The concept of optical bound states in the continuum (BICs) currently drives the field of dielectric resonant nanophotonics, providing an important physical mechanism for engineering highquality (high-Q) optical resonances in high-index dielectric nanoparticles and structured dielectric metasurfaces. For structured metallic metasurfaces, realization of BICs remains a challenge associated with strong dissipative losses of plasmonic materials. Here, we suggest and realize experimentally anisotropic plasmonic metasurfaces supporting high-Q resonances governed by quasi-BIC collective resonant modes. Our metasurfaces are composed of arrays of vertically oriented doublepillar meta-molecules covered by a thin layer of gold. We engineer quasi-BIC modes and observe experimentally sharp resonances in mid-IR reflectance spectra. Our work suggests a direct route to boost the resonant field enhancement in plasmonic metasurfaces via combining a small effective mode volume of plasmonic systems with engineered high-Q resonances provided by the BIC physics, with multiple applications to enhance light-matter interaction for nanooptics and quantum photonics.

Metamaterial-based perfect absorbers provide efficient ways for selective absorption of light with both linear or circular polarizations. Perfect absorption for an arbitrary polarization requires the development of subwavelength structures absorbing efficiently elliptically polarized light, but they remain largely unexplored. Here, we design and realize experimentally novel plasmonic metasurfaces for full-Stokes polarization perfect absorption in the mid-infrared. The metasurface unit cell consists of coupled anisotropic meta-atoms forming a diatomic metamolecule. The designed plasmonic metastructures provide a strong field enhancement by at least 1 order of magnitude higher than conventional perfect absorbers. In experiment, our plasmonic metasurfaces demonstrate sharp differentiation of spectral responses for an arbitrary pair of orthogonal polarization states (linear, circular, or elliptical) providing perfect absorption for one polarization with strong reflection for its counterpart. Our results suggest a novel route for efficient control of light polarization in metasurfaces offering numerous potential applications ranging from thermal imaging to chiral molecule detection.

An all-metal absorber consisting of an L-shaped stripe with two vertical pillars is constructed and investigated for triple-band absorption. The nanostructure reveals distinct absorption properties at resonant wavelengths selected by the polarization of incident waves. It achieves single-band absorptivity of 98.5% at 5.198 μm for 135° linearly polarized wave and dual-band absorptivity of 91.5% and 99.4% at 3.058 μm and 3.887 μm respectively for 45° linear polarization. We interpret the physical mechanism by using the charge-current multipolar expansion framework, which shows that the excitation of two transverse magnetic dipoles and a non-radiating anapole are responsible for these absorption resonances.

Gold-helix metamaterials play a crucial role in tailoring chiral light-matter interactions. However, conventional gold helices with weak mechanical stability are easy to break or collapse, resulting in device failure. Here, we propose a novel ultra-stable pyramid-shaped gold-helix metamaterial in which we utilize photoresist walls to support tapered gold helices to address this challenge. Numerical results show that the proposed ultra-stable metamaterial can work as an efficient mid-infrared circular polarizer in almost two octaves (from 5.82μm to16.76μm) with a maximum extinction ratio greater than 25dB. The device has many potential applications in circular polarization-dependent optical systems without worrying about vibrations.