Observation of Emergent Dirac Physics at the Surfaces of Acoustic Higher-Order Topological Insulators.

Using 3D sonic crystals as acoustic higher-order topological insulators (HOTIs), 2D surface states described by spin-1 Dirac equations at the interfaces between the two sonic crystals with distinct topology but the same crystalline symmetry are discovered. It is found that the Dirac mass can be tuned by the geometry of the two sonic crystals. The sign reversal of the Dirac mass reveals a surface topological transition where the surface states exhibit zero refractive index behavior. When the surface states are gapped, 1D hinge states emerge due to the topology of the gapped surface states. The zero refractive index behavior and the emergent topological hinge states are confirmed experimentally. This study reveals a multidimensional Wannier orbital control that leads to extraordinary properties of surface states and unveils an interesting topological mechanism for the control of surface waves.

Hybrid anisotropic plasmonic metasurfaces with multiple resonances of focused light beams.

Plasmonic metasurfaces supporting collective lattice resonances have attracted increasing interest due to their exciting properties of strong spatial coherence and enhanced light-matter interaction. Although the focusing of light by high-numerical-aperture (NA) objectives provides an essential way to boost the field intensities, it remains challenging to excite high-quality resonances by using high-NA objectives due to strong angular dispersion. Here, we address this challenge by employing the physics of bound states in the continuum (BICs). We design a novel anisotropic plasmonic metasurface combining a two-dimensional lattice of high-aspect-ratio pillars with a one-dimensional plasmonic grating, fabricated by a two-photon polymerization technique and gold sputtering. We demonstrate experimentally multiple resonances with absorption amplitudes exceeding 80% at mid-IR using an NA = 0.4 reflective objective. This is enabled by the weak angular dispersion of quasi-BIC resonances in such hybrid plasmonic metasurfaces. Our results suggest novel strategies for designing photonic devices that manipulate focused light with a strong field concentration.