Patent Application
20080063808
For a better understanding of the present invention, reference is made to the accompanying figures, wherein:
a,b is a schematic representation of POLIPHEM transmission grating after the holographic recording without and with an applied electrical field;
As outlined above, the films of the present invention are obtained by forming a homogeneous, optically isotropic film on a substrate or between two substrates, the film being prepared from a homogeneous and isotropic, photocurable mixture, containing or consisting of a photopolymerizable component (composed of one or more monomers or/and oligomers), photoinitiator, if required or desired, and liquid crystal (LC) molecules or a LC mixture, optionally in admixture with additional components. The initial mixture should be selected such that an optically clear, non-scattering isotropic film of good quality can be formed which contains a relatively high amount of LC. Homogeneity of the initial mixture should be present at least in a temperature range of about 15-25° C. The same applies for the requirement that the film is optically isotropic.
In order to obtain such a homogeneous, isotropic mixture, the two major components of the photocurable mixture, the photopolymerizable component and the liquid crystal component, should be completely miscible when provided at the above mentioned temperature range of about 15-25° C. and will normally be combined in the said temperature range. This means that the proportion of the photocurable monomers and/or oligomers will be selected such that they “dilute” the LC component in such a way that the properties of anisotropic LC alignment are suppressed. In specific, but exceptional cases, mixing may take place above the said temperature, in case no phase separation will occur upon cooling to the above mentioned temperature. It is preferred that the mixture contains the photocurable monomer(s)/oligomer(s) and the liquid crystal(s) in high, substantial amounts.
It is moreover preferred that the ratio of the components is chosen such that the liquid crystal component is not far from the isotropic-non-isotropic phase transition, e.g. that already small loss of energy or changes of chemistry would re-establish the (preferably nematic, but also possibly other, for example smectic or cholesteric) alignment of the LCs and would transfer the mixture into an anisotropic state .
In principle, monomers and/or oligomers useful for the photopolymerizable component can be selected without limitation, as long as the monomer(s)/oligomer(s) is/are photopolymerizable and its mixture with the LC or LC mixture will be homogeneous and isotropic. Therefore, the said monomers and/or oligomers should of course be themselves isotropic. It is preferred to use relatively polar educts, in order to provide surface forces within the mixture that are just too low and just not sufficient to provoke a phase separation of the mixture with the liquid crystals. However, this is not a mandatory measure since miscibility with the LC or LC mixture may be properly adapted with the aid of additives, e.g. surfactants or the like, as known in the art. The monomers and/or oligomers can be (and will be in most cases) pure organic molecules, but in some cases, they may instead or in addition comprise “hybrid” inorganic-organic molecules, e.g. organically modified silanes.
In one specific and important embodiment of the invention, the monomer(s) or oligomer(s) used for the present invention will reach their gelation point under polymerization only after at least about 30% thereof have been reacted, preferably only after about 50% monomer-polymer conversion degree, and more preferably, only after about 70% to 80% monomer-polymer conversion degree, the percentage being on a molar basis. The reason for this measure is that an early gel point will negatively affect or even prohibit complete phase separation: If the forming polymers attain at their gel point, remaining liquid crystal molecules can no longer escape from the forming network, but remain entrapped therein. The inventors found that if the gel point is only reached after the majority of the monomers or polymers has reacted, the liquid crystal molecules will have sufficient time to escape from the forming polymer network and to diffuse into the dark regions which are free from said network, i.e. a complete separation will take place, without the necessity to raise the temperature of the mixture above environmental temperature. Therefore, use of organic, polymerizable monomers or oligomers having a “late” gel point will result in a better separation and therefore in improved and superior optical properties of the resulting gratings, for example in better switching times.
On the other hand, it should be kept in mind that the method of the present invention will be most effective if photocrosslinking is performed such that a dense network is obtained. The dense network formation increases the liquid compound separation, resulting in areas enriched with this component (LC) and, finally after nematic ordering of LCs, macroscopically aligned liquid crystalline phase areas and between said areas polymer network areas are formed in which the reactive monomers are enriched and polymerized. It is to be noted that the said process will take place under environmental temperature conditions.
Under consideration of the above, It is preferred to use compounds having at least one C═C double bond. The said or at least one of the C═C double bonds may be part of a so called Michael system, e.g. containing the fragment C═C—C═O. Alternatively or in addition, so called thiol-ene combinations wherein a thiol compound is added to a compound having a C═C double bond, and/or compounds having epoxy groups may be used. The said combinations are preferred due to the fact that the gel point is only reached after a substantial amount thereof has been reacted. More preferably, a mixture of monomers and/or oligomers having a different number of double bonds is used, even more preferably together with one or more thiol compounds. One example is the combination of penta-acrylates with di- or tri-acrylate in order to optimize both the functionality and viscosity of the pre-polymer material. In a specifically preferred embodiment, a mixture of approximately 1:4 or 1:2 of di- to penta-acrylate is used in order to assist homogeneous mixing. With such a mixture, optically transparent recording films of desired thickness, e.g. 6-20 μm, can easily be prepared.
As an example, suitable monomers and oligomers may be selected from the group comprising acrylates and methacrylates, such as diethylene glycol diacrylate, diethylene glycol dimethacrylate, triethylene glycol diacrylate, triethylene glycol dimethacrylate, trimethylol propane, diallyl ether, trimethylolpropane triacrylate, pentaerythritol triacrylate, pentaerithritol tetracrylate, pentaerythol pentacrylate, and dipentaerythrytol hydroxy pentacrylate. Bifunctional or multifunctional acrylates as the photopolymerizable component have the advantage that they typically exhibit a high compatibility with low-molecular liquid crystal components which allows to use a high LC concentration and to prepare recording films having a high optical quality.
As an example of non-acrylate monomers, pentaerythritol tetrakis (3-mercapto) propionate can be mentioned, which may be added to a C═C-containing compound, e.g. vinyl compounds. For example, photocurable components normally used as adhesives and containing different thiol-ene monomers and oligomers in the combination with acrylate compounds, e.g. Norland NOAO-61,65,68,81, can be used. The proper selection will result in such a mixture which is initially homogenous, but which under the polymerization conditions will undergo a process of phase separation, separating the the liquid crystal component from the growing polymer network, by which the morphology of the resulting structures is attained. Use of thiol-ene polymerizable components are favorable, due to their specific photopolymerization kinetics. This is because the combination of step-growth and free-radical reaction between multifunctional aliphatic thiols and vinyl monomers containing “ene” groups result in a late “gelation—point”. Moreover, most of the double bonds are consumed while the precursor is still liquid. Due to these features, it is possible to compose the photopolymerizable mixture exclusively, essentially or substantially from the photopolymerizable component and the liquid crystal component only, without the addition of auxiliary components, e.g. surfactants, and to arrive at a two-phase structure as detailed above, upon proper irradiation as described.
The second component of POLIPHEM material is a liquid crystal (LC) component. This component allows the electro-optical response of the resulting structures. Suitable LCs, used in practice of the present invention may include any types of LC. Selection of a proper LC or LC mixture for the photocurable mixture is not critical. This means that e.g. any of nematic, cholesteric or smectic molecules can be used, wherein the liquid crystals may be used alone or in admixture with other components as known from the art. The LC mixtures may additionally contain dopants in order to obtain a further functionalisation of LCs and thus, resulting in additional desired properties of the final device. Such dopants may be e.g. dye molecules or photochromic molecules.
The concentration of LC utilized should be selected such that on one hand, it is sufficiently high to allow a significant phase separation of the mixture upon irradiation, but should not exceed the amount beyond which PDLC structures might be formed which, as outlined above, are opaque and hazy. If the liquid crystal component is present in an amount below 20% by weight of the prepolymer mixture or polymerizable mixture, a rather weak difference of the refractive indices in the first and second areas will occur upon irradiation at room temperature, even if a full phase separation occurs, and consequently, diffraction efficiency would be low. On the other hand, if the LC component is present in an amount of more than 45% by weight, the product will mostly become highly scattering, and its diffraction efficiency and transmittance decreases. Thus, it is preferred to incorporate the LC component in a proportion of 25 to 45% by weight, in relation to the weight of the prepolymer mixture, and more preferably in a proportion of approximately 35-40% by weight. Such mixtures typically result in a film or coating which shows high diffraction efficiency, optical clarity and good electro-optical parameters. In addition, the mixture should preferably be selected such that the addition of not more than about 10% by weight, preferably not more than 5% by weight, more preferably not more than about 2% by weight an most preferred not more than about 0.4 to 0.7% by weight of the liquid crystal component to the mixture as chosen for the invention would result in a opaque or scattering mixture wherein the liquid crystal component would arrange itself in its “liquid crystal” order. By this selection of the LC ratio, it is made sure that the liquid crystals or liquid crystal mixture are/is disturbed in the mixture by the presence of the other component(s) and therefore do/does not exhibit their “normal”, e.g. nematic behaviour, but that the liquid crystals in the mixture could be brought into said “normal” behaviour already upon small losses of energy or by slight amendments in the chemical conditions. In other words, the concentration of LC should be selected in order to obtain such conditions upon which the forces for a phase separation of the mixture prior to irradiation should be only slightly higher than the attraction forces.
Addition of a surfactant (e.g. octanoic acid, heptanoic acid, hexanoic acid and the like) has an effect on the miscibility of the components (i.e. increases miscibility), increases the optical performances of the film, inter alia the switching time, and decreases the switching voltage. For example, Fluorad RTM FC-430 or Fluorad RTM FC-431 can be used in a weight concentration 0.2-5%, related to the weight of the polymerizable mixture.
As far as the polymerizable mixture requires a photoinitator for starting polymerization, the sensitivity of the prepolymer materials to illumination light is determined by the presence of such a photoinitiator and its concentration. Type and concentration of photoinitiator will be selected considering the irradiation wavelength required or desired and the compounds to be polymerized. In this context, the expression “photopolymerizable” is used to include not only the possibility of polyaddition, but also of polycondensation, and the expression “photopolymerizable compound” or “photopolymerizable mixtures” shall encompass any compound having one or more functional groups which participate in the curing of the polymer matrix. Photoinitiators are activated by irradiation with actinic light and free active radical formation. Free radicals created usually start the polymerization process.
Conveniently, photoinitiators to be employed, if necessary or desired, are commercially obtainable substances. For example, Irgacurep®184, Irgacure®1700, Irgacure®500, Irgacure®369, Irgacure®1117 (Ciba—Geily), Michlers ketone, (1-hydroxycyclohexyl) phenyl ketone, benzophenone or similar derivatives can be used for UV irradiation recording. For blue-green and red light sources the known photoinitiator systems like Irgacure 784, dye rose bengal ester, rose Bengal sodium salt, campharphinone, methylene blue and like with electron donor (co-initiators) can be used. Coinitiators can be employed in order to control the rate of curing in the free radical polymerization of the original prepolymer. Suitable coinitiators are N-phenylglicine, triethylamine, thriethanolamine and like.
The Norland Products, NOA® adhesives, employed in the below examples, include their own photoinitiator (for UV region of spectra). Other initiators can also be used, such as benzophenone, 2,2-diethoxyacetophenone (DEAP), benzoin, benzil or Irgacure®1700, Irgacure®369 as additives to Norland NOA®-61,65,68,81 to increase the photosensitivity in UV region of the spectrum and to complete a phase separation process between polymer and LC components.
It has been found that the best results in respect to the quality of the final devices, their diffraction efficiency and their electrooptical behaviour were achieved using an initiator amount of 0.5-2% by weight and a coinitiator in an amount of up to 2-3% by weight.
As mentioned earlier, it is necessary that the mixture is homogeneous and isotropic at the processing temperature which is environmental temperature. A skilled person is able to select the components accordingly.
In many cases, mixing the two major components will result in an isotropic, homogeneous mixture, without any additional measure. This is for example the case if the photocurable monomers and/or oligomers are selected such and in such a relation that the anisotropy of the “liquid crystals” is slightly (but preferably only slightly, see above) disturbed and consequently, they do not behave as liquid crystals in the said mixture, and instead, their “LC” properties (alignment) is suppressed. Photocurable monomers or oligomers for this purpose are for example (e.g. for the example of nematic LCs) molecules having a globular or more or less coiled or bulky shape, and which at least do not have the rod-like structures of the liquid crystal molecules. Acrylates, and more preferably pentaacrylates are an example for such molecules, while the skilled artisan is aware of a multiplicity of others. On the other hand, it should be kept in mind that the properties of the photopolymerizable monomer/oligomer component and the LC(s) for a specific mixture should be preferably selected such that they show physicochemical similarity (e.g. polarity) in such a way that they function as a “good” solvent for each other. Such a requirement may be met by a proper selection of physical interaction of the molecules, e.g. attraction or repulsion forces (like ionic or Van-der-Waals forces, coulomb interactions, phase separation/phase segregation tendency, specific inermolecular interactions, ratio of polarities, . . . ). Alternatively or in addition, other measures may be taken in order to disturb the anisotropy of the LCs, e.g. photochemical means (e.g. irradiation) by which non-mesogenic isomers are reversibly generated for a limited time period, a proper selection of the temperature profile at the time of mixing the components and subsequent cooling, application of a magnetic field, or the addition of photochromic additives. Of course, that part of the molecule mixture which upon irradiation will constitute the liquid crystal areas of the film to be used as an optical element must then, again, exhibit “normal” anisotropy of liquid crystals, i.e. must have at least nematic (or other LC) properties.
In short, the skilled artisan will preferably select a combination of photocurable monomer(s)/oligomer(s) and LC(s) having similarities in those physicochemical properties which are relevant for attraction and miscibility, but being of different shape, wherein specifically the monomer(s)/oligomer(s) do not have the rod-like (in the case of nematic LCs) or disk-like (in the case of cholesteric) structure of the LC component.
All components are mixed together by suitable means (e.g. ulta-sonification). The thus obtained starting mixture is a homogeneous, in general optically transparent blend.
According to the present invention, the said mixture is brought into the shape of a film, either on a substrate or between two such substrates. The above described first and second areas are preferably formed in this film by irradiation under conditions which are selected in respect of the properties (and especially the reactivity) of the components of the mixture and their concentration. The aim of irradiation is to obtain a high network density and to stimulate a spatially periodical separation of LCs from a forming network.
In most cases, a relatively low exposure intensity will be used, the specific value of which will be selected dependent on the materials employed (mainly reactivity of the polymerizable component and its concentration or, rather, concentration of the polymerizable groups), as known to a skilled artisan, but of course is also interdependent on other factors, e.g. the kind and concentration of polymerization initiator and time of irradiation. The intensity is selected such that it will ensure a relatively low photopolymerization rate of the local monomers/oligomers present in the bright regions of the inhomogeneous light field. On the other hand, a dense polymer network should be obtained. As a consequence, the LC molecules and also possibly a minor part of polymerizable material, e.g. monomers or smaller oligomers, may diffuse from the bright regions to the dark regions which-results in a more or less complete phase separation of the initial mixture. The driving force for the formation of a periodic volume structure into initially liquid monomer-diluent (LCs) is believed, according to literature, to be based on built-in concentration gradient of monomer inside the initial film during the local photopolymerization process, which induces the diffusion of monomer to the bright region on the illumination pattern. Subsequently, diluent (or the LC) molecules counterdiffuse to the dark region of the pattern. Finally, two-phase line-like structure is formed.
Local polymerization rate is related to local light intensity (relative contrast of the interference (illumination) pattern), functionality of oligomer and photoinitiator type and concentration and others. Upon photopolymerization of the crosslinkable polymerizable material, a shrinkage of this material occurs, driving out smaller molecules in sponge like manner. Without being wanted to be bound to any theory, inventors believe that photopolymerization can best be obtained, according to the present invention, by preferably providing conditions which allow at least one of, and preferably a combination of a low polymerization rate, a “late” gelation point and provide a dense network.
The phase separation tendency is connected with compatibility of the component, their polarity ratio, specific interfacial interaction between molecules, temperature. Although the process of periodic volume structures (holograms) formation in monomer-liquid solvent (LCs) mixture is quite complex and need a careful consideration of chemical, thermodynamical, physical processes taking account and effecting on this process, inventors believe that best results can be obtained using the measures outlined above.
The appropriate choice of all components of the photopolymerizable mixture (photopolymerizable monomers, LC, initiator system and additives, if required) and adjustment of their concentration, as described above, ensure high optical and electrical switching parameters of the final structures. The first component (photopolymerizable monomer(s) and/or oligomer(s)) will supply the formation of a closely cross-linked network under irradiation which tends to force the second phase material, the LC component, to anisotropically diffuse out of the forming or formed polymeric areas (which build up in the bright, irradiated parts) into well-definable regions comprising merely, substantially or mainly LC molecules or a LC mixture. These regions may be well aligned (after ordering of LCs) into “line-like” areas. The photoinitiator system and the intensity and wavelength of the actinic light, and parameters of different steps of the polymerisation process (initiation, propagation and termination) should control the rate of polymerization, phase separation of components and the final two-component structure of POLIPHEM such that the rate of polymerization is preferably low, in order to allow a full phase separation as detailed above.
The resulting structure may be a POLIPHEM film contained within a cell comprising of two substrates preferably made of glass or plastic and coated with a conducting film (
Thus, POLIPHEM structures (preferably holographic transmission and reflection (non-slanted or slantwise) volume diffraction gratings) form during exposure of the originally homogeneous, preferably isotropical film under conditions by which first areas of the film are irradiated, while second ones are not or substantially not. This irradiation may be performed e.g. either by using a masks or by holographic recording, i.e. a recording of light using 2 or more monochromatic beams of actinic light in such a way that the light intereferes in the film plane. Typical “holographic” recording set-ups can be used to fabricate POLIPHEM structures. After spatial filtering, expanding and division of an initial laser beam on the beam-splitter, the resulting beams are intersected and form the interference picture that is recorded in the volume of the reactive layer (or, in other words, by which the reactive layer is irradiated). The recording field, consisting of the constructive (bright) and destructive (dark) interference regions within the expanded beams, establishes a periodic intensity profile through the thickness or volume of the film. Recording beams penetrate into the film from the same side of the film if transmission gratings are to be fabricated, and from different sides if reflection gratings shall be formed (symmetrically—in the case of nonslanted gratings and asymetrically—in the case of slantwise gratings). Such non-homogeneous illumination initiates a locally fast polymerization of photopolymerizable monomers and/or oligomers in the bright regions of the mask or interference field which, however, is not too fast to allow escape of the smaller monomers so that the brighter regions are completely, almost or mainly depleted from LC molecules. The monomer concentration gradient causes a diffusion of photopolymerizable units from the dark to the bright regions, due to shrinkage effects which develop a sort of drawing force, while and the the second component, such as nematic or other liquid crystals, are forced into the opposite direction, due to phase separation tendencies and photopolymerization forces which act like a sponge, pushing small molecules out of the growing polymer network. Therefore, the smaller molecules, i.e. liquid crystal molecules, but possibly also smaller unreacted monomers, diffuse into the dark regions of the interference field. Thus, alternating well-defined regions of more or less or completely pure LC or liquid crystal mixture, separated by well-defined, more or less or completely pure polymer regions can be produced in-situ in a controllable way, as shown in
No specific measures during irradiation are required. Normal room conditions, e.g. ambient temperature and pressure are sufficient. Proper exposure intensity allows controlling the rate of the monomer photopolymerization such that the diffusion of LC molecules or components is not prohibited or hindered in any moment of time. Low-rate diffusion of LC ensures the formation of the continuous LC structures, e.g. lines, fringes or the like.
The exposure intensity itself can be properly selected by a skilled artisan in the light of the physical properties of the selected chemical components. Specifically, the ratio of the polymerization rate to the diffusion rate is important to allow sufficient time for the smaller molecules and especially the LC molecules to escape from the forming polymer network. Clearly, the polymerization rate can be controlled by means explained above. Diffusion rate is determined by the diffusion coefficients of the smaller sized components, mainly LC molecules, which inter alia are dependent on the diameter and shape of the molecules, their hydrophilic/hydrophobic properties relative to that of the growing polymer network. Thus, for each mixture, the respective parameters will be individually selected as known in the art. Reference is made to relevant articles, e.g. J. R. Lawrence, F. T. O'Neil, J. T. Sheridan, Optik 112 (2001) 449, G. M. Karpov, V. V. Obukhovsky, T .N. Smirnova, V. V. Lemeshko Spatial transfer of matter as a method of holographic recording in photoformers Opt. Comm. 174 (2000) 391-400), or Smirnova T. N., Sakhno O. V. Kinetic peculiarities of holographic recording in photopolymers 1 Proc. SPIE, 1998, V. 3488, PP. 276-284.
Efficiency of the grating (refractive index modulation) or volume periodic structures is increased with the growth of the segregation of polymer and LCs, which is determined by their thermodynamic compatibility and kinetic parameters of the polymerization process and diffusion segregation of the components. The diffusion process will not limit the rate of the hologram recording, which (in such mixtures) is determined by the rate of polymerization.
The intensity of the illuminating light should be sufficiently low to provide a not very fast crosslinking of monomers in bright regions and to allow a maximal segregation of LCs from these areas. On the other hand, upon low illumination intensity (which gives a low relative contrast of the illumination pattern) the rates of polymer conversion into bright and dark regions will become almost equal, which can slow down the rate of diffusion mass-transport and decreases the segregation degree. Moreover, relatively high intensity in the bright regions (and a sufficiently high contrast of the pattern) is required to form a dense polymeric network in the said regions. Besides, the efficiency of the diffusion mass-transport and final grating efficiency is dependent on the period of structure. Thus, all points mentioned above should be taken into account to adjust the illumination conditions.
For example, if the basic mixture contains thiol-ene polymerizable components, an irradiation intensity of from about 0.1 to 200 mW/cm2 with UV actinic light, depending on the concrete material and the efficiency of phase separation, may be used. More preferably, the intensity does not exceed 100-150 mW/cm2 and is even more preferably in the range of about 20 to 50 mW/cm2.
If the intensity of irradiation exceeds an upper limit beyond which the polymerization rate is too high to allow monomers to sufficiently diffuse from the growing polymer network, the formation of PDLC structures are observed. These PDLCs are characterized by much stronger light-scattering and by the absence of anisotropy after the fabrication step.
The resulting polymer-LC material has a spatial distribution of LC (with a non-droplets morphology) within the photochemically cured polymer matrix. Due to the almost uniform LC director's alignment within the LC-rich planes of POLIPHEM, that takes place during the exposure, these structures are polarization dependent and hence the POLIPHEM reveals the anisotropy after the exposure step, in contrast to the usual (H)PDLC. Microscopic images of POLIPHEM gratings with Λ=1.2 μm and 0.85 μm, d=8 μm and part of Fresnel lens based on POLIPHEM (with a distance between fringes near 6 μm) between crossed polarizers are shown in
Evidence of the non-droplets morphology of LC inside the LC-rich regions of POLIPHEM can be given by the observation of the relief structure of POLIPHEM layers after the removal of one substrate.
Formation of POLIPHEM can be controlled by observing the shape of the kinetic curve during irradiation (through a mask or by holographic recording) One example of such a curve is given in
In the diffracted state (“off-state”), an electric field can be applied to the POLIPHEM structure which is able to change the final orientation of LC director in the LC regions therein which causes the refractive index modulation of the “fringes” to reduce and then the grating diffraction efficiency to decrease up to very low values (“on-state”)
In specific cases, POLIPHEM structures may be used as waveguide reflection Bragg gratings. For this purpose, the light should be penetrate into the film though the lateral edge of the POLIPHEM grating. In these cases, POLIPHEM are fabricated as transmission gratings and will act as reflection grating for the light in a waveguide mode propagation. The extremely large spectral shift of the narrow-width selectivity band can be achieved by the application of an electrical field or by temperature processing. The controlled reflectivity and transitions of the light by such type of the optical element could achieved by the following ( see
This opens the possibility to use POLIPHEM structures as or within different elements, e.g. for visual displays and optical telecommunication systems like mirrors, line filters, electro-optical switches and like.
Reflective holographic polymer-liquid crystal elements exhibit narrow wavelength bands with high reflection efficiency and can be controlled by the electric fields. Therefore, they are attractive candidates for numerous applications, for example, reflective display. Such switchable reflection HOE can be fabricated on the base of POLIPHEM structures as well, as outlined above. Under application of the field across such reflection grating it can be switched between a state where it is reflective and the state where it is transitive.
In the fabrication of the reflection POLIPHEM, some specific demands should be fulfilled. For example, nematic liquid crystals or liquid crystal mixtures having a negative dielectric anisotropy As must be utilized as LC component in the polymerizable mixture. In reflection structures, the fringes should be placed completely or almost parallel to the substrates, and the director of LC will be align along the gratings vector (which is almost perpendicular to the substrates surfaces in the “off-state”). To control the grating parameters at the electric field application, LC must be rotated perpendicular to the electrical field vector E, then As should be negative. Two-frequency LC can be used for the same aim (LC in which As is changed for different frequencies of the applied voltage).
Switching of the reflective grating in case of a positive anisotropy could be done with magnetic filed.
Numerous different monomers and LCs may be combined in solution and photopolymerized in accordance with the present invention, and the LC concentration within the solution may be varied considerably. Several specific examples will now be described, and these examples are intended to illustrate various implementations of the invention, and are not intended to be limiting.
Using the method of the present invention, e.g. the following subject-matter can be provided:
Further, the optical elements as defined under f) to m) can be operated or used, wherein the operation or use comprises orientation and/or reorientation of the liquid crystals in the second areas by application of an electric, electromagnetic or magnetic field such that the element may be switched between a diffracted state (off state) and a non-diffracted state and optionally any intermediate state between said states which intermediate states vary in refractive index of the second areas. This can e.g. be in an optical telecommunication device, wherein the optical element is preferably selected from bi-directional electro-optical switches, variable optical attenuators (VOAs), tunable Bragg grating filters (TBG filters), wavelenght division multiplexers, which can be realized either in free-space or planar waveguide architecture, switchable beam-steering devices for freespace optical coupling or other purposes, tuneable spectral filters and waveguide systems.
Samples of photopolymerizable mixture were fabricated using NOA®68 optical adhesive produced by Norland Products, Inc., New Brunswick, N.J., and 5CB (Merck) cyanobiphenyl liquid crystal mixture in a 60:40 ratio, by weight. Surfactant FC-431 2 wt. % and UV photoinitiator Irgacure®369 (Ciba) 0.5% were added. All components were thoroughly mixed and then used for drop filling between ITO (indium-tin oxide) coated glass substrates with 8-15 μm Mylar.RTM spacers. All operations were made at room temperature. The cells filled with isotropic liquid were exposed to an interference field of 365 nm Ar-laser wavelength with vertical (s-) polarization of the recording beams. The exposure time was varied in the range 100-300 sec, and the intensity of the curing laser light (for both beams) was in the range 200-250 mW/cm2. Volume transmission gratings with periods 1.5-0.3 μm were recorded in the test cells at 22° C. (room temperature). Real-time behaviour of the grating's diffraction efficiency (kinetic of holographic recording) was tested by Ne-He laser beam with horizontal (p-) and/or vertical (s-) polarization, which was input into the cell at the appropriate Bragg angle. Diffraction efficiency (DE) is determined as a ratio of the intensity of the diffracted beam to a sum of the diffracted and transmitted ones. Under the mentioned recording conditions the common holographic PDLC structures (prior art) were produced. This was proven by the presence of a light scattering in the samples which is due to LC droplet formation of the samples as described above, by the absence of anisotropy after holographic recording, and a rather low diffraction efficiency, not more than 60%.
Reactive mixture and samples were fabricated as for Example 1. The cells filled with isotropic liquid were exposed to the interference field with wavelength 365 nm and s-polarization of the recording beams to record the transmission gratings with periods 1.5-0.3 μm at 22° C. The total intensity of the laser light (for both beams) was varied in the range 40÷60 mW/cm2. As an example, kinetic curve of the holographic recording of grating with Λ=0.9 μm, d=8 μm is shown at FIG. 4.a; it has a two-stages behavior: the first stage is due to polymerization process and the second one is due to a phase separation process. As result, the POLIPHEM structure was created. DE was near 98% (on 632.8 nm for p-polarization of the tested beam). Almost full absence of the light-scattering and high anisotropy of the grating after the holographic exposure was observed. The electro-optical properties (dependence of DE via applied electric field) of such structures are shown on the
Reactive mixture and samples were fabricated as for Example 1,2. Volume transmission gratings with periods of 1.2-0.5 μm were recorded at 22° C. The cells were placed into the interference field with wavelength 365 nm. The used light intensity was varied in the range 60÷100 mW/cm2. Kinetics of the holographic recording is shown at
| Number | Date | Country | Kind |
|---|---|---|---|
| 04 015 240.7 | Jun 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/06950 | 6/28/2005 | WO | 00 | 10/1/2007 |
