Wenyong Su

Down-conversion luminescence from (Ce, Yb) co-doped oxygen-rich silicon oxides

We have studied down-conversion photoluminescence (PL) from (Ce, Yb) co-doped “oxygen rich” silicon oxide films prepared by sputtering and annealing. The Ce3+ ∼510 nm PL is sensitive to the Ce concentration of the films and is much stronger for 3 at. % Ce than for 2 at. % Ce after annealing at 1200 °C. The PL emission and excitation spectroscopy results indicate that the excitation of Yb3+ is mainly through an energy transfer from Ce3+ to Yb3+, oxide defects also play a role in the excitation of Yb3+ after lower temperature (∼800 °C) annealing. The Ce3+ 510 nm photon excites mostly only one Yb3+ 980 nm photon. Temperature-dependent PL measurements suggest that the energy transfer from Ce3+ to Yb3+ is partly thermally activated.

Electron dynamics in Na4 clusters under femtosecond laser irradiation

The electron dynamics in metal clusters under femtosecond laser irradiation yield excited nonlinear phenomena under the control of laser parameters. In this study, metal clusters Na4 as an example under femtosecond laser irradiation have been observed within the framework of time-dependent density functional theory molecular dynamics (TDDFT-MD), which self-consistently couples a quantum-mechanical TDDFT treatment of valence electrons with a classical molecular dynamics description of ionic cores. The specific excitation frequencies and polarization directions of the laser have been chosen to investigate the behavior of the dipole response and ionization process. We found that it is necessary to explore the electron dynamics of metal clusters by matching the laser frequency and polarization direction. In addition, resonant dipole oscillations induced by suitable laser parameters show pronounced enhancement of the ionization behavior.

The photoluminescence and structural properties of (Ce, Yb) co-doped silicon oxides after high temperature annealing

We studied the photoluminescence (PL) and structural properties of Ce and Yb co-doped silicon oxide films after high temperature annealing. The PL spectra of Ce3+ and Yb3+ ions were sensitive to the structural variation of the films, and the Yb PL intensities were significantly enhanced especially upon 1200 °C annealing. X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy, indicated that rare earth silicates and the CeO2 phase had formed in the oxides. The proportions of the phases varied with the “nominal Si-richness” of the films. Energy transfer from the excited Ce3+ to Yb3+ can be inferred from both PL excitation and decay spectra.

Theoretical investigation of the electron capture and loss processes in the collisions of He2+ + Ne

Based on the time-dependent density functional theory, a method is developed to study ion-atom collision dynamics, which self-consistently couples the quantum mechanical description of electron dynamics with the classical treatment of the ion motion. Employing real-time and real-space method, the coordinate space translation technique is introduced to allow one to focus on the region of target or projectile depending on the actual concerned process. The benchmark calculations are performed for the collisions of He(2+) + Ne, and the time evolution of electron density distribution is monitored, which provides interesting details of the interaction dynamics between the electrons and ion cores. The cross sections of single and many electron capture and loss have been calculated in the energy range of 1-1000 keV/amu, and the results show a good agreement with the available experiments over a wide range of impact energies.

Strong near band edge emission of (Ce, Yb) co-doped ZnO thin films after high temperature annealing

We studied the photoluminescence (PL) properties of (Ce + Yb) co-doped ZnO thin films as a function of high temperature annealing. The films were fabricated by magnetron sputtering. After 1000-1100°C annealing, the near band edge (NBE) emissions of the films were dozens to a hundred times stronger than that of undoped ZnO, while the Yb3+ emission (~980 nm) was quite weak, indicating that energy transfers from the ZnO host to Yb3+ ions in the films were not efficient. X-ray diffraction analysis and scanning electron microscopy observations demonstrated that the (Ce + Yb) co-doping had a large effect on the morphology and crystallinity of the films. The crystallinity enhancement of the films is considered to be the main reason for the strong NBE enhancements of the co-doped ZnO films.

Theoretical study of small silicon clusters: Optical absorption spectra and dipole polarizabilities of Si n (n=2−7)

The optical absorption spectra of Sin (n=2−7) clusters have been systematically calculated within the framework of time-dependent density functional theory (TDDFT). In addition, calculated dipole polarizabilities of Sin (n=2−7) clusters based on density functional theory (DFT) are very good agreement with other theoretical results. It is suggested that the geometry structure of clusters can be identified by optical absorption spectra and dipole polarizabilities.

The Photoluminescence from (Eu, Yb) Co-Doped Silicon-Rich Si Oxides

We report on photoluminescence (PL) properties of europium (Eu) and ytterbium (Yb) co-doped silicon oxide films with different Si excess. After annealing the films in N2, strong PL were observed from Eu and Yb3+ ions and their intensities are correlated. The PL intensity of Eu is mainly from 3+ for no and relatively low temperature anneals (<900 °C) while the Eu2+ emission is dominating for annealing at 1000 °C or above in the co-doped Si-rich oxide films. Transmission electron microscopy shows amorphous (Eu, Yb, Si, O)-containing precipitates in the Si-rich oxide during 1000-1200 °C annealing and these precipitates are considered to be responsible for the Eu2+-related luminescence.

QUANTUM CHEMICAL STUDY OF ELECTRONIC TRANSPORT PROPERTIES OF BIPHENYLDITHIOL MOLECULAR JUNCTION

Applying the elastic scattering Greens function theory in combination with the frontier molecular orbital theory for describing the surface-molecule coupling and hybrid density-function theory for geometrical and electronic structure calculations, we successfully reproduce the current–voltage properties of the 4,4-biphenyldithiol molecular junction, which has been measured using a lock-in technique by Lee et al.1 We also analyze the conductance characteristics of different dimensional electrodes in contact with the molecular device, and we think that the one-dimensional formula is consistent with the experiment, and that the interaction between neighboring molecules will decrease the molecular orbital energies and draw the conductance peak positions closer to experimental results.

Optical limiting performances of the methyl-red-dye-doped nematic liquid crystal films

The optical limiting characteristics of the methyl-red dye doped nematic liquid crystal (NLC) films have been tested, and an interesting dark spot phenomenon and a dynamic self-action diffraction ring pattern in the far field behind the sample cell have been observed. The time sequence of the dark spot formation and the dynamic self-action diffractino ring apperaance has also been investigated. We found that, with increasing the energy impinging on the sample cell, the dark spot accompanied by a single outer bright ring always appears and lasts until the multiple diffraction rings form. We also found that the dark spot with a single outer bright ring in the far field corresponds to the nonlinear section of the optical limiting characteristic curves of the doped NLC fimls, while the onset of the multiple diffraction rings corresponds to the input damage threshold of the doped NLC film limiters. Our experimental results show that the optical limiter based on the methyl-red dye doped NLC films has a quite low optical limiting threshold and an unusually low clamped output to eye safety. In this paper, we will present how we tested the optical limiting characteristics of the methyl-red dye doped NLC films and what kind of the typical test results we obtained in the experiments. We will theoretically analyze the optical limiting mechanisms of the methyl-red dye doped NLC films. We will also discuss the advantages and some limitations of the methyl-red dye doped NLC films. We will also discuss the advantages and some limitations of the methyl-red dye liquid crystal limiters, and propose several possible solutions to these limitations.

Comparing two evidences of quantum chaos

Abstract We compute the evolution of Wigner distribution in the case of dissipation in a classical chaotic system to analyze the quantum chaos. We find that the quantum chaos can be denoted in two ways. First, we can use the evolutions of χ2(t) and h(ρW,t) to denote quantum chaos: in regular case, χ2(t) and h(ρW,t) oscillate with constant amplitude; in chaotic case, they exponentially increase to equilibrium values. Second, when the coupling of environment exists, we can also use the evolution the Renyi entropy to denote chaos: in regular case, Renyi entropy decreases linearly; in chaotic case, Renyi entropy decreases exponentially. Our results show that besides coupling, quantum coherence can also be disturbed by chaos.