Negative terahertz conductivity in disordered graphene bilayers with population inversion

TL;DR

This paper demonstrates that disorder-induced indirect interband transitions in pumped graphene bilayers can produce negative terahertz conductivity, potentially overcoming intraband absorption and enabling efficient THz lasing.

Contribution

It reveals that disorder-related indirect interband transitions can significantly contribute to negative THz conductivity in graphene bilayers, offering new insights for THz laser development.

Findings

Disorder-induced indirect interband transitions can nearly cancel intraband absorption.

Extended defects can enhance the negative THz conductivity beyond fundamental limits.

Predictions impact strategies for graphene-based THz laser design.

Abstract

The gapless energy band spectra make the structures based on graphene and graphene bilayers with the population inversion created by optical or injection pumping to be promising media for the interband terahertz (THz) lasing. However, a strong intraband absorption at THz frequencies still poses a challenge for efficient THz lasing. In this paper, we show that in the pumped graphene bilayer structures, the indirect interband radiative transitions accompanied by scattering of carriers caused by disorder can provide a substantial negative contribution to the THz conductivity (together with the direct interband transitions). In the graphene bilayer structures on high-$\kappa$ substrates with point charged defects, these transitions almost fully compensate the losses due to the intraband (Drude) absorption. We also demonstrate that the indirect interband contribution to the THz conductivity in a graphene bilayer with the extended defects (such as the charged impurity clusters, surface corrugation, and nanoholes) can surpass by several times the fundamental limit associated with the direct interband transitions and the Drude conductivity. These predictions can affect the strategy of the graphene-based THz laser implementation.