Effect of Cr co-doping on the optical absorption and emission characteristics of Yb:YVO4 single crystals grown by OFZ technique

Cr co-doped Yb:YVO4 (Cr:Yb:YVO4) single crystals with concentrations of 1.0 and 1.5 at.% of Cr and 3.0 at.% of Yb were grown by the optical floating zone technique. X-ray diffraction analysis on the grown crystals revealed that the lattice parameters of Cr:Yb:YVO4 crystals do not depend on the Cr doping concentration. The dislocation etch pit density estimated from the etching study was found to be 103/cm2. The observed fringes of nearly uniform thickness and spacing in birefringence interferometry for [100] plate and conoscopy pattern for [001] plate indicate the compositional and optical homogeneity of the crystal. Further, the influence of the Cr doping concentration on the 2A1→2B2 transition (at ∼1100 nm) and 2A1→2E transition (at ∼640 nm) due to Cr5+ ion were analysed using two orthogonal polarized light with electric field perpendicular (π-polarization) and parallel (σ-polarization) to [001] axis. These absorption peaks of Cr ion were found to be sensitive to the polarization of incident light. The absorption transition 2A1→2B2 around 1100 nm are permitted for π-polarization and on the other hand, the absorption transition 2A1→2E at 640 nm are permitted only for σ-polarization. The characteristic broader absorption band around ∼1100 nm is of importance as it is useful for self-Q-switching in a laser gain medium. The absorption coefficient corresponding to Cr5+ ions at 1100 nm increases with the increase in Cr doping concentration. The band-gap for σ-polarized light is slightly higher than π-polarized light and decreases with Cr doping. The increase in the estimated Urbach energy (50–122 meV) with the increase in the Cr doping concentration confirms that the defect states close to the band edge increase thus decrease the band gap. Further the photoluminescence intensity is in-line with the absorption data and decreases with increase in the Cr concentration from 0 to 1.5 at.%.