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We theoretically study a tunable reflective focusing lens, based on graphene metasurface, which consists of rectangle aperture array. Dynamic control of either the focal intensity or focal length for terahertz circular polarized waves can be achieved by uniformly tuning the graphene Fermi energy. We demonstrate the graphene apertures with the same geometry; however, spatially varying orientations can only control the focal intensity. To change the focal length, the spatially varying aperture lengths are also required. A comparative study between the metalenses, which generate only geometric or both gradient and geometric phase changes, has shown that the apertures' spatially varying length distribution is the key factor for determining the modulation level, rather than the focal length's modulation range. This kind of metalens provides tunable, high-efficiency, broadband, and wide-angle off-axis focusing, thereby offering great application potential in lightweight and integrated terahertz devices.

We present diffractive optical elements with an extended depth of focus, namely, fractal spiral zone plates (FSZPs), which combine a fractal structure and spiral zone plates (SZPs) to generate a sequence of coaxial vortices in the focal volume along the propagation direction. The axial irradiance of the FSZPs is examined both experimentally and in a simulation and is compared with that of SZPs and that of fractal zone plates. The focusing properties of the FSZPs with different parameters are investigated, and a potential application to edge-enhancement images is also shown.

The development of new substrates for surface-enhanced spectroscopy is primarily motivated by the ability to design such substrates to provide the maximum signal enhancement. In this paper, we theoretically design and investigate a crisscross dimer array as a plasmonic substrate for enhancing coherent anti-Stokes Raman scattering (CARS). The plasmonic film-crisscross dimer array system can excite multiple resonances at optical frequencies. By properly designing structure parameters, three plasmon resonances with large field enhancements and same spatial hot spot regions can spectrally match with the pump, Stokes and anti-Stokes beams, respectively. The CARS signals are strongly enhanced by multi-resonance plasmon field enhancements. The estimated CARS factor can reach as high order as ~10 16 over conventional CARS without the plasmonic substrate.

Plasmonic metallic nanostructures have been demonstrated an effective way to enhance the light emission efficiency in LEDs. Here, we propose a design of white LEDs that combining dielectric silicon nanopillar array in the color-converting layer. By investigating theoretically the guided mode caused by the nanopillar array-waveguide system, we demonstrate that the silicon nanopillar arrays enable larger near-field enhancement and more efficient photons emission property than the plasmonic counterparts. These performances make the silicon nanopillar arrays have potential application in light converter for efficient white LEDs. We also show that the guided mode can be controlled by changing the period of nanopillar grating and the thickness of polymer layer. More significant performance can be achieved by further optimizing the shape and size of the silicon nanoparticles. Compared with the square nanoparticle arrays, the hexagonal nanopillar arrays are demonstrated to have larger field enhancement and emission enhancement. Our research is expected to give insights into the design and optimization of the solid-state lighting systems by using silicon nanostructures, and the all-dielectric metamaterials for gaining or lasing devices.

A novel plasmonic metamaterial consisting of the solid (bar) and the inverse (slot) compound metallic nanostructure for electromagnetically induced absorption (EIA) is proposed in this paper, which is demonstrated to achieve an ultra-narrow absorption peak with the linewidth less than 8 nm and the absorptivity exceeding 97% at optical frequencies. This is attributed to the plasmonic EIA resonance arising from the efficient coupling between the magnetic response of the slot (dark mode) and the electric resonance of the bar (bright mode). To the best of our knowledge, this is the first time that the plasmonic EIA is used to realize the narrow-band perfect absorbers. The underlying physics are revealed by applying the two-coupled-oscillator model. The near-perfect-absorption resonance also causes an enhancement of about 50 times in H-field and about 130 times in E-field within the slots. Such absorber possesses potential for applications in filter, thermal emitter, surface enhanced Raman scattering, sensing and nonlinear optics.

A novel planar plasmonic metamaterial for electromagnetically induced transparency and slow light characteristic is presented in this paper, which consists of nanoring and nanorod compound structures. Two bright modes in the metamaterial are induced by the electric dipole resonance inside nanoring and nanorod, respectively. The coupling between two bright modes introduces transparency window and large group index. By adjusting the geometric parameters of metamaterial structure, the transmittance of EIT window at 385 THz is about 60%, and the corresponding group index and Q factor can reach up to 1.2 × 10³ and 97, respectively, which has an important application in slow-light device, active plasmonic switch, SERS and optical sensing.

Double Fano resonant characteristics are investigated in planar plasmonic structure by embedding a metallic nanorod in symmetric U-shaped split ring resonators, which are caused by a strong interplay between a broad bright mode and narrow dark modes. The bright mode is resulted from the nanorod electric dipole resonance while the dark modes originate from the magnetic dipole induced by LC resonances. The overlapped dual Fano resonances can be decomposed to two separate ones by adjusting the coupling length between the nanorod and U-shaped split ring resonators. Fano resonances in the designed structure exhibit high refractive-index sensing sensitivity and figure of merit, which have potential applications in single or double-wavelength sensing in the near-infrared region.

A broad-band perfect absorber composing a two-dimensional periodic metal-dielectric-metal sandwiches array on dielectric/metal substrate is designed and numerically investigated. It is shown that the nearly-perfect absorption with a bandwidth of about 50 nm in visible region can be achieved by overlapping of two plasmon resonances: one originating from the coupling of electric dipoles between adjacent unit cells and another arising from magnetic dipole plasmon resonances. A capacitor-inductor circuit description is introduced to explain the dependence of resonance frequencies and band-width on geometrical parameters.

We demonstrate theoretically an analog of electromagnetically induced transparency (EIT) in the plasmonic metamaterial with the unit cell consisting of three parallel metallic rods. The electromagnetic mechanism for the EIT-like transmission is discussed based on our investigation of the localized surface plasmon resonances in three trirod configurations. We find that the transmission minima surrounding the transparency window on both sides correspond to two detuned magnetic resonances, which arise respectively from the antiphase plasmon couplings between a long rod and a short rod and between two short rods. A decrease of more than 10 times in the group velocity can be achieved for the trirod structure at the transparency window in the optical regime, and the EIT-like response can be well described by the theoretical model of two harmonic oscillators. This work not only reveals the EIT-like transmission in plasmonic metamaterial consisting of detuned magnetic "atoms," but also provides further insight into the plasmons' interactions, especially for metallic nanostructures comprising multiple metallic elements.