ORDER-DISORDER PHASE-TRANSITION AND CRITICAL SLOWING-DOWN IN PHOTOREFRACTIVE SELF-OSCILLATORS

We study self-oscillation in a photorefractive double phase-conjugate mirror and show that it undergoes an order-disorder phase transition at a critical gain-threshold point. We present theoretical and experimental results on two key properties of this phase transition: critical slowing down and the characteristic order parameter. We show that the correlation distance of the refractive index variation responsible for the double phase conjugation changes abruptly from very small values (disorder, below threshold) to a large unique value (order, above threshold) determined primarily by the boundary conditions. Finally, we point out similarities and important differences between this combined light and matter phase transition and the mean field and Ginzburg-Landau theories. © 1995 The American Physical Society.