Xingjun Wang

Numerical modelling of high efficiency InAs/GaAs intermediate band solar cell

Quantum Dots (QDs) intermediate band solar cells (IBSC) are the most attractive candidates for the next generation of photovoltaic applications. In this paper, theoretical model of InAs/GaAs device has been proposed, where we have calculated the effect of variation in the thickness of intrinsic and IB layer on the efficiency of the solar cell using detailed balance theory. IB energies has been optimized for different IB layers thickness. Maximum efficiency 46.6% is calculated for IB material under maximum optical concentration.

Size and shape dependent optical properties of InAs quantum dots

In this study Electronic states and optical properties of self assembled InAs quantum dots embedded in GaAs matrix have been investigated. Their carrier confinement energies for single quantum dot are calculated by time-independent Schrödinger equation in which hamiltonianian of the system is based on effective mass approximation and position dependent electron momentum. Transition energy, absorption coefficient, refractive index and high frequency dielectric constant for spherical, cylindrical and conical quantum dots with different sizes in different dimensions are calculated. Comparative studies have revealed that size and shape greatly affect the electronic transition energies and absorption coefficient. Peaks of absorption coefficients have been found to be highly shape dependent.

Terahertz broadband polarization converter based on metamaterials

Based on the metamaterial composed of symmetrical split resonant ring, a broadband reflective terahertz polarization converter is proposed. The numerical simulation shows that it can rotate the polarization direction of linear polarized wave 90° in the range of 0.7-1.8THz and the polarization conversion ratio is over 90%. The reflection coefficient of the two electric field components in the diagonal direction is the same and the phase difference is 180° ,which leads to the cross-polarization rotation.In order to further study the physical mechanism of high polarization conversion, we analyze the surface current distribution of the resonant ring. The polarization converter has potential applications in terahertz wave plate and metamaterial antenna design.

Structural and optoelectronic properties of ZnGaO thin film by pulsed laser deposition

ZnO has attracted much attention because of its high-energy gap and exciton binding energy at room temperature. Compared to ZnO thin films, ZnGaO thin films are more resistive to oxidation and have smaller deformation of lattice. In this study, the high purity ZnSe and Ga2O3 powders were weighted at a molar ratio of 18:1. Se was oxidized to Se2O3 and separated from the mixture powders by using conventional solid state reaction method in air, and the ZnGaO ceramic target was prepared. We fabricated the ZnGaO films on silica glass by pulsed laser deposition (PLD) method under different oxygen pressure at room temperature. The as-grown films were tested by X-ray diffraction and atomic force microscope (AFM) to diagnose the crystal structure and surface morphology. Moreover, we obtained the optical transmittance of ZnGaO film and found that the electrical conductivity capacity varied with the increase of oxygen pressure.

Nonreciprocal optical propagation by magnetic tamm plasmon polaritons

We have proposed and realized nonreciprocal light propagation by a structure consisting of graphene photonic crystal and magneto-optical semiconductor. The nonreciprocal transmission is caused by magnetic Tamm plasmon polaritons which exit on the interface of graphene photonic crystal and magneto-optical material. Transmission spectrum is investigated to analyze the influence of structural parameters, external magnetic field, and graphene chemical potential on magnetic Tamm plasmon polaritons. We show that it is possible to realize active control on unidirectional light by altering external magnetic field and graphene chemical potential. And one-way transmission can be easily switched from forward to backward propagation.

Design of polarization-independent transmission fused-silica grating with high diffraction efficiency

The high diffraction efficiency and high dispersion ability of diffraction grating plays a very important role in laser systems. Fused-silica transmission gratings not only have board band, high diffraction efficiency and high damage threshold, but also have the advantage of light path without shelter comparing to reflective gratings. In this paper, the study of polarization-independent transmission fused-silica grating is carried out, and the influence of rectangular and trapezoidal grating microstructures on the -1st diffraction efficiency of grating is analyzed. For trapezoidal groove structure, in the range of 80 to 90 degrees, the distributions of diffraction efficiency at different bottom angle are calculated and analyzed. The structure parameters of the grating are optimized by rigorous coupled wave theory. The designed grating groove density is 1440 lines/mm. The -1st diffraction efficiency of the grating is over 96% for both of TE and TM polarized waves at the Littrow angle (49.7 degrees) with the center wavelength of 1060nm. Within the bandwidth of 42nm (from 1039 to 1081nm), the -1st diffraction efficiency of the designed grating is theoretically greater than 90% for both of TE and TM polarized waves.

Organic-inorganic broadband photodetector

The capability to detect optical signals over a broad wavelength band is highly important for practical device applications. However, high speed responsive across entire wavelength band within a single photodetector remains challenge. Here we demonstrated a broadband photodetector using a single quantum-dot-doped polyaniline nanowire with a broadband responsive at 350-700 nm (see schematic). The high responsivity is attributed to the high density of trapping states at the enormous interfaces formed in polyaniline and quantum dots. The interface trapping can effectively reduce the recombination rate, promote the separation of photogenerated carriers, and then enhance the efficiency for optical detection.

SU-8 optical waveguides and devices for the 2μm wavelength range

SU-8 optical waveguides and devices are developed for the 2 μm wavelength range for the potential applications in the future high-capacity optical communications operating at 2 μm. The mode properties of the SU-8 optical waveguides are analyzed numerically. The SU-8 optical waveguides and devices are fabricated with the spin-coating process. The measured propagation losses for the fabricated SU-8 optical waveguides is about 10dB/cm. The demonstrated microring resonator (MRR) has a Q-value of 5000.

Design of bent waveguide semiconductor lasers using nonlinear equivalent chirp

Reconstruction equivalent chirp (REC) technique is widely used in the design and fabrication of semiconductor laser arrays and tunable lasers with low cost and high wavelength accuracy. Bent waveguide is a promising method to suppress the zeroth order resonance, which is an intrinsic problem in REC technique. However, it may introduce basic grating chirp and deteriorate the single longitudinal mode (SLM) property of the laser. A nonlinear equivalent chirp pattern is proposed in this paper to compensate the grating chirp and improve the SLM property. It will benefit the realization of low-cost Distributed feedback (DFB) semiconductor laser arrays with accurate lasing wavelength.

Light emission in quantum dots and dyes doped polymer nanofibers

Polymer nanofibers are cheap and flexible building blocks for nanophotonic components. For high density nanophotonic integration, both passive and active polymer nanofibers are desirable. In contrast to passive polymer nanofibers, active polymer nanofibers are more desirable because they can act as a light source and waveguide simultaneously. In this talk, light emission in quantum dots and dyes doped polymer nanofibers will be introduced.