Permittivity changes induced by injected electrons and field-induced phase transition in KTa_(1-x)Nb_xO_3 optical beam deflectors

摘要

A space-charge-controlled optical beam deflector made of a KTa_(1-x)Nb_xO_3 (KTN) single crystal utilizes electrons that are injected through the cathode by applying voltage. With the deflector made of lithium-doped KTN (K_(0.95)Li_(0.05)Ta_(0.73)Nb_(0.27)O_3, KLTN/0.05/0.27), we observed large increases in the capacitance of the deflector when we injected electrons. The increases were not caused by changes in the electrode interface but by changes in the permittivity of the bulk crystal. In the paraelectric phase, the KLTN/0.05/0.27 crystal exhibited nonlinearity in the dielectric response with double hysteresis loops in the D-E curves. We ascribed the permittivity change to this nonlinear phenomenon. We also discuss this nonlinearity in terms of the Landau-Devonshire phenomenological theory. The coefficient g_4 of the fourth power term in the expanded free energy was negative in the paraelectric phase near the phase transition temperature as it is for other materials that exhibit a first-order phase transition. However, g_4 depended on the temperature and its sign became positive about 15℃ above the phase transition temperature.