Microscopic mechanism for transient population inversion and optical gain in graphene

TL;DR

This paper presents a microscopic theory explaining transient population inversion and optical gain in graphene, highlighting the roles of optical pumping, carrier cooling, and Coulomb-induced Auger recombination in the process.

Contribution

It provides a detailed microscopic mechanism for transient optical gain in graphene, which was previously observed experimentally.

Findings

Transient gain occurs due to optical pumping and carrier cooling.

Relaxation bottleneck near the Dirac point causes population inversion.

Femtosecond decay driven by Coulomb-induced Auger recombination.

Abstract

A transient femtosecond population inversion in graphene was recently reported by Li et al., Phys. Rev. Lett. 108, 167401 (2012). Based on a microscopic theory we clarify the underlying microscopic mechanism: Transient gain and population inversion in graphene occurs due to a complex interplay of strong optical pumping and carrier cooling that fills states close to the Dirac point giving rise to a relaxation bottleneck. The subsequent femtosecond decay of the optical gain is mainly driven by Coulomb-induced Auger recombination.