Tunable and dual-broadband giant enhancement of second-harmonic and third-harmonic generation in an optimized graphene-insulator-graphene metasurface

We demonstrate a scheme to dramatically enhance both the second- and third-harmonic generation (SHG, THG) in a graphene-insulator-graphene metasurface. The key underlying feature of our approach is the existence of a double-resonance phenomenon, namely, the metasurface is designed to possess fundamental plasmon resonances at both the fundamental frequency and the higher harmonic. This dual resonant field enhancement, combined with a favorable spatial overlap of the optical near fields, lead to the increase of the THG and SHG by similar to 10(9) and similar to 10(6), respectively. We also demonstrate that by tuning the Fermi energy of the graphene gratings the dual-resonance property can be locked in over a remarkably broad spectral range of similar to 20 THz, which is more than three orders of magnitude larger than the spectral tunability achievable in metal-based plasmonic systems. Importantly, the enhanced nonlinear frequency generation process can be readily switched in the same system between the second and third harmonic. This type of graphene metasurface could open up new avenues towards the development of novel ultracompact and multifrequency active photonic nanodevices.