Anomalous optical saturation of low-energy Dirac states in graphene and its implication for nonlinear optics

TL;DR

This paper demonstrates that graphene exhibits optical saturation at arbitrarily weak fields due to diverging interband coupling at the Dirac point, significantly impacting its nonlinear optical properties and challenging conventional perturbative approaches.

Contribution

It introduces a novel understanding of optical saturation in graphene caused by diverging interband coupling, supported by semiconductor Bloch equations and many-body relaxation dynamics.

Findings

Optical saturation occurs at arbitrarily weak fields in graphene.

Carrier relaxation near the Dirac point is slow, enhancing saturation effects.

Conventional perturbative nonlinear optics is inadequate for graphene at low Fermi levels.

Abstract

We reveal that optical saturation of the low-energy states takes place in graphene for arbitrarily weak electromagnetic fields. This effect originates from the diverging field-induced interband coupling at the Dirac point. Using semiconductor Bloch equations to model the electronic dynamics of graphene, we argue that the charge carriers undergo ultrafast Rabi oscillations leading to the anomalous saturation effect. The theory is complemented by a many-body study of the carrier relaxations dynamics in graphene. It will be demonstrated that the carrier relaxation dynamics is slow around the Dirac point, which in turn leads to a more pronounced saturation. The implications of this effect to the nonlinear optics of graphene is then discussed. Our analysis show that the conventional perturbative treatment of the nonlinear optics, i.e., expanding the polarization field in a Taylor series of the electric field, is problematic for graphene, in particular at small Fermi levels and large field amplitudes.