Space Charge Waves In Photorefractive Crystals Research Articles

High-frequency branch of space-charge waves in photorefractive crystals

Space-charge waves have been investigated in the sillenite-type crystals Bi12SiO20, Bi12GeO20, and Bi12TiO20. For the experiments, oscillating light patterns and the effects of spatial and overall rectification have been utilized. Main attention has been paid to the high-frequency branch and to the detailed comparison of experiment and theory. It was found that the theoretical model describes the experimental results for the high- and low-frequency branches quite reasonably; in particular, the dispersion laws are confirmed. By use of the data obtained for both branches, the major characteristics of the crystals (lifetime of the carriers, mobility, Maxwell relaxation time, and screening factor) have been determined.

Second harmonic generation and rectification of space-charge waves in photorefractive crystals

A theory explaining the effects of second harmonic generation and rectification of space-charge waves (SCWs) in both space and time in photorefractive crystals and semi-insulating semiconductors is formulated for the first time. The theory predicts two mechanisms of SCW second harmonic excitation. An experimental technique is proposed for second harmonic detection, and the formulas required for interpreting the experimental results are derived. It is found that the effect of complete rectification of SCWs may lead to a change in the current passing through the sample by tens of percent. The results of calculations are in qualitative agreement with preliminary experimental results.

Overall rectification and second harmonic generation of space charge waves

The effects of overall rectification and second-harmonic generation of space-charge waves in photorefractive crystals were reliably detected. The effect of overall rectification results in variations of the dc current flowing in a circuit containing a photorefractive crystal and a voltage source when space-charge waves are excited in the crystal. In the experiments, variations in the dc current reached 20%. Two mechanisms responsible for second-harmonic generation were experimentally revealed. The theory of the observed effects is briefly described.

Phase modulation spectroscopy of space-charge wave resonances in Bi 12SiO 20

A new experimental method for the study of resonance effects and space-charge wave excitation in photorefractive Bi 12SiO 20 crystals by using a combination of frequency detuning and phase modulation technique has been developed. The accuracy of the method allows a detection of resonance peaks of diffraction efficiency within 0.5 Hz. Numerical simulations of the nonlinear differential equations describing the behaviour of the space-charge waves in photorefractive crystals have been performed and found to be in a good agreement with experiment. We have measured the photocurrent through the crystal and revealed its resonance dependence. A minimum of electric current through the sample corresponds to the main resonance detected by phase modulation technique.

Space-charge waves in photorefractive crystals and their parametric excitation

It is shown that weakly damped space-charge waves exist in photorefractive crystals with a sufficiently large lifetime–mobility product. The dispersion law, the damping constant, and the quality factor of these waves and their dependence on the crystal parameters and on the experimental conditions are found. The instability of the fundamental grating against excitation of these waves is investigated; the fundamental grating is excited either by a moving interference pattern or by a standing interference pattern plus an oscillating external electric field. The dependence of the threshold and the characteristic exponent of the instability on the wave vectors of the exciting waves and on the experimental parameters is found for the three-dimensional case. It turns out that the strongest instabilities correspond to the cases of optimal enhancement of photoinduced gratings. The theory is compared with the experimental data for BSO crystals.