Optical Image Storage Research Articles

Effects of Photo-Controlled Nanophase Segregation in a Re-entrant Nematic Liquid Crystal

Liquid crystals composed of photoactive groups, such as azobenzene, are promising materials for optical switching and image storage applications. On exposure to light LC materials commonly undergo an order-to disorder transition. Here, an unusual disorder-to-order transition is presented (the Figure shows a photo-induced smectic texture surrounded by a nematic phase), and a photo-controlled nanophase segregation mechanism is invoked to explain it.

Second-order nonlinearity and optical image storage in phenyl-silica hybrid films doped with azo-dye chromophore using optical poling technique

4-[ N-ethyl- N-(2-hydroxyethyl)]amino-4 ′-nitro-azobenzene (DR1)-doped phenyl group substituted silica films were prepared by a sol–gel method. The films were optically poled by the coherent superposition of 1064 and 532 nm beams from a Q-switched Nd:YAG laser. To discuss the effects of the modifier group, interaction between DR1 molecules and the matrix was investigated. The delocalization of π electrons occurred between DR1 molecules and the phenyl-silica hybrid matrix, and that consequently the polarized DR1 molecules could be stabilized. By use of the optimized optical poling technique, optical storage was successfully demonstrated for a phenyl-silica hybrid film doped with DR1.

Optical Switching and Image Storage by Means of Photochromic Liquid Crystals

Optical switching and image storage have been examined by means of photochromic polymer liquid crystals (LCs). The photoisomerization of photochromic moieties, which act as a photoresponsive switch, induced the LC-to-isotropic phase transition and the change in alignment of LCs, and they resulted in optical switching and image storage.

Light-scattering-mode optical switching and image storage in polymer/liquid crystal composite films by means of photochemical phase transition

Photochemical phase transition behavior triggered by photoisomerization of a guest azobenzene doped in nematic (N) liquid-crystalline (LC) domains was investigated in isotropic (I) acrylate polymers with alkyl side chains on their surfaces. These polymer/LC composite films were prepared by in situ thermal polymerization-induced phase separation method (thermal PIPS) from their starting mixtures containing a di- and a mono-functional acrylate monomer. Strong light-scattering states of the intrinsic composite films were transformed into complete transparent states by photochemical N–I phase transition of the LC domains within the polymer matrices that was induced by the trans–cis photoisomerization. Recovery process from the transparent state to the initial light-scattering state was strongly influenced by the alkyl side chains. While the composite film with short alkyl side chain acted as an all-optical switching material because the optical switching could be achieved reversibly and repeatedly between the two different optical states, the composite films with long alkyl side chain showed an ability of optical image storage with high contrast based on light-scattering mode.

Image Storage and Real-Time Distorted Image Correction by Using Photorefractivity in a Stable Photorefractive Polymer Composite

Using a highly stable and excellently performed three-component photorefractive (PR) polymer composite, poly(N-vinylcarbazole):1-n-butoxyl-2,5-dimethyl-4-(4'-nitrophenylazo)benzene:2,4,7-trinitro-9-fluorenone, optical image storage was demonstrated. The dark storage time was 5-7 h. The PR response time was measured to be about 200 ms at an intensity of 1 W/cm2 with an applied electric field of 84 V/μm. A proof-of-principle experiment on real-time correction of distorted images based on phase conjugattion of four-wave mixing geometry was also carried out.

Photoinduced changes in photorefractive PVA films doped with Cr3+ and VO2+: EPR and PAS investigations

In order to understand the mechanism of optical image storage in photorefractive polyvinyl alcohol (PVA) films, photo-EPR and photoacoustic spectral investigations were carried out on PVA films doped with Cr3+ and VO2+. The EPR spectrum of Cr3+ has shown reduction in intensity onin situ illumination with copper vapor laser (CVL). The decay and recovery of Cr3+ signal, with and without CVL illumination respectively, was monitored at different temperatures in 10–300 K region. These were found to obey a double exponential, with one time constant independent of temperature, and the other showing significant temperature dependence. From Τ(T), activation energy for the charge carrier transport in one of the processes was estimated to be 0.016 eV. The PA spectra showed shift towards lower wavelength side on consecutive runs. On the other hand, VO2+ doped PVA film has not shown any significant changes in intensity on laser illumination. These observations suggest (i) interaction of PVA matrix with excited Cr3+ and (ii) predominant non-radiative relaxation in VO2+: PVA system with no change in the oxidation state.

PHOTOREFRACTIVE POLYMER STACKS

By stacking multiple polymer films in a sandwichlike fashion, chemists at the University of California, San Diego, have prepared photorefractive samples that exhibit a high enough gain to demonstrate important optical phenomena [ Science , 277 , 549 (1997)]. Polymer-based photorefractive materials—a class of compounds known for less than a decade—have been the focus of intense research because of their expected ease of processing and low cost compared with their better known inorganic analogs. Possible applications for these hologram-producing materials—whether organic or inorganic—are numerous. Examples include optical image storage and processing, optical switching devices, and associative learning—a process by which a machine could identify a face, for instance, from just the image of an eye. Despite their potential, the organic polymers have generally performed poorly compared with inorganic crystals. Recently, synthesized polymer films have shown enormous improvement in one of the performance measure...

Liquid crystal photonics: optical switching and image storage by means of nematic liquid crystals and ferroelectric liquid crystals

Optical switching and image storage have been examined by means of liquid crystals (LCs), nematic LCs and ferroelectric LCs. Photochromic molecules were incorporated either by dispersion into host LCs or covalently attached to polymer main chains. These photochromic moieties act as a photoresponsive switch which induces phase transition and change in orientation of LCs, resulting in optical switching and image storage.

Liquid Crystal Photonics: Optical Switching and Image Storage by Means of Azobenzene Liquid-Crystal Films

AbstractOptical switching and image storage were explored by means of azobenzene liquid crystals (LCs). Azobenzene derivatives with monomeric and polymeric forms were prepared, which show nematic LC phase in the trans form while no LC phase in the cis form. On pulse irradiation at 355 nm, which causes trans-cis isomerization of the azobenzene moiety, these azobenzene LC films underwent nematic (N) to isotropic (1) phase transition in 100 ˜ 200 μs as probed by change in transmittance of a He-Ne laser through crossed polarizers (transmission-mode analysis) or change in reflectivity at the interface between the sample and glass substrate (reflection-mode analysis). In the reflection-mode analysis, the decay was faster than that observed in the transmission-mode analysis. The N phase recovered through the cis-trans thermal backisomerization process in the transmission-mode analysis, while it was restored through diffusion of the cis form, followed by reorientation of the trans form which replaced the cis form in the interface region, in the reflection-mode analysis. The photochemical N-I phase transition was also induced in 200 μs below Tg of the polymer azobenzene LC. Image could be stored into the polymer azobenzene LC film on photoirradiation to bring about the N-I phase transition and the stored image remained unchanged even after one year when it was kept below Tg of the polymer.

Anisotropic strong volume hologram in BaTiO 3

A photorefractive strong volume hologram by using anisotropic diffraction in a normal-cut BaTiO 3 is proposed and demonstrated. Experimental measurements fit the theoretical calculations well by using Kogelnik's formula with unclamped E-O coefficients. The maximum refractive index perturbation in the experiment is larger than 2.7 × 10 −4. In addition, we have observed that the output of the diffracted light are concentric circles. This pattern is shown as a good guide of the estimation of how large the refractive index perturbation is. The Bragg condition of the anisotropic strong volume hologram is so strict that an image-carried hologram cannot be directly read out. We proposed an address image method by erasure for solving this problem. Through this method, this anisotropic strong volume hologram has some potential for optical image storage.

Optical Switching and Image Storage by Means of Azobenzene Liquid-Crystal Films

Liquid crystals are promising materials for optical switching and image storage because of their high resolution and sensitivity. Azobenzene liquid crystals (LCs) have been developed, in which azobenzene moieties play roles as both mesogens and photosensitive chromophores. Azobenzene LC films showed a nematic phase in trans isomers and no LC phase in cis isomers. Trans-cis photoisomerization of azobenzene with a laser pulse resulted in a nematic-to-isotropic phase transition with a rapid optical response of 200 microseconds.

Spectral Hole‐Burning: Applications to Optical Image Storage

AbstractSpectral hole‐burning was used to record information in the inhomogeneous broadened absorption band of free base chlorin in polyvinylbutyral at 1.7 K. The wavelength multiplexing property of these materials allows to store many images in the form of holograms in a single spot of about 5 mm diameter. The gray level pattern of the image is translated into a pattern of a spatially varying hole depth. Such a storage device can be considered three‐dimensional, involving the two spatial dimensions of the thin sample film and the dimension of the optical wavelength. If in addition an electric field is applied, a four‐dimensional storage device is created having two spatial, one wavelength and one electric field dimensions. Addressing a specific image is achieved by merely adjusting the read‐out wavelength and the value of the electric field to the values used during the storage process. No moving parts are involved at all. It is demonstrated that in chlorin at least 25 images can be stored in a single 1 cm−1 interval and in the electric field range of ± 100 kV/cm.

Photochemical Image Storage in Polymer Liquid Crystals

Abstract Amplified optical image storage has been demonstrated in polymer liquid crystals (PLC). A small amount (< 5 mol%) of a low molecular weight mesogen with a photoresponsive moiety, 4-butyl-4′-methoxyazobenzene (BMAB), was doped in the films of polyacrylates with mesogenic side chain, poly(4′-methoxyphenyl-4-acryloyloxyalkoxybenzoate)(PAn), in which the alkyl spacer length (CH2)n was varied as n = 2, 3, 5, and 6. Photoirradiation of the PA3/BMAB film with a 366 nm light caused trans→cis isomerization of BMAB, which induced simultaneously the nematic (N)→isotropic phase transition of the PLC films. This photochemically induced phase transition was reversible and photoirradiation with a 525 nm light which induced cis→trans isomerization restored the system to the initial phase (N). The photo-induced phase transition was found to be strongly dependent on molecular weight of the host PLC and temperature.

The ODISS Project, Digital Imaging Research for Archival Institutions

AbstractIn 1985, the National Archives and Records Administration of the United States undertook the Optical Digital Image Storage System project to evaluate the feasibility and desirability of such a system to improve the preservation and reference operations for some of the institution’s more heavily used archival collections. The following year, the National Archives contracted for the development of a large-scale research system. In early 1988, the institution’s Research and Evaluation Staff will begin to scan, store, and reference a test sample of over 1.5 million documents representative of its holdings. The project will experiment with and examine all facets of the conversion operation including image enhancement capability, conversion throughput, and quality control.

Spectral hole burning: High resolution optical spectroscopy and image storage

The technique of holographic spectral hole burning was used to investigate electronic properties of potential recording materials. Applications in frequency and electric field domain image storage are demonstrated.

Spectral hole burning: High-resolution optical spectroscopy and image storage

Spectral hole burning was used to study electronic properties of organic molecules embedded in an amorphous host. Doped silica samples were prepared by the sol-gel method and investigated as potential recording materials for frequency and electric field selective optical storage. Multiple holographic image storage making use of inhomogeneous line broadening is demonstrated using an organic polymer film.

Optical image storage in ion implanted PLZT ceramics

We have demonstrated that optical images can be stored in transparent lead-lanthanum-zirconate-titanate (PLZT) ceramics by exposure to near-UV light with photon energies greater than the band gap energy of ∼3.35 eV. The image storage process relies on optically induced changes in the switching properties of ferroelectric domains (photoferroelectric effect). Stored images are nonvolatile but can be erased by uniform UV illumination and simultaneous application of an electric field. Although high quality images, with contrast variation of ⩾100 : 1 and spatial resolution of ∼10 μm, can be stored using the photoferroelectric effect, relatively high exposure energies (∼100 mJ/cm 2) are required to storethese images. This large exposure energy severely limits the range of possible applications of nonvoltile image storage in PLZT ceramics. We have recently found from studies of H, He and Ar implanted PLZT that thephotosensitivity can be significantly increased by ion implantation into the surface to be exposed. For example, the photosensitivity after implantation with 5 × 10 14 500 KeV Ar/cm 2 is increased by about three orders of magnitude over that of unimplanted PLZT. The image storage process and the effect of ion implantation is presented along with a phenomenological model which describes the enhancement in photosensitivity obtained by ion implantation. This model takes into account both light- and ion implantation-induced changes in conductivity and gives quantitative agreement with the measured changes in the coercive voltage with light intensity for ion implanted PLZT.

Laser holography in dentistry

I he purpose of this article is to illustrate some of the dental applications of nondestructive laser imaging or holography. The attempted analysis of biologic forces related to dental structures has been well documented in the dental literature.l-‘* The development of laser holography may lead to a further advancement in these studies. A laser is a device that generates and amplifies coherent electromagnetic energy at optical frequencies; it is a source of coherent, extremely bright light of a single color.23 The term laser stands for “light amplification by stimulated emission of radiation.” It is the analog of the maser, the forerunner of the laser. Maser stands for “microwave amplification by stimulated emission of radiation.” The laser was a natural outgrowth of the maser in the early 1960’s and for many years was called an optical maser. One important application of the laser has been in the field of optical image storage and image reconstruction known as holography. The principle of holography was discovered by Gaborz4 in 1948. When the laser light source was applied to holography in 1963 by Leith and Upatnieks,25 the practical application of holography became possible for three-dimensional recording of objects.

Reusable Optical Image Storage and Processing Device

The PROM (Pockels Readout Optical Modulator) described in this paper is a device for storing and processing optical images and information in recyclable, real time operating systems. The device consists of a single active layer of a cubic photosensitive, electrooptic material. ZnS, ZnSe, and Bi(12)SiO(20) devices have been fabricated and characterized demonstrating high writein sensitivity, high resolution, high contrast, high optical and diffraction efficiency, fast recycling times, long storage times, and freedom from fatigue. Operating modes and measured performance parameters are discussed.

An optical image storage and processing device using electrooptic ZnS

An image-sensing and memory device is described that photoconductively converts and electrostatically stores optical images at the interface between a UV sensitive ZnS film and an associated blocking electrode. Optical readout is accomplished by means of the Pockels electrooptic effect generated in the ZnS film by the electrostatic image. A high ratio of UV to visible photosensitivity (approximately 10 5 :1 between 340 and 630 nm) allows pseudonondestructive readout of the stored image pattern. A device has been constructed and tested having an active sensing and storage area of 2 cm2with an observed limiting resolution of 85 line pairs/mm. A 340-nm exposure of 20 erg/cm2gives a 2:1 change in readout light intensity (100 erg/cm2gives a 10:1 change). Dark storage periods of approximately 100 h have been observed under room temperature conditions. The device can be erased electrically or optically and recycled. The device structure will be described along with the theory of operation in the sensing, storing, and readout modes. The lumped parameter equivalent circuit models and the equations that predict device performance will be discussed. This device is applicable as a reusable memory plane for use in coherent processors. It has been evaluated for this use by storing phase gratings and measuring their diffraction efficiency. Such operations as real-time correlation, Fourier transformation, and spatial filtering are made possible by this device.