Conference on Lasers and Electro-Optics, Apr 25, 1988
In recent years much attention has been focused on the photorefractive effect in barium titanate.... more In recent years much attention has been focused on the photorefractive effect in barium titanate. The effect is not completely understood, but models have been developed that predict most of the observed behaviors.1,2
Intensity dependent absorption and photorefractive effects have recently been observed at low int... more Intensity dependent absorption and photorefractive effects have recently been observed at low intensities in barium titanate crystals [1, 2]. Also, there is some evidence that recombination centers other than iron impurities exist in BaTiO3 and contribute to the photorefractive effect [3, 4]. The model we present here assumes the existence of primary deep level traps such as iron and secondary shallow traps as shown in Fig. 1. We show that this model predicts the intensity dependence in BaTiO3.
The alternating electric field technique is attractive as it can significantly enhance the two-be... more The alternating electric field technique is attractive as it can significantly enhance the two-beam coupling gain over that of a dc field alone. In this paper we study the ac field frequency influence on the photorefractive characteristics. A finite difference technique was employed to numerically solve the material equations. These predictions are compared with experimental results for BSO crystals. We examine the frequency dependence of the magnitude of the space charge field as well as the temporal fluctuations in amplitude and phase. The frequency dependent characteristics are also found to be influenced by the modulation index.
The first active feedback experiments are performed with photorefractive Sn2P2S6, characterized b... more The first active feedback experiments are performed with photorefractive Sn2P2S6, characterized by inhibition of the steady-state gain factor because of superposition of two out-of-phase space-charge gratings (formed by movable charge carriers of different sign). The active feedback allows to suppress the compensating grating and to keep the steady-state gain factor as large as the transient one. One way to achieve this goal is to use the second harmonic of the modulation frequency to generate an error signal and to choose the modulation frequency larger than the reciprocal relaxation time of the fast grating but smaller than that of the slow grating.
A partially asymmetric quantum-well Fabry Perot uses unequal partial reflections from front and b... more A partially asymmetric quantum-well Fabry Perot uses unequal partial reflections from front and back interfaces and balances destructive interference upon reflection by relying on intrinsic absorption in the quantum wells. The antireflection condition produces large modulation contrast ratios for reflected beams, while the partial reflections simultaneously allow transmission through the device. When the device is operated as a holographic optical element by incorporating a photorefractive quantum well structure in the Fabry Perot, output diffraction efficiencies can approach 100%. These holographic devices are optically addressed and electrically switched. In this paper we present theoretical calculations of the properties of partially asymmetric Fabry-Perot quantum wells.
ABSTRACT Optical switching properties of a photorefractive BaTiO3 crystal are studied. The data s... more ABSTRACT Optical switching properties of a photorefractive BaTiO3 crystal are studied. The data shows a small but symmetric switching for ordinary polarized input beams and a larger but nonsymmetric switching for extraordinary polarized beams. Slow drifts were also observed in the outputs after switching with extraordinary beams. The extreme nonsymmetry and drifts are believed to be caused by coupling that is due to gratings produced by internal reflections. We suggest that these effects may make the photorefractive BaTiO3 crystal unsuitable for optical switching applications but do not affect its ability to be used as a memory storage device with optical readout.
The three-dimensional hydrodynamics of laser-induced, steady state vaporization of aluminum (2024... more The three-dimensional hydrodynamics of laser-induced, steady state vaporization of aluminum (2024) in vacuim was investigated with the aid of both a hydrodynamic computer code and a Mach-Zender interferometer. Provided the laser beam is sufficiently intense, a vapor plasma forms, resulting in a sustained region of high pressure near the target. The pressure is distributed asymmetrically over the footprint in the case of oblique incidence.
Sun-tracking microwave (ST-MW) radiometry is a ground-based technique where the Sun is used as a ... more Sun-tracking microwave (ST-MW) radiometry is a ground-based technique where the Sun is used as a beacon source to infer the atmospheric path attenuation in all-weather conditions. ST-MW radiometry shows an appealing potential for overcoming the difficulties to perform satellite-to-Earth radiopropagation experiments in the unexplored millimeter-wave and submillimeter-wave frequency region, especially where experimental data from beacon receivers are not available. The theoretical framework, the ad hoc procedures and data processing will be presented, together with the estimate of the overall error budget. The applications and challenges during field deployments, such as the recent WRad campaign in Italy based on ST-MW data analysis, funded by ESA and carried out together with AFRL (NY, USA), will be discussed.
The band-transport model of Kukhtarev [1] is commonly used to describe the photorefractive gratin... more The band-transport model of Kukhtarev [1] is commonly used to describe the photorefractive grating formation. According to this model the grating formation time should be inversely proportional to the laser intensity. However, experiments performed on barium titanate have shown a response time which has varied with intensity as I−x, where the exponential factor x has ranged from 0.5 to 1.0.[2] In this paper we show by computer modeling of the grating formation that the sublinear dependence of the response time is accounted for by the presence of secondary photorefractive centers.
In a recent letter Rytz et al[1] reported results of two-wave mixing experiments performed on BaT... more In a recent letter Rytz et al[1] reported results of two-wave mixing experiments performed on BaTiO3 crystals in the temperature range between 20° and 120°C. They observed a large decrease in the density of empty traps as well as a significant decrease in the response time at elevated temperatures. The physical mechanism for the observed effects was not well understood. In this paper we show that the temperature variation in the empty trap concentration is accounted for by secondary photorefractive centers and discuss the possible influence of these traps on the hole mobility.
Bi12TiO20 (BTO) is a photorefractive material in the same structural class (sillenite) as Bi12SiO... more Bi12TiO20 (BTO) is a photorefractive material in the same structural class (sillenite) as Bi12SiO20 (BSO) and Bi12GiO20 (BGO). However, BTO offers some unique advantages over BSO and BGO: (1) larger electro-optic coefficient (5.7 pm/V)1, and (2) lower optical activity (6°/mm at 633 nm)1, 2. Previous photorefractive measurements3,4 have shown that gain coefficients on the order of 10-15 cm−1 can be produced through the use of an applied AC field. In this work we show that the largest gain values can only be obtained for large values of the pump/probe intensity ratio β. As β approaches unity (large signal regime), higher spatial order gratings become prominent, and the gain is reduced from its large-β value.5−8 Our results are similar to those obtained by other researchers for BSO5 and GaAs6 with an applied field. We have analyzed this and related phenomena using a finite difference method to model the photorefractive grating formation. This method yields accurate numerical solutions which are valid for for all values of β.
Enhancements in the two-beam coupling gain can be achieved in cubic crystals of the BSO type by t... more Enhancements in the two-beam coupling gain can be achieved in cubic crystals of the BSO type by the use of nonstationary external conditions such as the application of a dc field along with a moving interference pattern or by the application of an alternating electric field[1,2,3,4,5]. However, for modulation index, m, greater than about 0.1 the performance falls off significantly with increasing m[1,5]. Some success has been achieved in obtaining steady-state solutions of the nonlinear equations in the presence of moving fringes[1,6,7,8]. These numerical calculations were accomplished by expansion of the variables into their Fourier components or by perturbation techniques. In this paper we model the photorefractive grating formation by a finite difference method. This approach is quite general. The numerical solutions predict the photorefractive behavior at large modulation for stationary and nonstationary photorefractive recording techniques.
The photorefractive properties of BaTiO3 are influenced by the presence of secondary centers. The... more The photorefractive properties of BaTiO3 are influenced by the presence of secondary centers. These are traps that are highly thermally ionized in the dark but can become populated when the crystal is illuminated with laser light. The photo-induced absorption is a signature of these traps. Two species of secondary centers are suggested by spectral and temporal characteristics of the induced absorption. The BaTiO3 sample was heated in a silicon oil bath to temperatures of up to 100°C. The dark decay rates of the photo-induced absorption had temperature dependencies of the Arrhenius form, with activation energies of 0.5 and 0.8 eV.
Recent experimental and theoretical results indicate that the photorefractive properties of bariu... more Recent experimental and theoretical results indicate that the photorefractive properties of barium titanate are influenced by the presence of secondary photorefractive centers.1-4 These are traps which are highly ionized at room temperature but can become populated by illumination with light. We have also attributed an intensity-dependence in the absorption coefficient to these traps.4 In this paper we study the secondary photorefractive centers by measuring the spectral and temporal characteristics of the photo-induced absorption.
Conference on Lasers and Electro-Optics Europe, 1996
ZnTe has the potential to be an outstanding photorefractive material. In earlier work(1) it was f... more ZnTe has the potential to be an outstanding photorefractive material. In earlier work(1) it was found that incorporation of vanadium in vapor grown ZnTe was very difficult. Even when vanadium was incorporated as an active impurity the doping usually was not uniform. In this paper we report on improved photorefractive response in ZnTe through co-doping with vanadium and manganese.
Conference on Lasers and Electro-Optics, Apr 25, 1988
In recent years much attention has been focused on the photorefractive effect in barium titanate.... more In recent years much attention has been focused on the photorefractive effect in barium titanate. The effect is not completely understood, but models have been developed that predict most of the observed behaviors.1,2
Intensity dependent absorption and photorefractive effects have recently been observed at low int... more Intensity dependent absorption and photorefractive effects have recently been observed at low intensities in barium titanate crystals [1, 2]. Also, there is some evidence that recombination centers other than iron impurities exist in BaTiO3 and contribute to the photorefractive effect [3, 4]. The model we present here assumes the existence of primary deep level traps such as iron and secondary shallow traps as shown in Fig. 1. We show that this model predicts the intensity dependence in BaTiO3.
The alternating electric field technique is attractive as it can significantly enhance the two-be... more The alternating electric field technique is attractive as it can significantly enhance the two-beam coupling gain over that of a dc field alone. In this paper we study the ac field frequency influence on the photorefractive characteristics. A finite difference technique was employed to numerically solve the material equations. These predictions are compared with experimental results for BSO crystals. We examine the frequency dependence of the magnitude of the space charge field as well as the temporal fluctuations in amplitude and phase. The frequency dependent characteristics are also found to be influenced by the modulation index.
The first active feedback experiments are performed with photorefractive Sn2P2S6, characterized b... more The first active feedback experiments are performed with photorefractive Sn2P2S6, characterized by inhibition of the steady-state gain factor because of superposition of two out-of-phase space-charge gratings (formed by movable charge carriers of different sign). The active feedback allows to suppress the compensating grating and to keep the steady-state gain factor as large as the transient one. One way to achieve this goal is to use the second harmonic of the modulation frequency to generate an error signal and to choose the modulation frequency larger than the reciprocal relaxation time of the fast grating but smaller than that of the slow grating.
A partially asymmetric quantum-well Fabry Perot uses unequal partial reflections from front and b... more A partially asymmetric quantum-well Fabry Perot uses unequal partial reflections from front and back interfaces and balances destructive interference upon reflection by relying on intrinsic absorption in the quantum wells. The antireflection condition produces large modulation contrast ratios for reflected beams, while the partial reflections simultaneously allow transmission through the device. When the device is operated as a holographic optical element by incorporating a photorefractive quantum well structure in the Fabry Perot, output diffraction efficiencies can approach 100%. These holographic devices are optically addressed and electrically switched. In this paper we present theoretical calculations of the properties of partially asymmetric Fabry-Perot quantum wells.
ABSTRACT Optical switching properties of a photorefractive BaTiO3 crystal are studied. The data s... more ABSTRACT Optical switching properties of a photorefractive BaTiO3 crystal are studied. The data shows a small but symmetric switching for ordinary polarized input beams and a larger but nonsymmetric switching for extraordinary polarized beams. Slow drifts were also observed in the outputs after switching with extraordinary beams. The extreme nonsymmetry and drifts are believed to be caused by coupling that is due to gratings produced by internal reflections. We suggest that these effects may make the photorefractive BaTiO3 crystal unsuitable for optical switching applications but do not affect its ability to be used as a memory storage device with optical readout.
The three-dimensional hydrodynamics of laser-induced, steady state vaporization of aluminum (2024... more The three-dimensional hydrodynamics of laser-induced, steady state vaporization of aluminum (2024) in vacuim was investigated with the aid of both a hydrodynamic computer code and a Mach-Zender interferometer. Provided the laser beam is sufficiently intense, a vapor plasma forms, resulting in a sustained region of high pressure near the target. The pressure is distributed asymmetrically over the footprint in the case of oblique incidence.
Sun-tracking microwave (ST-MW) radiometry is a ground-based technique where the Sun is used as a ... more Sun-tracking microwave (ST-MW) radiometry is a ground-based technique where the Sun is used as a beacon source to infer the atmospheric path attenuation in all-weather conditions. ST-MW radiometry shows an appealing potential for overcoming the difficulties to perform satellite-to-Earth radiopropagation experiments in the unexplored millimeter-wave and submillimeter-wave frequency region, especially where experimental data from beacon receivers are not available. The theoretical framework, the ad hoc procedures and data processing will be presented, together with the estimate of the overall error budget. The applications and challenges during field deployments, such as the recent WRad campaign in Italy based on ST-MW data analysis, funded by ESA and carried out together with AFRL (NY, USA), will be discussed.
The band-transport model of Kukhtarev [1] is commonly used to describe the photorefractive gratin... more The band-transport model of Kukhtarev [1] is commonly used to describe the photorefractive grating formation. According to this model the grating formation time should be inversely proportional to the laser intensity. However, experiments performed on barium titanate have shown a response time which has varied with intensity as I−x, where the exponential factor x has ranged from 0.5 to 1.0.[2] In this paper we show by computer modeling of the grating formation that the sublinear dependence of the response time is accounted for by the presence of secondary photorefractive centers.
In a recent letter Rytz et al[1] reported results of two-wave mixing experiments performed on BaT... more In a recent letter Rytz et al[1] reported results of two-wave mixing experiments performed on BaTiO3 crystals in the temperature range between 20° and 120°C. They observed a large decrease in the density of empty traps as well as a significant decrease in the response time at elevated temperatures. The physical mechanism for the observed effects was not well understood. In this paper we show that the temperature variation in the empty trap concentration is accounted for by secondary photorefractive centers and discuss the possible influence of these traps on the hole mobility.
Bi12TiO20 (BTO) is a photorefractive material in the same structural class (sillenite) as Bi12SiO... more Bi12TiO20 (BTO) is a photorefractive material in the same structural class (sillenite) as Bi12SiO20 (BSO) and Bi12GiO20 (BGO). However, BTO offers some unique advantages over BSO and BGO: (1) larger electro-optic coefficient (5.7 pm/V)1, and (2) lower optical activity (6°/mm at 633 nm)1, 2. Previous photorefractive measurements3,4 have shown that gain coefficients on the order of 10-15 cm−1 can be produced through the use of an applied AC field. In this work we show that the largest gain values can only be obtained for large values of the pump/probe intensity ratio β. As β approaches unity (large signal regime), higher spatial order gratings become prominent, and the gain is reduced from its large-β value.5−8 Our results are similar to those obtained by other researchers for BSO5 and GaAs6 with an applied field. We have analyzed this and related phenomena using a finite difference method to model the photorefractive grating formation. This method yields accurate numerical solutions which are valid for for all values of β.
Enhancements in the two-beam coupling gain can be achieved in cubic crystals of the BSO type by t... more Enhancements in the two-beam coupling gain can be achieved in cubic crystals of the BSO type by the use of nonstationary external conditions such as the application of a dc field along with a moving interference pattern or by the application of an alternating electric field[1,2,3,4,5]. However, for modulation index, m, greater than about 0.1 the performance falls off significantly with increasing m[1,5]. Some success has been achieved in obtaining steady-state solutions of the nonlinear equations in the presence of moving fringes[1,6,7,8]. These numerical calculations were accomplished by expansion of the variables into their Fourier components or by perturbation techniques. In this paper we model the photorefractive grating formation by a finite difference method. This approach is quite general. The numerical solutions predict the photorefractive behavior at large modulation for stationary and nonstationary photorefractive recording techniques.
The photorefractive properties of BaTiO3 are influenced by the presence of secondary centers. The... more The photorefractive properties of BaTiO3 are influenced by the presence of secondary centers. These are traps that are highly thermally ionized in the dark but can become populated when the crystal is illuminated with laser light. The photo-induced absorption is a signature of these traps. Two species of secondary centers are suggested by spectral and temporal characteristics of the induced absorption. The BaTiO3 sample was heated in a silicon oil bath to temperatures of up to 100°C. The dark decay rates of the photo-induced absorption had temperature dependencies of the Arrhenius form, with activation energies of 0.5 and 0.8 eV.
Recent experimental and theoretical results indicate that the photorefractive properties of bariu... more Recent experimental and theoretical results indicate that the photorefractive properties of barium titanate are influenced by the presence of secondary photorefractive centers.1-4 These are traps which are highly ionized at room temperature but can become populated by illumination with light. We have also attributed an intensity-dependence in the absorption coefficient to these traps.4 In this paper we study the secondary photorefractive centers by measuring the spectral and temporal characteristics of the photo-induced absorption.
Conference on Lasers and Electro-Optics Europe, 1996
ZnTe has the potential to be an outstanding photorefractive material. In earlier work(1) it was f... more ZnTe has the potential to be an outstanding photorefractive material. In earlier work(1) it was found that incorporation of vanadium in vapor grown ZnTe was very difficult. Even when vanadium was incorporated as an active impurity the doping usually was not uniform. In this paper we report on improved photorefractive response in ZnTe through co-doping with vanadium and manganese.
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