▎ 摘　　要

The exotic plasmonic properties of highly doped graphene monolayer have opened new doors to develop advanced nanophotonic devices. It has been successfully experienced that the incident field can become substantially intense enough to convert the fundamental laser frequency into both even and odd high order harmonics via a nonlinear phenomenon. In this work, using highly doped graphene nanorings with semimetallic behaviour, we analyzed the formation of high-harmonic generation using both finite element method (FEM) and finite-difference time-domain (FDTD) calculations. Employing the inherent anharmonic charge-carrier dispersion of graphene and choosing accurate geometrical sizes for the atomic scale nanorings allow for the generation of third-harmonic (similar to 730 nm) with high intensity, substantial quality factor (Q-factor), and significant conversion efficiency up to eta(THG)similar to 0.0014%. By realizing the hybridization of excited plasmons, we showed that the unique spectral response of graphene allows for the generation of gate-tunable and efficient higher order harmonic formation along the visible spectrum. We believe that the proposed atomic-scale platform has a strong potential to be used for advanced and integrated photonic devices. (C) 2017 Elsevier B.V. All rights reserved.