Real-time simulation of nonlinear photocurrents

We investigate the bulk photovoltaic effect, a phenomenon where photocurrent is generated in the absence of interfaces. Utilizing real-time simulations based on first-principles time-dependent density functional theory (rt-TDDFT), we examine the role of non-perturbative effects in controlling the bulk photovoltaic effect in systems with strongly-coupled charge, spin, and lattice degrees of freedom. The simulations reveal that ballistic photocurrents, generated by Coulomb scattering, can arise in non-equilibrium contexts under high fields and in low-dimensional materials. Notably, these ballistic currents are comparable to shift currents under experimentally accessible conditions, as demonstrated using monolayer GeS as an example. Furthermore, the simulations provide insights into recombination pathways that modify photocurrent magnitudes beyond predictions based on perturbation theory.

Next, we discuss photocurrents generated by novel quasiparticles, or in hidden photo-induced phases. We find that orbital-ordered phases that host excitations known as orbitons can generate a strong THz oscillatory photocurrent with a long lifetime, under the requirement that the orbitons are polar. Therefore, an experimental THz-emission study could use this long-lived THz photocurrent to detect orbitons, which have so far been elusive. These findings demonstrate the advantages of real-time simulations in in capturing non-perturbative effects, including saturation, strong-field effects, and photo-induced phase transitions, which lie beyond conventional response theory. rt-TDDFT simulations were performed using INQ, a software package for scalable real-time electronic structure calculations.