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Physical Review Letters

Observation of Nondegenerate Photorefractive Parametric Amplification

Henrik C. Pedersen, Per Michael Johansen

Published May 27, 1996• 10 citations
Publisher

Abstract

We report on the first experimental observation of so-called nondegenerate photorefractive parametric amplification. We show that due to this effect it is possible for a weakly modulated photoinduced grating to be parametrically amplified via nonlinear interaction with a strongly modulated photoinduced grating.
DOI
10.1103/physrevlett.76.4159

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We present for the first time, to our knowledge, a theoretical analysis of the nonlinear dynamic space-charge field buildup when a photorefractive medium is exposed simultaneously to two running gratings. It is shown that the two gratings are nonlinearly coupled, hence, for an appropriate choice of parameters, the two gratings interact, causing one grating to be amplified at the expense of the other. This process is referred to as degenerate parametric amplification. In principle an infinite amplification is possible because, even in cases for which no signal grating (i.e., the one to be amplified) is externally seeded into the crystal, a finite signal-grating amplitude is obtained after relaxation of the amplification process.
We present an analysis of what is called parametric oscillation in photorefractive media. It is shown theoretically that when a running grating and a constant electric field are applied to a photorefractive medium it is possible for two secondary gratings to grow spontaneously. These theoretical predictions are subsequently verified experimentally in the crystal Bi12SiO20, and excellent qualitative agreement is obtained. In previous subharmonic experiments, only grating vectors K/2, K/3, and K /4 were observed. Now, however, a whole continuum of grating vectors has been observed. Thus subharmonic generation in photorefractive materials is simply a special case of parametric oscillation in which the two secondary gratings are identical.
We show that a certain phase feedback between output and input changes dramatically the characteristics of photorefractive two-beam coupling. The diffraction efficiency of the dynamic phase grating, $\ensuremath{\eta}$, reaches 1 or 0 (depending on the feedback sign) within a finite time for a wide range of experimental parameters. The spatial-temporal behavior of the wave amplitudes is strongly nonlinear and the grating fringes for $\ensuremath{\eta}=1$ or 000 are bended but not tilted. The theory elucidates a number of empirical observations and reveals the great potential of active stabilization for applications.
We present a theoretical model of photorefractive parametric oscillation that covers, for the first time, to our knowledge, the occurrence of the whole spectrum of parametric processes from transversal over degenerate to longitudinal parametric oscillation. It is shown that inclusion of so-called noneigenwaves is essential for completing the model. We report on the first experiment that shows the transition from transversal over degenerate to longitudinal parametric oscillation. The experimental observations agree well with the theoretical predictions.