Using the distinctive features of the photorefractive nonlinearity, we derive a general self-consistent set of equations to describe two-wave coupling in the presence of fast and arbitrary strong-phase modulation. By considering a number of important particular cases, we show that phase modulation is a powerful and useful tool for shaping the characteristics of two-wave coupling such as the value of the energy exchange, the diffraction efficiency of the recorded grating, and the structure of the grating fringes. Finally, we analyze the role of the phase modulation in the active stabilization of wave coupling by means of an electronically introduced phase feedback.
We develop the idea of critical enhancement of the photorefractive response near the threshold of parametric excitation of space-charge waves (the spatial subharmonics) taking into account the vectorial character of beam coupling and a fairly strong broadening of the nonlinear resonance owing to light absorption. The results of our calculations are a description of the measurable characteristics of critical enhancement and optimization of the experimental conditions for detection of anomalously high amplification gain factors in cubic Bi(12)SiO(20) crystals.
We investigate here both theoretically and experimentally light-induced scattering in (111)-cut Bi12TiO20 crystals with an external ac field. Our simple analytic solution, which is nearly as precise as the numeric one, allows us to recognize the following otherwise hidden general features. Without the elasto-optic contribution, the scattering patterns are identical for the same value of xi=zeta(0)+(2/3)straight phi(p), where straight phi(p) is the initial polarization angle and zeta(0) is the angle of the external field. With the elasto-optic contribution, the scattering patterns for the same xi are still very similar. For xi not equal0, the scattering patterns depend differently on the elasto-optic coefficients p(12) and p(13) so that in principle p(12) and p(13) can be measured by purely holographic experiments. On the experimental side, we present scattering patterns for xi=0 and +/-30 degrees, showing thereby the similarity of the scattering patterns for equal values of xi. In all cases, we obtain good qualitative agreement of our analytic and numeric calculations with the experimental findings.
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