Population Inversion in Monolayer and Bilayer Graphene

TL;DR

This paper demonstrates long-lived population inversion in bilayer graphene using tr-ARPES, attributed to its small band gap, which could enhance optoelectronic applications like Terahertz light amplification.

Contribution

It provides the first evidence of extended population inversion in bilayer graphene and offers a microscopic model explaining this phenomenon.

Findings

Long-lived population inversion observed in bilayer graphene.

Small band gap in bilayer graphene contributes to longer inversion lifetime.

Potential for increased inversion lifetime with higher pump photon energy and fluence.

Abstract

The recent demonstration of saturable absorption and negative optical conductivity in the Terahertz range in graphene has opened up new opportunities for optoelectronic applications based on this and other low dimensional materials. Recently, population inversion across the Dirac point has been observed directly by time- and angle-resolved photoemission spectroscopy (tr-ARPES), revealing a relaxation time of only ~ 130 femtoseconds. This severely limits the applicability of single layer graphene to, for example, Terahertz light amplification. Here we use tr-ARPES to demonstrate long-lived population inversion in bilayer graphene. The effect is attributed to the small band gap found in this compound. We propose a microscopic model for these observations and speculate that an enhancement of both the pump photon energy and the pump fluence may further increase this lifetime.