Huang Yongqing

Flat Supercontinuum Generated in a Single-Mode Optical Fibre with a New Chromatic Dispersion Profile

A new chromatic dispersion profile of a single-mode optical fibre is proposed for generating a supercontinuum with a flatly broadened spectrum. The chromatic dispersion D(λ,z) is a convex function of wavelengths and has no zero-dispersion wavelengths over the whole part of the fibre as D(λ,z) is negative. It is shown that the flat supercontinuum spectrum is obtained when the pump wavelength is set in the vicinity of the wavelength at which the peak chromatic dispersion is near zero and the strong residual pump component is eliminated.

Defect reduction in GaAs/Si film with InAs quantum-dot dislocation filter grown by metalorganic chemical vapor deposition

The growth of GaAs epilayers on silicon substrates with multiple layers of InAs quantum dots (QDs) as dislocation filters by metalorganic chemical vapor deposition (MOCVD) is investigated in detail. The growth conditions of single and multiple layers of QDs used as dislocation filters in GaAs/Si epilayers are optimized. It is found that the insertion of a five-layer InAs QDs into the GaAs buffer layer effectively reduces the dislocation density of GaAs/Si film. Compared with the dislocation density of 5×107 cm−2 in the GaAs/Si sample without QDs, a density of 2×106 cm−2 is achieved in the sample with QD dislocation filters.

Fabrication and electrical properties of axial and radial GaAs nanowire pn junction diode arrays

We report on the fabrications and characterizations of axial and radial GaAs nanowire pn junction diode arrays. The nanowires are grown on n-doped GaAs (111)B substrates using the Au-catalyzed vapor—liquid—solid mechanism by metal—organic chemical vapor deposition (MOCVD). Diethyl—zinc and silane are used as p- and n-type dopant precursors, respectively. Both the axial and radial diodes exhibit diode-like J—V characteristics and have similar performances under forward bias. Under backward bias, the axial diode has a large leakage current, which is attributed to the bending of the pn junction interface induced by two doping mechanisms in Au-catalyzed nanowires. The low leakage current and high rectification ratio make the radial diode more promising in electrical and optoelectronic devices.

Parallel Prefix Network Based DQPSK Precoder for High-Speed Optical Transmission

This paper introduced the principle of parallel prefix network (PPN) and proved the feasibility of applying PPN to differential quadrature phase-shift keying (DQPSK) precoder. Kogge Stone (KS) adder is a typical parallel algorithm of PPN. A ameliorated KS adder was proposed to implement the precoder. With 78Mb/s and 128 parallel channels, 20Gb/s DQPSK precoder was achieved in ML523 (Xilinx Virtex V FPGA). Results presented that the clock minimum period was 1.154 ns and the maximum frequency was 866.551 MHz. About 2% of slice logic resource was employed in the implementation.

Defect Reduction in GaAs/Si Films with the a-Si Buffer Layer Grown by Metalorganic Chemical Vapor Deposition*

The growth of GaAs epilayers on silicon substrates with a thin amorphous silicon (a-Si) buffer layer by metalorganic chemical vapor deposition is investigated in detail. Combining with the two-step growth method, the growth conditions of the a-Si buffer layer are optimized for growth of high-quality GaAs/Si epilayers. The a-Si buffer layer exhibits the best effect with thickness of 1.8 nm and growth temperature of 620°C. It is found that the introduction of this a-Si layer on Si substrates effectively reduces the dislocation density in GaAs/Si films. As compared with the dislocation density of 5 × 107 cm−2 in the GaAs/Si sample without the a-Si layer, a density of 3 × 105 cm−2 is achieved in the sample with the a-Si layer, and the defect reduction mechanism is discussed in detail.

Modeling of Fano Resonance in High-Contrast Resonant Grating Structures

A new model is presented for Fano resonance in resonant grating structure based on the temporal coupled mode theory. By using this model, the reflection spectrum can be reproduced with the information of eigenmode of the structure, which can be numerically calculated by the finite element method. Therefore, the eigenmode plays a key role in determining the profile of the line shape of the Fano resonance in the resonant grating structure. When the space of two grating modulations is decreased, the line shape experiences a significant change. Such a drastic change can be attributed to the increase of quality factor of the eigenmodes. Thus, our model not only provides a simple and intuitive understanding on the mechanism of Fano resonance, but it also offers a convenient way to engineer the line shape of the Fano resonance. The proposed model can be used in many applications, such as biosensors, optical filters, and optical switchers.

Unintentional Doping Mechanisms in GaAs/Si Films Grown by Metalorganic Chemical Vapor Deposition

To explain different doping effects in a buffer layer, thermally annealed interface, and upper epilayers of GaAs/Si films grown by Metalorganic Chemical Vapor Deposition (MOCVD), the behaviors of unintentional doping in GaAs/Si films are investigated in detail. A third doping mechanism of arsine impurity incorporation during the growth process of GaAs/Si films, apart from conventional mechanisms of gas phase reaction and diffusion from the silicon substrate, is proposed. The experimental results reveal that the doping behavior in the buffer layer studied is determined by the three types of doping mechanisms together. However in the thermally annealed interface and upper epilayers, the third doping mechanism is dominant. According to the third mechanism, the background carrier concentration in GaAs/Si films grown by MOCVD could be properly controlled through the arsine flow rate.

Growth and characterization of GaAs/InxGa1?xAs/GaAs axial nanowire heterostructures with symmetrical heterointerfaces

We report on the Au-assisted vapour—liquid—solid (VLS) growth of GaAs/InxGa1−xAs/GaAs (0.2 ≤ x ≤ 1) axial double-heterostructure nanowires on GaAs (111) B substrates via the metal-organic chemical vapor deposition (MOCVD) technique. The influence of the indium (In) content in an Au particle on the morphology of nanowires is investigated systematically. A short period of pre-introduced In precursor before the growth of InxGa1−xAs segment, coupled with a group III precursor interruption, is conducive to obtaining symmetrical heterointerfaces as well as the desired In/Ga ratio in the InxGa1−xAs section. The nanowire morphology, such as kinking and tapering, are thought to be related to the In composition in the catalyst alloy as well as the VLS growth mechanism.

Growth of Self-Catalyzed InP Nanowires by Metalorganic Chemical Vapour Deposition

The fabrication of self-catalyzed InP nanowires (NWs) is investigated under different growth conditions. Indium droplets induced by surface reconstruction act as nucleation sites for NW growth. Vertical standing NWs with uniform cross sections are obtained under optimized conditions. It is confirmed that the growth rate of NWs is strongly affected by the surface diffusion adatoms while contributions from the direct impingement of vapor species onto the In droplets can be negligible. The results indicate that the droplet acts as an adatom collector rather than a catalyst. Moreover, the diffusion flow rate of adatoms increases with time at the beginning of growth and stabilizes as the growth proceeds.

Fabrication and characterization of novel high-speed InGaAs/InP uni-traveling-carrier photodetector for high responsivity*

A top-illuminated circular mesa uni-traveling-carrier photodetector (UTC-PD) is proposed in this paper. By employing Gaussian graded doping in InGaAs absorption layer and InP depleted layer, the responsivity and high speed response characteristics of the device are optimized simultaneously. The responsivity up to 1.071 A/W (the external quantum efficiency of 86%) is obtained at 1550 nm with a 40-μm diameter device under 10-V reverse bias condition. Meanwhile, the dark current of 7.874 nA and the 3-dB bandwidth of 11 GHz are obtained with the same device at a reverse bias voltage of 3 V.