Browsing by Author "Bahalla, A.S."

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Hypersonic anomalies and optical properties of RbTiOAsO4 and KTiOPO4 single crystals
(1999-01-01) Tu, Chi-Shun; Katiyar, R.S.; Schmidt, V. Hugo; Guo, R.; Bahalla, A.S.
The longitudinal (LA) Brillouin back-scattering spectra along the [001] phonon direction have been measured as a function of temperature for RbTiOAsO4 (RTA) and KTiOPO4 (KTP) single crystals. As temperature increases, the acoustic-phonon frequencies of RTA and KTP show a clear softening (which reaches a turning point at Tc∼800°C for RTA). We conclude that the transitions in RTA and KTP are either weakly first order or of second order. It seems likely that the alkali ions (K+ or Rb+) are strongly involved in the soft mode. A broad damping evolution (which attains a maximum at Tc∼800 °C in RTA) was observed for both crystals and can be attributed to the dynamic order-parameter fluctuations. The optical transmissions, refractive indices (nx,ny,nz) and the Cauchy equations (for RTA) were obtained as a function of wavelength. The LA[001] sound velocities VLA and elastic constants C33+(e233/ɛs33) were also calculated at room temperature for both crystals.
Hypersonic studies and refractive indices of CsTiOAsO4 and KTiOAsO4 single crystals
(1997-10-01) Tu, Chi-Shun; Yeh, Y.-L.; Schmidt, V. Hugo; Chien, R.-M.; Katiyar, R.S.; Guo, Ruqian; Guo, Ruyan; Bahalla, A.S.
The longitudinal Brillouin backscattering spectra along the [001] phonon direction have been measured as a function of temperature (24-896 °C) for both CsTiOAsO4 (CTA) and KTiOAsO4 (KTA). As the temperature approaches the ferroelectric transition temperature Tc, the acoustic phonon frequencies of both CTA and KTA show a strong softening (which reaches a minimum at Tc∼688°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 η2μ-type coupling is the dominant mechanism for the acoustic soft modes in CTA and KTA. Here η is the order parameter and μ is the strain. The refractive indices (nx,ny,nz) and Cauchy’s equations were obtained as a function of wavelength (0.4–1.7 μm) for KTA. The phase matching angles (θ,φ) of second harmonic generation for the fundamental wavelengths of 1.064 (type I) and 1.32 (types I and II) were also calculated.