Optical conductivity of a topological system driven using a realistic pulse

TL;DR

This paper investigates how a realistic, pulse-shaped drive affects the optical conductivity of topological systems, revealing that the system's response retains memory of its initial state and involves Floquet band populations influenced by the initial equilibrium.

Contribution

It introduces a method to analyze driven topological systems with pulse-shaped drives using real-time evolution and compares it to Floquet theory, highlighting the memory effect in optical response.

Findings

Optical conductivity retains memory of initial equilibrium state.

Floquet band populations are influenced by initial state overlap.

Response at band inversion points shows population inversion.

Abstract

The effect of a time-periodic perturbation, such as radiation, on a system otherwise at equilibrium has been studied in the context of Floquet theory with stationary states replaced by Floquet states and the energy replaced by quasienergy. These quasienergy bands in general differ from the energy bands in their dispersion and, especially in the presence of spin-orbit coupling, in their states. This may, in some cases, alter the topology when the quasienergy bands exhibit different topological invariants than their stationary counterparts. In this work, motivated by advances in pump-probe techniques, we consider the optical response of driven topological systems when the drive is not purely periodic but is instead multiplied by a pulse shape/envelope function. We use real time-evolved states to calculate the optical conductivity and compare it to the response calculated using Floquet theory. We find that the conductivity bears a memory of the initial equilibrium state even when the pump is turned on slowly and the measurement is taken well after the ramp. The response of the time-evolved system is interpreted as coming from Floquet bands whose population has been determined by their overlap with the initial equilibrium state. In particular, at band inversion points in the Brillouin zone the population of the Floquet bands is inverted as well.