arXiv subjects

Wei Hu

Publications and source records attributed to Wei Hu.

At least 379 records · Page 21Linked to original sources

arXiv2014

A digital CDS technique and the performance testing

Readout noise is a critical parameter for characterizing the performance of charge-coupled devices (CCDs), which can be greatly reduced by the correlated double sampling (CDS) circuit. However, conventional CDS circuit inevitably introduces new noises since it consists of several active analog components such as operational amplifiers. This paper proposes a digital CDS circuit technique, which transforms the pre-amplified CCD signal into a train of digital presentations by a high-speed data acquisition card directly without the noisy CDS circuit first, then implement the digital CDS algorithm through numerical method. The readout noise of 3.3 e$^{-}$ and the energy resolution of 121 eV@5.9keV can be achieved via the digital CDS technique.

physics.ins-det↗

arXiv2014

Electronic Structure of Large-Scale Graphene Nanoflakes

With the help of the recently developed SIESTA-PEXSI method [J. Phys.: Condens. Matter \textbf{26}, 305503 (2014)], we perform Kohn-Sham density functional theory (DFT) calculations to study the stability and electronic structure of hexagonal graphene nanoflakes (GNFs) with up to 11,700 atoms. We find the electronic properties of GNFs, including their cohesive energy, HOMO-LUMO energy gap, edge states and aromaticity, depend sensitively on the type of edges (ACGNFs and ZZGNFs), size and the number of electrons. We observe that, due to the edge-induced strain effect in ACGNFs, large-scale ACGNFs' cohesive energy decreases as their size increases. This trend does not hold for ZZGNFs due to the presence of many edge states in ZZGNFs. We find that the energy gaps $E_g$ of GNFs all decay with respect to $1/L$, where $L$ is the size of the GNF, in a linear fashion. But as their size increases, ZZGNFs exhibit more localized edge states. We believe the presence of these states makes their gap decrease more rapidly. In particular, when $L$ is larger than 6.40 $nm$, we find that ZZGNFs exhibit metallic characteristics. Furthermore, we find that the aromatic structures of GNFs appear to depend only on whether the system has $4N$ or $4N+2$ electrons, where $N$ is an integer.

cond-mat.mtrl-sci↗

arXiv2014

Aging research of the LAB-based liquid scintillator in stainless steel container

Stainless steel is the material used for the storage vessels and piping systems of LAB-based liquid scintillator in JUNO experiment. Aging is recognized as one of the main degradation mechanisms affecting the properties of liquid scintillator. LAB-based liquid scintillator aging experiments were carried out in different material of containers (type 316 and 304 stainless steel and glass) at two different temperature (40 and 25 degrees Celsius). For the continuous liquid scintillator properties tests, the light yield and the absorption spectrum are nearly the same as that of the unaged one. The attenuation length of the aged samples is 6%~12% shorter than that of the unaged one. But the concentration of element Fe in the LAB-based liquid scintillator does not show a clear change. So the self aging has small effect on liquid scintillator, as well as the stainless steel impurity quenching. Type 316 and 304 stainless steel can be used as LAB-based liquid scintillator vessel, transportation pipeline material.

physics.ins-det↗

arXiv2014

Real-time Decolorization using Dominant Colors

Decolorization is the process to convert a color image or video to its grayscale version, and it has received great attention in recent years. An ideal decolorization algorithm should preserve the original color contrast as much as possible. Meanwhile, it should provide the final decolorized result as fast as possible. However, most of the current methods are suffering from either unsatisfied color information preservation or high computational cost, limiting their application value. In this paper, a simple but effective technique is proposed for real-time decolorization. Based on the typical rgb2gray() color conversion model, which produces a grayscale image by linearly combining R, G, and B channels, we propose a dominant color hypothesis and a corresponding distance measurement metric to evaluate the quality of grayscale conversion. The local optimum scheme provides several "good" candidates in a confidence interval, from which the "best" result can be extracted. Experimental results demonstrate that remarkable simplicity of the proposed method facilitates the process of high resolution images and videos in real-time using a common CPU.

cs.GR↗

arXiv2014

Proton irradiation effect on SCDs

The Low Energy X-ray Telescope is a main payload on the Hard X-ray Modulation Telescope satellite. The swept charge device is selected for the Low Energy X-ray Telescope. As swept charge devices are sensitive to proton irradiation, irradiation test was carried out on the HI-13 accelerator at the China Institute of Atomic Energy. The beam energy was measured to be 10 MeV at the SCD. The proton fluence delivered to the SCD was $3\times10^{8}\mathrm{protons}/\mathrm{cm}^{2}$ over two hours. It is concluded that the proton irradiation affects both the dark current and the charge transfer inefficiency of the SCD through comparing the performance both before and after the irradiation. The energy resolution of the proton-irradiated SCD is 212 eV@5.9 keV at $-60\,^{\circ}\mathrm{C}$, while it before irradiated is 134 eV. Moreover, better performance can be reached by lowering the operating temperature of the SCD on orbit.

physics.ins-det↗

arXiv2014

Nematicons in liquid crystals with negative dielectric anisotropy

We report a theoretical and experimental work on the nematicon in the planar cell containing the nematic liquid crystal with negative dielectric anisotropy, aligned homeotropically in the presence of an externally applied voltage. The formation of the soliton is resulted from the balance between the linear difrraction and the nonlocal nonlinearity due to molecular reorientation.

physics.optics↗

arXiv2014

Measurement of the liquid scintillator nonlinear energy response to electron

Nonlinearity of the liquid scintillator energy response is a key to measuring the neutrino energy spectrum in reactor neutrino experiments such as Daya Bay and JUNO. We measured in laboratory the nonlinearity of the Linear Alkyl Benzene based liquid scintillator, which is used in Daya Bay and will be used in JUNO, via Compton scattering process. By tagging the scattered gamma from the liquid scintillator sample simultaneously at seven angles, the instability of the system was largely cancelled. The accurately measured nonlinearity will improve the precision of the $θ_{13}$, $Δm^2$, and reactor neutrino spectrum measurements at Daya Bay.

physics.ins-det↗

arXiv2013

A Constant Factor Approximation Algorithm for Fault-Tolerant k-Median

In this paper, we consider the fault-tolerant $k$-median problem and give the \emph{first} constant factor approximation algorithm for it. In the fault-tolerant generalization of classical $k$-median problem, each client $j$ needs to be assigned to at least $r_j \ge 1$ distinct open facilities. The service cost of $j$ is the sum of its distances to the $r_j$ facilities, and the $k$-median constraint restricts the number of open facilities to at most $k$. Previously, a constant factor was known only for the special case when all $r_j$s are the same, and a logarithmic approximation ratio for the general case. In addition, we present the first polynomial time algorithm for the fault-tolerant $k$-median problem on a path or a HST by showing that the corresponding LP always has an integral optimal solution. We also consider the fault-tolerant facility location problem, where the service cost of $j$ can be a weighted sum of its distance to the $r_j$ facilities. We give a simple constant factor approximation algorithm, generalizing several previous results which only work for nonincreasing weight vectors.

cs.DS↗

arXiv2012

Large phase shift of spatial soliton in lead glass by cross-phase modulation in pump-signal geometry

We investigate the large phase shifts of the bi-color spatial soliton pair in a cylindrical lead glass rod. The theoretical study suggests a synchronous propagation of a strong pump beam and a weak signal beam under the required initial condition. We experimentally obtain a $π$ phase shift of the signal beam by changing the power of the pump beam by about 14 mW around the soliton critical power, which agrees qualitatively with our theoretical result. The ratio of the phase shift rate of the signal soliton to that of the pump soliton shows a close match to the theoretical estimation.

physics.optics↗

arXiv2012

Defect Solitons in Parity-Time Symmetric Optical Lattices with Nonlocal Nonlinearity

The existence and stability of defect solitons in parity-time (PT) symmetric optical lattices with nonlocal nonlinearity are reported. It is found that nonlocality can expand the stability region of defect solitons. For positive or zero defects, fundamental and dipole solitons can exist stably in the semi-infinite gap and the first gap, respectively. For negative defects, fundamental solitons can be stable in both the semi-infinite gap and the first gap, whereas dipole solitons are unstable in the first gap. There exist a maximum degree of nonlocal nonlinearity, above which the fundamental solitons in the semi-infinite gap and the dipole solitons in the first gap do not exist for negative defects. The influence of the imaginary part of the PT-symmetric potentials on soliton stability is given. When the modulation depth of the PT-symmetric lattices is small, defect solitons can be stable for positive and zero defects, even if the PT-symmetric potential is above the phase transition point.

physics.optics↗

arXiv2012

Defect solitons supported by nonlocal PT symmetric superlattices

The existence and stability of defect solitons supported by parity-time (PT) symmetric superlattices with nonlocal nonlinearity are investigated. In the semi-infinite gap, in-phase solitons are found to exist stably for positive or zero defects, but can not exist in the presence of negative defects with strong nonlocality. In the first gap, out-of-phase solitons are stable for positive or zero defects, whereas in-phase solitons are stable for negative defects. The dependence of soliton stabilities on modulation depth of the PT potentials is studied. It is interesting that solitons can exist stably for positive and zero defects when the PT potentials are above the phase transition points.

physics.optics↗

arXiv2012

A variational approach to Schroedinger equation with parity-time symmetry Gaussian complex potential

A variational technique is established to deal with the Schrodinger equation with parity-time(PT) symmetric Gaussian complex potential. The method is extended to the linear and self-focusing and defocusing nonlinear cases. Some unusual properties in PT systems such as transverse power flow and PT breaking points can be analyzed by this method. Following numerical simulations, the analytical results are in good agreement with the numerical results.

nlin.PS↗

arXiv2011

Solitons in thermal media with periodic modulation of linear refractive index

We address the existence and properties of solitons in thermal media with periodic modulation of linear refractive index. Many kinds of solitons in such optical lattices, including symmetric and antisymmetric lattices, are found under different conditions. We study the influence of the refractive index difference between two different layers on solitons. It is also found that there do not exist cutoff value of propagation constant and soliton power for shifted lattice solitons. In addition, the solitons launched away from their stationary position may propagate without oscillation when the confinement from lattices is strong.

physics.optics↗

arXiv2011

Solitons supported by complex PT symmetric Gaussian potentials

The existence and stability of fundamental, dipole, and tripole solitons in Kerr nonlinear media with parity-time symmetric Gaussian complex potentials are reported. Fundamental solitons are stable not only in deep potentials but also in shallow potentials. Dipole and tripole solitons are stable only in deep potentials, and tripole solitons are stable in deeper potentials than for dipole solitons. The stable regions of solitons increase with increasing potential depth. The power of solitons increases with increasing propagation constant or decreasing modulation depth of the potentials.

physics.optics↗

arXiv2011

Multiple-type solutions for multipole interface solitons in thermal nonlinear media

We address the existence of multipole interface solitons in one-dimensional thermal nonlinear media with a step in the linear refractive index at the sample center. It is found that there exist two types of solutions for tripole and quadrupole interface solitons. The two types of interface solitons have different profiles, beam widths, mass centers, and stability regions. For a given propagation constant, only one type of interface soliton is proved to be stable, while the other type can also survive over a long distance. In addition, three types of solutions for fifth-order interface solitons are found.

physics.optics↗

arXiv2011

Bound states of spatial optical dark/gray solitons in nonlocal media

It is shown that three or more dark/gray solitons can form bound states in nonlocal media. More over dark/gray solitons can form bound states in several balance distances. Numerical simulations indicate that some of such bound states are unstable and will decay into a group of fundamental solitons, while others may be stable. There exist degenerate bound states with the same velocity, Hamiltonian, particle numbers and momentum but decaying in different ways and having different lifetimes.

physics.optics↗

arXiv2011

Interface solitons in thermal nonlinear media

We demonstrate the existence of fundamental and dipole interface solitons in one-dimensional thermal nonlinear media with a step in linear refractive index. Fundamental interface solitons are found to be always stable and the stability of dipole interface solitons depends on the difference in linear refractive index. The mass center of interface solitons always locates in the side with higher index. Two intensity peaks of dipole interface solitons are unequal except some specific conditions, which is different from their counterparts in uniform thermal media.

physics.optics↗

arXiv2011

Large phase shift of spatial solitons in lead glass

The phenomenon of the large phase shift of the strongly nonlocal spatial optical soliton was predicted by Guo et al. within the phenomenological framework [Q. Guo, et al., Phys. Rev. E 69, 016602 (2004)], but has not been experimentally confirmed so far. We theoretically and experimentally investigate the large phase shift of that propagating in the lead glass. It is verified that the change of the optical power carried by the optical beam about 10 mW around the critical power for the soliton can lead to a π phase shift, which would be of its potential in the application of all-optical switchings.

nlin.PS↗