Comparative study of perturbation theory and first-principles density matrix dynamics for nonlinear photocurrent

The photogalvanic effect (PGE) is a second-order phenomenon where non-centrosymmetric materials generate a steady direct current (DC) under illumination. Besides electric charge current, recent research in spintronics and orbitronics has also sparked interest in the generation of spin and orbital photocurrents. Despite decades of experimental investigation, a fully ab-initio quantum kinetic theory to describe nonlinear photocurrent at both transient and steady state remains to be developed. In this work, we compare second order perturbation theory with our real-time density matrix dynamics approach to study the nonlinear photocurrent in solids. Our methodology goes beyond photoexcitation by incorporating electron-phonon and electron-electron scatterings and spontaneous emission processes explicitly through our density matrix dynamics approach. We examine the charge and spin photocurrent responses from both linear and circular polarized light as well as the role of scatterings. Our study offers a comprehensive understanding of nonlinear photocurrent mechanisms and provides a general framework for investigating a range of nonlinear optical effects.