Terahertz driven extremely nonlinear bulk photogalvanic currents in non-resonant conditions

TL;DR

This paper demonstrates the generation of controllable, nonlinear bulk photocurrents in various materials driven by non-resonant terahertz fields, revealing a universal mechanism with potential applications in ultrafast electronics and photovoltaics.

Contribution

It introduces a universal, non-resonant mechanism for bulk photocurrent generation in diverse materials using terahertz fields without phase stabilization.

Findings

Photocurrents are transverse and controllable in multiple materials.

The mechanism relies on cycle-to-cycle asymmetries in nonlinear response.

High laser powers can break time-reversal symmetry, affecting photocurrent behavior.

Abstract

We report on the generation of bulk photocurrents in materials driven by non-resonant bi-chromatic fields that are circularly polarized and co-rotating. The nonlinear photocurrents have a fully controllable directionality and amplitude without requiring carrier-envelope-phase stabilization or few-cycle pulses, and are generated with photon energies much smaller than the band gap (reducing heating in the photo-conversion process). We demonstrate with ab-initio calculations that the photocurrent generation mechanism is universal and arises in gaped materials (Si, diamond, MgO, hBN), in semi-metals (graphene), and in two- and three-dimensional systems. Photocurrents are shown to rely on sub-laser-cycle asymmetries in the nonlinear response that build-up coherently from cycle-to-cycle as the conduction band is populated. Importantly, the photocurrents are always transverse to the major axis of the co-circular lasers regardless of the material's structure and orientation (analogously to a Hall current), which we find originates from a generalized time-reversal symmetry in the driven system. At high laser powers (~10^13 W/cm^2) this symmetry can be spontaneously broken by vast electronic excitations, which is accompanied by an onset of carrier-envelope-phase sensitivity and ultrafast many-body effects. Our results are directly applicable for efficient light-driven control of electronics, and for enhancing sub-band-gap bulk photovoltaic effects.