Theory of graphene saturable absorption

TL;DR

This paper presents a theoretical study of graphene's saturable absorption at very low light intensities, revealing its potential for nonlinear optics and optoelectronic applications through a non-perturbative Dirac equation approach.

Contribution

We develop a non-perturbative single-particle model using the Dirac equation to describe saturable absorption in graphene, including both interband and intraband effects, and demonstrate electrical tunability.

Findings

Interband absorption dominates saturable absorption in graphene.

Saturable absorption occurs at very low light intensities.

Modulation depth can be controlled via gate voltage.

Abstract

Saturable absorption is a non-perturbative nonlinear optical phenomenon that plays a pivotal role in the generation of ultrafast light pulses. Here we show that this effect emerges in graphene at unprecedentedly low light intensities, thus opening avenues to new nonlinear physics and applications in optical technology. Specifically, we theoretically investigate saturable absorption in extended graphene by developing a non-perturbative single-particle approach, describing conduction-electron dynamics in the atomically-thin material using the two-dimensional Dirac equation for massless Dirac fermions, which is recast in the form of generalized Bloch equations. By solving the electron dynamics non-perturbatively, we account for both interband and intraband contributions to the intensity-dependent saturated conductivity and conclude that the former dominates regardless of the intrinsic doping state of the material. The results are in excellent agreement with atomistic quantum-mechanical simulations including higher-band effects. Additionally, we find that the modulation depth of saturable absorption in graphene can be electrically manipulated through an externally applied gate voltage. Our results are relevant for the development of graphene-based optoelectronic devices, as well as for applications in mode-locking and random lasers.