A five-band absorber based on graphene metamaterial for terahertz ultrasensing

We design and propose a five-band absorber based on graphene metamaterial for the terahertz (THz) sensing field. The localized surface plasmon resonances (LSPR) of patterned graphene are excited, contributing to five tunable ultra-narrow absorption peaks, which are specified by the electric field distributions. Moreover, the absorber is insensitive to different polarization modes and incident angles. When increasing the Fermi level of the patterned graphene, which is composed of a round ring and a square ring connected by four thin wires, the resonant frequencies exhibit distinct blue shifts. For refractive index sensing, due to the addition of a continuous dielectric groove, the theoretical results show that the maximum averaged normalized sensitivity, Q factor, and FOM can reach 0.647 RIU-1 (refractive index unit, RIU), 355.94, and 215.25 RIU-1, indicating that the sensing performances are further enhanced compared with previous works. As a result, the proposed structure may provide a new method to realize ultrasensing in the THz region.

High-efficiency and tunable circular dichroism in chiral graphene metasurface

Circular dichroism (CD) response is extremely important for dynamic polarization control, chiral molecular sensing and imaging, etc. Here, we numerically demonstrated high-efficiency and tunable CD using a symmetry broken graphene-dielectric-metal composite microstructure. By introducing slot patterns in graphene ribbons, the metasurface exhibits giant spin-selective absorption for circularly polarized (CP) wave excitations. The maximum CD reaches 0.87 at 2.78 THz, which originates from the localized surface plasmon resonances (LSPRs) in patterned graphene. Besides, the operating frequency and magnitude of CD are dynamically manipulated by gating graphene's Fermi energies. The proposed chiral graphene metasurface with high- efficiency and tunable capability paves a way to the design of active CD metasurfaces.

On the Localized Surface Plasmonic Resonances of AgPd Alloy Nanoparticles by Experiment and Theory

Ag/Pd multilayers and AgPd alloyed ultrathin films were deposited on Corning glass by magnetron sputtering. After being annealed in a furnace in air at 460 °C, self-assembled nanoparticles were formed. Localized surface plasmon resonances were observed only for the Ag-rich samples in the full range of the visible light spectrum. The resonance position was found to depend on the initial film thickness. In order to gain further physical insight, rigorous theoretical calculations were carried out via the rigid coupled-wave analysis method for the entire compositional range between Ag and Pd. Theoretical calculations were proven to be in suitable agreement with the experimental results.

Second-Order Dispersion Sensor Based on Multi-Plasmonic Surface Resonances in D-Shaped Photonic Crystal Fibers

This paper proposes a scheme to determine the optical dispersion properties of a medium using multiple localized surface plasmon resonances (SPR) in a D-shaped photonic crystal fiber (PCF) whose flat surface is covered by three adjacent gold layers of different thicknesses. Using computational simulations, we show how to customize plasmon resonances at different wavelengths, thus allowing for obtaining the second-order dispersion. The central aspect of this sensing configuration is to balance miniaturization with low coupling between the different localized plasmon modes in adjacent metallic nanostructures. The determination of the optical dispersion over a large spectral range provides information on the concentration of different constituents of a medium, which is of paramount importance when monitoring media with time-varying concentrations, such as fluidic media.

Space- and time-resolved second harmonic spectroscopy of coupled plasmonic nanocavities

Localized surface plasmon resonances of individual sub-wavelength cavities milled in metallic films can couple to each other to form a collective behavior. This coupling leads to a delocalization of the plasmon field at the film surface and drastically alters both the linear and nonlinear optical properties of the sample. In periodic arrays of nanocavities, the coupling results in the formation of propagating surface plasmon polaritons (SPP), eigenmodes extending across the array. When artificially introducing dislocations, defects and imperfections, multiple scattering of these SPP modes can lead to hot-spot formation, intense and spatially confined fluctuations of the local plasmonic field within the array. Here, we study the underlying coupling effects by probing plasmonic modes in well-defined individual triangular dimer cavities and in arrays of triangular cavities with and without artificial defects. Nonlinear confocal spectro-microscopy is employed to map the second harmonic (SH) radiation from these systems. Pronounced spatial localization of the SPP field and significant enhancements of the SH intensity in certain, randomly distributed hot spots by more than an order of magnitude are observed from the triangular arrays as compared to a bare silver film by introducing a finite degree of disorder into the array structure. Hot-spot formation and the resulting enhancement of the nonlinear efficiency are correlated with an increase in the lifetime of the localized SPP modes. By using interferometric SH autocorrelation measurements, we reveal lifetimes of hot-spot resonances in disordered arrays that are much longer than the few-femtosecond lifetimes of the localized surface plasmon resonances of individual nanocavity dimers. This suggests that hot spot lifetime engineering provides a path for manipulating the linear and nonlinear optical properties of nanosystems by jointly exploiting coherent couplings and tailored disorder.

Plasmonically enhanced photoluminescence of monolayer MoS2 via nanosphere lithography-templated gold metasurfaces

We demonstrate a simple, cost-effective method to enhance the photoluminescence intensity of monolayer MoS2. A hexagonal symmetric Au metasurface, made by polystyrene nanosphere lithography and metal coating, is developed to enhance the photoluminescence intensity of monolayer MoS2. By using nanospheres of different sizes, the localized surface plasmon resonances of the Au metasurfaces can be effectively tuned. By transferring monolayer MoS2 onto the Au metasurface, the photoluminescence signal of the monolayer MoS2 can be significantly enhanced up to 12-fold over a square-centimeter area. The simple, large-area, cost-effective fabrication technique could pave a new way for plasmon-enhanced light-mater interactions of atomically thin two-dimensional materials.