Aperiodic oscillations in thermal focussing by single crystals of lead magnesium niobate

Optical bistability in monocrystalline lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃, (PMN) has been studied at temperatures near the ferroelectric phase transition under 514.5 nm illumination. At moderate laser power densities above a threshold but less than 100 kW/cm², this material exhibits reproducible aperiodic oscillations following the rapid switching on of a constant intensity Gaussian laser beam. With increasing incident power the oscillations change from the symmetric type observed in lanthanum modified lead zirconium titanate (PLZT) to a sequence of rapid jumps followed by a slow relaxation between two metastable thermal lens configurations, similar to those observed in ceramic samples of PMN at low power densities (about 1 kW/cm²). The duration and number of the oscillations depend reproducibly on incident power and ambient temperature, with different spatial structures from those observed in ceramic samples, depending not only on incident beam power but also on incident beam width and wavefront curvature.