Near-field nanoscopy of THz split-ring resonators with a localized graphene layer in the gap: a new platform for efficient third harmonic generation in graphene

The development of efficient and miniaturized photonic and optoelectronic devices at terahertz (THz) frequencies, is fundamental for addressing the plethora of applications in this spectral range. Micro-resonators, capable of manipulate and shape the wavefront of incident waves constitute a fundamental building block in this respect. When implemented in metamaterial or metasurface patterns, they can efficiently operate as beam steerers, modulators, or, if embedded with a layered nanomaterial, as resonators that once optically pumped, can allow light emission via nonlinear optical processes. Here, we investigate by scattering-type scanning near-field optical microscopy (s-SNOM) single-layer graphene (SLG)-embedded split-ring resonator (SRR) metasurfaces. By implementing a polarization-sensitive holographic approach through either single or multimode quantum cascade lasers, we trace the field enhancement spatial distribution in a detectorless configuration. The demonstrated field enhancement opens perspectives for highly efficient harmonic generation at high (> 6 THz) frequencies.