The structural origin of the 15 μm residual absorption in the BaGa4Se7 crystal

Abstract

The BaGa₄Se₇ (BGSe) crystal is an excellent mid- and far-IR nonlinear optical crystal, but often shows an unexpected residual absorption peak around 15 μm which substantially deteriorates the crystal performance. The structural origin of residual absorption is still under debate. In this work, Raman spectroscopy, factor group analysis and density functional theory computations were used to study the IR/Raman spectra of the perfect BGSe crystal and four defect BGSe crystals in order to clarify the structural origin of the residual absorption. The perfect BGSe crystal has 72 lattice phonons, including three acoustic phonons (2A′ + 1A′′), nine translational phonons (4A′ + 5A′′), six librational phonons (3A′ + 3A′′) and 54 internal vibrational phonons (27A′ + 27A′′). Except for the three acoustic phonons, all the phonons are IR- and Raman-active. The maximum-frequency phonon of the perfect BGSe crystal (A′′), with a frequency of 295 cm⁻¹, is attributed to the stretching vibration of Ga–Se bonds. The two-phonon absorption of the 295 cm⁻¹ phonon corresponds to the crystal IR absorption edge, rather than the residual absorption peak. Density functional theory computations show that the residual absorption of the BGSe crystal originates from the O_Se defect (Se is substituted by O). The deforming vibrations of Ga–O–Ga bonds in the defect crystal produce two IR absorption bands located around 665 cm⁻¹, corresponding to the crystal residual absorption. The result will help us to eliminate the residual absorption and improve the crystal quality.