# Enhanced third harmonic generation with graphene metasurfaces

## TL;DR

This paper presents ultrathin graphene-based metasurfaces that significantly enhance third harmonic generation at THz frequencies by exploiting localized plasmons and tunable resonances, enabling efficient, adjustable nonlinear optical sources.

## Contribution

The work introduces a novel design of graphene metasurfaces that dramatically boosts THG efficiency and allows frequency tuning without changing the physical structure.

## Key findings

- THG efficiency increased by several orders of magnitude
- Resonant frequency tunable via Fermi energy adjustment
- Potential applications in THz spectroscopy and imaging

## Abstract

The nonlinear responses of different materials provide useful mechanisms for optical switching, low noise amplification, and harmonic frequency generation. However, the nonlinear processes usually have an extremely weak nature and require high input power to be excited. To alleviate this severe limitation, we propose new designs of ultrathin nonlinear metasurfaces composed of patterned graphene micro-ribbons to significantly enhance third harmonic generation (THG) at far-infrared and terahertz (THz) frequencies. The incident wave is tightly confined and significantly boosted along the surface of graphene in these configurations due to the excitation of highly localized plasmons. The bandwidth of the resonant response becomes narrower due to the introduction of a metallic substrate below the graphene micro-ribbons, which leads to zero transmission and standing waves inside the intermediate dielectric spacer layer. The enhancement of the incident field, combined with the large nonlinear conductivity of graphene, can dramatically increase the THG conversion efficiency by several orders of magnitude. In addition, the resonant frequency of the metasurface can be adjusted by dynamically tuning the Fermi energy of graphene via electrical or chemical doping. As a result, the third harmonic generated wave can be optimized and tuned to be emitted at different frequencies without the need to change the nonlinear metasurface geometry. The proposed nonlinear metasurfaces provide a new way to realize compact and efficient nonlinear sources at the far-infrared and THz frequency ranges, as well as new frequency generation and wave mixing devices which are expected to be useful for nonlinear THz spectroscopy and noninvasive THz imaging applications.

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Source: https://tomesphere.com/paper/1704.04506