Retardation-film-compensated reflective bistable twisted nematic liquid-crystal displays

The reflective bistable twisted nematic liquid-crystal-displays (RBTN-LCDs) with a single front polarizer have been compensated by front half-wave or integer-wave plates or both. The compensation has been optimized through maximizing the contrast ratio. The optimized RBTN-LCDs with one or two retardation plates showed great improvement in contrast ratio. The wavelength-dependent reflectance, viewing-angle properties, and effect of the retardation variation of these optimized RBTNs with retardation compensation were also investigated. The simulation was carried out by using realistic parameters on RBTN modes that have retardation values from 0.30 to 0.85 microm and a low contrast ratio. The viewing-angle properties did not change much before and after the retardation compensation. The reflectance varied little, but the contrast ratio varied significantly with the variation of the retardations of the bistable twisted nematic cells, for both compensated and uncompensated cases. It is shown that a high contrast ratio (100:1), high reflectance (30%), and low dispersion are possible for such RBTN-LCDs with proper retardation compensations. Black-white operations can be realized by compensating RBTN-LCDs.     

Estimation of the possible scale for holographic switches with liquid-crystal displays

Simulating the positions of output beams under the assumption that a liquid-crystal display acts as a binary phase modulator reveals that the number of the outputs increases almost linearly with the square root of the number of pixels assigned to an input. This result is confirmed by experiments, and it is estimated that 1016 outputs can be obtained when the number of pixels is 700 × 700. Holographic switches with liquid-crystal displays are therefore suitable for large-scale switches.     

Applications of multidirectional asymmetrical microlens-array light-control films on reflective liquid-crystal displays for image quality enhancement

The multidirectional asymmetrical microlens-array light-control film (MAMA-LCF) is developed for enhancing the image brightness and contrast ratio of various reflective liquid-crystal displays. By use of index-matching material, the interface reflection is greatly reduced. Through optimized designs, the surface-scattering effect is also suppressed; thus the contrast ratio is much enhanced. From experimental results, the MAMA-LCF leads to a approximately 1.5 x gain in brightness over the MgO standard white and a 15:1 contrast ratio for the reflective color super-twist nematic liquid-crystal display, 2.8 x MgO and a 23:1 contrast ratio for the polymer-dispersed liquid-crystal, and 2.8 x MgO and a 13:1 contrast ratio for the cholesteric liquid-crystal display. Potential applications of this low-cost plastic thin film for reflective liquid-crystal displays are foreseeable.     

Evanescent wave holographic recording in a photopolymer

We present a study of holographic recording with an evanescent reference and a homogeneous (plane) object wave. The grating step was 324 nm. The dependence of the diffraction efficiency on exposure was investigated. The maximum value obtained was 0.01% at 5 mJ cm^?2 exposure. The optimal pre-exposure, needed for grating adhesion to the glass substrate, was 1 μJ cm^?2.     

Multilayer holographic recording using a two-color-absorption photopolymer

We developed a sensitive two-color-absorption photopolymer in which holograms are recorded by simultaneous irradiation with a 660 nm interference light and a 410 nm gate light. Doped with bis(silyl)pentathiophene as a two-color-photosensitive dye and 2,2-dimethoxy-2-phenylacetophenone as a radical photopolymerization initiator; its matrix contains low-refractive index binding polymers and high-refractive index monomers. The sensitivity and diffraction efficiency of 25 mum thick layers are from 1.2 x 10(-9) to 3.7 x 10(-9) cm2/mJ and from 1% to 4%. We made a three-photopolymer-layer waveguide structure, where each photopolymer layer and high-refractive index adhesive layer serves as a core layer and is sandwiched between two low-refractive index glass substrates that serve as clad layers. Gate light propagated through the adhesive layers, reference and object beams intersected the photopolymer layers, and different diffraction patterns could be written in each layer.     

Polarization eigenstates for twisted-nematic liquid-crystal displays

We derive theoretical expressions for the eigenvalues and the eigenvectors for a twisted-nematic liquid-crystal display (LCD) as a function of the twist angle and the birefringence by use of the Jones-matrix formalism. These polarization eigenvectors are of particular interest for phase-only transmission because they propagate unchanged through the display. We find that the eigenvectors are elliptically polarized and that the ellipticity changes as a function of the birefringence of the LCD (which is proportional to the external voltage applied to the display). We can define an average eigenvector over a desired range for the applied voltage. We show, using Jones matrices, how this average eigenvector can be generated using a quarter-wave plate and a linear polarizer having appropriate orientation angles. Using this average eigenvector, we show that superior phase-only operation can be obtained over a given operating range for the LCD compared with other approaches.     

Improvement of photopolymer materials for holographic data storage

Photopolymer materials are practical materials for use as holographic recording media due to the fact that they are inexpensive, self-processing materials with the ability to record low loss, highly diffraction efficient volume holographic gratings. In general these materials absorb light of an appropriate wavelength, causing photo-polymerization of the local monomer, inducing a change in the material’s refractive index. These small changes in refractive index enable the storage of large quantities of data using holographic techniques. In an attempt to further develop the data storage capacity and quality of the information stored, i.e., resolution, in such materials, a deeper understanding of the photochemical mechanisms present during the formation of holographic gratings has become ever more crucial. From this understanding the response of an acrylamide/polyvinylalcohol based photopolymer to high spatial frequency information is improved through the addition of a chain transfer agent, i.e., sodium formate, HCOONa.     

Properties of DMDs for holographic displays

Digital micromirror device’s (DMD) properties as being a display device for holographic displays are investigated. High speed, a large separation between reconstructed image and reconstruction beam, two symmetric diffraction patterns, and low intensity (0,0)th-order beam at a blazed grating condition are the desired properties for the displays. The blazed grating condition of a DMD can reconstruct images with higher diffraction efficiency than the line grating condition. DMD’s high speed enables to present colors and gray levels to the reconstructed image. However, reconstructed images from a gray-level computer-generated hologram (CGH) and its binary form hologram reveal no noticeable difference between them, except the background noise in the image from the CGH.     

Characterization of the DuPont photopolymer for three-dimensional holographic storage

DuPont's HRF-150 photopolymer film is investigated for use in three-dimensional holographic memories. Measurements of sensitivity, hologram persistence, the lateral spread of the photoinitiated reaction, and the variation of diffraction efficiency with modulation depth, spatial frequency and tilt angle, and intensity are reported. We observed that the diffraction efficiency of the HRF-150 photopolymer for a given exposure decreases with increases in intensity and grating tilt angle. The holograms were nondestructively reconstructed for long periods of time at room temperature. The photoinitiated reaction spread less than 100 μm over a period of 16 h.     

Holographic characteristics of a 1-mm-thick photopolymer to be used in holographic memories

Poly(vinyl alcohol-acrylamide) photopolymers are materials of interest in the field of digital information storage (holographic memories). We analyzed the behavior of a 1-mm-thick photopolymer. Using a standard holographic setup, we recorded unslanted diffraction gratings. The material has high angular selectivity (0.4 degrees), good sensitivity (88 mJ/cm2), and small losses caused by absorption and scattering of light. It also has a high maximum diffraction efficiency (70%). A significant induction period was seen in the material. The authors hypothesize that, during most of this induction period, polymerization does in fact take place but is not reflected in the appearance of the diffracted light until a certain threshold value of exposure is reached.     

Optical response of photopolymer materials for holographic data storage applications

We briefly review the application of photopolymer recording materials in the area of holographic data storage. In particular we discuss the recent development of the Non-local Polymerisation Driven Diffusion model. Applying this model we develop simple first-order analytic expressions describing the spatial frequency response of photopolymer materials. The assumptions made in the derivation of these formulae are described and their ranges of validity are examined. The effects of particular physical parameters of a photopolymer on the material response are discussed.     

Narrow-band emitters in LED backlights for liquid-crystal displays

Over the last decade, narrow-band emitters have been recognized as key enablers for light emitting diodes (LEDs) backlights in liquid-crystal displays (LCDs) by competing with other display technologies. Today, efforts have been devoted to the exploration of narrow-band green/red luminescent materials with high quantum efficiency and excellent stability to optimize the performance of LED backlights. This review first presents an overview of the significant progress made in the development of narrow-band emitters used in LED backlights for LCDs with the emphasis on the versatile materials databases from doped phosphors to luminescent II–VI, III-V semiconductor quantum dots, and the recent halide perovskites nanocrystals and bulk metal halides. Subsequently, the correlation of structure-luminescence properties, and the device performance optimization of these emitters have been analyzed. The focus is placed on summarizing and comparing the remarkable examples of outdated and new narrow-band luminescent materials as potential candidates in LED backlights. Finally, the outlooks and challenges in discovering new narrow-band emitters have been proposed.     

Nonlocal-response diffusion model of holographic recording in photopolymer

The standard one-dimensional diffusion equation is extended to include nonlocal temporal and spatial medium responses. How such nonlocal effects arise in a photopolymer is discussed. It is argued that assuming rapid polymer chain growth, any nonlocal temporal response can be dealt with so that the response can be completely understood in terms of a steady-state nonlocal spatial response. The resulting nonlocal diffusion equation is then solved numerically, in low-harmonic approximation, to describe grating formation. The effects of the diffusion rate, the rate of polymerization, and a new parameter, the nonlocal response length, are examined by using the predictions of the model. By applying the two-wave coupled-wave model, assuming a linear relationship between polymerized concentration and index modulation, the resulting variation of the grating diffraction efficiency is examined.     

Electronic speckle pattern shearing interferometer with a photopolymer holographic grating

A photopolymer holographic grating is used to produce the two sheared images in an electronic speckle pattern shearing interferometer. A ground glass screen following the grating eliminates unwanted diffraction orders and removes the requirement for the CCD camera to resolve the diffraction grating's pitch. The sheared images on the ground glass are further imaged onto the CCD camera. The fringe pattern contrast was estimated to be above 90%. A validation of the system was done by comparing the theoretical phase difference distribution with the experimental data from the three-point bending test.     

Multichannel holographic recording method for three-dimensional displays

A multichannel holographic recording method is presented for three-dimensional (3D) displays, utilizing pixel-based recording instead of image-based recording in order to realize parallel processing. The proposed approach is composed of two main stages. In the first stage, each two-dimensional (2D) image acquired from multiple viewpoints is partitioned by holographic recording channels (HRC) into nonoverlapping subimages. In the second stage, the corresponding pixels of the subimages are rearranged to constitute an encoding image. The encoding images are recorded simultaneously by each HRC, respectively, so the recording speed is improved significantly. The experimental results have demonstrated that the three-channel system is feasible and the full-parallax hologram reconstructed with white light is acceptable in quality. The three-channel system saves approximately 60% of the recording time in comparison with the single-channel system. More importantly, the proposed method can accomplish a large-scale final hologram composed of multichannel holograms without sacrificing the hologram quality. Several 3D imaging applications such as medical diagnosis and advertisements could benefit from this research.     

Effects of apodization on a holographic demultiplexer based on a photopolymer grating

An investigation of the effects of apodization on a holographic demultiplexer that is based on a photopolymer grating is presented. Uniform and Gaussian apodized gratings are fabricated in a DuPont HRF-150-38 photopolymer. From the theoretical and experimental results, the spectral response of the apodized grating has a larger main lobe but lower sidelobes than those in the uniform-grating case. A 42-channel demultiplexer that is based on the Gaussian apodized grating with an 0.4-nm channel spacing is demonstrated. A cross-talk level of -30 dB and an interchannel uniformity of 1.5 dB are archived in the wavelength range of approximately 1550 nm.     

对绿光敏感的全息存储光致聚合物性能的研究

报道了一种对绿光敏感的以丙烯酰胺为单体的光致聚合物，该光聚物由单体、光引发剂、共引发剂和成膜物组成。可以得到材料的最大衍射效率为55％。将光聚物用不同温度进行烘烤加热，可以看到材料的折射率调制度会增加。使用角度复用技术在光聚物中存储10幅全息图，说明所研究材料有希望于大容量体全息存储。     

High efficiency panchromatic photopolymer recording material for holographic data storage systems

Studies carried out to gauge the potential of a metal-ion doped panchromatic photopolymer recording material for application in real-time holographic data storage is presented. The photopolymer films are spin coated on glass plates to ensure better surface uniformity. Volume holographic transmission gratings with peak diffraction efficiency of 80% could be stored in the photopolymer films of 100 μm thickness. An efficiency of 70% is achievable even for gratings recorded with exposure energy as low as 10 mJ/cm². A checkerboard pattern data page recorded in the photopolymer film using a defocused 4-f recording geometry could be reconstructed with good image quality. The experimental results illustrate the competency of the developed photopolymer for holographic data storage applications.     

Real-time holographic vibrometry with a ferroelectric liquid-crystal spatial light modulator

A fast ferroelectric liquid-crystal spatial light modulator, originally developed for optical computing, has found a new application in vibrometry. A new scheme of vibration-synchronized double-exposure holographic interferometry is proposed that makes full use of the speed of the ferroelectric liquid-crystal spatial light modulator. Preliminary experiments were performed that demonstrate virtually continuous real-time vibrometric data acquisition.     

Phasor analysis of eigenvectors generated in liquid-crystal displays

We analyze optical systems for creating phase-only operation of a liquid-crystal display (LCD). We introduce a new formalism in which any input or output polarization state can be described in terms of a linear combination of the eigenvectors of the LCD. We use a phasor analysis to examine the linear combination of the eigenvectors and show how improved performance can be obtained with a new configuration. We experimentally compare the intensity and phase operation for two configurations with the same spatial light modulator and show the improved performance with the new configuration.