Spatio-temporal dynamics of shift current quantum pumping by femtosecond light pulse

TL;DR

This paper investigates the nonperturbative quantum dynamics of shift current in two-terminal devices under femtosecond light pulses, revealing superballistic transport, two-photon absorption effects, and nonadiabatic quantum charge pumping.

Contribution

It introduces a nonperturbative Green function approach to analyze shift current in realistic devices, uncovering new phenomena beyond traditional perturbative theories.

Findings

Superballistic transport of photoexcited charges

Quadratic photocurrent dependence on light intensity due to two-photon absorption

Shift currents as nonadiabatic quantum charge pumping

Abstract

Shift current---a photocurrent induced by light irradiating noncentrosymmetric materials in the absence of any bias voltage or built-in electric field---is one of the mechanisms of the so-called bulk photovoltaic effect. It has been traditionally described as a nonlinear optical response of periodic solids to continuous wave light using a perturbative formula, which is linear in the intensity of light and which involves Berry connection describing the shift in the center of mass position of the Wannier wave function associated with the transition between the valence and conduction bands. We analyze realistic two-terminal devices, where paradigmatic Rice-Mele model is sandwiched between two metallic electrodes, using recently developed time-dependent nonequilibrium Green function algorithms scaling linearly in the number of time steps and capable of treating nonperturbative effects in the amplitude of external time-dependent fields. This unveils novel features: superballistic transport, signified by time dependence of the displacement, $\sim t^\nu$ with $\nu > 1$, of the photoexcited charge carriers from the region where the femtosecond light pulse is applied toward the electrodes; and photocurrent quadratic in light intensity at subgap frequencies of light due to two-photon absorption processes that were missed in previous perturbative analyses. Furthermore, frequency dependence of the DC component of the photocurrent reveals shift currents as a realization of nonadiabatic quantum charge pumping enabled by breaking of left-right symmetry of the device structure. This demonstrates that a much wider class of systems, than the usually considered polar noncentrosymmetric bulk materials, can be exploited to generate nonzero DC component of photocurrent in response to unpolarized light and optimize shift-current-based solar cells and optoelectronic devices.