Optical Conductivity Signatures of Floquet Electronic Phases

TL;DR

This paper explores how optical conductivity measurements can reveal unique signatures of Floquet electronic phases in graphene antidot lattices, highlighting phenomena like negative conductivity and non-zero Hall responses induced by periodic driving.

Contribution

It provides a theoretical framework for identifying Floquet phases via optical conductivity, including signatures like negative real parts and non-zero Hall responses, with practical implications for experiments.

Findings

Distinct peaks in conductivity characterize different Floquet phases.

Negative real conductivity indicates power amplification by the Floquet drive.

Floquet Hall conductivity can be significant even at equilibrium-zero levels.

Abstract

Optical conductivity measurements may provide access to distinct signatures of Floquet electronic phases, which are described theoretically by their quasienergy band structures. We characterize experimental observables of the Floquet graphene antidot lattice (FGAL), which we introduced previously [Phys. Rev. B 104, 174304 (2021)]. On the basis of Floquet linear response theory, the real and imaginary parts of the longitudinal and Hall optical conductivity are computed as a function of probe frequency. We find that the number and positions of peaks in the response function are distinctive of the different Floquet electronic phases, and identify multiple properties with no equilibrium analog. First, for several intervals of probe frequencies, the real part of the conductivity becomes negative. We argue this is indicative of a subversion of the usual Joule heating mechanism: The Floquet drive causes the material to amplify the power of the probe, resulting in gain. Additionally, while the Hall response vanishes at equilibrium, the real and imaginary parts of the Floquet Hall conductivity are non-zero and can be as large as the longitudinal components. Lastly, driving-induced localization tends to reduce the overall magnitude of and to flatten out the optical conductivity signal. From an implementation standpoint, a major advantage of the FGAL is that the above-bandwidth driving limit is reached with photon energies that are at least twenty times lower than that required for the intrinsic material, allowing for significant band renormalization at orders-of-magnitude smaller intensities. Our work provides the necessary tools for experimentalists to map reflectance data to particular Floquet phases for this novel material.