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

In the incipient ferroelectric crystal KTaO3 due to the inversion symm etry of the lattice no second harmonic generation (SHG) is expected. H owever, even in nominally pure crystals a weak intensity S-zzz of freq uency 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. Ph ys. A51, 361 (1990)] and previous results [Fujii and Sakudo, Phys. Rev . B13, 1161 (1976)] under applied electric field E. Doping of KTaO3 wi th off-center Li-ions leads, at low temperature, to higher SHG intensi ty compared with nominally pure KTaO3. Various samples K1-xLixTaO3 wit h Li-concentrations x = 0.008-0.063 are measured and the spatial distr ibution and the total integrated SHG intensity S-zzz (calibrated with respect to LiNbO3) was determined as a function of temperature, with o r without applied electric field. For small Li-concentrations x < 0.02 , a gradual increase of S-zzz with decreasing temperature appears. Hea ting and cooling cycles follow the same S-zzz curve without thermal hy steresis for these small Li-concentrations. The spatial distribution o f S-zzz shows a narrow forward scattered peak. When cooling under appl ied electric field E the intensity S-zzz(E) increases drastically at l ow temperatures indicating the alignment of dipolar regions. The maxim um intensity saturates at relatively low electric field E(sat) with ra tios S-zzz(max)(E(sat))/S-zzz(max)(0) varying from 250 for x = 0.008 t o 50 for x = 0.022. The fact that these ratios S-zzz(max)(E(sat))/S-zz z(max)(0) of the SHG intensity are much larger than the corresponding birefringence ratios Delta n(max)(E)/Delta n(max)(0) indicates the ali gnment of antiparallel ordered regions. S-zzz(max)(E(sat)) as a functi on 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 suppo rted 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, a t low temperatures, a periodicity pointing to dipolar regions with tet ragonal 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) ap proximate 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-concentrat ions both fast (tau less than or equal to 0.1 s) and slow relaxation p rocesses. 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 e qual to 0.022).