Bo Han Cheng

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.

Vertical split-ring resonator based anomalous beam steering with high extinction ratio

Metasurfaces created artificially with metal nanostructures that are patterned on surfaces of different media have shown to possess “unusual” abilities to manipulate light. Limited by nanofabrication difficulties, so far most reported works have been based on 2D metal structures. We have recently developed an advanced e-beam process that allowed for the deposition of 3D nanostructures, namely vertical split-ring resonators (VSRRs), which opens up another degree of freedom in the metasurface design. Here we explore the functionality of beam steering with phase modulation by tuning only the vertical dimension of the VSRRs and show that anomalous steering reflection of a wide range of angles can be accomplished with high extinction ratio using the finite-difference-time-domain simulation. We also demonstrate that metasurfaces made of 3D VSRRs can be made with roughly half of the footprint compared to that of 2D nano-rods, enabling high density integration of metal nanostructures.

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.

Multi-level surface enhanced Raman scattering using AgOx thin film.

Ag nanostructures with surface-enhanced Raman scattering (SERS) activities have been fabricated by applying laser-direct writing (LDW) technique on silver oxide (AgOx) thin films. By controlling the laser powers, multi-level Raman imaging of organic molecules adsorbed on the nanostructures has been observed. This phenomenon is further investigated by atomic-force microscopy and electromagnetic calculation. The SERS-active nanostructure is also fabricated on transparent and flexible substrate to demonstrate our promising strategy for the development of novel and low-cost sensing chip.

Actively controlled super-resolution using graphene-based structure

A super-resolution (with λ/50 resolution ability at mid-infrared region) device that consists of a monolayer graphene sandwiched between two dielectric materials with two alternate chemical potentials in graphene (which can be obtained by alternately applying two biased voltages to graphene) is proposed and analyzed. When the subwavelength resolution is achieved, the graphene-based device can be viewed as an effective optical medium with alternate arrangement of positive and negative refractive indices. And the isofrequency dispersion curves of the effective optical medium have the hyperbolic form. Furthermore, the super-resolution at different desired frequencies can be reached by merely changing the chemical potentials of graphene. The proposed devices have potential applications in multi-functional material, real-time subwavelength imaging, and high-density optoelectronic components for using the abnormal diffraction feature.

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.

Achieving planar plasmonic subwavelength resolution using alternately arranged insulator-metal and insulator-insulator-metal composite structures

This work develops and analyzes a planar subwavelength device with the ability of one-dimensional resolution at visible frequencies that is based on alternately arranged insulator-metal (IM) and insulator-insulator-metal (IIM) composite structures. The mechanism for the proposed device to accomplish subwavelength resolution is elucidated by analyzing the dispersion relations of the IM-IIM composite structures. Electromagnetic simulations based on the finite element method (FEM) are performed to verify that the design of the device has subwavelength resolution. The ability of subwavelength resolution of the proposed device at various visible frequencies is achieved by slightly varying the constituent materials and geometric parameters. The proposed devices have potential applications in multi-functional material, real-time super-resolution imaging, and high-density photonic components.

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.

Classical Analog of Electromagnetically Induced Transparency in the Visible Range With Ultra-Compact Plasmonic Micro-Ring Resonators

We theoretically show that it should be possible to demonstrate Electromagnetic-Induced-Transparency-like (EIT-like) effects in the visible range by using ultra-compact plasmonic micro-ring resonators with μm2 order foot print. By using the finite-difference time-domain (FDTD) numerical method and the coupled mode theory (CMT) collaboratively, the transmission intensity, phase, and group delay spectra of the coupled plasmonic ring resonators are theoretically calculated with the inclusion of metallic loss and dispersion through the Drude model. The interference induced transmission peak can be successfully achieved through suitable design of the plasmonic ring resonator structure.