Polar Configurations in Pure and Li-Doped KTaO3 Studied by Second Harmonic Generation and its Variation under Electric Field

Abstract

A weak intensity of frequency doubled light (SHG) is observed at low temperature in the inversionsymmetric crystal KTaO3. The SHG intensity increases under applied electric field. Results are compared with theoretical model calculations [1]. Doping of KTaO3 with off-center Li-ions leads to a high SHG intensity at low temperature. Various K1-xLixTaO3 samples with Li-concentrations x = 0.008-0.063 are measured. The spatial distribution and the integrated SHG intensity S-zzz (calibrated with respect to LiNbO3) is determined as a function of temperature, with or without applied electric field. For small Li-concentrations x < 0.02 a gradual increase of S-zzz with decreasing temperature, no thermal hysteresis and a narrow forward scattered distribution of S-zzz is observed. At higher Li-concentrations x > 0.02 a thermal hysteresis of S-zzz and lateral peaks in the spatial distribution appear. The time dependence of Szzz after a fast switch-on or -off of an electric field (E -> 0) reveals for the small Li-conentrations both fast (r <= 0.1 sec) and slow relaxation processes. The slow relaxation process can be described by an Arrhenius-like behavior with a concentration dependent activation energy (E-A = k . 170 K for x = 0.008 to E-A = k . 1000 K for x >= 0.022).