Electro-optic Response in Germanium Halide Perovskites

Electro-optic materials that can be solution-processed and provide high-crystalline quality are sought for the development of compact, efficient optical modulators. Here we present density functional theory investigations of the linear electro-optic coefficients of candidate materials cesium and methylammonium germanium halide perovskites. As with their lead halide counterparts, these compounds can be solution-processed, but in contrast, they possess the noncentrosymmetric crystal structures needed to provide a linear electro-optic effect. We find substantial electro-optic responses from these compounds; in particular, for the r₅₁ tensor element of CsGeI₃, we predict an electro-optic coefficient of 47 pm·V^-1 at the communications wavelength of 1550 nm, surpassing the strongest coefficient of LiNbO₃ at 31 pm·V^-1. The strong electro-optic responses of the germanium compounds are driven by high nonlinear susceptibilities and dynamics of the germanium atoms that ultimately arise from the distorted crystal structures. Alongside the electro-optic coefficient calculations, we provide the frequency responses for the linear and nonlinear electronic susceptibilities.