Theory of subcycle time-resolved photoemission: application to terahertz photodressing in graphene

TL;DR

This paper develops a gauge-invariant theoretical framework for subcycle time-resolved photoemission spectroscopy, applied to terahertz-pumped graphene, enabling more accurate simulations of transient electronic dynamics.

Contribution

It derives a nonequilibrium Green's function approach for photoemission in different gauges, incorporating the photoelectron continuum and applying it to graphene with first-principles tight-binding models.

Findings

Gauge invariance of the photoemission signal is demonstrated.

The formalism accurately captures subcycle effects in time-resolved photoemission.

Application to graphene reveals characteristic pump-induced electronic modifications.

Abstract

Motivated by recent experimental progress we revisit the theory of pump-probe time- and angle-resolved photoemission spectroscopy (trARPES), which is one of the most powerful techniques to trace transient pump-driven modifications of the electronic properties. The pump-induced dynamics can be described in different gauges for the light-matter interaction. Standard minimal coupling leads to the velocity gauge, defined by linear coupling to the vector potential. In the context of tight-binding (TB) models, the Peierls substitution is the commonly employed scheme for single-band models. Multi-orbital extensions -- including the coupling of the dipole moments to the electric field -- have been introduced and tested recently. In this work, we derive the theory of time-resolved photoemission within both gauges from the perspective of nonequilibrium Green's functions. This approach naturally incorporates the photoelectron continuum, which allows for a direct calculation of the observable photocurrent. Following this route we introduce gauge-invariant expressions for the time-resolved photoemission signal. The theory is applied to graphene pumped with short terahertz pulses, which we treat within a first-principles TB model. We investigate the gauge invariance and discuss typical effects observed in subcycle time-resolved photoemission. Our formalism is an ideal starting point for realistic trARPES simulations including scattering effects.