Patent Application
20080089073
The invention relates to dynamic holograms formed in liquid crystal materials. By applying an electric field, the orientation of part of the liquid crystals can be altered and the hologram can be turned on and off. The invention is suitable in e.g. dynamic holographic optical components whereby an optical function can be included/excluded in a beam path without introducing or removing elements.
Conventional holograms known in the literature are static holograms. Once the hologram is made its optical characteristics cannot be changed. Holograms that can be electrically controlled have been made by combining the advantages of liquid crystals with volume holographic gratings. First, a holographic transmission grating is formed by exposing a photo polymerizable material with a conventional two-beam apparatus for forming interference patterns inside the material. After exposure, the material is processed to produce voids in regions of the greatest exposure and the voids are infused with liquid crystals. Unfortunately, these materials are complex to manufacture and do not offer flexibility for in situ control over liquid crystal domain size, shape, density, or ordering.
Switchable liquid crystal holograms have also been fabricated in polymer dispersed liquid crystal (PDLC) materials. U.S. Pat. No. 5,942,157 discloses a PDLC material comprising a homogeneous mixture of a nematic liquid crystal and a multifunctional pentaacrylate monomer, in combination with photoinitiator, coinitiator and cross-linking agent. The PDLC material is exposed to coherent light to produce an interference pattern inside the material. Photopolymerization of the PDLC material produces a hologram of clearly separated liquid crystal domains and cured polymer domains.
It would be advantageous if it was possible to provide improved dynamic holographic elements having simple fabrication, easy operation, high transparency, low diffraction in an off-state, high diffraction efficiency in an on-state, and well defined birefringent properties.
PDLC materials applied in switchable holographic elements in the prior art are isotropic systems with no macroscopic alignment. Although a PDLC solution comprises LC components, the mixture is not a LC since the other components disturb the LC characteristics and make the molecules randomly oriented.
Contrary to PDLC materials, LC gel materials are in an anisotropic liquid crystal phase before polymerization. In the present text, the LC gel material before polymerization will also be referred to as the LC pre-gel mixture. LC gel systems are polymer-stabilized, anisotropic liquid crystal phases wherein none of the constituents, in their concentration and state before polymerisation, had the ability to disturb the (refractive index) anisotropy or liquid crystal state of the phase.
The inventors of the present invention have found that by using an anisotropic LC pre-gel mixture instead of an isotropic pre-PDLC mixture, a holographic LC gel element that perform differently than holographic PDLC elements can be produced. The production involves a two-step illumination process, which results in new and surprising characteristics of the holographic element.
Accordingly, a first aspect of the invention provides a hologram formed by exposing an interference pattern of polymerizing light inside an anisotropic LC pre-gel material and thereafter exposing the bulk LC pre-gel material to flood polymerizing light.
A LC pre-gel mixture comprises the following components
The individual components do not need to be in a liquid crystal phase as long as they do not disturb the liquid crystal ordering of the overall mixture before or after polymerization. The LC host can be any commercially available LC mixture. Various types of functional groups may be chosen for polymerization of the monomers. The monofunctional polymerizable polymer may e.g. be mono acrylate, mono epoxy, mono vinyl ether. The multi functional polymer for crosslinking may e.g. be a di- or tri- (multi) acrylate, epoxy or vinylether. Thioleene systems with a functionality higher than three reactive groups may also be used. In a preferred embodiment, reactive molecules are chosen as mesogenic molecules which show the tendency to form liquid crystal phases. The photoinitiator may be any molecule, which initiate free-radical, cationic/anionic polymerization upon exposure to light.
LC pre-gel mixtures can be photo-polymerized by illuminating the material with polymerizing radiation, typically ultraviolet (UV) light. During polymerization, networks of cross-linked polymer chains are formed which reduces the tendency of the LC molecules to align in an exterior electric field. In the present context, polymerizing as a verb means to undergo or be subject to polymerization and, as an adjective, means the ability to cause or induce polymerization in a polymerizable medium.
According to a second aspect, the invention provides a method for forming a dynamic holographic element, the method comprising the steps of:
During the first illumination step, the formation of high and low intensity regions in the LC gel phase, several processes are initiated. First, the photoinitiator molecules are split into radicals by the incident radiation. This reaction has a higher rate in the high intensity regions. This starts the polymerization reaction between the monomers and the cross-linking molecules. This reaction, as a consequence of the photoinitiator reaction, also have a higher rate in the high intensity regions, and a gradual depletion of monomers and/or the cross-linking molecules in the high intensity regions is initiated. If one of the polymerizing components is substantially less abundant than the other(s), it is only the concentration of this component which is significantly affected. This again results in a net diffusion of the less abundant polymerizing component from the low intensity regions to the high intensity regions according to Fick's law.
During the first illumination step, the initially homogeneous LC mixture become inhomogeneous with a larger concentration of polymerizing components in the high intensity regions. The composition of the LC gel phase and a scale of intensity variations in fringes of the interference pattern are preferably adapted to allow for efficient diffusion of polymerizing components from the low or no intensity regions to the high intensity regions. Also, the illuminating light of the first step is preferably applied with an average intensity and duration which allow for efficient diffusion of polymerizing components from the low or no intensity regions to the high intensity regions. Appropriate parameters depend on the given constitution of the LC gel, typical parameters are polymerization wavelength of about 350-450 nm, typically 360 nm, intensity μW-10 mW/cm2, typically 0.1 mW, and a polymerization time 1-30 min., typically 10 min. Due to the weak average intensity, the polymerization described in the above is slow and not complete.
In the second illumination step, the cell containing the pre gel mixture is illuminated with flood radiation of high average intensity. Here, the polymerization is completed in all regions. Due to the inhomogeneity created in the first step, the resulting polymer stabilization of the LCs are different in the high and low intensity regions of the first step which thereby form LC gel regions with high polymer network density and LC gel regions with low polymer network density respectively. Throughout this text, these regions will be referred to simply as high/low-network density regions.
Parameters such as the intensity of the polymerizing light and the concentration of the multifunctional reactive monomer are important for obtaining a transparent gel in the field off state and high diffraction efficiency in the field on state. Preferred concentration range of mono-functional monomer is 0-50% and multifunctional is in the range 0-3%. In the most typical embodiment mono-functional is in the range 10-30% and multifunctional in the range 0.5-1%.
In a preferred embodiment, the LC phase further comprises non-linear photo absorber having a nonlinear absorption of the polymerizing light, typically a UV absorber or a dye. The nonlinear absorption component shows a non-linear absorption behavior and above certain intensity, absorption decreases. Thereby, in the most ideal case the nonlinear absorption component reduces the amount of radiation impinging the photoinitiator in the low intensity regions while leaving the high intensity region unaffected. This will increase the effective intensity contrast between lowly and highly illuminated regions and provides high diffraction efficiency in the system. The non-linear photo absorber may e.g. be a photochromic or photo bleaching dye. Examples of such dyes can be found in
In a third aspect, the invention provides the use of anisotropic liquid crystal gel materials for the fabrication of dynamic holograms.
In a LC gel element wherein the bulk phase has been polymerized, the gel is highly transparent due to the ordered molecular alignment. When an applied voltage exceeds a threshold voltage, the exerted torque from the electric field exceeds the resistance by the polymer network. As a result, LC molecules start reorienting in the direction of the applied electric field. The threshold voltage (Vc) of a uniaxially oriented system is given by the equation below.
Vc=π(K1/ε0Δε)0.5 (1)
where K1 is the splay elastic constant, ε0 is the permittivity of the free space and Δε is the dielectric anisotropy of the material.
In the dynamic holographic elements according to the invention, regions forming an ordered structure are illuminated first where after the bulk phase is illuminated. The resulting phase contains regions of polymer networks with different crosslink density and thereby different elastic constants and threshold voltage for reorienting the LC molecules.
Hence, in the first and second aspects, polymerized anisotropic LC gel materials preferably comprise low-network density LC gel regions and high-network density LC gel regions formed by exposing the interference pattern inside the LC gel material so that the high-network density LC gel regions form an ordered structure in the low-network density LC gel regions.
Also, in a fourth aspect, the invention provides a dynamic holographic element comprising a cell holding an anisotropic liquid crystal (LC) gel phase, the cell comprising orientation layers to induce macroscopic alignment of the pre gel mixture positioned on top of first and second electrodes positioned on opposite sides of the cell to impose an electric field over the LC gel phase, the LC gel phase comprising low-network density LC gel regions and high-network density LC gel regions,
The threshold voltage is the voltage which must be applied to the first and second electrodes to induce a realignment of the LC molecules. The network densities in the LC gel influence their ability to change alignment when influenced by an external force and is therefore closely related to the threshold voltage.
When there is no electrical field imposed over the anisotropic LC gel phase, the LC molecules of the low-network density and high-network LC gel regions have at least substantially the same orientation. When a voltage is applied to the electrodes, the electric field will cause a change in the alignment of the LC molecules, which is larger for the low-network density regions than for the high-network density regions. If the applied voltage is lower than the threshold voltage of high-network density regions but higher than the threshold voltage of low-network density regions the change in alignment of the molecules only takes place in low-network density regions.
This is to be seen in contrast to the different regions in PDLC elements. In PDLC, the polymer phase is isotropic and the LC molecules within the system are not macroscopically aligned with respect to each other at zero electric field. As mentioned previously, the pre-PDLC solution was not in a LC state prior to polymerization due to the added components. This means that the LC host molecules were randomly oriented upon polymerization leading to an isotropic polymer matrix. If the polymerization was performed with an interference pattern, droplets of LC host are formed between the isotropic polymer-rich regions. Upon imposing an electric field, the LC droplets align leading to a diffraction contrast between the aligned droplets and the isotropic polymer matrix.
In short, in the LC gel element of the present invention, different regions of already macroscopically aligned (and thereby anisotropic) LCs change alignment differently upon application of an electrical field. In the PDLC element of the prior art, regions (droplets) of non-aligned LCs will start aligning upon application of an electrical field whereas other regions (isotropic polymer-rich matrix) will not.
The ordered structure preferably forms a pattern or a grating representing a reflection, refraction or transmission of light by an object or a component. The structure is ordered so that the low- or high-network density regions are not randomly distributed throughout the LC gel phase.
Preferably, the ordered structure of the high-network density LC gel regions have been formed by exposing an interference pattern inside the LC gel. The ordered structure of the high-network density LC gel regions may be arranged to form a diffraction pattern or grating in the low-network density LC gel regions. The ordered structure of high-network density LC gel regions may form a hologram of an optical component in the low-network density LC gel regions.
Also, the first and second electrodes may each comprise a number of individually addressable electrode parts so that an electrical field can be applied to a selected volume of the LC gel phase
In a fifth aspect, the invention provides a dynamic light emitting setup comprising a dynamic holographic element according to the fourth aspect and one or more first light sources positioned so that light to be emitted from the one or more light source will be transmitted by the dynamic holographic element.
The first light source emits a beam and may include passive optics such as a reflector or a lens to shape the beam. In a preferred embodiment, a first light source is a light emitting diode (LED) having a first primary color.
The light emitting setup may also contain more light emitting diodes emitting other primary colors and their intensity can be controlled individually. In this way the color and/or the color temperature of the light source can be changed by color mixing. When such dynamic holographic element is combined with such a light source, a set up with color as well as beam control is obtained.
The basic differences between PDLCs and LC gels outlined above give rise to a number of advantages of the present invention over the prior art. These advantages of the present invention solve a number of disadvantages of the prior art solution which was realized by the inventors of the present invention.
Due to the anisotropy prior to polymerization, all regions are aligned in the electric field off-state in the LC gel element of the present invention. Upon application of an electrical field, different regions change orientation differently. This provides the advantage that the element will show polarization dependence or birefringence at all times in all states, regardless of the electric field. This is often a requirement in optical set-ups.
In the PDLC element of the prior art, droplets of non-aligned LCs will start aligning upon application of an electrical field whereas the polymer matrix will remain isotropic. Therefore PDLC based holograms are not macroscopically aligned and do not show polarization dependence. This makes them unsuitable if polarization dependent operation is required.
The present invention suggests the use of holography in order to produce structures in LC gels. During holographic illumination, various areas of a LC pre-gel mixture will be illuminated at another intensity leading to formation of regions with different cross-link density in the pre-gel mixture. Cross-link density in the pre-gel mixture determines the threshold voltage (Vc) of the corresponding high/low-network density LC gel regions after completed polymerization.
The high- and low-network density LC gel regions of the element according to the invention have different concentrations of polymerized components. Thus, the high- and low-network LC gel regions are different regions of essentially the same phase and have at least substantially the same refractive index along any given axis. This means that it is possible to make the element transparent in the state of zero electrical field so that the hologram appears only during application of an electric field.
In PDLC elements, the LC droplets and the polymer matrix are essentially different phases with different refractive indices. In such an element a hologram is visible in the state of zero electrical field. It is therefore necessary to apply a voltage to reach both the optimum on-state and the optimum off-state so that it is always necessary to use an electric field, which is considered a major disadvantage. Further, such hologram is mostly not in the optimum state as the refractive index difference between the different regions of the hologram is difficult to control. If e.g. the operation temperature is changed, the bias voltage in the field off and field on states will need to be altered.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
FIGS. 18 A-C show schematic representations of dynamic holographic elements according to the invention in combination with a light source.
FIGS. 19 A-G show structures of some non-linear absorbers applicable in the present invention.
Figures are preferably schematically drafted in order to facilitate the understanding of the invention. Therefore other designs that could be drafted in the same schematic way are implicitly also disclosed in this document.
Furthermore it is important to have high diffraction efficiency. In order to get high diffraction efficiency from such a grating, the duty ratio should be 50% (i.e. x1=x2) and the phase difference needs to be half a wave (i.e. d*[nH−nL]=λ/2).
In
Here, the cross linker is C6M, a diacrylate shown in
It can also be seen that the threshold voltage remained constant up to a certain cross linker concentration, above which the threshold voltage rapidly increases. The fact that the threshold voltage shows an increase above a critical concentration indicates that the elastic constant in the expression (1) for the threshold voltage shows an increase above this concentration, corresponding to the gel-point of the system. At this concentration a three-dimensional network is created by the side-chain polymers formed by the monoacrylate molecules cross-linked by the diacrylate molecules. It can be seen from
In the following, we describe two different anisotropic gel systems used to study holographic recording. One of the systems is uniaxially oriented gel with a positive birefringence “Gel 1”. The other system is a gel with a negative birefringence “Gel 2”. Gel 2 is obtained using a chiral system with a very short pitch comparable that of the wavelength of light. Such a twisted configuration gives the system negative birefringence. Furthermore such a negative birefringent system has the property of showing no polarization direction dependence for light falling perpendicular to the cell.
The Gel 1 system comprises
The Gel 2 system comprises
We produced gratings using the holographic set up shown in
A series of measurements of how much diacrylate (cross linker) was necessary with a given monoacrylate (monomer) concentration for a system to reach the onset of good diffraction efficiency was conducted.
A linear regression of the curve of
Ccrosslink−1=0.08·Cmonomer+0.13 (2)
which may be used as a guideline for determining proper relative amounts of cross linker and monomer.
It was also determined that the intensity of the zero order peak from gratings could be decreased further when the system was provided with a nonlinear photo absorber, e.g. a dye, in the LC pre-gel mixture.
As can be seen, the addition of dye increases the diffraction efficiency considerably; from I0=8.5 at zero dye concentration to I0=3.5 at 0,2% dye concentration. Adding more dye slowly deteriorates the extinction of the zero order, most likely by introducing more scattering in the system. It appears that the optimum dye concentration is to be in the interval 0<Cd≦0.2%, at least for dye molecule 11646. Another dye molecule 457 was also found be working effectively. The structure of these dyes is shown in
The effect of the nonlinear absorption component is attributed to its strong absorption at low intensities and weak absorption at high intensities. Thereby, in the fringe pattern shown in
There are a large number of molecules, which can be used as the liquid crystal host in a LC pre-gel mixture. Structures of a non-exclusive list of applicable LC molecules are shown in of the structures in
In the above description, the fabrication of dynamic LC gel holographic elements of a grating and a lens is shown. It is possible for the person skilled in the art to produce dynamic LC gel holographic elements representing any other optical components.
Such optical elements can be used in combination with a light source with or without beam shaping optics. The holographic element can be placed in such a system in order to dynamically alter the shape or direction of the light beam.
Upon switching the holographic element 20 on (V≠0) using a voltage source, the ordered structure of the hologram cause the incident beam to diverge as shown in
The light source may emit a white light. However it may also consist of a plurality of light sources emitting different primary colors, typically light emitting diodes. If the intensity of the light sources emitting the different colors can be individually controlled, then the color and/or the color temperature of the light can also be adjusted. When such light source is combined with a dynamic hologram a dynamic light source with color and beam control can be obtained.
In the above description the term “comprising” does not exclude other elements or steps and “a” or “an” does not exclude a plurality. Furthermore the terms “include” and “contain” does not exclude other elements or steps.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04106070.8 | Nov 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB05/53846 | 11/21/2005 | WO | 11/12/2007 |
