Optical response of graphene under intense terahertz fields

TL;DR

This paper investigates how intense terahertz fields affect graphene's optical properties, revealing dynamic energy gaps, multi-step optical conductivity features, and polarization-dependent effects using Floquet theory.

Contribution

It provides a detailed analysis of the quasi-energy spectrum and optical conductivity of graphene under intense terahertz fields, highlighting new gap behaviors and spectral features.

Findings

Gaps open in the quasi-energy spectrum due to photon resonances.

Optical conductivity shows multi-step structures and dips at specific frequencies.

Circular polarization induces unique peaks in the optical response.

Abstract

Optical responses of graphene in the presence of intense circularly and linearly polarized terahertz fields are investigated based on the Floquet theory. We examine the energy spectrum and density of states. It is found that gaps open in the quasi-energy spectrum due to the single-photon/multi-photon resonances. These quasi-energy gaps are pronounced at small momentum, but decrease dramatically with the increase of momentum and finally tend to be closed when the momentum is large enough. Due to the contribution from the states at large momentum, the gaps in the density of states are effectively closed, in contrast to the prediction in the previous work by Oka and Aoki [Phys. Rev. B {\bf 79}, 081406(R) (2009)]. We also investigate the optical conductivity for different field strengths and Fermi energies, and show the main features of the dynamical Franz-Keldysh effect in graphene. It is discovered that the optical conductivity exhibits a multi-step-like structure due to the sideband-modulated optical transition. It is also shown that dips appear at frequencies being the integer numbers of the applied terahertz field frequency in the case of low Fermi energy, originating from the quasi-energy gaps at small momentums. Moreover, under a circularly polarized terahertz field, we predict peaks in the middle of the "steps" and peaks induced by the contribution from the states around zero momentum in the optical conductivity.