Strong Second Harmonic Generation from Bilayer Graphene with Symmetry Breaking by Redox-Governed Charge Doping

TL;DR

This paper demonstrates that redox-controlled charge doping can break the centrosymmetry of bilayer graphene, enabling strong second harmonic generation, and shows SHG spectroscopy as a tool for probing molecular doping in 2D materials.

Contribution

It introduces a novel method of symmetry breaking in bilayer graphene via charge doping, resulting in significant second harmonic generation.

Findings

Charge doping effectively breaks centrosymmetry in bilayer graphene.

Redox couples induce strong SHG comparable to monolayer MoS2.

SHG spectroscopy can probe molecular doping in 2D materials.

Abstract

Missing second-order nonlinearity in centrosymmetric graphene overshadows its intriguing optical attribute. Here, we report redox-governed charge doping could effectively break the centrosymmetry of bilayer graphene (BLG), enabling a strong second harmonic generation (SHG) with a strength close to that of the well-known monolayer MoS2. Verified from control experiments with in situ electrical current annealing and electrically gate-controlled SHG, the required centrosymmetry breaking of the emerging SHG arises from the charge-doping on the bottom layer of BLG by the oxygen/water redox couple. Our results not only reveal that charge doping is an effective way to break the inversion symmetry of BLG despite its strong interlayer coupling but also indicate that SHG spectroscopy is a valid technique to probe molecular doping on two-dimensional materials.