The Nonlinear Optical Effects of Opening a Gap in Graphene

TL;DR

This paper investigates how opening an energy gap in graphene influences its nonlinear optical properties, revealing the emergence of even harmonics and the suppression of odd harmonics due to Berry phase effects.

Contribution

It introduces a theoretical framework to analyze the impact of a spectral gap and Berry phase on graphene's nonlinear optical responses, especially harmonic generation.

Findings

Gapping the spectrum induces Berry phase effects in carrier dynamics.

Berry phase suppresses odd-harmonic generation in gapped graphene.

Tuning pump energy to the gap enables interference among harmonics, producing even harmonics.

Abstract

Graphene possesses remarkable electronic, optical and mechanical properties that have taken the research of two-dimensional relativistic condensed matter systems to prolific levels. However, the understanding of how its nonlinear optical properties are affected by relativistic-like effects has been broadly uncharted. It has been recently shown that highly-nontrivial currents can be generated in free-standing samples, notably leading to the generation of even harmonics. Since graphene monolayers are centrosymmetric media, for which such harmonic generation is deemed inaccessible, this light-driven phenomenon is both startling and promising. More realistically, graphene samples are often deposited on a dielectric substrate, leading to additional intricate interactions. Here, we present a treatment to study this instance by gapping the spectrum and we show this leads to the appearance of a Berry phase in the carrier dynamics. We analyse the role of such a phase in the generated nonlinear current and conclude that it suppresses odd-harmonic generation. The pump energy can be tuned to the energy gap to yield interference among odd harmonics mediated by interband transitions, allowing even harmonics to be generated. Our results and general methodology pave the way for understanding the role of gap-opening physical factors in the nonlinear optics of hexagonal two-dimensional lattices.