Hyperspectral Nanoimaging of van der Waals Polaritonic Crystals

Abstract

Phonon polaritons (PhPs) in van der Waals (vdW) crystal slabs enable nanoscale infrared light manipulation. Specifically, periodically structured vdW slabs behave as polaritonic crystals (vdW-PCs), where the polaritons form Bloch modes. Because the polariton wavelengths are smaller than that of light, conventional far-field spectroscopy does not allow for a complete characterization of vdW-PCs or for revealing their band structure. Here, we perform hyperspectral infrared nanoimaging and analysis of PhPs in a vdW-PC slab made of h-BN. We demonstrate that infrared spectra recorded at individual spatial positions within the unit cell of the vdW-PC can be associated with its band structure and local density of photonic states (LDOS). We thus introduce hyperspectral infrared nanoimaging as a tool for the comprehensive analysis of polaritonic crystals, which could find applications in the reconstruction of complex polaritonic dispersion surfaces in momentum-frequency space or for exploring exotic electromagnetic modes in topological photonic structures.

Anisotropy and Modal Hybridization in Infrared Nanophotonics Using Low-Symmetry Materials

“…However, it is important to note that modes in nanostructured films are no longer the exact same electromagnetic modes excited in planar films. Importantly, the photonic bands can be formed and altered when the hBN and/or the underlying structure are patterned and the (subdiffractional) photonic crystals have been studied in both infrared (polaritonic) − and visible (dielectric) . The choice of technique to probe a hyperbolic polariton therefore depends on whether it is more critical to accurately determine the spatial or the momentum information from the system.…”

Optical Metasurfaces for Energy Conversion

“…Arrays of hBN cones supporting localized HPhPs have been investigated in the far-field and near-field. − HPhPs are internally reflected at specific angles in the cones, as it can be seen from electromagnetic simulations and near-field measurement performed with scattering–scanning near-field optical microscopy (sSNOM) as shown in Figure d for the UTM 13 resonance of the structure (U stands for the upper band, TM indicates the transverse magnetic nature of the mode, l = 1 and m = 3 indicate the z -axis angular momentum and orbital index, respectively). Similar near and far-field studies have been carried out with different hBN metasurface geometries. − An extremely high Q factor of up to 500 was obtained with image HPhPs, where the hBN layer is separated from a metal mirror by a nanometric gap. Coupling with far-field radiation was obtained by patterning the underlying metallic mirror in a metasurface geometry .…”

Twisted Polaritonic Crystals in Thin van der Waals Slabs

“…Remarkably, the strongest resonant feature in Figure 5a matches with the Bragg condition for the (±1,±2) resonance, corresponding to the nearly asymptotic regime of the hyperbolic IFC (a straight line), where the PhP momentum is already rather high. We speculate that apart from the Bragg scattering processes, the high efficiency of the resonant PhP excitations in (±1,±2) diffraction orders can be related with the dipolar resonance of the holes, as well as with the flat bands arising in polaritonic crystals (11). However, the detailed analysis of such anomaly goes beyond the scope of this manuscript and will be addressed in future works.…”

“…PCs based on anisotropic polar vdW materials (such as h-BN) have demonstrated interesting possibilities for enhanced light-matter interactions (11)(12)(13) and topological photonics (14). On the other hand, PCs in in-plane anisotropic materials, where the manipulation of polaritons can exhibit more degrees of freedom, have not yet been addressed.…”