Refractive-index studies in Ca_2Ga_2SiO_7 and SrLaGa_3O_7 melilite-type compounds

Refractive-index dispersion and anisotropy were measured in tetragonal Ca2Ga2SiO7 (CGSM; Ga-gehlenite) and SrLaGa3O7 (SLGM) single crystals at room temperature between 308 and 1064 nm. Large single crystals were grown by the Czochralski method. Bireflectance in a crystallographically oriented single-crystal prism was used for an accurate measurement of the anisotropy. CGSM is negative uniaxial. At the wavelength λ=632.8 nm the ordinary and the extraordinary refractive indices no and ne were no=1.7214 ± 0.0004 and ne=no-0.0112 ± 0.0001. The temperature dependence was dno/dT=(12.7±0.2) × 10-6 °C-1 and dne/dT=(5.4±0.4)×10-6 °C-1. SLGM is positive uniaxial. At λ=632.8 nm, no = 1.8249±0.0004 and ne=no+0.0097±0.0001. The temperature dependence was dno/dT=(15.3 ± 0.2)×10-6 °C-1 and dne/dT=(10.5±0.5)×10-6 °C-1. The refractive-index dispersions were fitted to a Sellmeier function. Both materials can be considered for use in nonlinear optics (frequency doubling, optical parametric oscillations).