Characterization of Nonlinear Optical Properties of Crystal RbGeCl3•x(H2O) in Infrared Region

The nonlinear optical (NLO) property of hydrated rubidium germanium chloride (HRGC), RbGeCl3 center dot x(H2O), is identified. Infrared absorbtion data support structural evidence that HRGC contain co-ordinated water molecules with strong hydrogen bond. The infrared spectrum indicated HRGC is transparent in most of the infrared region with only little influence from water. Calculations base on density functional theory shows that the band gap of the RbGeCl3 (RGC) crystal is at least 3.84 eV, which is larger than that of the infrared (IR) NLO crystal CsGeCl3. Single crystals of HRGC, sized up to 3 x 2 x 1cm(3), were grown in aqueous solution by a slow dehydrate technique. The synthetic, structural and optical propertiels of an off-centrosymmetric IR nonlinear optical (NLO) RbGeCl3 center dot x(H2O) crystal were investigated experimentally. Powder second harmonic generation (PSHG) measurement indicated that the crystal structure of HRGC becomes off-centrosymmetric. Precise X-ray diffraction measurements showed that [100] family diffraction peaks split slightly. Unlike the RGC crystal structure whose space group is P2(1)(m) over bar, the HRGC crystal loses the inversion symmetry. Comparisons with known NLO material KH2PO4 (KDP), indicate that HRGC's NLO susceptibility, x((2)), is about one third of that for KDP. The absorption edge of HRGC occured at 310 nm (approximate to 4.0ev), which indicated NLO HRGC crystal can have larger laser damage threshold. According to the Fourier transform infrared (FTIR) measurement, HRGC has a transparent region from 0.31 to 30 mu m, thus it can be applied to wider optical spectrum from ultraviolet, visible to mid-IR. (C) 2009 The Japan Society of Applied Physics