Nonlinear nonreciprocal photocurrents under phonon dressing

Abstract

Nonlinear optical (NLO) effects have attracted great interest recently. However, most of the computational studies on NLO use the independent particle approximation and ignore many-body effects. Here we develop a generic Green's function framework to calculate the NLO response functions, which can incorporate various many-body interactions. We focus on the electron-phonon coupling and reveal that phonon dressing can make significant impacts on nonlinear photocurrent, such as the bulk photovoltaic (BPV) and bulk spin photovoltaic (BSPV) currents. BPV and BSPV should be zero for centrosymmetric crystals, but when phonons are driven out of equilibrium by, for example, a temperature gradient $\ensuremath{\nabla}T$, the optical selection rules are altered and phonon-pumped BPV and BSPV currents can be nonzero in nominally centrosymmetric crystal. Moreover, we show that such NLO responses under nonequilibrium phonon dressing can be nonreciprocal, as the direction of the current does not necessarily get reversed when the direction of the temperature gradient is reversed.