Metasurfaces provide a versatile platform for manipulating the wavefront of light using planar nanostructured surfaces. Transmissive metasurfaces, with full 2π phase control, are a particularly attractive platform for replacing conventional optical elements due to their small footprint and broad functionality. However, the operational bandwidth of metasurfaces has been a critical limitation and is directly connected to either their resonant response or the diffractive dispersion of their lattice. While multiwavelength and continuous band operation have been demonstrated, the elements suffer from either low efficiency, reduced imaging quality, or limited element size. Here, we propose a platform that provides for multiwavelength operation by employing tightly spaced multilayer dielectric metasurfaces. As a proof of concept, we demonstrate a multiwavelength metalens doublet (NA = 0.42) with focusing efficiencies of 38% and 52% at wavelengths of 1180 and 1680 nm, respectively. We further show how this approach can be extended to three-wavelength metalenses as well as a spectral splitter. This approach could find applications in fluorescent microscopy, digital imaging, and color routing.

Surface phonon polaritons (SPhPs), the surface-bound electromagnetic modes of a polar material resulting from the coupling of light with optic phonons, offer immense technological opportunities for nanophotonics in the infrared (IR) spectral region. However, once a particular material is chosen, the SPhP characteristics are fixed by the spectral positions of the optic phonon frequencies. Here, we provide a demonstration of how the frequency of these optic phonons can be altered by employing atomic-scale superlattices (SLs) of polar semiconductors using AlN/GaN SLs as an example. Using second harmonic generation (SHG) spectroscopy, we show that the optic phonon frequencies of the SLs exhibit a strong dependence on the layer thicknesses of the constituent materials. Furthermore, new vibrational modes emerge that are confined to the layers, while others are centered at the AlN/GaN interfaces. As the IR dielectric function is governed by the optic phonon behavior in polar materials, controlling the optic phonons provides a means to induce and potentially design a dielectric function distinct from the constituent materials and from the effective-medium approximation of the SL. We show that atomic-scale AlN/GaN SLs instead have multiple Reststrahlen bands featuring spectral regions that exhibit either normal or extreme hyperbolic dispersion with both positive and negative permittivities dispersing rapidly with frequency. Apart from the ability to engineer the SPhP properties, SL structures may also lead to multifunctional devices that combine the mechanical, electrical, thermal, or optoelectronic functionality of the constituent layers. We propose that this effort is another step toward realizing user-defined, actively tunable IR optics and sources.

Polaritonic materials that support epsilon-near-zero (ENZ) modes offer the opportunity to design light–matter interactions at the nanoscale through extreme subwavelength light confinement, producing phenomena like resonant perfect absorption. However, the utility of ENZ modes in nanophotonic applications has been limited by a flat spectral dispersion, which leads to small group velocities and extremely short propagation lengths. Here, we overcome this constraint by hybridizing ENZ and surface plasmon polariton (SPP) modes in doped cadmium oxide epitaxial bilayers. This results in strongly coupled hybrid modes that are characterized by an anticrossing in the polariton dispersion and a large spectral splitting on the order of 1/3 of the mode frequency. These hybrid modes simultaneously achieve modal propagation and ENZ mode-like interior field confinement, adding propagation character to ENZ mode properties. We subsequently tune the resonant frequencies, dispersion, and coupling of these polaritonic-hybrid-epsilon-near-zero (PH-ENZ) modes by tailoring the modal oscillator strength and the ENZ-SPP spectral overlap. PH-ENZ modes ultimately leverage the most desirable characteristics of both ENZ and SPP modes, allowing us to overcome the canonical plasmonic trade-off between confinement and propagation length.

Strong coupling between optical modes can be implemented into nanophotonic design to modify the energy− momentum dispersion relation. This approach offers potential avenues for tuning the thermal emission frequency, line width, polarization, and spatial coherence. Here, we employ three-mode strong coupling between propagating and localized surface phonon polaritons, with zone-folded longitudinal optic phonons within periodic arrays of 4H-SiC nanopillars. Energy exchange, mode evolution, and coupling strength between the three polariton branches are explored experimentally and theoretically. The influence of strong coupling upon the angle-dependent thermal emission was directly measured, providing excellent agreement with theory. We demonstrate a 5-fold improvement in the spatial coherence and 3-fold enhancement of the quality factor of the polaritonic modes, with these hybrid modes also exhibiting a mixed character that could enable opportunities to realize electrically driven emission. Our results show that polariton−phonon strong coupling enables thermal emitters, which meet the requirements for a host of IR applications in a simple, lightweight, narrow-band, and yet bright emitter.

Epsilon near zero modes offer extreme field enhancement that can be utilized for developing enhanced sensing schemes. However, demonstrations of enhanced spectroscopies have largely exploited surface polaritons, mostly due to the challenges of coupling a vibrational transition to volume-confined epsilon near zero modes. Here we fabricate high aspect ratio gratings (up to 24.8 µm height with greater than 5 μm pitch) of 4H-SiC, with resonant modes that couple to transverse magnetic and transverse electric incident fields. These correspond to metal-insulatormetal waveguide modes propagating downwards into the substrate. The cavity formed by the finite The electromagnetic field confinement offered by surface polaritons 1-2 and epsilon-near-zero (ENZ) 3-7 modes have long been discussed for applications in surface-enhanced sensing [8][9][10][11][12][13] , and vibrational coupling [14][15] . Whilst surface plasmon polaritons (SPPs) have been extensively explored, demonstrating enhanced spectroscopies using ENZ modes has remained challenging. This is largely because at optical frequencies ENZ modes are often realized by coupling light into a material where epsilon is close to zero 16-18 , or via waveguides that are not hollow and therefore incompatible with confining the analyte of interest within the region of highly confined electromagnetic fields [3][4] . In this letter we investigate high-aspect-ratio grating (HAG) structures designed to support surface phonon polaritons (SPhPs) 1 at the interface between the polar crystal grating surfaces and the surrounding environment 19 . We show that the modes supported by this structure behave like metal-insulator-metal (MIM) waveguide modes in a short cavity [20][21][22] .Furthermore, due to this architecture, these structures support an ENZ mode in the gap between the grating teeth. This enables the first colocation of strongly confined ENZ fields with an analyte of interest, including liquids, with large surface area. As proof of this, we demonstrate that the ENZ fields can coherently couple to vibrational transitions in a liquid. Thus, this constitutes a platform for studying ENZ and SPhP strong coupling at infrared (IR) frequencies, with potential applications in surface enhanced spectroscopies [8][9][10][11][12][13]23 as well as light-controlled chemistry [14][15] .This study exploits high aspect ratio gratings, which have a height (h) that is much larger than their period (Λ) (see Fig 1). In HAGs and nanopillars, surface polariton modes are supported between the teeth, propagating as MIM waveguide modes downwards into the grating [20][21][22][24][25] .The frequency of the modes can be controlled by changing the effective index of refraction (neff) of the polariton wave using the size of the air gap (g) between the teeth, or the height of the grating (h, see Fig. 1 a-c). Furthermore, polaritonic modes in these structures have been demonstrated to