Raman spectroscopy of fluorophosphate and fluorovanadate laser crystals

Abstract

Fluorophospates and fluorovanadates are efficient solid state laser materials with promising new applications, especially for diode pumping of microchip lasers. High quality, low loss Nd doped Sr<SUB>5</SUB>(PO<SUB>4</SUB>)<SUB>3</SUB>F, Ca<SUB>5</SUB>(PO<SUB>4</SUB>)<SUB>3</SUB>F, Sr<SUB>5</SUB>(VO<SUB>4</SUB>)<SUB>3</SUB>F and Ca<SUB>5</SUB>(VO<SUB>4</SUB>)<SUB>3</SUB>F crystals were grown by the Czochralski technique. The lattice vibrations of the host material are studied by polarized Raman spectroscopy. The Raman active modes are determined by the irreducible representations of the space group of the crystal, and they provide insight to the crystal symmetry and chemical bonding. In all four compounds the internal and external vibrations are separated in the Raman spectra indicating that the internal binding of the (PO<SUB>4</SUB>)<SUP>-3</SUP> and (VO<SUB>4</SUB>)<SUP>-3</SUP> tetrahedra is different from the other ionic forces in the crystals. The group of internal modes in the fluorovanadates is shifted to lower frequencies by more than 100 cm<SUP>-1</SUP> compared with the fluorophosphates, though the x-ray structures are isomorphous. In addition there are remarkable changes in intensity, which may be attributed to interaction among the tetrahedral subunits in the hexagonal unit cell. The substitution of Sr for Ca causes frequency shifts in the vibrational modes. These are discussed with respect to the mass effect and changes in the coupling strength due to the differences between alkaline-earth cation binding and oxygen binding in the tetrahedral subunits. A comparison of the various substitutions makes assignments of the vibrational modes possible.