Threshold of Terahertz Population Inversion and Negative Dynamic Conductivity in Graphene Under Pulse Photoexcitation

TL;DR

This paper theoretically investigates the conditions for achieving population inversion and negative dynamic conductivity in graphene under pulse photoexcitation, revealing that lower photon energies significantly reduce the required pulse energy.

Contribution

It introduces a theoretical model showing how pulse photon energy and graphene quality influence population inversion and negative conductivity thresholds.

Findings

Lower photon energies reduce the threshold pulse energy for inversion.

Negative dynamic conductivity occurs in both high- and low-quality graphene.

Threshold energy can be orders of magnitude lower with lower photon energy.

Abstract

We theoretically study the population inversion and negative dynamic conductivity in intrinsic graphene in the terahertz (THz) frequency range upon pulse photoexcitation with near-/mid-infrared wavelength. The threshold pulse energy required for the population inversion and negative dynamic conductivity can be orders-of-magnitude lower when the pulse photon energy is lower, due to the inverse proportionality of the photoexcited carrier concentration to the pulse photon energy and to the weaker carrier heating. We also investigate the dependence of the dynamic conductivity on the momentum relaxation time. The negative dynamic conductivity takes place either in high- or low-quality graphene, where the Drude absorption by carriers in the THz frequency is weak.