Na ₂Ge ₂Se ₅: A highly nonlinear optical material

We report an integrated experimental and theoretical study of the two-dimensional polar selenogermanate compound Na ₂Ge ₂Se ₅, which exhibits strong nonlinear optical (NLO) second harmonic generation response in the visible and infrared region. The compound is type-I phase-matchable with a large SHG coefficient χ ⁽²⁾≈290 pm V ^-1, which is the second highest among the phase-matchable NLO materials. It also performs as an NLO frequency mixer to produce radiation via difference frequency generation. The compound is optically transparent from the visible (0.521 μm) to the far IR (18.2 μm) and melts congruently. Ab initio density functional theory band structure calculations show that the unusually large second-order optical nonlinearity is attributed to the two-dimensional character of the crystal structure. Raman, IR and optical absorption spectroscopy, differential thermal analysis, NLO properties of derivatives of Na ₂Ge ₂Se _4.55Te _0.45 and Na ₂Ge _1.64Sn _0.36Se ₅ and thermal expansion behavior studies are also reported.