Valley polarization has recently been adopted in optics, offering robust waveguiding and angular momentum sorting. The success of valley systems in photonic crystals suggests a plasmonic counterpart that can merge topological photonics and topological condensed matter systems, for instance, two-dimensional materials with the enhanced light-matter interaction. However, a valley plasmonic waveguide with a sufficient propagation distance in the near-infrared (NIR) or visible spectral range has so far not been realized due to ohmic loss inside the metal. Here, we employ gap surface plasmons for high index contrasting and realize a wide-bandgap valley plasmonic crystal, allowing waveguiding in the NIR-visible range. The edge mode with a propagation distance of 5.3 μm in the range of 1.31-1.36 eV is experimentally confirmed by visualizing the field distributions with a scanning transmission electron microscope cathodoluminescence technique, suggesting a practical platform for transferring angular momentum between photons and carriers in mesoscopic active devices.

Atomic-scale defects/disorded states induced by sulfur sublimation are responsible for reduced lattice thermal conductivity of thermoelectric colusite.

Plasmonic resonator arrays have attracted a great interest as a platform to enhance light-matter interaction and have been examined for their applicability to various types of optical devices, such as sensors, light emitter, and photocatalyst, to name a few. In a plasmonic resonator array, localized and propagating plasmon modes can hybridize, which is known to result in an anticrossing of the plasmon bands in the dispersion curves. However, it was so far unclear how the modal symmetry affects such a hybridization, especially when it occurs at a specific reciprocal lattice point with a high degree of symmetry, for example, the Gamma point. In this work, we used momentum-resolved cathodoluminescence-scanning transmission electron microscopy to comprehensively characterize the modal hybridization at the Gamma point. Our study reveals theoretically and experimentally the existence of mode symmetry selection rules that specify hybrid pairs of the lattice mode and localized mode.

Electron beam spectroscopy has recently attracted much attention in the modal analysis of nanophotonic and plasmonic systems. In principle, electron energy-loss spectroscopy (EELS) provides information about the electromagnetic local density of states of all sorts of electromagnetic modes as well as nonradiative modes. However, there have not been many examples related to the EELS analyses for electromagnetic Bloch modes. Herein, EELS measurements are performed for the characterization of nonradiative band-edge modes facing the first bandgap in a plasmonic crystal with a triangular lattice, which is well-known for its full bandgap formation. The obtained spectrum images clearly determine the characteristics of the lower and upper band-edge modes, compared with an analytical model based on group theory and a numerical simulation by the finite-difference time-domain method. The EELS spectra also reveal differences in the plasmonic density of states between the lower and upper band-edges, which provides information about the band deformation near the bandgap.