Optical properties of RbTiOAsO4 single crystal

Abstract

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.