Hypersonic studies and refractive indices of CsTiOAsO4 and KTiOAsO4 single crystals

The longitudinal Brillouin backscattering spectra along the [001] phonon direction have been measured as a function of temperature (24-896 degrees C) for both CsTiOAsO4 (CTA) and KTiOAsO4 (KTA). As the temperature approaches the ferroelectric transition temperature T-c, the acoustic phonon frequencies of both CTA and KTA show a strong softening (which reaches a minimum at T-c similar to 688 degrees C for CTA). For both CTA and KTA, a broad damping peak was observed and can be attributed to the dynamic order-parameter fluctuations. This anomaly implies that an eta(2) mu-type coupling is the dominant mechanism for the acoustic soft modes in CTA and KTA. Here eta is the order parameter and mu is the strain. The refractive indices (n(x),n(y),n(z)) and Cauchy's equations were obtained as a function of wavelength (0.4-1.7 mu m) for KTA. The phase matching angles (theta phi) of second harmonic generation for the fundamental wavelengths of 1.064 (type I) and 1.32 (types I and II) were also calculated.