Experimental study of second harmonic generation by dipolar configurations in pure and Li-doped KTaO 3 and its variation under electric field

Abstract

In the incipient ferroelectric crystal KTaO 3, 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 KTaO 3 with off-center Li-ions leads, at low temperature, to higher SHG intensity compared with nominally pure KTaO 3. Various samples K 1−xLi xTaO 3 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 LiNbO 3) 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 th S zzz max(E sat) S zzz max(0) of the SHG intensity are much larger than the corresponding birefringence rati Δn max(E) Δn max(0) indicates the alignment of antiparallel ordered regions. S zzz max( E sat) as a f Li-concentration is maximum at x ≈ 0.03. The ratios S zzz max(E sat) S zzz max(0) decrease drastically 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 tensor components in all cases turn out to be ( d 33 d 31 ) 2 = 6.3 ± 0.6 and ( d 31 d 15 ) 2 ≈ 1 . The time dependence of S zzz after a electric field ( E 3 0) reveals for the small Li-concentrations both fast (τ ⩽ 0. l 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 ⩾ 0.022).