JP2007538293A - Optical switching device using holographic polymer dispersed liquid crystal - Google Patents

Abstract

A thin holographic optical switch (100) used in a liquid crystal display device (800) includes an opposing transparent substrate (102), a transparent electrode (104) between the substrates (102), and an electrode. And (104) a diffraction grating (105). The diffraction grating (105) includes a transparent polymerized photopolymer region and a cholesteric liquid crystal microdroplet region. The refractive index of the photopolymer and the refractive index of the liquid crystal are substantially the same when the electrodes have the same potential. The holographic optical switch (100) transmits broadband LED light and is not polarization dependent when the potential difference between the electrodes (104) is zero. The holographic optical switch (100) diffracts broadband LED light when the potential difference between the electrodes (100) is non-zero and is not polarization dependent.
[Selection] Figure 1

Description

  The present invention generally relates to an optical device capable of switching the direction of collision light. More specifically, the present invention relates to a holographic optical switching device incorporating a polymer-dispersed liquid crystal in a liquid crystal display device and other electronic devices.

  Liquid crystal display devices (LCDs) have a number of advantages over other types of display devices. LCDs are small, high in image quality, and lightweight. Furthermore, the LCD consumes relatively little power, depending on the type of LCD. For these reasons, LCDs are used exclusively in the portable electronic device market for applications such as small portable TVs, mobile phones and other communication products, video recording devices, notebook computers, and desktop monitors. Yes.

  The active LCD is the most popular LCD among the LCDs in use, and is provided on one of a substrate, a liquid crystal layer through which light passes, and a light guide by supplying an electric field to the liquid crystal layer. And a pixel electrode forming a panel. The metal used to fabricate the pixel electrode depends on the type of LCD used. Since a reflective LCD uses a natural or artificial light source located outside the LCD, the material used for the pixel electrode must be a reflective conductive material such as metallic aluminum. However, when the intensity of the external light is not sufficiently high, the image displayed on the reflective LCD is low quality.

  In order to overcome the above problem, an internal light source called a backlight is added to the light guide panel. The backlight is supplied to the liquid crystal layer from a fluorescent lamp, a light emitting diode (LED), or an electroluminescent diode (EL). Since the backlight is provided behind the display device, the material used as the pixel electrode must be a transparent conductive material such as indium tin oxide (ITO). However, in addition to increasing the size of the LCD (and adding weight and cost), the backlight consumes the most power in the LCD, which can significantly reduce battery life when used constantly. .

  Therefore, reducing power consumption has become an urgent research subject, and transflective LCDs have been developed. In the transflective LCD, the pixel electrode is a foldable type in which aluminum is disposed at a certain location and ITO is disposed at another location. In this way, if the external light has enough intensity to provide a good quality image, use the external light as a light source, and if the external light does not have sufficient intensity, use the backlight as a light source. Will be able to use. However, the area where the image is displayed is narrow in both the transmissive mode and the reflective mode in the transflective LCD.

  Polymer dispersed liquid crystal (PDLC) layers are employed for switching in optical fiber communication applications. Although there are few examples of use, the PDLC layer is used in a transflective LCD to widen the display area. PDLC is a photovoltaic material that allows light to pass through the material when a voltage is applied to the structure and renders the structure relatively opaque by scattering incident light when no voltage is applied. PDLC is a mixture of monomers or oligomers and liquid crystal molecules, after which the polymer / oligomer is polymerized to form a polymer. The liquid crystal molecules aggregate into fine droplets and are dispersed within the polymer matrix under certain conditions. Since the PDLC layer is disposed between the backlight and the liquid crystal panel, directly below the liquid crystal panel, in the reflection mode in which no voltage is applied to the PDLC layer, external light is scattered and the liquid crystal display unit emits light. However, in the transmissive mode in which a voltage is applied to the PDLC layer, the PDLC becomes transparent and light from the backlight (spread over the surface area) strikes the liquid crystal layer directly from below.

  In addition, there is a transflective LCD in which the backlight is not directly below the liquid crystal panel. Alternatively, the backlight may be adjacent to the light guide layer below the liquid crystal layer. The light guide layer guides light to the liquid crystal layer. A PDLC layer has never been used with the above structure. In addition, with the advent of technology with increased complexity, a new usage of a light source built into a transflective LCD has also emerged. A straightforward example is the use of an internal light source that illuminates the area in front of the electronic device without the need for an additional light source. More specifically, in the case of a camera-equipped mobile phone, the backlight is used, for example, to illuminate a subject so that a practical photograph or continuous photograph can be taken. However, there are no structures or optical switches that can provide the above capabilities within acceptable limits required for small electronic devices such as cost, size, weight, durability, and minimum power consumption.

  First, as an overview, the present embodiment provides an optical switch that is preferably used in small electronic devices and can be used anywhere. Optical switches transmit light from a broadband light source, such as a light emitting diode (LED), without applying a voltage to the switch and without substantial attenuation over the entire wavelength range of the light source. The switch is so compact that it can be mounted on, for example, a camera-equipped mobile phone without being visible to additional dimensions, and is light enough not to add a noticeable weight. The switch is polarization independent and transmits and diffracts both S and P polarized light without substantial loss. That is, the material of the switch compensates for the various polarizations so that the switch acts substantially similarly on both the S and P polarizations of light from the light source without depending on the voltage applied to the switch. . The switch is non-mechanical as long as it does not require the use of shutters or other elements, so it is a sturdy component that does not easily slip or break when the electronic device is dropped or subjected to a physical shock. Can be provided.

  An optical switch according to an embodiment includes an opposing substrate and an electrode disposed between the two substrates, and a Bragg grating is provided between the electrodes. The Bragg grating includes a region of polymerized photopolymer and a region where liquid crystals are aggregated. When the electrodes have the same potential, the refractive index of the photopolymer and the liquid crystal is substantially the same.

  A liquid crystal display device including one optical switch according to an embodiment of the present invention is disposed as a liquid crystal layer on opposite sides of a substrate, electrodes on the substrate, a liquid crystal layer between the electrodes, and opposite sides of the substrate. And a liquid crystal display unit having a light guide unit disposed on one of the polarizing plates. The optical switch is disposed between the light guide unit and the light source. The optical switch includes an opposing optical switch substrate, an optical switch electrode disposed between the optical switch substrates, and a Bragg grating disposed between the electrodes. The Bragg grating has a region of a polymerized photopolymer and a region of a liquid crystal aggregate, and is substantially independent of polarization.

  In another embodiment, a method of manufacturing a liquid crystal device includes placing an LED and a holographic polymer dispersed liquid crystal (HPDLC) adjacent to each other, and light entering the HPDLC from the light source applies a voltage to the HPDLC. And adjusting the HPDLC so that it can be directly transmitted through the HPDLC without being substantially diffracted.

  In another embodiment, a method of making a holographic polymer dispersed liquid crystal (HPDLC) comprises combining a monomer and liquid crystal to form a mixture; and the resulting mixture between two bonded glass substrates. Filling the cavities; starting the polymerisation of the bonded substrates in the high-intensity region of the interference pattern, the liquid crystal diffuses into the low-intensity region, saturates, settles into aggregates, and the liquid crystal and polymer Exposing it to a cross-coherent radiation beam having sufficient intensity to cause phase separation by concentration for a sufficient period of time; irradiating the exposed mixture with a uniform radiation beam to cure the liquid crystal aggregates And without causing voltage to be applied to the mixture that has been overfilled, and allowing the refractive index of the aggregate to be the same as the refractive index of the matrix.

  The above summary has been provided as an overview. Nothing in this section should be considered limiting to the claims that define the scope of the invention.

  As described above, in the transflective LCD, light from the light source is supplied to the LCD through the light guide layer of the LCD and the transparent substrate. Usually, no switch is provided between the light source and the liquid crystal panel. Although mechanical switches can be employed, such switches are relatively bulky and add considerable weight to the LCD and the device in which the switch is housed. Also, the size of the LCD is greatly increased and becomes relatively fragile (ie, if the device is dropped or some physical shock is applied, the switch can easily be broken, disconnected, or displaced) and requires large power / current This is a serious drawback for the use of mechanical switches. In order to switch and alleviate the above problems, HPDLC is integrated into the device. The HPDLC according to the present invention dramatically shortens the switching response time and the response time of the mechanical switch is typically 10-20 milliseconds, whereas with the HPDLC of the present invention, the response time ranges from 10 μs to 1− 2 milliseconds, 1 to 3 orders of magnitude faster. Since there are no moving parts in the optical switch based on HPDLC, the time until failure occurs is much longer.

  There are a number of problems when using conventional HPDLC. Although not limited to this, one major problem is that the entire amount of light from the internal light source cannot be used in the conventional HPDLC. That is, since the conventional HPDLC has polarization dependency, for example, the amount of light transmitted through the conventional HPDLC is immediately halved. In any configuration in which the HPDLC is arranged between the light source and the light guide panel, only half the amount of light can be used. Therefore, in order to make the amount equal to the amount of light hitting the light guide without the conventional HPDLC, a larger amount Power is required.

  In addition, since it is increasingly necessary to make electronic devices, particularly portable devices, thinner, it is less desirable to add more multilayer elements to the liquid crystal panel. Therefore, the HPDLC is disposed between the light source and the light guide unit, and the liquid crystal display unit reflects the liquid crystal panel, the light guide unit, the HPDLC, the light source, and the light source for reflecting the light from the light source back toward the HPDLC. The lower opaque reflector can be included, but other embodiments may be more suitable for minimizing electronics.

  In some embodiments, the HPDLC is positioned adjacent to the light guide, as shown in FIG. 4 and described below. In other words, the HPDLC is arranged in the lateral direction near the end of the light guide, not under the light guide. The arrangement of the end portions of the light guide is hereinafter referred to as end mounting HPDLC, while the arrangement under the light guide is referred to as surface mounting HPDLC. This reduces the thickness of the structure and uses HPDLC without substantially increasing the overall device dimensions (if any) by placing other electronic components nearby in the same manner. Will be able to. Such a structure makes it possible to make use of diverted light somewhere if necessary.

  However, if the conventional HPDLC is used, the above polarization problem still occurs. Another problem with the conventional HPDLC is that light cannot be transmitted directly to the optical switch unless a voltage is applied. This means that the conventional HPDLC transmits light only when a voltage of several volts or more (usually 1 V to 20 V) is applied to the HPDLC. Therefore, in the PDLC of the end-mounted light source, a large amount of power is consumed to place the optical switch in the transmission state, which is a normal operation state of the apparatus.

  An example of such a conventional HPDLC is shown in FIG. 9A. In this example, the response of the grating depends on the polarization. As shown in the figure, in the S-polarized laser beam of 422 nm, when the applied voltage increases from 0V to 40V, the diffraction efficiency decreases from about 15% to less than 5%. However, with P-polarized light, the diffraction efficiency drops from nearly 100% to almost 0% even in the same voltage range. In other words, the transmission efficiency of the conventional HPDLC increases from 85% to over 95% and from 0% to almost 100% for S and P polarized light, respectively. It is necessary to apply a relatively high voltage.

  By using the HPDLC disclosed herein, an optical switch that is mechanically stable and uses less power can be formed. Holographic optical switches are ideal for use in portable electronic devices or other devices where a thin, low power switch is required. For example, such an optical switch can be used to provide an additional light source for a camera phone, so that relatively short distance photographs can be taken without using an external power source. Appropriate selection of the photopolymer matrix and liquid crystal and the grid dimensions encapsulated in it can maximize the transmission of the switch without applying a voltage to the structure, while at the same time being substantially polarization dependent. You will be able to provide no switch. Thus, light from a broadband light source such as an LED can be used for the liquid crystal display unit. Light can be coupled to the light panel relatively easily with the exposed edge structure. The gratings exposed here are relatively thin and have a thickness of less than 10 μm, typically 2 μm to 4 μm, while having sufficient diffraction efficiency at the desired applied voltage (eg, maximum transmittance). Providing a relatively broad passband (sufficient to adequately transmit light from the light source, with substantially no attenuation, such as 10% of at least about 10 μm).

  Holographic polymer dispersed liquid crystal (HPDLC) is an optical device formed by recording a Bragg grating (also known as a three-dimensional phase grating, hologram or holographic grating, or diffraction grating) in a polymer dispersed liquid crystal mixture. is there. As shown in FIG. 1, the HPDLC device 100 uses a two parallel substrates 102 whose side surfaces are sealed in a cavity formed by combination in the same manner as in the manufacturing process of the liquid crystal panel of the liquid crystal display unit. Fabricated by filling a mixture 106 of polymer and liquid crystal material. Both substrates 102 are spaced evenly using spacers (not shown) such as spherical inserts or prismatic protrusions.

  The substrate 102 is made of glass or other material that is substantially transparent to a specific wavelength range, such as the visible wavelength range of about 400 nm to about 800 nm. The substrate 102 may be provided with an anti-reflective coating to improve the transmission in the specific wavelength range of interest to 99% or higher.

  Examples of one or more glasses forming the substrate 102 include BK7, FK51, and LAK6, although certain glasses have little effect on the outcome of switching. Other materials that can be used in place of or in addition to glass include, for example, quartz and plastic. Examples of the type of plastic substrate include polyesters such as polyethylene terephthalate (PET) or polyvinyl alcohol (PVA), polycarbonate (PC), or triacetyl cellulose (TAC). For a birefringent substrate, for example, a uniaxially stretched plastic film may be used. The substrate may be coated with abrasive polyimide. When a birefringent substrate or polishing polyimide is used, the polarization dependency of the optical switch is lowered. If a polarizing material is used as the thin layer 106, the substrate may or may not be removed after the polymerization. If the substrate is not removed from the polymer film after polymerization, an isotropic substrate may be used.

  One or both substrates support a transparent electrode 104. The electrode 104 may be formed of any of a wide variety of transparent films such as indium tin oxide or indium zinc oxide films. A voltage is supplied between the electrodes 104 and an electric field is applied between the HPDLC layers 106.

  When the grating 100 is manufactured, as described above, the liquid crystal material 106 that can be polymerized is filled in the space between the two substrates including the transparent electrode 104 so that the liquid crystal material is uniformly oriented. To align. Here, the orientation of the liquid crystal material is permanently fixed by the solid polymer structure after formation.

  Polymerization of the polymerizable liquid crystal material is realized by, for example, exposure to coherent radiation. As a radiation source, a laser is generally used. UV, visible, or IR wavelengths can be used, and X-rays, gamma rays, or other high-energy particles such as ions and electrons can also be used. Thus, the radiation may be photolithography radiation, ie, radiation used in standard photolithography processes including exposure through a phase mask. Depending on the application and formulation, the mixture may additionally contain photoinitiators, surfactants, and other ingredients. When there is a photoinitiator, the photoinitiator is absorbed at the wavelength of radiation when polymerized. For example, when polymerizing using a UV laser, one or more photoinitiators can be used that generate free radicals or ions that decompose under UV irradiation to initiate the polymerization reaction. Examples of commercially available photoinitiators include Irgacure 651, Irgacure 184, Darocure 1173, or Darocure 4265 (above Ciba Geigy) or UVI 6974 (Union Carbide). In any photoinitiator, the photoinitiator included in the total composition amount is from about 0.01% to about 10% by weight.

  The curing time depends on the reactivity of the polymerizable material, the thickness of the coating layer, the type of polymerized initiator, and the intensity of the radiation source. The curing time should be as short as possible when high throughput processing is desired. In general, the curing time is at most several minutes.

  In addition to the polymerizing initiator, the polymerisable material may include one or more other suitable components, such as catalysts, stabilizers, chain transfer agents, or co-reacting monomers. In particular, the addition of a stabilizer prevents, for example, the polymerizable material from unintentionally spontaneously polymerizing during storage.

  As stabilizers, all compounds suitable for this purpose known to those skilled in the art can be used. These compounds are widely available commercially. Representative examples of stabilizers include 4-ethoxyphenol or butylated hydroxytoluene (BHT).

  As other additives, for example, a chain transfer agent may be added to the polymerizable material in order to modify the physical properties of the polymer film. When a chain transfer agent such as a monofunctional thiol compound such as dodecanethiol or a polyfunctional thiol compound such as trimethylpropane tri (3-mercaptopropionate) is added to the polymerizable material, It is possible to control the length of the free polymer chain and / or the length of the polymer chain between two crosslinks of the polymer membrane of the invention. When the amount of the chain transfer agent is increased, the length of the polymer chain of the resulting polymer film is shortened.

  As shown in FIG. 2, a typical fabrication apparatus 200 includes a UV laser 202 that emits a laser beam 201 having a wavelength shorter than about 400 nm. The laser beam 201 is controlled by a shutter 204 that opens or closes depending on whether the presence of the laser beam 201 is desired. The shutter 204 may be placed basically anywhere along the path of the laser beam, as long as the beam hits the grating structure containing the liquid material 214. The laser beam 201 then strikes the polarizer 206, becomes a polarized laser beam 201 and enters the beam expander 208. The beam expander 208 increases the radius of the laser beam 201 to form the expanded beam 203. By enlarging the laser beam 201, the entire area of the grating structure 214 can be covered by the expanded beam 203, which in one embodiment is a beam column with a cross-sectional area of about 100 cm2. A large exposure area provides other advantages, such as more precise control of the power density of the exposure and thus loose manufacturing tolerances and high efficiency. The beam splitter 210 divides the expanded laser beam 203 into two coherent beams, and the divided beams are directed by the mirror 212 and irradiated on the grating structure 214. The two beams may or may not be identical. High quality front mirrors are used. Typical laser power used to produce the HPDLC of the present invention is about 10 mW to about 500 mW when the wavelength is in the UV region. Typical cure times for the HPDLC structures of the present invention are from about 1 second to about 300 seconds.

  The Bragg grating 214 is formed in the beam crossing region by the interference between the divided enlarged laser beams 203. FIG. 3 shows an enlarged view of the state in which the object beam and the reference beam collide at the intersection region. The diffraction grating 214 is a result of generating an interference pattern by the interaction of polarized beams in the photopolymer dispersed in the liquid crystal. When the photopolymer is exposed to this interference pattern, the photopolymer itself is patterned in the same way, so that the interference pattern is actually embedded in the cell.

  More specifically, a homogeneous mixture of liquid crystals and monomers and / or oligomers, and a polymer syrup cell containing a photoinitiator and a surfactant are placed in the center of the interference region. During the recording process, the photopolymer becomes polymerized and the mixture undergoes phase separation, creating areas with densely packed liquid crystal droplets, interspersed with transparent polymer areas. That is, the electrically switchable grating is formed from a polymerized organic matrix having a periodic pattern defined by holography by microphase separation of a small molecule liquid crystal. Polymerization is initiated by exposure to high intensity regions of the interference pattern. The term liquid crystal droplet simply refers to a mass of liquid crystal, not a specific shape such as a teardrop or sphere.

  The interference of two plane waves in the medium can be described as the sum of two electric fields. The optical intensity (square of electric field amplitude) is quadrupled in the high intensity region. This means that when constructive interference occurs, the intensity is 4I0, and when destructive interference occurs, the intensity is zero. In this way, constructive interference occurs when the wavelength amplitude is added, and the intensity is maximum in the high intensity region, and destructive interference occurs when the wavelength amplitudes cancel each other, and the intensity is minimum in the low intensity region. It becomes.

  In the high strength region, the monomers begin to cross each other and form polymer chains. In the low-strength region, polymerization hardly occurs. Other monomers or oligomers diffuse into these bright areas and rapidly crosslink into polymer chains. At the same time, the liquid crystal diffuses into the low density region, saturates and settles the droplets, which grow larger as the diffusion process continues. A time shutter is used to remotely control the exposure time. When the diffusion process reaches an appropriate stage, the entire mixture is flooded with uniform light, and the entire liquid crystal droplet is covered with a completely cured polymer matrix to form a solid lattice layer. A configuration in which regions having a large amount of liquid crystal and regions having no liquid crystal are alternately arranged forms a striped surface of the lattice. Typical sources of uniform light include lasers or conventional UV light.

Bragg gratings exhibit very high diffusion efficiency, which is controlled by the strength of the electric field applied between the HPDLC layers. A collection of droplets “looks” as a homogeneous region having an effective refractive index (n _LCM ) that is different from the effective refractive index (n _p ) of the interspersed polymer region. The refractive index modulation is Δn = n _LCM −n _p . The basic parameters are shown in the examples of FIGS. 4A and 4B, where the grating period Λ (also referred to as the grating spacing), refractive index modulation, tilt angle Φ, and grating vector K, However, these will be described later.

  The created HPDLCs are classified into two types, transmission holograms and reflection holograms, and are shown in FIGS. 5 (a) and 5 (b), respectively. In a transmission grating, the incident beam and the diffracted beam are located on opposite sides of the grating. In a reflective grating, the incident beam and the diffracted beam are located on the same side of the grating.

Unlike conventional gratings, the refractive index modulation Δn = n _LCM -n _p changes based on the electro-optic effect. In other words, the liquid crystal in the droplet responds to the application of an external electric field through the electrode. When an external electric field is applied, the director of the liquid crystal molecules aligns with the electric field, so that the HPDLC grating switches between the two states. The natural orientation of the liquid crystal droplets changes from an uneven orientation when no electric field is applied to a state aligned with the applied electric field. In one state, the droplet and the polymer matrix have the same refractive index (n _p = n _LCM ) and the entire composition appears to be a uniform material with one refractive index. In the other state, the liquid crystal rich region and the matrix differ in refractive index by Δn = n _LCM −n _p and the entire composition exhibits a well-defined lattice.

  Although the grating is shown as containing one material layer containing a polymerized photopolymer and liquid crystal aggregates, it may be a multilayer containing different materials. Such layers include one or both of different polymeric photopolymers and liquid crystal aggregates, each layer being formed of a different material and / or composition than one or more other layers. .

In the HPDLC grating, as shown in FIGS. 6 and 7, the grating switches between two states because liquid crystal molecules align when an external electric field is applied. In one state, the droplet and the polymer matrix have the same refractive index (n _p = n _LCM ) and the entire composition appears to be a uniform material with one refractive index. In the other state, the region rich in liquid crystal and the matrix differ in refractive index by Δn = n _LCM −n _p and the entire composition exhibits a well-defined lattice.

  In the conventional HPDLC, the refractive index of the liquid crystal droplets arranged in a direction perpendicular to the LC-polymer interface is different from the refractive index of the polymer region, so that the refractive index of the structure is modulated. When an electric field is applied, the liquid crystal droplets are uniformly oriented in the same direction, and the refractive indexes of the liquid crystal droplets and the polymer region are finally equal. In other words, the refractive index of the polymer region and the average refractive index of the liquid crystal droplets match only when a specific non-zero electric field is applied to the structure (hereinafter referred to as the maximum voltage). Thus, when a voltage is applied to the electrodes, the refractive index modulation of the stripes is reduced, and the hologram diffraction efficiency is lowered to a very low level. In other words, the maximum voltage is applied between the electrodes. When the light hits the device, it basically passes completely through the device as shown in FIGS. 6 (a) and 6 (b).

Specifically, the electrically switchable grating is basically a three-dimensional phase grating whose diffraction characteristics can be predicted well using the Kogelnik model. The diffraction properties can be modeled as the index modulation is given by the following equation:

Here, n ₀ is an average refractive index, K is a lattice vector, and r is a position coordinate. In equation ( ₁ ), n ₁ is the refractive index modulation amplitude given by:

Where f _c is the volume fraction of the phase separated liquid crystal in the lattice, n _p is the refractive index of the polymer, n _LCM is the average refractive index of the liquid crystal droplets, and α is the ratio of the liquid crystal droplets to the lattice period Λ. is there. For inline reflective gratings, the equation for diffraction efficiency for TE and TM polarization is also the same as that predicted by Kogelnik and is given by:

Here, R is peak reflection, d is the grating length, λ is the Bragg wavelength, and n ₁ = n _p + f (n _LCM −n _p ).

  One parameter that affects performance characteristics is refractive index modulation. The refractive index modulation is usually in the range of 0.01 to 0.2 in order to achieve high diffraction efficiency (about 100%). The HPDLC cell thickness (interaction length) is in the range of 1 to 25 microns. The optimized manufacturing process employs a proportion that balances diffusion and polymerisation.

Based on the requirements of the application, different film thicknesses are designed and placed on the target substrate for lattice recording. For reflective gratings, the passband width of the filter can be calculated according to the following formula:

  Here, λ is a recording wavelength, γ is a spatial frequency of the recording interference measurement grating, and T is a film thickness. Therefore, when the recording wavelength and the spatial frequency are approximately the same size and the film thickness is approximately 4 mm, the passband frequency is approximately 300 nm, which is a sufficient width to allow light from the LED to pass without substantial attenuation. .

  However, unlike conventional HPDLC gratings, if the grating elements of the HPDLC optical switch of the present invention are carefully selected, the refractive index of the polymer region matches the average refractive index of the liquid crystal droplets without applying an external electric field between the matrices. Can be made. Accordingly, fringe refractive index modulation and hologram diffraction efficiency are minimized when no voltage is applied to the electrodes and maximized when a maximum voltage is applied between the electrodes. This is shown in FIGS. 7 (a) and 7 (b).

  Furthermore, since both the grating structure and the liquid crystal droplet are polarization sensitive, polarization sensitivity is compensated by adjusting the grating geometry and structural engineering. For example, the grating structure is taken so that the diffraction angles for both polarizations are close, and then the material is chosen to compensate. In some embodiments, a polymer having a relatively high refractive index and the ability to polymerize well is selected prior to selecting the liquid crystal material. The relatively high refractive index polymers used herein have a refractive index greater than about 1.55, desirably greater than about 1.58. The relatively high refractive index of the polymerized photopolymer is sufficient to match the effective refractive index of the liquid crystal aggregate when the electrodes have the same potential. Cholesteric liquid crystal is generally selected instead of nematic liquid crystal because of its refractive index insensitive to polarized light.

  Specifically, due to the elongated shape of the liquid crystal, light will propagate at a different speed when propagating parallel to the direction of the elongated molecule than when propagating perpendicular to the elongated molecule. So molecular birefringence means that two refractive indices explain this behavior. A nematic liquid crystal that is arranged in parallel but not in the lateral direction and thus has a constant director cannot be used alone because the refractive index depends on the focal length ae of the main axis. On the other hand, cholesteric liquid crystals are arranged slightly at an angle to each other (not parallel like nematic). Each successive molecule is slightly rotated with respect to each adjacent molecule, so it has a director that rotates in a spiral, and the refractive index depends on the average focal length of the principal and semi-major axes (2ao + ae / 3) To do. However, as will be described later, the combination can be used by adding a chiral dopant to the nematic liquid crystal and twisting the director of the nematic liquid crystal.

  Photopolymers with different refractive indices were formulated from a wide variety of commercially available monomers, oligomers, photoinitiators, and optional adhesion promoters, surfactants, and the like. Since photopolymerization kinetics directly affects the phase separation of the liquid crystal from the polymer matrix, one idea is to use a photopolymer formulation with a reasonable cure rate. In this regard, acrylate photopolymers are preferred because they have better cure speeds than other photopolymers such as epoxies and vinyl ethers. High refractive index monomers and / or oligomers useful in the present invention include one or a sulfur component, a bromine component, or a bisphenol A derivative.

  Two examples of photopolymers are given below. List each component of the polymer and list the weight percent after each component.

  Photopolymer 1: 52% 2-ethylhexyl acrylate, 2% ethoxylated bisphenol A diacrylate, 17% polyester tetraacrylate, 10% phenylthioethyl acrylate, 16% octafluoropentyl acrylate, diphenyl (2,4,6-trimethylbenzoyl) ) Phosphine oxide 1%, Dalacure 1173 1%, Methacryloyl trimethoxysilane 1%.

  Photopolymer 2: 16.5% ethoxylated bisphenol A diacrylate, 64.5% phenylthioethyl acrylate, 16% octafluoropentyl acrylate, 1% diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide, DaraCure 1173 1 %, Methacryloyl trimethoxysilane 1%.

  A wide variety of commercially available cholesteric liquid crystals are suitable for the fabrication of HPDLC devices. Merck offers a wide range of cholesteric liquid crystals for HPDLC and can be used in the HPDLC device of the present invention when combined with suitable photopolymers. Examples of commercially available cholesteric liquid crystals include Merck BL112, BL118, or BL126, commercially available from EM Industries, Hawthorne, NY. A cholesteric liquid crystal can also be produced by adding a chiral dopant to a nematic liquid crystal. Examples of chiral dopants include Merck C15, CB15, ZLI-811, ZLI3786, ZLI-4571, ZLI-4572, MLC-6247, and MLC-6248, which are also commercially available from EL Industries, Hawthorne, NY . The HPDLC syrup was obtained by combining the photopolymer formulation with the liquid crystal. The liquid crystal level as a weight percent of the total syrup is in the range of about 35-75%, more precisely in the range of about 45-60%.

The material composition and performance data are listed in Table 1 below. In the table, Example 1 is a conventional HPDLC cell, and Example 2 is a conventional HPDLC cell based on a commercially available cholesteric liquid crystal BL118. is there. Example 3 is the HPDLC cell of the present application based on the commercially available BL118. As the light source, a 460 nm laser was used for measurement.
Table 1 Examples of HPDLC samples and obtained diffraction characteristics

  As shown in Table 1, the use of the third syrup allows a simple holographic optical switch to be obtained from a conventional HPDLC grating without increasing material and manufacturing costs. The third syrup includes two kinds of liquid crystals with different weight percentages, but three or more kinds may be used as necessary, or the weight percentage may be the same. The holographic cell has a thickness of 6 μm and exhibits substantially uniform transmission and diffraction characteristics for both S-polarized light and P-polarized light at the same voltage. Note that as the grating thickness decreases, the refractive index increases, resulting in a wider usable wavelength range. If the thickness is as thin as about 2 to 4 μm, a sufficient wavelength range (see Equation 4) can be used.

  For HPDLC of Example 2 (with chiral agent added), the diffraction efficiency is shown in FIG. 9 (b) as a function of applied voltage. FIG. 9A shows the first embodiment. As shown, the diffraction efficiency is generally the same for both S-polarized light and P-polarized light of 442 nm laser light. As in the conventional HPDLC shown in FIG. 9A, the diffraction efficiency basically decreases monotonously with increasing voltage. The diffraction efficiency of HPDLC in Example 3 is shown in FIG. In this HPDLC, the grating response does not depend on the polarization of the incident light. As shown, the diffraction efficiency of 422 nm S-polarized and P-polarized laser light is substantially the same. Unlike the conventional HPDLC shown in FIG. 9 (a) or FIG. 9 (b), the diffraction efficiency basically increases from about 5% at 0V to about 25% at 100V (ie, the transmission efficiency is From about 95% to about 75%.)

  FIG. 8A and FIG. 8B show the case where no voltage is applied to the optical switch and the case where the maximum voltage is applied, in the LCD display device of an embodiment. Specifically, the LCD display device 800 includes a light source 802. The light source 802 may be any backlight used in LCD display devices such as LEDs, EL, or fluorescent lamps. LEDs are widely used because they are efficient in terms of power and size. Using a broadband light source such as a white light LED covers at least about 400 nm to about 800 nm, ie the visible wavelength range. As a result, a color LCD can be used (a color filter is included but not shown in the drawing), and white light can be used externally as necessary. A single LED, such as a red, blue, or green LED, or a combination of these LEDs may be used. When using a blue (or green) LED, a fluorescent material is provided on one or more surfaces of the optical switch or inside the optical switch to absorb the light and other colors such as yellow You may make it light-emit. In this case, the LED light passes through the polymer matrix and liquid crystal droplets and is either transmitted or diffracted before impinging on the fluorescent material. The grating is designed to transmit and / or diffract between about 190 nm and about 2 μm.

  Light from the LED 802 impinges on a holographic optical switch 804 disposed adjacent to the LED 802. A reflector, converging lens, or some type of housing (not shown) surrounds the LED 802 and reflects or converges light from the LED 802 to direct it away from the holographic optical switch 804 or back to the holographic optical switch 804. When no voltage is present between the electrodes of the holographic optical switch 804 (off state), the light from the LED 802 is transmitted through the holographic optical switch 804 and a voltage is applied between the electrodes of the holographic optical switch. The case (on state) is diffracted. When in the on state, the diffracted light does not impinge on the LCD. Although not shown, when there is undiffracted light, it is sent to the LCD for display or other purposes.

  The LED 802 and the holographic optical switch 804 are configured such that the light transmitted through the holographic optical switch 804 is sent toward the light guide unit 806 so that at least a part of the light finally collides with the light guide unit 806. It is aligned with the light guide 806 of the LCD in the horizontal direction. In other words, at least a part of the LED 802, the holographic optical switch 804, and the light guide 806 are substantially in the same plane. The device shown in FIG. 8 easily couples light to the LCD. The light guide 806 may be formed of any other material used for plastic or LCD, and may be formed using any known light guide structure. Although the LED 802 and the LCD are shown as separate bodies, they may be attached to each other and integrated. Light incident on the light guide unit 806 enters the LCD panel from the lower side of the LCD panel that is in contact with the upper surface of the light guide unit 806. As shown, the light in the light guide 806 is reflected several times before being fully introduced into the LCD panel.

  The LCD panel includes a polarizing plate 808, a liquid crystal layer 810, and many other known layers that are not shown. Although not limited to the above layers, a pair of transparent substrates in which a polarizing plate 808 is disposed on the surface and a liquid crystal layer 810 is disposed between them, a transparent and reflective layer that functions to apply an electric field to the liquid crystal layer 801. , For example, a layer for forming and protecting a thin film transistor, and a color filter layer for dividing an LCD unit cell into multicolor (usually red, green, blue, and / or white) pixels.

  A power supply device 812 such as a battery is built in the LCD display device 800. The power supply 812 provides power to the LED 802, the electrical driver of the optical switch 804 (which supplies voltage between the electrodes of the optical switch), and the LCD panel. The power supply is adjustable and can change the supply voltage, for example, the interstitial electric field. Instead, power may be supplied to various components of the LCD display device 800 by an external power supply device. The LCD display device 800 or other portable electronic device also includes, among other components of the LCD display device 800, an LED 802, an optical switch 804, and a casing (or housing) 814 that encloses the LCD. The casing 814 includes a visual recognition unit (not shown) for allowing the user to view the LCD, and an illumination unit (not shown) that emits diffracted light out of the casing 814. This means that the light from the optical switch is directed to illuminate the outside of the casing. Although not shown, a lens may be added in the casing 814 and / or in the illumination section of the casing 814 to collimate or focus the diffracted light within the casing 814 and / or outside the casing 814.

  Thus, as shown in the drawing, the light source 802, the optical switch 804, and the liquid crystal display unit are configured such that, when the potential difference between the electrodes of the optical switch is not substantially zero, the light in the optical switch from the light source 802 It is arrange | positioned so that it may face in the direction which leaves | separates substantially from. In other words, when manufacturing the device, the angle of the optical switch is such that when a non-zero voltage at which light from the light source is refracted to the maximum is applied to the optical switch, the light coming from the light source and exiting the optical switch is It is adjusted so that it is directed to the vicinity of the liquid crystal panel without colliding with the liquid crystal panel.

  A multiple grating may be arranged between the light source and the light guide plate. The gratings may be optical switches with different properties, completely physically separated, or may be multiple layers integrated side by side or in the thickness direction within the same optical switch. If the optical switch is a separate type, the physical position of the grating, and other grating characteristics including, for example, physical tilt angle and spatial frequency may be different. The material used for the HPDLC layer may be the same or different when the optical switch is a separation type, and may be different when a plurality of layers are integrated into one optical switch. In this way, one or more light beams can be deflected to different positions as required. Regardless of whether the optical switch is a separated type or an integrated type, the same or different voltage may be applied to the HPDLC layer. For example, in an integrated optical switch, one or more transparent electrodes may be connected at different potentials, for example using a voltage divider.

  Furthermore, although only one light source is shown, multiple light sources may be used. These light sources may collide with the same grating or different gratings, may interact with the same or different light guides, and are adjacent to each other on the same side of the liquid crystal display or opposite sides of the liquid crystal display May be arranged.

  Accordingly, in one aspect, the liquid crystal display device includes a liquid crystal display unit, a light source, and an optical switch. The liquid crystal display unit includes an opposing substrate, electrodes on the substrate, a liquid crystal layer between the electrodes, a polarizing plate disposed as a liquid crystal layer on opposite sides of the substrate, and a light guiding unit disposed on one of the polarizing plates. Have. The optical switch is disposed between the light guide unit and the light source. The optical switch further includes opposing optical switch substrates, an optical switch electrode disposed between the optical switch substrates, and a diffraction grating disposed between the electrodes. This diffraction grating is a liquid crystal polymer having an effective refractive index that is substantially the same as the refractive index of the polymerized photopolymer when the electrode has the same potential as the polymerized photopolymer having a certain refractive index. A collection area. The power supply device is connected to the electrode of the optical switch. The grating is not substantially polarization dependent.

  The light source, the optical switch, and the liquid crystal display unit are arranged so that light from the light source passes through the optical switch and travels to the light guide unit when there is substantially no potential difference between the electrodes of the optical switch. ing. Similarly, in the light source, the optical switch, and the liquid crystal display unit, when the potential difference between the electrodes of the optical switch is substantially different from zero, the light in the optical switch coming from the light source is substantially from the liquid crystal display unit. It is pointed away. Generally, at least a part of the light source and the light guide are in the same plane.

  Accordingly, the above detailed description is to be regarded as illustrative rather than restrictive, and is intended to define the spirit and scope of the invention as defined by the appended claims and It should be understood that all of them are equivalent. The above description is not intended to deny the scope of the claimed invention or its equivalents. For example, other low power optical switches such as other optical systems or electronic ink (e-ink) may be used.

1 illustrates an embodiment of the present invention. FIG. 2 shows a schematic diagram of an experimental setup of an optical system for manufacturing the HPDLC according to the present embodiment with a mirror attached to a rotary stage. FIG. 3 shows an enlarged view of an intersecting region of split beams on the grating structure of FIG. 2. FIG. 4A and FIG. 4B show a detailed lattice structure and design parameters. FIG. 5A and FIG. 5B show a transmission optical switch and a reflection optical switch, respectively. FIG. 6A and FIG. 6B show a state where different voltages are applied to the conventional optical switch. FIGS. 7A and 7B show a state in which different voltages are applied to the optical switch according to an embodiment of the present invention. FIGS. 8A and 8B show a state in which different voltages are applied to the LCD according to an embodiment of the present invention. 9 (a), 9 (b), and 9 (c) show diffraction efficiency versus applied voltage for conventional HPDLC and HPDLC according to an embodiment of the present invention.

Claims (27)

In liquid crystal display devices,
An opposing substrate, an electrode on the substrate, a liquid crystal layer between the electrodes, a polarizing plate disposed as the liquid crystal layer on opposite sides of the substrate, and a conductive material disposed on one of the polarizing plates. A liquid crystal display section having a light section;
A light source;
An optical switch disposed between the light guide unit and the light source, the optical switch substrate facing each other, an optical switch electrode disposed between the optical switch substrate, and the electrode. An optical switch having a region of a polymerized photopolymer and a liquid crystal aggregate, wherein the grating is substantially non-polarization dependent. A liquid crystal display device provided. In liquid crystal display devices,
An opposing substrate, an electrode on the substrate, a liquid crystal layer between the electrodes, a polarizing plate disposed as the liquid crystal layer on opposite sides of the substrate, and a conductive material disposed on one of the polarizing plates. A liquid crystal display section having a light section;
A light source;
An optical switch disposed between the light guide unit and the light source, the optical switch substrate facing each other, an optical switch electrode disposed between the optical switch substrate, and the electrode. A diffraction grating, wherein the diffraction grating has a refractive index of the polymerized photopolymer having a certain refractive index and the polymerized photopolymer when the electrodes have the same potential. And a liquid crystal aggregate region having an effective refractive index substantially the same as the refractive index. In liquid crystal display devices,
An opposing substrate, an electrode on the substrate, a liquid crystal layer between the electrodes, a polarizing plate disposed as the liquid crystal layer on opposite sides of the substrate, and a conductive material disposed on one of the polarizing plates. A liquid crystal display section having a light section;
A light source;
An optical switch disposed between the light source and the liquid crystal display unit, the optical switch substrate facing each other, an optical switch electrode disposed between the optical switch substrate, and the electrode. A diffraction grating, wherein the diffraction grating has a refractive index of the polymerized photopolymer having a certain refractive index and the polymerized photopolymer when the electrodes have the same potential. A liquid crystal aggregate region having substantially the same effective refractive index as the refractive index, wherein the grating is substantially non-polarization dependent, and an optical switch. In optical switch,
An opposing substrate;
Electrodes disposed between the substrates;
A diffraction grating disposed between the electrodes, wherein the polymerized photopolymer having a certain refractive index and substantially the refractive index of the photopolymer when the electrodes have the same potential And a diffraction grating comprising a region of liquid crystal aggregates having equal effective refractive index. In optical switch,
An opposing substrate;
Electrodes disposed between the substrates;
A diffraction grating disposed between the electrodes, wherein the polymerized photopolymer having a certain refractive index and substantially the refractive index of the photopolymer when the electrodes have the same potential A region of the liquid crystal aggregate having an effective refractive index equal to, and the refractive index modulation of the liquid crystal aggregate is substantially the same for both P-polarized light and S-polarized light impinging on the diffraction grating An optical switch comprising a diffraction grating.   A fluorescent material disposed on or inside one or more surfaces of the optical switch, which absorbs impinging light and emits at least one other color light The liquid crystal display device according to claim 1, further comprising a fluorescent material.   In the light source, the optical switch, and the liquid crystal display unit, when there is substantially no potential difference between the electrodes of the optical switch, light from the light source passes through the optical switch and guides the light. The liquid crystal display device according to claim 1, wherein the liquid crystal display device is disposed so as to face the portion.   When the potential difference between the electrodes of the optical switch is not substantially zero, the light source, the optical switch, and the liquid crystal display unit emit light from the light source from the liquid crystal display unit. The liquid crystal display device according to claim 1, wherein the liquid crystal display device is disposed so as to be directed in a substantially separated direction.   When the potential difference between the electrodes of the optical switch is not substantially zero, the light source, the optical switch, and the liquid crystal display unit have both S light and P light in the optical switch coming from the light source. The liquid crystal display device according to claim 1, wherein the liquid crystal display device is arranged so as to be directed in a direction substantially away from the liquid crystal display unit.   The optical switch according to claim 1, wherein the grating is not substantially polarization dependent.   11. The polymerized photopolymer has a relatively high refractive index sufficient to match the effective refractive index of the liquid crystal aggregate when the electrodes have the same potential. An optical switch according to any one of the above.   The optical switch according to claim 1, wherein the refractive index modulation of the liquid crystal aggregate is substantially the same for both P-polarized light and S-polarized light impinging on the diffraction grating.   13. An optical switch according to any preceding claim, wherein one or more layers containing the polymerized photopolymer and liquid crystal aggregate are present in the lattice.   14. The optical switch of claim 13, wherein the liquid crystal aggregate is formed from at least two different cholesteric liquid crystals with different weight percentages. In a method of manufacturing a liquid crystal device,
Disposing a broadband light source and a holographic polymer dispersed liquid crystal (HPDLC) adjacent to each other;
Adjusting the HPDLC such that light coming from the light source and entering the HPDLC is directly transmitted through the HPDLC without being refracted without applying a voltage to the HPDLC. . Disposing a light guide portion laterally adjacent to the light source;
Aligning the light guide and the HPDLC such that light coming from the light source and exiting the HPDLC without applying a voltage to the HPDLC collides with the light guide. 16. The method of claim 15, comprising.   The method further includes aligning the light guide unit and the liquid crystal panel so that light impinging on the light guide unit is substantially directed to the liquid crystal panel, wherein the light guide unit and the liquid crystal panel are liquid crystal display units. The method of claim 16, wherein:   When a non-zero voltage that diffracts the light from the light source to the maximum is applied to the HPDLC, the light coming from the light source and exiting the HPDLC does not collide with the liquid crystal panel. The method of claim 17, further comprising adjusting an angle of the HPDLC to be directed in an adjacent direction.   The method of claim 18, further comprising encasing the liquid crystal display, the light source, and the HPDLC in a housing of a portable electronic device.   The method of claim 19, further comprising directing light from the HPDLC in a direction to illuminate the exterior of the casing.   A material that compensates for different potentials so that the HPDLC affects both S-polarized and P-polarized light from the light source in substantially the same manner independent of the voltage applied to the HPDLC; The method of claim 16, further comprising using in the HPDLC. In a method of manufacturing an optical switching device using holographic polymer dispersed liquid crystal (HPDLC),
Mixing liquid crystal and photopolymer to form a mixture;
Filling the mixture formed by a substrate with the mixture;
From the mixture, the liquid crystal aggregates and the polymerized photopolymer are periodically arranged alternately, and when no voltage is applied to the polymerized photopolymer and the liquid crystal aggregate, Forming a region where the effective refractive index of the liquid crystal aggregate is substantially equal to the refractive index of the matrix.   The forming step further comprises the step of exposing the filled cavities to uniform radiation to cure the polymer after forming the periodically interleaved regions. 23. The method according to 22.   23. The method of claim 22, wherein the forming comprises exposing the filled cavity to an interference pattern of a coherent laser beam.   Using a light source, it is determined whether or not light from the light source passes through the HPDLC when no voltage is applied to the HPDLC, and a voltage at which the maximum amount of light is diffracted by the HPDLC is obtained. 23. The method of claim 22, further comprising: testing the HPDLC.   Contains the liquid crystal aggregates and polymerized photopolymer, in which the S-polarized light and P-polarized light of the light incident on the HPDLC influences in substantially the same manner without depending on the voltage applied to the HPDLC 23. The method of claim 22, further comprising forming a matrix.   Further comprising disposing a fluorescent material on one or more surfaces of the optical switch or inside the optical switch that absorbs the impinging light and emits at least one other color light. 23. The method of claim 22, wherein

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