Xia Yu-Xing

Second-harmonic generation with broadened flattop bandwidth in aperiodic domain-inverted gratings

We report on a theoretical analysis for the broadened bandwidth of the fundamental waves in aperiodic gratings. The sequences and the length of the domains are optimized to realize the pre-designed wide bandwidth in the quasi-phase-matched (QPM) second-harmonic generation (SHG) by the simulated annealing (SA) method. About 3 nm prescribed flattop bandwidth as the full width at 95% maximum with 25% reduction of the nonlinear coefficient relative to that of a perfectly periodic QPM grating are obtained. Domains overgrown owing to the room-temperature electric poling technique show little influence on the broad bandwidth of the fundamental waves.

Temporal Behaviour of Harmonics from One-Dimensional H2+ in an Ultrashort Laser Pulse

With the method of wavelet transform, we consider the temporal behaviour of high-order harmonic generation from one-dimensional H2+ exposed to an ultrashort laser pulse with a duration of tens of femtoseconds. The results, which are calculated by numerically solving the corresponding time-dependent Schrodinger equation with the split-operator method in the non-Born-Oppenheimer approach, show that: (1) the high-order harmonics in the cut-off range emitted as a train of pulses have better coherence than those in the plateau; (2) the harmonics are emitted early in time when the intensity of the laser pulse increases.

Large Relative Raman Shift for Molecules Adsorbed on Metallic Nano-particles

The enhancement of two order-of-magnitudes is observed in surface-enhanced Raman spectroscopy (SERS) of gases (CO, C2H2, C2H4, etc) adsorbed on nitric acid-roughened metal foil In addition, some Raman lines of gases adsorbed on these active substrates show larger frequency shifts and linewidth broadening, compared with the Raman spectroscopy of free gases. Using the two-oscillator electromagnetic model, we explain this phenomenon. It is related to the large non-regular particles on the active substrate we prepared. It is found that the parameters of the surface-plasmon dispersion, the distance of molecules from the surface and the radius of particles play crucial roles on the relative large Raman shifts.

Diagnosis of Multiple Gases Separated from Transformer Oil Using Cavity-Enhanced Raman Spectroscopy

The Raman signal of gas molecules is very weak due to its small scattering cross section. Here, a near-confocal cavity-enhanced Raman detection system is demonstrated. In the cavity, a high power light of 9W is achieved by using a cw 200 mW 532 nm laser, which greatly enhances the detection sensitivity of gas species. A photomultiplier tube connected to a spectrometer is used as the detection system. The Raman spectra of the mixed gases separated from transformer oil has been observed. The relationship of absolute Raman intensity and gas pressure is also obtained. To our knowledge, this is the first Raman system to detect the gases separated from transformer oil.

Optimized Second Harmonic Generation of Femtosecond Pulse by Phase-Blanking Effect in Aperiodically Optical Superlattice

In order to minimize the effect of the unconsidered frequency components on the generated compression pulse, the phasing-blanking effect is taken into account of designing the one-dimensional aperiodic domain reversal structure. Hierarchic genetic algorithm for the design of a domain reversal grating to modulate the spectrum and phase of the generated SH pulse simultaneously are presented. Our simulation shows that the quality of an output pulse is fairly improved.

Electrical Properties of Nanostructured Magnetic Colloid and Influence of Magnetic Field

We investigate the electrical properties of the nanostructured magnetic colloid without and with magnetic field. The competition between the directional motion of the charged magnetic nanoparticles and other minor nonmagnetic impurities (also small amount of ions) under applied voltage and their random orientation due to thermal activation is implemented to elaborate the electrically conduction mechanism under zero magnetic field. Two equivalent electric circuits are employed for explaining the charging and discharging processes. The tunnelling conduction mechanism upon application of externally magnetic field may exist in the nanostructured magnetic colloid. The alternation of the two conduction mechanisms accounts for the current spikes when the magnetic field is switched on or off. This work presents the peculiar electrical phenomena of the magnetically colloidal system.

Evolution of Dark Spatial Soliton in Quasi-phase-matched Quadratic Media

We theoretically investigate the evolvement of dark spatial soliton with cascading quadratic nonlinearity in quasi-phase-matched second harmonic generation. It is shown that the dark solitary wave can propagate stably when background intensity is large enough, in which diffraction of beam can be balanced by the cascading quadratic nonlinearity. We also analyze the influence of phase-mismatch on the stability of dark soliton propagation.

Two-Colour Effect on the Ionization and Harmonic Generation of One-Dimensional He+

Using the one-dimensional ion mode, we theoretically investigate the dynamics ionization process and the high-order harmonic generation of the He+ subject to different kinds of two-colour laser pulses with the same fundamental frequency but various harmonic frequencies. It is found that from the results calculated by numerically solving the time-dependent Schrodinger equation, the two-colour laser pulse with the fundamental and harmonic frequencies enables the power spectrum of high-order harmonic generation to increase by a few orders of magnitude and the yield of He2+ to enhance significantly when the harmonic frequency of the laser pulse is close to the energy interval between the ground state and the first excited state of He+; even the harmonic field added into the pulse is much weaker than the fundamental one.

Recovery of Graded Index Profile of Planar Waveguide by Cubic Spline Function

A method is proposed to recover the refractive index profile of graded waveguide from the effective indices by a cubic spline interpolation function. Numerical analysis of several typical index distributions show that the refractive index profile can be reconstructed closely to its exact profile by the presented interpolation model.