Thermal focusing and optical bistability in ferroelectrics

A review is presented of phenomena involving thermal focusing and optical b istability arising from the temperature dependence of the refractive index in ferroelectrics. Aperiodic spatio-temporal oscillations have been observe d in the light transmitted and reflected by parallel-sided crystal and cera mic samples of the thermo-optic ferroelectrics PMN (lead magnesium niobate, Pb(Mg1/3Nb2/3)O-3), PLZT (lanthanum doped lead zirconium titanate, Pb(Zr1- xTix)O-3:La), BNN (barium sodium niobate, Ba2NaNb5O15) and Ce:SBN (cerium d oped strontium barium niobate, Ce3+ :Sr-x Ba1-x Nb2O6) under steady illumin ation by an incident c.w. laser beam of finite beamwidth, Such materials fo cus the incident beam due to the temperature dependence of the refractive i ndex and the establishment of a radial temperature gradient. The shape: of the resulting thermal lens varies in time as a result of variation in the a bsorbed energy arising from both thermal focusing and optical bistability ( Fabry-Perot resonance). Whereas PMN and PLZT exhibit relaxation oscillation s, the beam oscillations produced by BNN and Ce:SBN have equal rise;md fall times. We compare the predictions of theoretical models with experimental results for ferroelectric ceramics and crystals. The principal conclusions from the present work are that (1) there are two distinct mechanisms for ap eriodic oscillation in dispersively nonlinear plates, viz., focusing oscill ations and Fabry-Perot switching oscillations, of which only the latter inv olves bistability; (2) in Fabry-Perot etalons with a diffusive nonlinearity , as cc,contrasted with the systems (e.g. laser tubes) originally described by Gordon et al, the diffusive quantity (i.e. temperature) can exhibit bis table and multistable switching behaviour, leading to regenerative oscillat ions in other variables (e.g, light output); (3) there are two characterist ic relaxation times involved in these oscillations, differing by several or ders of magnitude; (4) the spatio-temporal characteristics of the transmitt ed beam patterns in the near and far fields can be quantitatively predicted and (5) comparison between theory and experiment can provide information o n the temperature dependence of conductivity and thermo-optic coefficient n ear the ferroelectric phase transition.