Metamaterial absorbers have attracted much attention due to their unique ability to achieve nearly perfect absorption in compact structures. Most of the reported metamaterial absorbers exhibit only one absorption band in a specific frequency range, but absorbers with two or more absorption bands in the visible and near-infrared regions are desired in many applications. Here, we propose and demonstrate a dual-band metamaterial absorber comprising an aluminum film, a silica spacer, and a periodic array of aluminum nanodiscs. The two absorption bands in the visible and near-infrared range are, respectively, attributed to the excitation of a propagating surface plasmon polariton and a localized magnetic polariton in the proposed nanostructure. Large area and high coverage samples of the designed structure were fabricated using a simple and cost-effective colloidal lithography nanofabrication method. We show that the positions of the absorption bands can be separately adjusted by changing the diameter or the lattice constant of the aluminum nanodiscs on the top of the metamaterial absorbers. Our design of compact and efficient absorbers would find great utility in a variety of applications including filters, sensors, solar cells, thermal emitters, and imaging devices.

Metamaterials have attracted considerable attention owing to their extraordinary ability in controlling the propagation of electromagnetic waves. These materials can be realized using artificial composites consisting of subwavelength metallic resonators, but losses of the metallic components may significantly degrade the performance of metamaterials, especially in the visible region. Here, we propose low-loss all-dielectric metasurfaces, comprised of a monolayer of titanium dioxide (TiO 2 ) nanoparticles, to achieve perfect reflection band at visible wavelengths. Using the Mie scattering theory, we explore the electromagnetic scattering features of one single TiO 2 nanosphere and show that both electric and magnetic dipole resonances can be excited inside the sphere in the visible range. Then, a semi-infinite medium of TiO 2 nanospheres is studied using Lewin effective-medium model, and we find that the effective permeability or permittivity becomes negative around the magnetic or electric resonance wavelength, leading to the perfect reflection of light. On the basis of these results, we design a monolayer of TiO 2 nanocylinder array to achieve a flat-top perfect reflection band by optimizing the wavelength interval between the magnetic and electric resonances. In addition, it is shown that the position of the perfect reflection band can be adjusted across the whole visible spectrum by changing the dimensions and lattice period of the TiO 2 nanocylinder array. Our design of all-dielectric metamaterial reflectors may find applications in diverse fields such as filter, color printing, spectroscopy, and so on.

We propose a novel method to achieve tunable plasmon focusing in graphene/photonic-crystal hybrid structure exhibiting all-angle negative refraction at terahertz frequencies. A two-dimensional photonic crystal composed of a square lattice of dielectric rods is constructed on the substrate of a graphene sheet to provide the hyperbolic dispersion relations of the graphene plasmon, giving rise to the all-angle plasmonic negative refraction. Plasmon lensing induced from the negative refraction is observed. We show that the ultracompact graphene-based system can produce sub-diffraction-limited images with the resolution significant smaller than the wavelength of the incident terahertz wave. Moreover, by adjusting the Fermi energy of the graphene, the imaging performance of the proposed system can remain almost invariant for different frequencies. Our results may find applications in diverse fields such as subwavelength spatial light manipulation, biological imaging, and so forth.

Optical bound states in the continuum (BICs) are spatially localized states with vanishing radiation, despite their energy embedded in the continuum spectrum of the environment. They are expected to greatly enhance light–matter interaction due to their long lifetime and high quality factor. However, the BICs in all-dielectric structures generally exhibit large mode volumes and their properties are difficult to manipulate. In this paper, we propose a metal–dielectric hybrid nanostructure where a silver film is inserted into the silicon (Si) substrate under the Si nanopillar array. We show that symmetry-protected BIC in this system can couple with surface plasmon polaritons (SPPs) to form a hybridized mode. Compared with previous symmetry-protected BICs in all-dielectric structures, the SPP-coupled BIC has a significantly decreased mode volume, and its corresponding electric field is strongly localized below the Si nanopillars. We also show that the SPP mode makes the original polarization-independent symmetry-protected BIC become polarization-dependent. In addition, we demonstrate that the silver film in the considered structure can induce a metal mirror effect. The destructive interference between the magnetic dipole inside the Si nanopillars and the mirror magnetic dipole in the silver film can lead to the formation of accidental BICs. Our hybrid structure provides a versatile platform for the manipulation of light–matter interaction in the nanoscale.

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