Transient Charge and Energy Balance in Graphene Induced by Ultrafast Photoexcitation

TL;DR

This paper investigates ultrafast photoexcitation in monolayer graphene, revealing transient optical gain and negative conductivity due to a dense, inverted Dirac fermion population, modeled with a two-chemical-potential approach.

Contribution

It introduces a two-chemical-potential model to describe the transient, dense, inverted electronic state in graphene under ultrafast excitation, explaining observed optical gain.

Findings

Negative optical conductivity observed in near-infrared region for 200 fs

Complete bleaching of absorption at saturation density

Reproduction of stimulated emission and optical gain in models

Abstract

Ultrafast optical pump-probe spectroscopy measurement on monolayer graphene observes significant optical nonlinearities. We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200fs, where stimulated emission completely compensates absorption loss in the graphene layer. To quantitatively investigate this transient, extremely dense photoexcited Dirac-fermion state, we construct a two-chemical-potential model, in addition to a time-dependent transient carrier temperature above lattice temperature, to describe the population inverted electronic state metastable on the time scale of tens of femtoseconds generated by a strong exciting pulse. The calculated transient optical conductivity reveals a complete bleaching of absorption, which sets the saturation density during the pulse propagation. Particularly, the model calculation reproduces the negative optical conductivity at lower frequencies in the states close to saturation, corroborating the observed femtosecond stimulated emission and optical gain in the wide near-infrared window.