Shao-Ji Jiang

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.

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.

Optical fiber sensor for tensile and compressive strain measurements by white-light Fabry–Perot interferometry

Based on the theory of optical fiber coupling and Fabry–Perot interference, a model of extrinsic Fabry–Perot interferometric (EFPI) optical fiber sensor for measuring strain is analyzed, and the theoretical model is demonstrated. A formula of white-light interfered EFPI optical fiber strain sensor is obtained. The system of the optical fiber sensor is designed using an LED source as light source to obtain reflected spectrum. The experimental results show that it coincides well with the computational simulation of theoretical model, which means the model is accurate.

Hardware architecture for full analytical Fraunhofer computer-generated holograms

Abstract. Hardware architecture of parallel computation is proposed for generating Fraunhofer computer-generated holograms (CGHs). A pipeline-based integrated circuit architecture is realized by employing the modified Fraunhofer analytical formulism, which is large scale and enables all components to be concurrently operated. The architecture of the CGH contains five modules to calculate initial parameters of amplitude, amplitude compensation, phases, and phase compensation, respectively. The precalculator of amplitude is fully adopted considering the “reusable design” concept. Each complex operation type (such as square arithmetic) is reused only once by means of a multichannel selector. The implemented hardware calculates an 800×600u2009u2009pixels hologram in parallel using 39,319 logic elements, 21,074 registers, and 12,651 memory bits in an Altera field-programmable gate array environment with stable operation at 50 MHz. Experimental results demonstrate that the quality of the images reconstructed from the hardware-generated hologram can be comparable to that of a software implementation. Moreover, the calculation speed is approximately 100 times faster than that of a personal computer with an Intel i5-3230M 2.6 GHz CPU for a triangular object.

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.

Real 3D Imaging/Video Based on Fraunhofer Computer-Generated Hologram

Three-dimensional imaging using full analytical triangular-based holographic computations are present. Holographic videos with both real- and virtual-life contents are of high performance, such as lighting and shading, surface texture, and wide viewing angle.

Robust Absorption in a Four-Layer Dielectric-Metal Structure

This letter predicts that robust absorption will appear in a four-layer dielectric-metal structure, and our experimental results verify this prediction well. The mechanisms are as follows: 1) The four layers form a cavity, and zero phase shift at the interface between the first metallic layer and the second dielectric layer is designed to make the backward light form a destructive interference and minimize the reflection. 2) The slow-light effect near the cutoff frequency in this structure increases the absorptive length many times. 3) It also can be explained as an asymmetric resonator, which satisfies the critical coupling condition. Many applications can be developed from this work, such as ensuring laser welding high quality for infrared transparent optoelectronic components.

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.

Highly uniform silver coated hydrogen silsesquioxane nanopost arrays as excellent surface-enhanced Raman scattering substrate

Periodic silver coated hydrogen silsesquioxane nanopost arrays (HSQ@Ag NPAs) with various diameters were fabricated as surface-enhanced Raman scattering (SERS) active substrates, and the SERS performance of the substrates were studied both experimentally and numerically. Raman signals of Rhodamine 6G molecules absorbed on the HSQ@Ag NPAs were measured and showed excellent SERS performance with significant enhancement and high uniformity. The enhancement factor under 514.5 nm excitation wavelength increased firstly and then decreased, but increased monotonically under 633 nm excitation wavelength. Finite-difference time-domain simulations of electric field distribution and far field absorption demonstrated that SERS enhancement is related to strong electric enhancements under both excitation and Stokes wavelengths, and the strongest enhancement occurred in HSQ@Ag NPAs with localized surface plasmon resonance (LSPR) wavelength located in the region between excitation wavelength and Stokes wavelength. Thus, by tuning the LSPR wavelength to the region between excitation wavelength and Stokes wavelength via reasonably designing the parameters of the nanostructure, SERS substrate with excellent performance could be obtained. Our work could be helpful in understanding the fundamental mechanism of SERS and provides a possible way to reasonably design excellent SERS substrates.