Positive solutions of the system for nth-order singular nonlocal boundary value problems

The paper deals with the existence and multiplicity of positive solutions to systems of nth-order singular nonlocal boundary value problems. The main tool used in the proof is fixed point index theory in cone. Some limit type conditions for ensuring the existence of positive solutions are given.     

Study of acoustic field modulation in the regenerator by double loudspeakers method

A model to modulate acoustic field in a regenerator of a thermoacoustic system by the double loudspeakers method is presented in this paper. The equations are derived for acoustic field modulation. They represent the relations among acoustic field (complex pressure p(0), complex velocity u(0), and acoustic impedance Z(0)), driving parameters of loudspeakers (voltage amplitude and its phase difference), and operating parameters involved in a matrix H (frequency, temperature of regenerator). The range of acoustic field is adjustable and limited by the maximal driving voltages of loudspeakers according to driving parameters. The range is simulated and analyzed in the amplitude-phase and complex coordinate planes for a given or variable H. The simulated results indicate that the range has its intrinsic characteristics. The expected acoustic field in a regenerator can be obtained feasibly by the modulation.     

Effects of nanostructure on optoelectronic properties of β phase polyoctylfluorene thin film

β phase polyoctylfluorene thin films were obtained by exposure to toluene vapor for various annealing times or dipping into a THF/methanol mixture. The photoluminescence and electroluminescence of PFO thin films decrease with increasing annealing time. Grazing incident X-ray diffraction of the thin films indicates that more and larger β phase crystallites will be generated in thin film exposed for longer time, which will lead to more defects that reduce photoluminescence and electroluminescence. By analyzing the mechanism of formation of β phase, we assume that the defects mainly come from the formation of out-of-plane crystalline structure. The assumption is confirmed by higher photoluminescence of β phase polyoctylfluorene thin film achieved by dipping into a THF/methanol mixture that has less out-of-plane crystalline structure.     

A full-duplex radio-over-fiber system based on dual quadrupling-frequency

We proposed and simulatedly demonstrated a novel full-duplex radio-over-fiber system using an external modulator and an optical interleaver to generate dual quadrupling-frequency optical millimeter waves for carrying two base station downstream data and wavelength reuse for uplink connection. The simulation results reveal that the power penalties for the downstream and upstream signals of both base stations are less than 0.8 dB. In this new scheme, the configuration of the both base stations is simplified further because there is no additional laser at two base stations. The frequency of local oscillator signal is reduced due to frequency quadrupling. The cost of the new system is largely reduced.     

Surface plasmon tunneling through a touching gold nanocylinder array on a thin gold film

The optical property of a structure composed of a touching gold nanocylinder array on a thin gold film is investigated using finite-difference time-domain (FDTD) method. It is discovered that the transmission behavior can be tuned by tuning the geometry of the structure. As the film thickness increases, the transmission mode associated with the localized surface plasmon resonance blue shifts accompanied with a decrease of magnitude and full width at half maximum, and a second transmission appear due to the interaction of the plasmons on the cylinder with their images induced on the film. The localized waveguide resonance diminishes but the second resonance peak is intensified and broadened noticeably with the separation of the cylinder array and film increase. The cylinder radius size influences the localized surface plasmon resonance mode obviously. These results may be helpful for the design of a novel optical device.     

Generation of atomic entangled states in a bi-mode cavity via adiabatic passage

We propose schemes to prepare atomic entangled states in a bi-mode cavity via stimulated Raman adiabatic passage (STIRAP) and fractional stimulated Raman adiabatic passage (f-STIRAP) techniques. Our scheme should be realizable in the near future because of the existence of all experimental ingredients. Our numerical simulation shows we can entangle the atoms with high fidelities by choosing proper laser pulses.     

Controlling of symmetric and asymmetric mode coupling in long-period fiber gratings singe-side induced by long-pulse CO2 laser

We report a simple fabricating approach to control the mode couplings in long-period fiber gratings (LPFGs) through side exposing fiber to long-pulse-10.6-μm laser from a cheap, internally modulated CO₂ tube. By tuning focused-spot size on fibers, not only circularly symmetric mode coupling but also asymmetric mode couplings can be effectively achieved. Simulation of mode profiles in grating cross-section with Finite Element Method (FEM), and LPFG-cladding etching experiment with hydrofluoric acid (HF), support our explanation that asymmetric mode coupling in LPFGs depends on local refractive-index (RI) change within an azimuthally thin cladding layer, resulted from large-spot method induced deep melt flow on fiber surface during CO₂ laser irradiation.     

Directional coupler design based on coupled cavity waveguides in photonic crystals

We propose a directional coupler design based on coupled cavity waveguide in photonic crystals. The plane wave expansion is used to give the dispersion of the coupled cavity waveguides and two parallel such waveguides. The couple length is got from the dispersion curves. Based on the research of the dispersion, we present a directional coupler and the transmission property is given. This structure is potentially important for highly efficient directional coupler in integrate optical circuit.     

Mott physics and topological phase transition in correlated dirac fermions

We investigate the interplay between the strong correlation and the spin-orbit coupling in the Kane-Mele-Hubbard model and obtain the qualitative phase diagram via the variational cluster approach. We identify, through an increase of the Hubbard U, the transition from the topological band insulator to either the spin liquid phase or the easy-plane antiferromagnetic insulating phase, depending on the strength of the spin-orbit coupling. A nontrivial evolution of the bulk bands in the topological quantum phase transition is also demonstrated.     

Cavity phase matching via an optical parametric oscillator consisting of a dielectric nonlinear crystal sheet

We experimentally demonstrate cavity phase matching for the first time using a sheet optical parametric oscillator which is made of an x-cut KTiOPO(4) crystal sheet. This microcavity presents 220 kW peak power capability for near-frequency-degenerate parametric outputs with up to 23.8% slope efficiency. It also features unique spectral characteristics such as single-longitudinal-mode and narrow linewidth. These attractive properties predict broad applications of such a mini-device, such as terahertz generation, photonic integration, spectroscopy, and quantum information, etc.