Wide-angle and broadband solar absorber made using highly efficient large-area fabrication strategy.

High performance and cost-effective solar absorbers are crucial for various optical applications, such as solar collection and thermophotovoltaic devices. This study designs and experimentally demonstrates a wide-angle and broadband solar absorber. The proposed absorber is composed of tapered polyimide substrate and Al-Cr-SiO2-Cr-SiO2 thin-film based on the optical interference of the multilayer thin film and excited magnetic resonance of light-trapping structures. The composite process of the colloidal lithography method and magnetron sputtering is employed for efficient fabrication in a large area. The average absorbance is more than 93% from 300 nm to 2500 nm and shows an angular tolerance of up to 60°. The high efficiency and large-area fabrication capability demonstrated by the proposed solar absorber presents future application potential in flexible solar collection devices.

High-performance and cost-effective absorber for visible and near-infrared spectrum based on a spherical multilayered dielectric-metal structure.

A broadband, polarization-insensitive, wide-angle absorber based on a spherical multilayered dielectric-metal structure is numerically designed and experimentally demonstrated in this paper. This absorber has average absorbance of 0.98 between 380 and 1910 nm, indicating a spectral width of 1530 nm with absorbance exceeding 0.9, and covering the entire visible and near-infrared spectrum. The physical mechanism leading to this broadband absorption is discussed along with the effect of structural parameters on the absorber performance. Importantly, the absorbance is hardly affected by incident angle below 45° and it still stays at a high level with incident angle up to 60°, for both transverse magnetic and transverse electric plane waves.

Morphological and Near-Field Properties of Silver Columnar Thin Film for Surface-Enhanced Raman Scattering.

Noble metal sculptured thin films have attracted great research interest last decade as competitive surface-enhanced Raman scattering (SERS) substrates. However, the influences of the deposition conditions and the morphology on the plasmonic properties and SERS performance of the metal sculptured thin films have not been well understood due to the complexities of the morphology. In this work, the influences of the deposition angle and the height are investigated in both experiment and numerical simulation. A more accurate geometrical model based on the binarized scanning electron microscope images has been utilized to study the near-field plasmonic properties of Ag column thin films by taking account of the geometry irregularities, size distributions and random arrangement of the columns. It's found that the cross-sectional electric field enhancement is mainly dominated by the column density. When the deposition angle increases from 68° to 82° the SERS enhancement factors increases monotonously due to the increase of the self-shadow effect. While with the increase of height the SERS enhancement factors firstly increase to the largest value of 3.05 × 108 at the thickness of 694 nm then decrease because of competitive growth mechanism during the deposition. The detection limit of the optimized sample is found to be lower than 10-12 M. Our work could be helpful in understanding the SERS mechanism and useful to the optimization of metal sculptured thin films as SERS substrates.

Optimization of Ag coated hydrogen silsesquioxane square array hybrid structure design for surface-enhanced Raman scattering substrate.

A computer-automated design process for a surface-enhanced Raman scattering (SERS) substrate using a particle swarm optimization algorithm is proposed. Nanostructured Ag coated hydrogen silsesquioxane nanopillar arrays of various sizes for SERS substrate applications are fabricated by direct Ag film deposition on substrates patterned by electron beam lithography and are investigated systematically. Good agreement is demonstrated between experimental and simulation results. The absorption spectra, charge distributions, and electric field distributions are calculated using finite-difference time-domain simulations to explain the field enhancement mechanism and indicate that this enhancement originates from plasmon resonance. Our work provides a guide towards optimum SERS substrate design.

Alternative long-ranged charge optimized many-body potential for aluminium.

A new COMB3 potential was developed for aluminium, which focuses on long-range interaction and phase transition. The potential was developed by fitting the equilibrium lattice properties of different phases and defects to ensure its transferability to general systems. The quality of the potential was tested in several problems and compared with the EAM potential as well as the published COMB3 potential, the effect of the cutoff method was studied in detail to demonstrate the necessity to extend the cutoff region. Systems of strong deformations along the Bain path, under a trigonal strain and with planar stacking faults were calculated and the present potential performed as well as the EAM potential. At last, a surface process that involves adsorption and diffusion was studied using the present potential.

Image quality improvement of polygon computer generated holography.

Quality of holographic reconstruction image is seriously affected by undesirable messy fringes in polygon-based computer generated holography. Here, several methods have been proposed to improve the image quality, including a modified encoding method based on spatial-domain Fraunhofer diffraction and a specific LED light source. Fast Fourier transform is applied to the basic element of polygon and fringe-invisible reconstruction is achieved after introducing initial random phase. Furthermore, we find that the image with satisfactory fidelity and sharp edge can be reconstructed by either a LED with moderate coherence level or a modulator with small pixel pitch. Satisfactory image quality without obvious speckle noise is observed under the illumination of bandpass-filter-aided LED. The experimental results are consistent well with the correlation analysis on the acceptable viewing angle and the coherence length of the light source.

Experimental realization of photonic topological insulator in a uniaxial metacrystal waveguide.

Photonic analogue of topological insulator was recently predicted by arranging ε/µ (permittivity/permeability)-matched bianisotropic metamaterials into two-dimensional superlattices. However, the experimental observation of such photonic topological insulator is challenging as bianisotropic metamaterial is usually highly dispersive, so that the ε/µ-matching condition can only be satisfied in a narrow frequency range. Here we experimentally realize a photonic topological insulator by embedding non-bianisotropic and non-resonant metacrystal into a waveguide. The cross coupling between transverse electric and transverse magnetic modes exists in metacrystal waveguide. Using this approach, the ε/µ-matching condition is satisfied in a broad frequency range which facilitates experimental observation. The topologically non-trivial bandgap is confirmed by experimentally measured transmission spectra and calculated non-zero spin Chern numbers. Gapless spin-filtered edge states are demonstrated experimentally by measuring the magnitude and phase of the fields. The transport robustness of the edge states is also observed when an obstacle was introduced near the edge.

Robust flow of light in three-dimensional dielectric photonic crystals.

Chiral defect waveguides and waveguide bend geometry were designed in diamond photonic crystal to mold the flow of light in three dimensions. Propagations of electromagnetic waves in chiral waveguides are robust against isotropic obstacles, which would suppress backscattering in waveguides or integrated devices. Finite-difference time-domain simulations demonstrate that high coupling efficiency through the bend corner is preserved in the polarization gap, as it provides an additional constraint on the polarization state of the backscattered wave. Transport robustness is also demonstrated by inserting two metallic slabs into the waveguide bend.

Viewing-angle enlargement in holographic augmented reality using time division and spatial tiling.

Viewing angle enlargement is essential for SLM-based 3D holographic display. An idea of constructing equivalent-curved-SLM-array (ECSA) is proposed by linear phase factor superimposition. Employing the time division and spatial tiling (TDST) techniques, an ECSA-based horizontal 4f optical system is designed and built. The horizontal viewing angle of a single SLM is increased to 3.6 times when retaining the same hologram area. An interlaced holographic display technique is developed to remove the flicker effect. Holographic augmented reality is performed using the TDST system. Floating holographic 3D image with parallax and accommodation effects is achieved. Both TDST and interlaced technique may extend to multiple SLMs system to achieve larger viewing angle.

Optically tunable chirped fiber Bragg grating.

This work presents an optically tunable chirped fiber Bragg grating (CFBG). The CFBG is obtained by a side-polished fiber Bragg grating (SPFBG) whose thickness of the residual cladding layer in the polished area (D(RC)) varies with position along the length of the grating, which is coated with a photoresponsive liquid crystal (LC) overlay. The reflection spectrum of the CFBG is tuned by refractive index (RI) modulation, which comes from the phase transition of the overlaid photoresponsive LC under ultraviolet (UV) light irradiation. The broadening in the reflection spectrum and corresponding shift in the central wavelength are observed with UV light irradiation density of 0.64mW/mm. During the phase transition of the photoresponsive LC, the RI increase of the overlaid LC leads to the change of the CFBG reflection spectrum and the change is reversible and repeatable. The optically tunable CFBGs have potential use in optical DWDM system and an all-fiber telecommunication system.

Fabrication of non-planar silver nano-arc-gap arrays.

We developed a method to fabricate an array of silver non-planar nano-arc-gaps via inverted hemispherical colloidal lithography and shadow metal evaporation methods. It is found that there is a localized surface plasmon mode which results in extraordinary optical transmission. The electric field is strongly localized at the nano-arc-gap region and therefore induces a resonance that has an ultra-small mode volume of less than 2.44 × 10(-6) µm(3). The ratio of the quality factor to the mode volume is as high as 1.44 × 10(6) µm(-3). This would be valuable for the design of optoelectric circuits.

Optimization of holographic storage with modulated recording beams in a thick polyvinyl alcohol/acrylamide photopolymer.

In a holographic photopolymer system, the storage properties were often limited due to the attenuation in depth of light during the recording step. To obtain smaller values of the depth attenuation profiles in 1 mm thick polyvinyl alcohol/acrylamide (PVA/acrylamide) photopolymers, we used a triangle prism, sitting one face tilted at 13.7 degrees to the axis within the focus of a lens, to modulate the distribution of recording beams. Doing this permitted larger refractive index modulation depth to be achieved, and larger dynamic range (M#=9.2) was obtained in the PVA/acrylamide photopolymers.

Design and fabrication of narrow-frequency sharp angular filters.

The theoretical deduction for the frequency variations of the defect mode and its dependence on the incident angle in one-dimensional defective photonic crystals is discussed. Filters with the dual function of narrow frequency passbands and sharp angular pass breadths in visible and near-infrared wave bands, in which both the defect mode and the periodic structure are layers of integral times of quarter-wavelengths, were designed. This kind of filter was fabricated with commonly used coating machines.