Gain and stability in photorefractive two-wave mixing - art. no. 033806

We demonstrate that the negative amplitude gain in a photorefractive two-wa ve-mixing experiment under applied electric field measurably reduces the ch aracteristic instability of the recorded hologram. In this sense photorefra ctive materials behave like electronic amplifiers with feedback. We analyze the case of stationary and running holograms both under an externally appl ied electric field. A continuous phase-modulation method is used to simulta neously measure diffraction efficiency and phase shift. Measurements carrie d out on a Bi12TiO20 crystal at 514.5 nm wavelength confirm the occurrence of continuous oscillations in both the diffraction efficiency and the phase shift. The perturbations in the diffraction efficiency increase considerab ly with increasing applied field and are reduced when energy is transferred from the weaker to the stronger beam (negative gain). Our results indicate that the perturbations in our experiments are probably due to resonantly e xcited transient effects.