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 K_1-xLi_xTaO₃ 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 S_zzz 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).