EXPERIMENTAL-STUDY OF 2ND-HARMONIC GENERATION BY DIPOLAR CONFIGURATIONS IN PURE AND LI-DOPED KTAO3 AND ITS VARIATION UNDER ELECTRIC-FIELD

Abstract

In the incipient ferroelectric crystal KTaO3 due to the inversion symmetry of the lattice no second harmonic generation (SHG) is expected. However, even in nominally pure crystals a weak intensity S-zzz of frequency doubled light is observed at low temperatures. The SHG intensity S-zzz increases with applied electric field. Our results are compared with theoretical predictions [Prusseit-Elffroth and Schwabl, Appl. Phys. A51, 361 (1990)] and previous results [Fujii and Sakudo, Phys. Rev. B13, 1161 (1976)] under applied electric field E. Doping of KTaO3 with off-center Li-ions leads, at low temperature, to higher SHG intensity compared with nominally pure KTaO3. Various samples K1-xLixTaO3 with Li-concentrations x = 0.008-0.063 are measured and the spatial distribution and the total integrated SHG intensity S-zzz (calibrated with respect to LiNbO3) was 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 appears. Heating and cooling cycles follow the same S-zzz curve without thermal hysteresis for these small Li-concentrations. The spatial distribution of S-zzz shows a narrow forward scattered peak. When cooling under applied electric field E the intensity S-zzz(E) increases drastically at low temperatures indicating the alignment of dipolar regions. The maximum intensity saturates at relatively low electric field E(sat) with ratios S-zzz(max)(E(sat))/S-zzz(max)(0) varying from 250 for x = 0.008 to 50 for x = 0.022. The fact that these ratios S-zzz(max)(E(sat))/S-zzz(max)(0) of the SHG intensity are much larger than the corresponding birefringence ratios Delta n(max)(E)/Delta n(max)(0) indicates the alignment of antiparallel ordered regions. S-zzz(max)(E(sat)) as a function of Li-concentration is maximum at x approximate to 0.03. The ratios S-zzz(max)(E(sat))/S-zzz(max)(0) decrease drastically with increasing Li-concentration corresponding to a strong increase of the mean size of ordered dipolar regions for E = 0 at low temperature. This is supported by the appearance (for x > 0.02) of a thermal hysteresis of S-zzz and of lateral peaks in the spatial distribution. The variation of S-zzz with the polarization direction of the Nd:YAG laser light shows, at low temperatures, a periodicity pointing to dipolar regions with tetragonal symmetry. The ratios of the SHG tenser components in all cases turn out to be (d(33)/d(31))(2) = 6.3 /- 0.6 and (d(31)/d(15))(2) approximate to 1. The time dependence of S-zzz after a fast switch-on or -off of an electric field (E<->0) reveals for the small Li-concentrations both fast (tau less than or equal to 0.1 s) 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 greater than or equal to 0.022).