Beam Coupling Gain Research Articles

High photorefractive sensitivity in an n-type 45°-cut BaTiO_3 crystal

The beam-coupling properties of a cobalt-doped oxygen-reduced n-type barium titanate crystal are reported in the 0 deg and 45 deg crystallographic orientations. Oxygen reduction improved the response time of the 0-deg-cut crystal by a factor of about 4.5 without diminishing the beam-coupling gain. The 45-deg-cut crystal has a peak gain of about 38.7/cm, a response time of about 21 ms, and a photorefractive sensitivity of 3.44 cu cm/kJ. From response time measurements an equivalent percentage change in the dc dielectric constant and the mobility with respect to crystallographic orientation is inferred.

Photorefractive gain enhancement in InP:Fe using band-edge resonance and temperature stabilization

We report photorefractive two beam coupling gain coefficients as high as Γ=19 cm−1 and net gains of Γ-α=14 cm−1 in InP:Fe at around 970 nm. This enhancement was achieved by combining band-edge resonant nonlinearities with a second resonant enhancement derived from the bipolar transport in this material. Measurements of gain as a function of pump intensity, applied field, grating spacing, wavelength, and temperature are presented. No moving gratings were required.

Photorefractive properties of BaTiO_3:Co

We measure and analyze beam-coupling gain and response time in BaTiO3 crystals doped with cobalt at levels of 50 and 100 parts in 106 for various oxidation and reduction states. We discuss the effect of cobalt doping and oxidation–reduction on the electron paramagnetic resonance and optical absorption of BaTiO3. We compare the BaTiO3:Co samples with nominally undoped and transition-metal- (chromium, manganese, iron, and iron + nickel) doped BaTiO3. We conclude that cobalt-doped BaTiO3 in the as-grown state displays high photorefractive gain that is correlated with the cobalt concentration in a reproducible manner. Cobalt appears to be the most promising transition-metal dopant for BaTiO3 investigated so far.

Photorefractivity at 1.5 μm in CdTe:V

We have for the first time demonstrated two-beam coupling energy transfer at a wavelength of 1.5 μm. Beam coupling gain coefficients of 0.6 cm−1 have been obtained in vanadium -doped CdTe with only 5 mW/cm2 incident intensity. These gain coefficients exceed typical gain coefficients in GaAs at 1.06 μm wavelength by 50%. In preliminary measurements using the moving grating technique, we have measured a gain coefficient of 2.4 cm−1. Through adjustment of the doping level, CdTe:V can be used as a sensitive photorefractive material through the 0.9–1.5 μm spectral range.

Band-edge photorefractive effect in semiconductors

We report here for the first time, two-beam coupling energy transfer using the Franz–Keldysh electrorefractive effect in photorefractive semiconductors. A large beam coupling gain coefficient (Γ=2.8 cm−1) exceeding the absorption coefficient (α=2.0 cm−1) is obtained in undoped semi-insulating GaAs using the new photorefractive process. The new photorefractive process differs from conventional photorefractivity in that the direction of energy transfer is dictated by the direction of an externally applied electric field. Using a moving grating technique and combining the electrorefractive grating with the conventional electro-optic grating, a very large gain Γ=16.3 cm−1 (α=3 cm−1, crystal length l=4mm) resulting in net amplification by more than a factor of 200 has been demonstrated.

Mobility-lifetime product of photoexcited electrons in GaAs

We show that photorefractive beam coupling gain as a function of grating period measured in semi-insulating GaAs with an external ac electric field or with a dc field and moving fringes can be explained theoretically by a mobility-lifetime product about four orders of magnitude smaller than that obtained in low-field conditions. This reduction is caused by enhanced occupation of the L band at high fields coupled with the low L band mobility and by the increase in recombination due to the cascade capture process that occurs for electric fields above a few kV/cm.

Temperature dependence of photorefractive properties of strontium-barium niobate (Sr0.6Ba0.4Nb2O6)

Beam coupling gain and response time together with reflectivity and response time of an external ring phase-conjugate mirror are studied as a function of temperature for a crystal of strontium-barium niobate. The photorefractive sensitivity is increased by a factor of 5 when the crystal is heated to 65 °C. Self-pumped reflectivities up to 19% are observed at room temperature. The reflectivity is smaller at elevated temperature.

Effects of crystal growth and processing conditions on the photorefractive properties of BaTiO3: A theoretical model

Abstract We present a model which can be used to predict the effects of doping and oxidation-reduction treatment on the photorefractive behavior of BaTiO3, crysta1s.1 The densities of photorefractive centers are calculated from a thermodynamic point defect model. These centers may exist in multiple charge states; the occupation of each the temperature and oxygen partial pressure of processing and dopant concentration. The beam coupling gain and response time are calculated using the electron-hole model, where the photorefractive donor and acceptor densities are derived from the defect model.

Photorefractive incoherent to coherent optical conversion using electro-chemically reduced KNbO3-crystals

Abstract With the electro-chemical reduction method developed recently we are able to improve the photorefractive properties of KNbO3 such as photoconductivity, beam coupling gain and response time. The preparation of large monodomain crystals with good optical and photorefractive homogeneity for photorefractive incoherent optical conversion (PICOC) is described. The image conversion was achieved by the anisotropic self-diffraction technique. The optimal crystal configuration for this application was verified experimentally. A resolution of up to 10 linepairs per mm was achieved for modulating a laser beam by a white light image.

Optimization of the photorefractive properties of KNbO3crystals

Abstract Photorefractive effects of pure and iron doped KNbO3 crystals with concentrations between 150 ppm and 300 ppm have been investigated. In order to optimize the photorefractive recording times and the amplitude of the refractive index changes the crystals were thermally treated with simultaneous photorefractive testing. The photoconductivity data and the photorefractive beam coupling gain dependent on the grating spacing show that the relative influence of electron and hole contribution is altered depending on the reduction treatment. The experimental results are discussed in terms of a two carrier model.

The Role of EL2 for the Mobility-Lifetime Product of Photoexcited Electrons in GaAs

abstractThe mobility lifetime products for photo-electrons in semi-insulating GaAs, which fit successfully the results of photorefractive studies undertaken in the presence of electric fields are three orders of magnitude smaller than those inferred from transport measurements or from the photorefractive effect with no applied electrical field. Consideration of enhanced recombination via EL2 effective-mass states linked to the L-conduction band minimum allows us to fit the dependence of the photorefractive beam coupling gain coefficient on the grating period for both AC fields and moving gratings. A cascade-capture process, which is three orders of magnitude faster than recombination by multiphonon emission from the Γ band to EL2, leads to greatly reduced mobility-lifetime products for field strengths greater than 1 kV/cm. Our results establish the dominant influence of the EL2 defect properties on the recombination processes essential for modelling and optimizing the photorefractive effect in semi-insulating GaAs.

Photorefractive measurement of photoionization and recombination cross sections in InP:Fe

The photorefractive effect combined with standard Hall measurements is used to determine photoionization and recombination cross sections and deep-level number densities in a crystal of iron-doped InP. The results are well fit by iron densities [Fe2+]=6×1016 cm−3 and [Fe3+]=2.5×1015 cm−3, photoionization cross sections at 1.06 μm, se= 4×10−18 cm2, and sh= 3.1×10−16 cm2, and recombination cross sections σe= 10−14 cm2 and σh= 3×10−16 cm2. The sign of the photorefractive beam coupling gain implies that holes dominate this process even though the dark conductivity and photoconductivity are dominated by electrons.

High-accuracy, high-reflectivity phase conjugation at 106 μm by four-wave mixing in photorefractive gallium arsenide

We have used a 20-kHz ac square-wave electric field to enhance the beam-coupling gain and the degenerate four-wave mixing reflectivity of photorefractive GaAs at 1.06 μm. The largest measured four-wave mixing reflectivity in the steady state was 15% at a grating period of 9 μm and an applied voltage of 3.2 kV across 0.4 cm. Theoretical results predict substantially higher reflectivity values for a field of 8 kV/cm; the origin of this discrepancy is thought to be space-charge effects that prevent us from obtaining the full field inside the crystal. We have also measured a transient reflectivity of 510% during the switch on of the backward pump beam. Finally, we have determined the accuracy of wave-front reversal through measurements of piston-error correction and conjugation fidelity.

Optimization of the photorefractive properties of KNbO3 crystals

Photorefractive effects of nominally pure and iron-doped KNbO3 crystals with concentrations between 150 and 300 ppm have been investigated. In order to optimize the photorefractive recording times and the amplitude of the refractive changes, the crystals were thermally treated with simultaneous photorefractive testing. The photoconductivity data and the photorefractive experiments (grating spacing dependence of the beam coupling gain) show that the relative influence of electron and hole contribution is altered depending on the reduction treatment. The experimental results are discussed with a two-carrier model.

Thermodynamic point defect model of barium titanate and application to the photorefractive effect

We present a model that provides a means for understanding the effects of crystal growth and processing conditions on the photorefractive properties of BaTiO3. The densities of photorefractive centers are calculated from a thermodynamic point defect model. This model allows for the existence of transition-metal dopants in multiple valence states and for the incorporation of more than one species with levels in the band gap. Beam-coupling gain and response time are calculated as a function of temperature and oxygen partial pressure of treatment for various dopant types and concentrations. The predicted behavior is compared with existing experimental data on BaTiO3 crystals, and implications for improving the photorefractive performance are discussed.

High-efficiency fast response in photorefractive BaTiO3 at 120 °C

Measurements of beam coupling gain and response time in BaTiO3 show a decrease by two orders of magnitude in response time and an unchanged gain coefficient as the temperature is increased from 20 to 120 °C.

Intensity-dependent absorption coefficient in photorefractive BaTiO_3 crystals

It is shown experimentally that the nonsymmetric behavior of the beam-coupling gain coefficient of BaTiO3 crystals with respect to the sense of the c axis is caused by an intensity-dependent absorption coefficient. It is also found that the actual coupling gain coefficient is dependent on the intensity, even after it is compensated for by the nonsymmetric component. The temperature dependence of the absorption coefficient is measured.

Beam coupling in BaTiO3 at 442 nm

Steady-state beam coupling is used to measure the concentration of empty traps, the sign of the dominant photocarrier, and the effective electro-optic coefficient in seven crystals of BaTiO3. The measured value of the effective electro-optic coefficient gives the product of the fractional poling of the crystal, the relative conductivity of the crystal, and the electro-optic coefficient. Anomalously low beam coupling gain measured in an eighth crystal suggests that this crystal is nearly perfectly compensated; i.e., the photoconductivities due to electrons and holes are nearly equal.