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Physical Review A

Phase-modulation effects in self-diffraction

Qiguang Yang, Jinhai Si, Yougui Wang, Peixian Ye

Published Aug 1, 1996• 9 citations
Publisher

Abstract

Phase-modulation effects in self-diffraction with two pump beams incident on a nonlinear medium were studied theoretically and experimentally. Based on a two-level model and considering the influence of the phase modulation on the phase matching, expressions describing the first-order and second-order scattered intensities were derived. The influences of phase modulation on the pump-intensity dependence and the sample-concentration dependence of the scattered intensities were discussed via numerical calculations. Experiments were performed with tetrahydrofuran solution of 1,4-diethinyl benzene to demonstrate the theoretical results. Some interesting results predicted by the theory, such as the fact that the scattered intensities of the sample with lower concentration can be larger than that with higher concentration at suitable high intensity of the pump, have been observed. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.54.1702

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Phase-modulation effects in self-diffraction with two pump beams incident on a nonlinear medium were studied theoretically and experimentally. Based on a two-level model and considering the influence of the phase modulation on the phase matching, expressions describing the first-order and second-order scattered intensities were derived. The influences of phase modulation on the pump-intensity dependence and the sample-concentration dependence of the scattered intensities were discussed via numerical calculations. Experiments were performed with tetrahydrofuran solution of 1,4-diethinyl benzene to demonstrate the theoretical results. Some interesting results predicted by the theory, such as the fact that the scattered intensities of the sample with lower concentration can be larger than that with higher concentration at suitable high intensity of the pump, have been observed. \textcopyright{} 1996 The American Physical Society.
The effects of side diffractions, generated from the interaction of a weak probe beam and its phase conjugate with two strong counterpropagating pump waves, and the effect of various intensity-dependent self- and mutual-phase modulations on the intensities of the probe beam and its phase conjugate are analytically studied in the nonpump-depletion limit. The results show that the conditions for the existence of, and threshold pump intensity parameters for, the divergence in the intensities of the probe and other generated beams are highly dependent on the wave-mixing angles and are different from the usual four-wave mixing phase conjugation process where the diffractions are not taken into account.>
A systematic study of degenerate two-wave mixing in the stationary regime in thin nonlinear local-response media is presented. The model is based on the coherent interaction of four waves, the two incident beams, and the two main diffracted waves. Analytical results are obtained in all cases investigated under the assumption of weak-beam amplification. It is demonstrated that the energy transfer arises only from the forward degenerate four-wave mixing terms. In particular, the theoretical results show two new features: (1) the gain factor of the interaction is substantially reduced because of the phase change associated with the two-wave mixing terms and (2) a phase mismatch can compensate for this phase change, leading to an appreciable gain enhancement. Moreover, the general case is solved numerically. Particular attention is given to the influence of separate two-wave and forward degenerate four-wave mixing terms. Self-diffraction effects and the gain dependence versus the ratio of the pump-beam intensity to the signal-beam intensity are investigated.
We present a nonperturbative theory of self-diffraction in thick, pseudostationary population gratings induced by the interference of two nearly collinear incident laser beams of arbitrary relative and absolute intensities. At large saturation parameters, the grating becomes nonsinusoidal, and high-order diffraction occurs efficiently. We obtain an exact expression for the Fourier coefficients of the grating that significantly simplifies the interpretation of the various coherent coupling mechanisms and permits efficient computer simulations. In the case of small depletion of the pump waves, we obtain simple analytic expressions for the scattered intensity into any order of self-diffraction. We present our results in a manner that should aid an experimenter in choosing the optical thickness of the unsaturated medium and the sum and ratio of the pump intensities to obtain optimum diffraction intensity in a given order.
Two-wave mixing amplification of a weak probe beam interfering with a powerful pump beam of the same linear polarization is demonstrated when the probe beam enters the semiconductor sample before the pump pulse. This mixing of two beams of the same frequency is made possible by the difference between self-phase modulation and cross-phase modulation, which modify differently the instantaneous frequencies of both beams. Net gain coefficients up to 5.5 cm (-1) are obtained for experiments performed in CdTe. Theoretical calculations quantitatively confirm this interpretation and predict much higher gain for more resonant configurations or higher pump energies, making this process of potential interest for optical processing.
Full space-time numerical modeling of near-resonance self-action of a beam, whose frequency is harmonically modulated in time, is presented. Maxwell-Bloch equations describing the propagation of the beam through a two-level medium with saturable absorption and refraction are solved using an original stable numerical scheme for multidimensional nonlinear Schrodinger equations. Delayed redistribution of intensity over the beam cross-section due to nonlinear self-focusing and self-phase-modulation effects is shown to affect the phase and the amplitude dynamics of the output signal in a rather nontrivial way, depending on the carrier laser frequency.