Doping-Induced Second-Harmonic Generation in Centrosymmetric Graphene from Quadrupole Response.

Area of Science:

• Nonlinear optics
• Condensed matter physics
• Materials science

Background:

• Centrosymmetric materials, like graphene, typically do not exhibit optical second-harmonic generation (SHG) due to the electric-dipole approximation.
• Breaking inversion symmetry is usually required for SHG in 2D materials.

Purpose of the Study:

• To investigate the possibility of SHG in centrosymmetric graphene.
• To explore the mechanisms and tunability of this phenomenon.

Main Methods:

• Experimental observation of SHG in monolayer graphene.
• Tuning SHG by varying carrier doping and chemical potential.
• Analysis of the SHG response in relation to Fermi-edge resonances and electron-hole symmetry.

Main Results:

• Observed strong, doping-induced SHG in graphene, comparable to SHG in noncentrosymmetric materials.
• Identified the SHG as an electric-quadrupole response due to optical dressing and effective symmetry breaking.
• Demonstrated wide tunability of SHG by carrier doping, with enhancement at Fermi-edge resonances and vanishing at the charge neutral point.

Conclusions:

• Graphene exhibits a novel, doping-tunable electric-quadrupole SHG, challenging previous expectations for centrosymmetric materials.
• This phenomenon expands the understanding of nonlinear optics in 2D Dirac materials.
• The findings suggest potential for new optoelectronic and photonic devices based on graphene.