Hyperbolic and ENZ Metasurfaces

Metamaterials have provided a well-established platform to realize unusual light-matter interactions, allowing to manipulate wave propagation in unusual ways. Practical examples include zero permittivity (ENZ) and hyperbolic metamaterials (HMTMs), which have enabled exciting applications such as hyperlensing, negative refraction, and broadband, giant enhancement of the spontaneous emission rate (SER) of nearby emitters. However, fabrication challenges and volumetric losses have so far hindered ENZ and HMTMs applicability in optics. These challenges can be overcome by introducing ultrathin metasurfaces, able to provide superior electromagnetic performance over an ultrathin, planarized and easily accessible platform that is fully compatible with optoelectronic components and circuit integration. 

By merging the concepts of ENZ, HMTMs and 2D materials, we recently envisioned ultrathin uniaxial metasurfaces supporting hyperbolic plasmon propagation and extreme conductivity responses. Compared to surface plasmons (SPPs) propagating over conventional surfaces, which are characterized by elliptic propagation properties (panel a), hyperbolic plasmons (b) are guided toward specific directions, are ideally confined well below diffraction – limited in practice only by losses and granularity – and support a strong enhancement of the local density of states. In addition, extremely anisotropic σ-near-zero metasurfaces (c) support dispersion-free SPPs, thus implementing the canalization regime typical of volumetric ENZ structures. In contrast to waves propagating in bulk media, plasmons possess evanescent fields in the direction perpendicular to the metasurface, offering more resilience to losses and a dramatic enhancement of light-matter interactions (d), with its maximum at the topological transition between closed elliptic and open hyperbolic regimes. We envision nanostructured graphene (e) – modelled by an effective conductivity tensor – to realize dynamic metasurfaces able to modify their dispersion topology by applying a gate bias, thus tuning topological transitions in real-time. These structures can pave the way towards ultrathin devices able to strongly interact with the incoming light, allowing dynamic processing of extremely confined and easily accessible SPPs, with exciting applications in imaging, sensing, quantum optics, and on-chip networks.