Method for investigating generation of the second harmonic in micromonocrystals and its application to BaTiO3 and KNbO3 ferroelectrics

A version has been elaborated of the investigation techniques of the laser radiation second harmonics which by the nature of the objects studied occupies the intermediate position between known methods of a large monocrystal and powder. A nonpolarized laser radiation is used, focussed to the most perfect part of a miniature crystal which for the optimal utilization of the techniques has the volume of 0.1 to 1 mm3. The investigation of mono- and polydomain crystals BaTiO3 and KNbO3 is an example showing that the temperature dependdence of the second harmonic Isub(2ω) intensity obtained for noncentrosymmetric crystals involves a wide information on the position and nature of the phase transitions, including the information on the Curie points of ferroelectric materials and their domain structure presence and variation with temperature. It has been determinated that the nonlinear susceptibility of KNbO3 is very high and that at 20 deg C the nonlinear coefficients d32 approximately d33 of the compound exceed the coefficient d31(BaTiO3) as much as 1.6 times. The temperature dependence of d31(BaTiO3) has been determined which proved similar to the temperature dependence of the spontaneous polarization Psub(s) and Psub(s) in potassium niobate has been evaluated