Jensen Tsan Hang Li

Direct and Seamless Coupling of TiO2 Nanotube Photonic Crystal to Dye-Sensitized Solar Cell: A Single-Step Approach

A TiO(2) nanotube layer with a periodic structure is used as a photonic crystal to greatly enhance light harvesting in TiO(2) nanotube-based dye-sensitized solar cells. Such a tube-on-tube structure fabricated by a single-step approach facilitates good physical contact, easy electrolyte infiltration, and efficient charge transport. An increase of over 50% in power conversion efficiency is obtained in comparison to reference cells without a photonic crystal layer (under similar total thickness and dye loading).

Analysis of the Drude model in metallic films

A method, believed to be new, to simulate Drude parameters for collective oscillation of the free carriers in metallic films is proposed. Plasma resonance frequency and relaxation were simulated simultaneously from both the real and the imaginary parts of the dielectric function of a metallic film after consideration of their correlation in the Drude model. As examples, the contributions of the electrons in Ag films and of the free carriers in metallic silicide, NbSi(2) and TaSi(2), films have been studied.

Fabrication and optical characterization of gold-infiltrated silica opals

We report the fabrication of metal-infiltrated silica opals for optical studies. Highly mono-dispersed silica microspheres are fabricated and assembled by a force packing method to form opals with large domain sizes. The opals are then infiltrated with gold by an electroplating technique. The optical properties of the infiltrated opals in the visible range are studied and model calculations based on a multiple-scattering formalism are used to interpret the experimental results. The calculated position of the directional gap of the silica opal agrees very well with experimental observation. We found that the optical properties of the infiltrated sample can be explained using a model system in which the voids in the silica opal are partially filled with Au and the surface of the slab is covered with a thin layer of Au.

Subwavelength focusing using a hyperbolic medium with a single slit

A hyperbolic dispersion medium with a planar surface that can be used for subwavelength focusing is proposed. By combining the hyperbolic medium in a single slit with diffraction limit width, a laser beam could be focused to a subwavelength spot in the near field. Compared to a conventional superlens, the subdiffraction focusing in this work has higher optical throughput. Using a planar hyperbolic medium, which is actually alternating silver/dielectric multilayers, we showed that the focusing resolution of the designed device is down to ~λ/5 using green light illumination (at a wavelength of 514.5 nm).

Optical properties of metallo-dielectric microspheres in opal structures

We report the fabrication of opal structure using metallo-dielectric silica microspheres. Mono-dispersed silica microspheres were coated with silver using an electrode-less wet-plating technique. Thin slabs of opal were obtained by assembling the silver-coated microspheres between two glass plates using a forced-packing method. The optical properties of the resulting opal structure were studied in the infrared range. Good agreement is obtained with the predictions of a multiple scattering approach, provided that the silver layer is modelled as a silver composite.

Effective parameters retrieval for complex metamaterials with low symmetries

We have derived a closed-form expression relating the S-parameters and the general constitutive matrix as a radiatively corrected local effective medium formula, which can work for thin metamaterials with low symmetries at normal incidence. We apply our approach to typical metamaterials possessing negative permeability or negative refractive index, and then to metamaterials with generally low symmetries. Our results show that anti-resonance for the extracted constitutive parameters from our formula is avoided while the effective medium is found to well represent a metamaterial with low symmetry except very near to resonance. Our approach will be useful for modelling metamaterials with low symmetries using an effective medium, as well as for serving as a starting point for designing future complex metamaterials with desired optical properties.

Near field imaging with resonant cavity lens

We showed that a Ag-SiO(2)-Ag Fabry-Pérot cavity can be used in near-field imaging based on omnidirectional resonance tunneling. The omnidirectional resonance was experimentally demonstrated in the Ag-SiO(2)-Ag resonant cavity working at a wavelength of 365 nm. The resonant cavity lens with high transmittance and high image fidelity was fabricated using standard photolithography method. Grating source with 190 nm line resolution was imaged through the resonant cavity lens with a total thickness of 128 nm.