Erasure Of Holograms Research Articles

Photorefractive parameters in GaAs:Cr with externally applied magnetic field

Using band charge transport model, photorefractive (PR) parameters in semiconductors in presence of applied magnetic field have been studied. It is found that external magnetic field significantly affect the photorefractive space charge electric field (ESC), convenient gain coefficient and diffraction efficiency ESC ∼105V/m, normalized gain coefficient ∼0.9m−1 and diffraction efficiency ∼90% observed at B0=2T. This value of gain coefficient and diffraction efficiency is nearly nine times higher than the value obtained in the absence of B0. A typical behavior of recording and erasure of hologram with respect to time has also been investigated. Results manifest that GaAs:Cr is efficient for ultrafast writing and erasing media for PR-grating.

HOLOGRAM RECORDING AND ERASURE IN GaAs:Cr WITH SIMULTANEOUSLY APPLIED ELECTRIC AND MAGNETIC FIELDS

Externally applied fields play crucial role in the enhancement of space charge electric field (Esc) in photorefractive (PR) materials. Using band-charge transport model, Esc (~105V/m) is obtained in presence of externally applied dc electric (E0) and magnetic (B0) fields simultaneously. Numerical estimation of GaAs:Cr shows that in presence of externally applied fields (optimum value of E0 = 5 × 102 V/m and B0 = 640 gauss), diffraction efficiency ~90% can be observed. To further elaborate the above result, a typical behavior of recording and erasure of hologram with respect to time has been investigated. Result manifest that GaAs:Cr is efficient, ultrafast writing and erasing media for PR-grating.

Improvement of thermal stability in photochromic holograms

Significant improvement of the thermal stability of the hologram recorded on photochromic materials had been achieved via covalent bonding of the photochromic dye to the polymer matrix as compared to the host-guest systems. For the photochromic materials with covalent bonding of the dye to the polymer matrix the level of thermal stability is demonstrated such that at temperatures (T) ca. 100 degrees C there was no detectable diffusion-type degradation of the hologram's diffraction efficiency after 6 h of exposure to elevated T. The same heating of the hologram in photochromic host-guest polymer led to a full hologram erasure within 40 min.

Enhancement of photochromic hologram thermal stability

Significant enhancement of the thermal stability of the hologram recorded on photochromic materials had been achieved via covalent bonding of the photochromic dye to the polymer matrix as compared to the host-guest systems. One time partial reduction of the hologram’s initial diffraction efficiency due to thermal exposure was observed for both—polymers with attached dye as well as hostguest materials. Such one time reduction is interpreted to be due to the thermal relaxation of the polymer network induced with photochromic transition in the dye molecule. A gradual hologram erasure at elevated temperatures was observed for the host-guest system, which is assumed to be due to dye’s diffusion between highly lit and dark areas. For the photochromic materials with covalent boding of the dye to the polymer matrix the level of thermal stability is demonstrated such that at temperatures ca. 100C there was no detectable diffusion type degradation of the hologram after 6 hours of exposure to elevated temperature. Same heating of the hologram in photochromic host-guest polymer led to a full hologram erasure within 40 minutes. In both cases the one time initial DE reduction due to polymer matrix relaxation at elevated temperatures had comparable value of about 0.5 of hologram’s initial DE.

Selective Erasure of Holograms in Photorefractive and Photopolymer Materials for Rewritable and Secure Data Storage

A selective erasure/update method of multiplexed photorefractive and photopolymer holograms by use of the double Mach–Zehnder (DMZ) interferometric arrangement is proposed. The DMZ arrangement is used as the optical phase-control device for holographic recording/erasure/update operations, which can produce a pair of π-phase-shifted interference patterns with a simple optical setup. We intend to erase the holograms recorded in not only the inherently rewritable photorefractive crystal but also the photopolymer material generally used for the write-once (i.e., unerasable) holographic recording. It is experimentally confirmed that our method can erase the write-once type photopolymer holograms as well as the photorefractive holograms, although there will be a certain degree of restriction on the rewrite cycles limited by the monomer density, the degree of multiplexing and so on.

X-ray-induced conductivity in iron-doped lithium niobate crystals

Illumination of photorefractive, iron-doped lithium niobate crystals $({\mathrm{LiNbO}}_{3}:\mathrm{Fe})$ with x-rays generates a conductivity that we determine from the speed of hologram erasure. The doping levels of the crystals and the acceleration voltage of our x-ray tube are varied. A theoretical model is presented, which describes the obtained results. A decrease of the conductivity with increasing ${\mathrm{Fe}}^{2+}$ concentration can be explained by assuming that holes are the dominant charge carriers for this short-wavelength illumination.

Photorefractive trapping and the correlation between recording and erasure dynamics in a polymer composite

The fast time constants of holographic contrast growth and erasure for various photorefractive polymer composites were measured under a wide range of experimental conditions. Either of two sensitizers, 2,4,7-trinitro-9-fluorenone (TNF) or (2,4,7-trinitro-9-fluorenylidene)malonodinitrile (TNFDM), were used in a degenerate four-wave mixing experiment under varying conditions of field, wavelength and intensity. Collected together, the fast growth and erasure time constants were found to be correlated with a coefficient of 0.96. From this correlation we conclude that the charge generation process during holographic recording and the optical de-trapping process during hologram erasure are indistinguishable. Hence in accordance with the photorefractive crystal model, hole recombination with the sensitizer anions forms the primary charge trapping mechanism in the majority of photorefractive polymer composites.

Spatial differentiation of optical images using selective erasure of photorefractive holograms

We propose and demonstrate a novel technique of spatial differentiation of optical images based on the selective erasure of holograms in photorefractive crystals using a composite filter which consists of an amplitude and a phase filter. The net filter is realized by controlling the modulation depth, amplitude of a photorefractive grating using a one-dimensional Gaussian erasing beam in conjunction with a phase filter in the reading beam's path. Experimental results are presented and compared with other type of filter.

A hybrid neural network for image classification

Principles of the photorefractive perceptron learning algorithm are described. The influences of the finite response time and hologram erasure of the photorefractive gratings on the convergence property of the photorefractive perceptron learning are discussed. A novel neural network which could resolve these constraints is presented. It is a hybrid system which utilizes the photorefractive holographic gratings to implement the inner product between the input image and the interconnection matrix. A personal computer is used for storing the interconnection matrix and the updating procedure, and it also functions as a feedback means during the learning phase. After training the weight vectors are recorded in the volume hologram of an optical processor. This novel method combines the advantages of the massive parallelism of optical systems and the programmability of electronic computers. Experimental results of image classification are presented. It shows that the system could correctly classify the input patterns into one of the two groups after training on four examples in each group during successive iterations. The system has been extended to perform multi-category image classification.

Generalized perceptron learning rule and its implications for photorefractive neural networks

We consider the properties of a generalized perceptron learning network, taking into account the decay or the gain of the weight vector during the training stages. A mathematical proof is given that shows the conditional convergence of the learning algorithm. The analytical result indicates that the upper bound of the training steps is dependent on the gain (or decay) factor. A sufficient condition of exposure time for convergence of a photorefractive perceptron network is derived. We also describe a modified learning algorithm that provides a solution to the problem of weight vector decay in an optical perceptron caused by hologram erasure. Both analytical and simulation results are presented and discussed.

Conditional convergence of photorefractive perceptron learning

We consider the convergence characteristics of a perceptron learning algorithm, taking into account the decay of photorefractive holograms during the process of interconnection weight changes. As a result of the hologram erasure, the convergence of the learning process is dependent on the exposure time during the weight changes. A mathematical proof of the conditional convergence, as well as computer simulations of the photorefractive perceptrons, is presented and discussed.

Quasipermanent hole-photorefractive and photochromic effects in Bi12TiO20 crystals

We report the presence of quasipermanent effects of photochromic and hole-photorefractive nature in Bi12TiO20 (BTO) at low irradiance at 514.5 nm. A new selective two-wave mixing (S2WM) detection technique was used that allows to separately detect phase and amplitude gratings. S2WM is used in this work for operating a self-stabilized recording setup which allows to produce unusually large quasipermanent effects. We also show that stabilized recording is necessary to obtain reliable information from hologram erasure experiments. Results reported in this paper show the complex nature of recording in BTO and the capabilities of stabilized recording and S2WM for fundamental and applied research on photorefractive crystals.

Angular selectivity and coherent erasure of quadratic polarizability holograms in fiber waveguides

Picosecond pulses of neodymium laser radiation and its second harmonic were used in recording of holograms. These holograms exhibited an angular selectivity. Coherent erasure of holograms by second exposure to signals with the opposite sign of the phase shift between them was demonstrated.

Reversible recording and erasure of holograms in photodarkened As_2S_3 thin film

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text Seung Gol Lee and Sang Soo Lee, "Reversible recording and erasure of holograms in photodarkened As2S3 thin film," Appl. Opt. 25, 4512-4514 (1986) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Optical erasure of one- and two-photon holograms in lithium niobate

Abstract The dynamics of optical erasure of holographic gratings formed by one-photon and two-photon photorefractive processes in iron-doped (150 ppm) LiNbO3 have been studied as a function of wavelength of the erasing light. At a given erasing wavelength and intensity, the time constant for erasure of holograms recorded at 0.61 μm by a two-photon process is similar to that for erasure of one-photon holograms of the same grating spacing recorded at 0.488 μm; thus the same ultimate traps appear to be involved in both recording processes. The minimum photon energy required to empty these traps is not greater than 1.9 eV. Rapid erasure occurs when the photon energy is above 2.35 eV. These results will be compared with erasure dynamics in nominally undoped LiNbO3, and with the recording dynamics in the doped and undoped materials; and the implications for photorefractive recording mechanisms will be discussed.

Optical erasure of one- and two-photon holograms in fe-doped linbo3

Abstract The dynamics of erasure of holographic gratings formed by one-photon and two-photon photorefractive processes in iron-doped (150 ppm) LiNbO3 have been studied as a function of wavelength of the erasing light. For a given wavelength and intensity of this light, the time constant for erasure of holograms recorded at 0.61 μm by a two-photon process is similar to that for erasure of one-photon holograms of the same grating spacing recorded at 0.488 μm. Thus the same ultimate traps appear to be involved in both recording processes. The minimum photon energy required to empty these traps is not greater than 1.9 eV. Rapid erasure occurs when the photon energy is above 2.35 eV.

Reading and optical erasure of holograms stored by the photorefractive effect in lithium niobate

The reading and optical erasure (on and off the Bragg angle) of holograms stored in lithium niobate and similar materials by the photorefractive effect have been modeled using numerical methods for the case of a single trapping level and with the further restrictions of uniformly and completely illuminated crystals, short transport lengths, and constant applied voltage. For the first time, the model allowed simultaneously for finite dark conductivity, feedback effects due to space charge on the redistribution of photoliberated electrons, writing of a new hologram by interference between the reconstructed and the reading waves (causing enhancement or disenhancement of the original hologram according to the conditions), and nonuniformity of the hologram through the thickness of the crystal. The model generates all the reported types of behavior, and it is believed to be accurate enough for a meaningful comparison with experiment. Excluded are those effects due to the large scale field associated with the envelope of the illumination when this is nonuniform.

Coherent properties of a copper vapor laser and dynamic holograms in vanadium dioxide films

Dynamic holography in vanadium dioxide films was used to study experimentally the coherence of the radiation emitted from a copper vapor laser. It was established that the temporal coherence length exceeded 12 cm and the spatial coherence length was at least equal to the laser beam diameter. An investigation was made of the holographic characteristics of vanadium dioxide using the copper vapor laser to record and reconstruct holograms. The investigation included the dependence of the diffraction efficiency on the power of the laser pulses and their repetition frequency. The possibility of optical modulation of the semiconductor–metal phase transition and of the corresponding recording and erasure of holograms in VO2 at frequencies above 10 kHz was demonstrated for the first time. There was no deterioration in the holographic characteristics of VO2 after more than 108 recording–erasure cycles.

Optical erasure of holograms in lithium niobate crystals

In this paper we intend to present an experimental investigation of a volume phase hologram erasure under stationary optical illumination.

Electrical Control in Photoferroelectric Materials for Optical Storage

After defining photoferroelectrics as photosensitive and nonlinear electrooptic materials, we report electrical control of holographic diffraction efficiency in PLZT ceramics and electrical fixation and erasure of holograms recorded in SBN single crystals. On the basis of experimental data, these effects are explained in terms of the local field amplitude, computed as the sum of the applied field and the photoinduced space charge field and high nonlinear electrooptic effects associated with ferroelectric phase transitions.