Photorefractive Response Time Research Articles

Photorefractive response time of B-ion-implanted KNbO3:Rb crystal

An experiment investigating the photorefractive response time of Rb-doped KNbO/sub 3/ crystal has been performed after proton implantation. Only one type of photorefractive grating was observed in this experiment. The experimental results show that B-ion implantation is more effective in speeding up the photorefractive response of KNbO/sub 3/:Rb crystals than proton implantation. This phenomenon has been analysed in terms of the carrier density.

Photorefractive Fe:LiNbO_3 crystal thin plates for optical information processing

The effects of the donor acceptor concentration ratio in iron-doped lithium niobate (Fe:LiNbO3) crystal plates on photorefractive response time and on grating diffraction efficiency are studied both theoretically and experimentally. The results provide a useful guide for designing photorefractive plates for optical information-processing applications. Two devices for real-time image recognition have been demonstrated: a photorefractive joint-transform correlator and a VanderLugt correlator. The former emphasizes fast response time, and the latter emphasizes high diffraction efficiency. By appropriate adjustment of the dopant concentration and the ratio of the donor acceptor levels, photorefractive Fe:LiNbO3 crystals that facilitate specific applications have been designed and fabricated. Shift-invariant image correlations have been achieved.

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.

Different temperature dependences of photorefractive parameters of Ce-doped and Rh-doped BaTiO3

Temperature dependences of the total effective trap density Neff and the electro-optic coefficient r33 in Ce-doped and Rh-doped BaTiO3 were determined by two-beam coupling measurements. It was found that the effective trap density Neff of BaTiO3:Ce increases whereas that of BaTiO3:Rh decreases with increasing temperature. The electro-optic coefficient r33 of both crystals increases with temperature. The photorefractive response times were also measured and found to decrease with different rates as temperature increases in the two crystals. The results were discussed by using the two-centre model for BaTiO3:Ce and three-charge-state model for BaTiO3:Rh. We found that the different temperature dependence of Neff in the two crystals was due to the fact that the deep- and shallow-trap levels in BaTiO3:Ce are caused by different impurity centres whereas those in BaTiO3:Rh are caused by different charge states of the same impurity centre.

Photoinduced electron transfer and its applications—Study on photochemical solar energy storage and polymeric photorefractive materials

The applications of photoinduced electron transfer, one in solar energy storage and the other in polymeric photorefractive materials are reported. In the former case, two new kinds of polymers containing norbornadiene and carbazole pendants were synthesized. The photoisomerization of norbornadiene pendants were achieved by irradiation with the light above the wavelength of 350 nm and a photoinduced electron transfer mechanism was proposed via fluorescence quenching, chemically induced dynamic nuclear polarization and thermodynamic discussion. In the latter case, a fast photorefractive response time of a polymer film composed of poly(N-vinylcarbazole), 2,4,7-trinitro-9-fluorenone and 4-(dicyanovinyl)-N,N-diethylaniline was observed by four-wave mixing diffraction measurement and the function of photoinduced electron transfer process in this photorefractive polymer film was also illustrated.

Build up mechanisms of (1+1)-dimensional photorefractive bright spatial quasi-steady-state and screening solitons

A (1+1)-dimensional model is studied numerically to evidence the build up mechanisms of photorefractive solitons, from the characteristic carrier recombination time, through the quasi-steady-state soliton, to the screening soliton. Three different build up regimes are evidenced and their domain of existence are computed. The transient quasi-steady-state soliton is shown to be characterized by two constants: its normalized width and its normalized build up time response multiplied by its peak intensity over dark irradiance. This latter assertion allows us to predict the photorefractive soliton response time for various optical powers. It is thus compared to existing experimental results.

Near-infrared incoherent coupling and photorefractive response time of ‘blue’ Rh:BaTiO 3

We have investigated the time constants of writing and decay of photorefractive gratings in ‘blue’ Rh:BaTiO 3 at visible and near-infrared wavelengths. High reflectivity (>200%) double phase conjugation has been achieved at near-infrared wavelengths (730–809 nm) and efficient double colour pumped oscillation has been demonstrated between wavelengths separated by 150 nm.

Photorefractive Beam Fanning Optical Limiter

AbstractWe have investigated the potential for using photorefractive phenomena in optical limiting devices. The major obstacle preventing the photorefractive limiter from reaching this potential has been speed of response. In this paper we report on several techniques which are successful in enhancing the photorefractive beam fanning response time. In particular, we demonstrate and characterize the use of applied electric fields, gratings to seed the beam fan, and cylindrical focusing. We also report the first demonstration of a prototype beam fanning optical limiter encompassing many of the proposed improvements.

Photorefractive effect in proton-implanted Fe-doped KNbO_3 waveguides at telecommunication wavelengths

We report on photorefractive two-beam coupling in proton-implanted Fe-doped KNbO3 waveguides at wavelengths from 632.8 to 1550 nm. Exponential gain coefficients of 11 and 0.9 cm-1 were measured at wavelengths of 632.8 nm and 1550 nm, respectively. Photorefractive response times as low as 120 µs are reported at 632.8 nm for a pump power of 4 mW. It is shown that proton irradiation increases the effective trap density, decreases the response time, and extends the photorefractive sensitivity of Fe-doped KNbO3 crystals toward near-infrared wavelengths.

Experimental investigation of the relationship between photorefractive and two-beam coupling response times

Quantitative dependences of two-beam coupling and hologram buildup time constants normalized by the photorefractive response time on a beam coupling gain and a pump-to-probe intensity ratio are investigated experimentally. We show that these normalized time constants increase with an increase of the beam coupling gain when the pump depletion can be negligible. However, they decrease and can be comparable to or less than unity in the strongly depleted pump regime. Numerical simulations also support these experimental results.

Phase modulated two-wave mixing in crystals with long photocarrier lifetimes

Abstract The temporal frequency response is derived for phase modulation in two-wave mixing for photorefractive crystals with long charge photocarrier lifetimes. Theoretical analysis shows that when the photorefractive response time is comparable with the photocarrier lifetime, the effects of the latter on the frequency transfer function must be considered. The present analysis is applied for Bi12SiO20-type photoreafractive crystals.

Phase modulated two-wave mixing in crystals with long photocarrier lifetimes

Abstract The temporal frequency response is derived for phase modulation in two-wave mixing for photorefractive crystals with long charge photocarrier lifetimes. Theoretical analysis shows that when the photorefractive response time is comparable with the photocarrier lifetime, the effects of the latter on the frequency transfer function must be considered. The present analysis is applied for Bi12SiO20-type photoreafractive crystals.

Nonlinear self-channeling of a laser beam at the surface of a photorefractive fiber

We experimentally observed the self-channeling of the light beam in photorefractive Bi12TiO20 fiberlike sample owing to the strong fanning effect. It results in significant speed up of the photorefractive response time in waveguides and in anomalous dependence of the response time on the applied ac electric field amplitude: the higher electric field, the faster response.

Light-induced dark decay and sublinear intensity dependence of the response time in cobalt-doped barium titanate

We report how the photorefractive properties of cobalt-doped barium titanate (BaTiO3) are influenced by deep and shallow traps. The light-induced dark decay of the space-charge field has two distinct time constants. The faster decay is due to the thermalization of charges from shallow traps, whereas the longer decay is that from deep traps formed when BaTiO3 is doped with cobalt. This dopant substantially decreases the dark conductivity of BaTiO3 and thus increases the dark storage time. The power coefficient of the sublinear intensity dependence of the photorefractive response time progressively increases with the cobalt-dopant concentration. Apparently cobalt doping of the deep level eliminates the effect from the shallow level in BaTiO3. Although cobalt-doped BaTiO3 has an increased effective trap density and thus an increased electrooptic gain, the response time and absorption are also increased, which thus decreases the photorefractive sensitivity.

Observation Of Photorefractive Effect In A Second Order Nonlinear Optical Chromophore Functionalized Polythiophene Derivative

ABSTRACTWe report the observation of photorefractive (PR) effect in a solution processable, thiophene copolymer with a side chain nonlinear optical (NLO) chromophore, poly(3- octylthiophene-co-N-(3-thenyl)-4-amino-2-nitrophenol) [POMDT]. The copolymer exhibits moderately fast photoconductivity. When doped with second order NLO active azo dye, this copolymer exhibited large second order NLO activity, and low asymmetric two beam coupling (TBC) gain. Enhanced TBC gain was observed, when this composite material was plasticized. The observed photorefractive response time is of the order of seconds. The slower response is attributed to the presence of large volume fraction of the plasticizer in the composition, which reduces the charge carrier mobility. The results from photoconductivity, second harmonic generation and two beam coupling experiments are discussed.

Photorefractive response time measurement in GaAs crystals by phase modulation in two-wave mixing

We describe a new method for photorefractive response time measurement in fast photorefractive crystals based on continuous phase modulation in two-wave mixing. We report experimental results for undoped semi-insulating GaAs that are in good agreement with theory. Values obtained for the response time and photoelectron generation quantum efficiency are consistent with previously published data.

Submillisecond photorefractive response time of KNbO_3:Rb^+

We investigate the photorefractive response time of diffraction gratings in Rb(+)-doped KNbO(3) crystals. Two components of charge carriers, with different response times, are found to contribute to the photoinduced index gratings. One component, which contributes to approximately 70% of the index grating, shows a submillisecond rise time when the total intensity of the writing beams is larger than 5 W/cm(2). The dependence of the response time on the light intensity and the grating wave vector is also investigated.

Frequency-dependent photorefractive response in the presence of applied ac electric fields

We consider the process of nonstationary enhancement of the photorefractive effect through the application of time-varying (square-wave) electric fields. By examining the full second-order equation governing the photorefractive response, we show that this response is strongly dependent on the frequency of the applied ac field and exhibits a maximum when the period of oscillation is between the photorefractive response time, τP, and the charge-recombination time in the medium, tR. Furthermore, we show that the diffraction efficiency, the gain, and the phase of the response fluctuate in tandem with the oscillating applied field, and we examine the dependence of these fluctuations on the driving frequency. These frequency-dependent phenomena are described both by numerical integration of the charge-transport equations and through an analytic solution thereof. From this analysis we obtain simple expressions for the magnitude of the efficiency, the gain, and the phase fluctuations and the conditions for maximum average diffraction efficiency. In particular, we show that the oscillation period yielding the maximum response behaves as a weighted geometric average of the response and the recombination times of the form T = constant × (τP2tR)1/3. These results are compared with those of earlier analyses of ac-field enhancement and are then verified in a BSO crystal to which ac fields over a range of frequencies are applied.

Photoconductivity and grating response time of a photorefractive polymer

We report the photoconductive properties and photorefractive grating response time of a polymer mixture composed of 40-wt. % dissolved diethylamino-benzaldehyde diphenyl hydrazone (DEH) and the non-cross-linking epoxy polymer Bisphenol A 4,4′-nitroaminostilbene. The films have improved photoconductive sensitivities as high as 2.1 × 10−10 cm/(W Ω) at a wavelength of 650 nm with a corresponding reduction of the grating response time constant to 0.11 ± 0.02 s at an intensity of 1 W/cm2. The nitro-aminostilbene chromophore is deduced to be the source of photogenerated charge carriers on the basis of a comparison of the wavelength dependence of the photoconductivity and absorption coefficient. Degradation of the photoconductivity and the dark conductivity as well as of the photorefractive speed with sample age is attributed to precipitation of the DEH; this explanation is supported by x-ray diffraction observations of crystal growth in the polymer.

Photorefractive frequency converter and phase-sensitive detector

A photorefractive frequency converter and phase-sensitive detector is proposed and demonstrated. Dynamic photorefractive theory is used to examine device characteristics for inputs from available types of spatial light modulator to determine which reference modulations are optimal for a given signal modulation. The output linearity is determined for pairs of signal and reference modulation types, and both spatial and temporal intensity variations are analyzed. Photorefractive response times are measured from the frequency-conversion bandwidth.