Linear and nonlinear optical probe of the ferroelectric-like phase transition in a polar metal, LiOsO₃

LiOsO₃ is one of the first materials identified in the recent literature as a "polar metal," a class of materials that are simultaneously noncentrosymmetric and metallic. In this work, the linear and nonlinear optical susceptibility of LiOsO₃ is studied by means of ellipsometry and optical second harmonic generation (SHG). Strong optical birefringence is observed using spectroscopic ellipsometry. The nonlinear optical susceptibility extracted from SHG polarimetry reveals that the tensor components are of the same magnitude as in the isostructural insulator LiNbO₃, except the component along the polar axis d₃₃ is suppressed by an order of magnitude. Temperature-dependent SHG measurements in combination with Raman spectroscopy indicate a continuous order-disorder type polar phase transition at 140 K. Linear and nonlinear optical microscopy measurements reveal 109°/71° ferroelastic domain walls, like in other trigonal ferroelectrics. No 180° polar domain walls are observed to emerge across the phase transition.