Nonequilibrium plasmons with gain in graphene

TL;DR

This paper develops a theory for non-equilibrium plasmons in photo-inverted graphene, revealing conditions for plasmon amplification and highlighting the competition with spontaneous emission, which is faster than previously thought.

Contribution

It provides an exact complex-frequency dispersion relation for non-equilibrium plasmons in graphene, including effects of doping, temperature, and collisions, advancing understanding of active plasmonic devices.

Findings

Plasmon amplification is possible in photo-inverted graphene.

Spontaneous emission in intrinsic graphene is 5 times faster than earlier estimates.

The theory enables prediction of plasmon gain under realistic conditions.

Abstract

Graphene supports strongly confined transverse-magnetic sheet plasmons whose spectral characteristics depend on the energetic distribution of Dirac particles. The question arises whether plasmons can become amplified when graphene is pumped into a state of inversion. In establishing a theory for the dynamic non-equilibrium polarizability, we are able to determine the exact complex-frequency plasmon dispersion of photo-inverted graphene and study the impact of doping, collision loss, and temperature on the plasmon gain. We calculate the spontaneous emission spectra and carrier recombination rates self-consistently and compare the results with approximations based on Fermi's golden rule. Our results show that amplification of plasmons is possible under realistic conditions but inevitably competes with ultrafast spontaneous emission, which for intrinsic graphene, is a factor 5 faster than previously estimated. This work casts new light on the nature of non-equilibrium plasmons and may aid the experimental realization of active plasmonic devices based on graphene.