You Zhe Ho

Plasmon inducing effects for enhanced photoelectrochemical water splitting: X-ray absorption approach to electronic structures.

Artificial photosynthesis using semiconductors has been investigated for more than three decades for the purpose of transferring solar energy into chemical fuels. Numerous studies have revealed that the introduction of plasmonic materials into photochemical reaction can substantially enhance the photo response to the solar splitting of water. Until recently, few systematic studies have provided clear evidence concerning how plasmon excitation and which factor dominates the solar splitting of water in photovoltaic devices. This work demonstrates the effects of plasmons upon an Au nanostructure-ZnO nanorods array as a photoanode. Several strategies have been successfully adopted to reveal the mutually independent contributions of various plasmonic effects under solar irradiation. These have clarified that the coupling of hot electrons that are formed by plasmons and the electromagnetic field can effectively increase the probability of a photochemical reaction in the splitting of water. These findings support a new approach to investigating localized plasmon-induced effects and charge separation in photoelectrochemical processes, and solar water splitting was used herein as platform to explore mechanisms of enhancement of surface plasmon resonance.

Design of plasmonic toroidal metamaterials at optical frequencies

Toroidal multipoles are the subject of growing interest because of their unusual electromagnetic properties different from the electric and magnetic multipoles. In this paper, we present two new related classes of plasmonic metamaterial composed of purposely arranged of four U-shaped split ring resonators (SRRs) that show profound resonant toroidal responses at optical frequencies. The toroidal and magnetic responses were investigated by the finite-element simulations. A phenomenon of reversed toroidal responses at higher and lower resonant frequencies has also been reported between this two related metamaterials which results from the electric and magnetic dipoles interaction. Finally, we propose a physical model based on coupled LC circuits to quantitatively analyze the coupled system of the plasmonic toroidal metamaterials.

Three‐Dimensional Plasmonic Micro Projector for Light Manipulation

photovoltaics, [ 5 ] super-resolution imaging, [ 6 ] and various twodimensional plasmonic lens. [ 7 ] Besides, using nanostructures to project SPP plane waves into the adjacent free space is also an important issue. The interactions of plasmonic nanostructure on SPP wave involve not only the in-plane behavior, but also out-of-plane scattering which is captured as the far-fi eld radiated light. [ 8 ] A few theoretical approaches to convert the confi ned surface plasmons into radiated waves have been proposed. [ 9 ] It is highly desirable to extend the application range of plasmonic devices into the domain of three-dimensional light manipulation. [ 10 ] Recently, three-dimensional focusing and diverging of SPP waves by a quarter circular structure composed of gold (Au) nanobumps were studied. [ 11 ] The forward and backward scattering from individual Au nanobump are observed above and below Au surface, respectively. Hence, the Au nanobumps confer additional three-dimensional propagating wave vectors ( k x , k y , k z ) on SPP wave for departing from surface. Therefore, it is possible to manipulate the three-dimensional plasmonic scattering into specifi c geometry by arranging the Au nanobumps, which is schematically depicted in Figure 1 a. In this paper, we manipulate the scattering of SPP waves by various plasmonic structures composed of arranged nanobumps on a gold thin fi lm. Upon controlling the geometry of the plasmonic structures, the height, position, and pattern of scattered light can be modifi ed as desired. It provides a simple and effi cient way to project a specifi c light pattern into free space, and demonstrate the capability of three-dimensional light manipulation.

Gain-assisted Hybrid-superlens Hyperlens for Nano Imaging

We propose an innovative active imaging device named gain-assisted hybrid-superlens hyperlens and examine its resolving power theoretically. This semi-cylindrical device consists of a core of semi-cylindrical super-lens and a half cylindrical outer shell of hyperlens. Both the superlens and hyperlens parts of the device are appropriately designed multi-layered metal-dielectric structures having indefinite eigenvalues of dielectric tensors. The dielectric layers of the hyperlens are doped with Coumarin, which play the role of gain medium. The gain medium is analyzed thoroughly using a generic four-level system model, and the permittivity of the gain medium is extracted from this analysis for simulating the imaging characteristics of the device. According to our simulation at wavelength of 365 nm, an excellent resolution power much better than the diffraction limit value can be achieved.

Optical Hybrid-Superlens Hyperlens for Superresolution Imaging

This study proposes an innovative device called “Hybrid-Superlens Hyperlens” with superresolution imaging ability and confirms it by using simulation. This device consists of two multilayered metal-dielectric anisotropic metamaterials: the upper planar superlens and the lower cylindrical hyperlens with different signs in their dielectric tensors and different isofrequency dispersion curves. In our simulation, 100-nm center-to-center resolution is obtained by using an incident wavelength of 405 nm, which is smaller than the optical diffraction limit.

Tunable plasmonic resonance arising from broken-symmetric silver nanobeads with dielectric cores

We investigate the plasmonic resonance modes and coupling effects of single silver nanobeads and silver nanobead dimers. Numerical investigation using the three-dimensional finite element method indicates that silver nanobeads exhibit two plasmonic resonances corresponding to the bonding and anti-bonding modes, respectively. The boundary symmetry on the inner and outer surfaces of the silver nanobeads can be broken by increasing the refractive indices of the cores filling the dielectric holes. It is shown that only the bonding mode can be found for low-refractive index cores, whereas both bonding and anti-bonding modes can be found for high-refractive index cores.

Anomalous reflection from metasurfaces with gradient phase distribution below 2π

Metasurfaces are artificial structures that have been demonstrated to possess the ability to manipulate light within a subwavelength spatial region. Here, we explore another unraised functionality of the energy redistribution of a metasurface by tuning the phase difference over a supercell. We also propose a practical nanorod-based design to achieve an anomalous steering reflection using the finite element method simulation. The proposed phenomena have potential applications in ultracompact nanophotonic systems and high-efficiency flat devices.

Three-dimensional light manipulation by gold nanobumps

The scattering of surface plasmon polariton (SPP) waves can be manipulated by various plasmonic structures. The plasmonic structure composed of arranged subwavelength nanobumps on a gold thin film is the promising structure to manipulation SPP wave. By controlling the geometric shape of the structures, the height, position, and pattern of scattered light from SPP wave can be modulated as desired. A clear single focusing spot can be reconstructed at a specific altitude by a particular curved structure with appropriate curvature and adjacent interspacing of nanobumps. The designed light patterns reconstructed by the focusing spot from the arranged curved structures at a specific observation plane are clearly demonstrated.

Plasmonic toroidal resonance at optical frequencies

Simulations of differentially oriented split-ring resonator arrays irradiated by visible light indicate induction of magnetic multipoles, opening up new avenues for high-capacity data storage.