Post-transient relaxation in graphene after an intense laser pulse

TL;DR

This paper investigates how hot, dense, population-inverted electrons in graphene relax back to equilibrium after intense laser excitation, highlighting the different timescales of temperature and population inversion relaxation.

Contribution

It combines hydrodynamic and kinetic theory to analyze post-transient relaxation in graphene, revealing the distinct relaxation rates of temperature and population inversion.

Findings

Population inversion relaxes slower than electron temperature.

Relaxation rates are determined by electron-phonon scattering processes.

Insights may impact graphene's use as an optical gain medium.

Abstract

High intensity laser pulses were recently shown to induce a population inverted transient state in graphene [T. Li et al. Phys. Rev. Lett. 108, 167401 (2012)]. Using a combination of hydrodynamic arguments and a kinetic theory we determine the post-transient state relaxation of hot, dense, population inverted electrons towards equilibrium. The cooling rate and charge-imbalance relaxation rate are determined from the Boltzmann-equation including electron-phonon scattering. We show that the relaxation of the population inversion, driven by inter-band scattering processes, is much slower than the relaxation of the electron temperature, which is determined by intra-band scattering processes. This insight may be of relevance for the application of graphene as an optical gain medium.