Browsing by Author "Guo, Ruqian"

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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.
Optical properties of RbTiOAsO4 single crystal
(1998) Tu, Chi-Shun; Yeh, Y.-L.; Katiyar, R. S.; Guo, Ruqian; Schmidt, V. Hugo; Chien, R.-M.; Guo, Ruyan; Bhalla, A.S.
Rubidium titanyl arsenate (RbTiOAsO4) belongs to the family of nonlinear optical crystals with the general formula M1+TiOX5+O4, where M = K, Rb, Tl, Cs and X = P, As. [1–6] The high damage threshold and broad angular acceptance have made such crystals attractive materials for frequency doubling of Nd-based lasers at λ=1.064 and 1.32 µm, and for optical parametric oscillators (OPO). In addition, the ion exchange properties also make them one of the best candidates for waveguide applications. Potassium titanyl phosphate, KTiOPO4 (KTP), is the most popular among such materials and has been used successfully in diﬀerent applications. However, the orthophosphate absorption at ∼4.3 and ∼3.5 µm in KTP severely limits the oscillator output power. In contrast, RTA has a broad infrared transparency (∼0.35-5.3 µm) and exhibits no overtone absorption between 3 and 5 µm. [4] This makes the RTA crystal a potential candidate for nonlinear optical applications. At room temperature, KTP-type crystals have an orthorhombic structure with non- centrosymmetric point group C2v (mm2) and space group Pna2 (Z=8). The crystal framework is a three-dimensional structure made from corner-linked TiO6 octahedra and PO4 tetrahedra. Four oxygen ions of the TiO6 belong to PO4 tetrahedral groups which link the TiO6 groups. In our earlier Raman results, a slight softening was exhibited by several LO and TO vibrational modes of RTA. [1,2] However, there is no typical soft mode observed in the lowfrequency modes of the Raman spectra. This motivated usto carryoutBrillouinscatteringmeasurements tolook for softening in the acoustic modes. We report here both the temperature-dependent acoustic phonon spectra and wavelength- dependent refractive indices. The Cauchy equations [n(λ) = A + B/λ2 + C/λ4] of nx, ny and nz are obtained. In particular, the ﬁrst direct evidence for acoustic phonon soft mode is presented.