflecting left-handed circularly polarized light. It has been mentioned that the polarized material used in the information medium according to the invention is aligned according to a photochemical principle. However, it will also be apparent to a person skilled in the art that other physical principles are applicable. For example, an electrical alignment of the material contained in the marks is possible in a direction substantially perpendicular to the information layers or very locally for a predetermined mark.

Any reference sign in the following claims should not be construed as limiting the claim. It will be obvious that the use of the verb "to comprise" and its conjugations do not exclude the presence of any other steps or elements besides those defined in any claim. The word "a" or "an" preceding an element or step does not exclude the presence of a plurality of such elements or steps.

Claims

1 An information medium comprising a first information layer (Ll) and a second information layer (L2), said first and second information layers (Ll, L2) comprising marks intended to store binary data, each mark being intended to be read by a light spot polarized according to a first direction (dl) or to a second direction (d2), wherein: - the first information layer (Ll) comprises first marks (101, 201, 301, 401, 501) sensitive to the first polarization direction, and second marks (102, 202, 302, 402, 502) not sensitive to said first polarization direction, said first and second marks being both not sensitive or equally sensitive to the second polarization direction, the second information layer (L2) comprising third marks (103, 203, 303, 403, 503) sensitive to the second polarization direction, and fourth marks (104, 204, 304, 404, 504) not sensitive to said second polarization direction, said third and fourth marks being both not sensitive or equally sensitive to the first polarization direction.

2 An information medium as claimed in claim 1, wherein said second marks and fourth marks are made of a non-polarized material, or of a polarized material polarized along a third axis (z) perpendicular to said first and second direction.

3 An information medium as claimed in claim 2, wherein the polarized material comprises a photo-alignment layer, at least one liquid crystal monomer and at least one dichroic dye.

4 An optical information medium as claimed in claim 2, wherein the polarized material comprises at least one liquid crystal monomer and at least one dichroic dye.

5 A method of manufacturing an information medium as claimed in claim 1, 2, 3 or 4, said method comprising the steps of: spin-coating a first photo-alignment layer on a substrate, exposing said layer with polarized Ultra-Niolet light having the first polarization direction, spin-coating a mixture containing liquid crystal monomers and dichroic dyes, locally curing said mixture by Ultra-Niolet exposure, resulting in the first marks of the first information layer, repeating the preceding steps to manufacture the third marks of the second information layer, a second alignment layer being spin-coated and exposed with polarized Ultra-Niolet light having the second polarization direction.

6 A device for reading information on an information medium as claimed in claim 1, 2,

3 or 4, said device comprising: - a light source unit for supplying at least one light spot, means for controlling a direction of polarization of said at least one light spot, so that the first information layer is read for a first polarization direction and that the second information layer is read for a second polarization direction.

7 A device for reading information on an information medium as claimed in claim 1, 2,

3 or 4, wherein means for controlling comprise a controllable polarizer for polarizing the at least one light spot according to the first and to the second polarization direction.