Moving metasurfaces support guided waves exhibiting unusual optical properties, including strong anisotropy, nonreciprocity and hyperbolic dispersion. However, for these phenomena to be noticeable, high speeds are typically required, challenging their practical implementation. Here, we show a viable route towards the realization of strongly nonreciprocal hyperbolic propagation in metasurfaces placed synthetically in motion through traveling-wave space-time modulation. In addition to non-reciprocal hyperbolic propagation, the proposed time-modulation scheme induces additional exotic opportunities for nanophotonic systems, such as efficient non-reciprocal frequency conversion and mode transfer.

In this work, we investigate the effects of two distinct physical mechanisms, the antenna effect and plasmon resonance, on electric field enhancement in representative geometrical structures and at the apex of a sharp tip. The three-dimensional (3D) apertureless surface near-field optical microscopy (a-SNOM) configuration is numerically investigated using a finite element method (FEM). Our results show good agreement with analytical predictions and prior numerical benchmarks. Accounting for both antenna effects and plasmon resonances is crucial for accurate modeling of local field amplification in nanostructured probes. Through a systematic parametric numerical study, we show that field enhancement is strongly influenced by illumination conditions, polarization angle, and tip geometry. Finally, these results clarify how intrinsic material properties, extrinsic geometric features, and optical excitation govern field enhancement in nanostructures.

Polaritons are quasiparticles describing the coupling between light and matter. In two-dimensional materials, polaritonic phenomena are abundant and unique, owing to their low dimensionality and the extraordinary properties of the supported quasiparticles. In this work, we predict the existence of hyperbolic exciton polaritons (HEPs) in few-layer transition-metal dichalcogenides (TMDs) at visible frequencies. We show that hyperbolicity can be induced in the TMD under certain conditions owing to the resonant behavior of the supported excitons, leading to the existence of HEPs. We derive the HEPs’ dispersion relation under these conditions and analyze their confinement and loss properties, incorporating nonlocal corrections stemming from the high momentum of the modes. In addition, we show that owing to the valley properties of TMDs, the HEPs are coupled to the valley degree of freedom, leading to a hyperbolic spin-valley Hall effect. Such highly confined and valley-polarized HEPs provide opportunities to control strong light-matter interaction at the atomic scale.

Metasurfaces are ultrathin planar arrays of carefully tailored subwavelength particles that enable agile and flexible manipulation of the impinging waves. Originally introduced in optics, their application to acoustic waves has recently opened exciting opportunities for exotic sound control. In conventional acoustic inclusions, the interactions with the impinging pressure and velocity are decoupled, limiting the functionalities that arrays of them can achieve. While the coupling between these two quantities in symmetry-breaking inclusions, known as Willis coupling, has been discussed for several years, only recently has it been realized that these phenomena can become nonperturbative in suitably tailored resonant scatterers. Here, we explore the opportunities that these Willis meta-atoms open in the context of acoustic metasurfaces, offering additional knobs to manipulate and tailor sound. The general response of Willis metasurface is analytically derived, yielding fundamental bounds and optimal surface responses enabling full control of the impinging acoustic wave front.

Tailored time variations can enable efficient control over signal flows, giving rise to exotic wave phenomena. In this work, we demonstrate how abrupt temporal switching of the coupling between two cavities can tailor the energy flow between them beyond the limitations of static scenarios, enabling unitary excitation transfer. The proposed scheme is robust with respect to a wide range of nonidealities, with implications for classical and quantum phenomena, from computing to nanophotonic systems.

Controlling and guiding optical spins and pseudospins over an interface is of great importance in the emerging fields of spintronics and valleytronics, holding the promise for new degrees of freedom in the quest for efficient information transport. Here we explore the general possibilities offered by metasurfaces to route optical momentum, spin and helicity through surface waves. We show how anisotropy and bianisotropy can be engineered to impart extreme directionality to local spin excitations. While most previous works have focused on out-of-plane spins, in-plane excitations have become particularly important for valleytronic applications relying on two-dimensional materials, and they offer specific challenges addressed here with suitably engineered nonlocality in metasurfaces.