Nonlinear Nonreciprocal Photocurrents under Phonon Dressing

TL;DR

This paper develops a Green's function framework to study nonlinear optical responses, revealing how phonon interactions can induce nonreciprocal photocurrents in centrosymmetric crystals under non-equilibrium conditions.

Contribution

It introduces a comprehensive method to include many-body effects in NLO calculations and uncovers phonon-driven nonreciprocal photocurrents in centrosymmetric materials.

Findings

Phonon dressing significantly affects nonlinear photocurrents.

Non-equilibrium phonons can induce non-zero BPV/BSPV in centrosymmetric crystals.

Photocurrents can be nonreciprocal under temperature gradients.

Abstract

Nonlinear optical (NLO) effects have attracted great interest recently. However, by far 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/BSPV should be zero for centrosymmetric crystals, but when phonons are driven out-of-equilibrium, by for example, a temperature gradient $\nabla T$, the optical selections rules are altered and phonon-pumped BPV/BSPV currents can be non-zero in nominally centrosymmetric crystal. Moreover, we elucidate that such NLO responses under non-equilibrium 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.