Realizing tunable multi-band perfect absorption from ultraviolet to near-infrared range in nanophotonics and metamaterials is extremely significant for solar energy harvesting and optical sensors. Here, five narrowband and broadband perfect absorbers (PA) are simultaneously achieved in a simple 3D metamaterial consisting of aluminum (Al) standing U-shaped split-ring resonators (SUSRRs) on SiO ₂ /Al layer, which stems from the plasmonic hybridization between three high-order surface plasmon polarization (SPPs) and multiple localized surface plasmon (LSPs) localized both within the individual SUSRR as well as within the dielectric layer underneath the SUSRR. These penta-band perfect absorption peaks can be tuned by varying the temperature of the sealed ethanol with a temperature tuning performance of 0.331 nm/°C. Due to the two narrow perfect absorption at 473 nm and 157 nm being sensitive to the surroundings, our proposed Al SUSRR array as a high-sensitivity biosensor displays the ultra-large figure of merit ( FoM *) of 3.0×10 ⁴ and 1.55×10 ⁴ , which could be applied to gas sensing and the detection of the sub-nanometer thickness of analyte film.

We numerically demonstrate an ultraviolet graphene ultranarrow absorption in a hybrid graphene-metal structure. The full-width at half maximum of the absorption band being 9 nm in ultraviolet range is achieved based on the coupling of lattice plasmon resonances of the metallic nanostructure to the optical dissipation of graphene. The position, absorbance and linewidth of the hybridized narrow resonant mode tuned by controlling geometrical parameters and materials are systematically investigated. The proposed structure possesses high refractive index sensitivity of 288 nm/RIU and figure of merit of 72, and can also be used to detect small molecules layer of sub-nanometer thickness and refractive index with small changes, providing promising applications in ultra-compact efficient biosensors.

Exceptional points (EPs) are degeneracies in open wave systems with coalescence of at least two energy levels and their corresponding eigenstates. In higher dimensions, more complex EP physics not found in two-state systems is observed. We consider the emergence and interaction of multiple EPs in a four coupled optical waveguides system by non-Hermitian coupling showing a unique EP formation pattern in a phase diagram. In addition, absolute phase rigidities are computed to show the mixing of the different states in definite parameter regimes. Our results could be potentially important for developing further understanding of EP physics in higher dimensions via generalized paradigm of non-Hermitian coupling for a generation of parity-time devices.

Achieving ultra-broadband and completely modulated absorption enhancement of monolayer graphene in near-infrared region is practically important to design graphene-based optoelectronic devices, however, which remains a challenge. In this work, by spectrally designing multiple magnetic plasmon resonance modes in metamaterials to be adjacent to each other, near-infrared light absorption in monolayer graphene is greatly improved to have an averaged absorption efficiency exceeding 50% in a very broad absorption bandwidth of about 800 nm. Moreover, by exerting an external bias voltage on graphene to change Fermi energy of graphene, the ultra-broadband absorption enhancement of monolayer graphene exhibits an excellent tunability, which has a nearly 100% modulation depth and an electrical switching property. This work is promising for applications in near-infrared photodetectors, amplitude modulators of electromagnetic waves, etc.

We theoreticallydemonstrate the realization of the ultranarrow dual-band perfect absorption at visible range in 3D metamaterials comprising Au vertical split-ring resonators (VSRRs) array above a thick Au film for high-sensitivity sensing. By placing the 3D metamaterial of Au VSRRs directly onto the Au substrate to remove the dielectric substrates effect, the simulated results reveal that ultranarrow dual-band perfect absorption with the two narrow absorption bandwidth of 7.3 nm and 1.1 nm in the visible regime can be excited from pure magnetic plasmon (MP) resonance and via the strong coupling between the MP resonance of the VSRRs and the surface plasmons polaritons (SPPs) propagating on the Au film, respectively. Moreover, coupling effects in the VSRRs with different structural parameters also are studied in detail. Because of the ultranarrow dual-band perfect absorption of the VSRRs being extremely sensitive to the changes of the material surrounding, our proposed 3D metamaterial exhibits ultrahigh sensitivity ( S = 918 nm/RIU, S ^* = 680/RIU) and figure of merit ( FoM = 835, FoM ^* = 4.5×10 ⁶ ) in near-infrared regime, which hold potential applications in label-free biosensing.

Silicon (Si) plasmonics in the ultraviolet (UV) region owing to its strong UV interband transitions is emerging in the fields of UV nanophotonics and metamaterials. Here, we discuss the narrow plasmon induced transparency (PIT) of a lifted Si nanostrip array and the optical sensing and slow light properties in the UV range. The strong plasmonic hybridization between the broad Si localized surface plasmon (LSP) supported by a lifted Si nanostrip and the narrow optical diffraction mode propagated in air caused by Wood anomaly of a periodic array is achieved and leads to a sharp PIT with the quality ( Q ) factor being 40 in the UV range. Our calculated PIT effect of the proposed lifted Si nanostrip array is well demonstrated by the coupled oscillator theory. The position and lineshape of the UV PIT tuned by varying structural sizes are further discussed. The large group index of our designed lifted Si nanostrip array is achieved to be about 320 within the UV region. Moreover, the sensitivity ( S ) and figure of merit ( FoM ) are obtained to be as large as 295 nm/RIU and 84, respectively. These research results could have significant impact on the realization of multifunctional UV plasmonic nanodevices for such as optical sensor and slow light.