Duration-Dependent Asymmetry in Photoionization of H atoms in Few-Cycle Laser Pulses

The photoionization of H atoms irradiated by few-cycle laser pulses is studied numerically. The variations of the total ionization, the partial ionizations in opposite directions, and the corresponding asymmetry with the carrier-envelope phase in several pulse durations are obtained. We find that besides a stronger modulation on the partial ionizations, the change of pulse duration leads to a shift along carrier-envelope （CE） phase in the calculated signals. The phase shift arises from the nonlinear property of ionization and relates closely to the Coulomb attraction of the parent ion to the ionized electron. Our calculations show good agreement with the experimental observation under similar conditions.     

Effect of Laser Pulse Shape on the Wake Field in Relativistic Intense Laser—Plasma Interaction

We study the effect of laser pulse shape on the plasma wake field driven by an intense laser beam.It is found that the modified asymmetric laser pulse shape with a sharply rising front and broad falling back,due to some nonlinear interactions of an intense laser pulse with plasmas,can significantly enhance the magnitude of the wake field.     

Interference Dynamics of Hydrogen Atoms in High—Frequency Dichromatic Laser Fields

We investigate the ionization and high-order harmonic generation of a hydrogen atom in high-frequency (several atomic units) super strong (up to several tens of atomic units) dichromatic laser fields.An effective iterative method in the framework of high-frequency Floquet theory is used in the calculations.We have considered two kinds of dichromatic laser field,i.e.1ω-2ω and 1ω-3ω.We find that,in both the cases,the ionization and high-order harmonic generation show evident dependence on the relative phase and strength of the additional harmonic field.The dynamical origin of these interference effects is also discussed.     

Investigation of Electron Injection Parameters in Vacuum Laser Acceleration

We investigate the parameters such as the energy gains, comparisons for linearly and circularly polarized laser fields, the electron incident polarization azimuth angle, the electron incident crossing angle, and the electronincident momentum, etc., which are crucial for experimental test of the vacuum laser acceleration scheme in capture and acceleration scenario (CAS) [see, e.g., Phys. Rev. E 66 (2002) 066501]. Physical explanation of these features has been presented based on the CAS scheme. The results show that all those parameters are in the reasonable ranges obtainable by presently experimental facilities.     

Modelling of Laser—Generated Guided Waves in Bonded Plates with a Weak Interface by the Two—Layer Normal Mode Expansion Method

The two-layer normal mode expansion method is employed to analyse the laser-generated elastic guided waves in bonded plates accounting for the adhesive bond layer in terms of a weak interface with the spring model.The sensitivity of dispersion and transient characteristics of the guided wave to the stiffness coefficients characterizing the cohesive quality is analysed in details.This Method provides a new prominsing way for the characterization of the cohesive quality in bonded plates.     

Compression of Intense Laser Pulse through Filamentation in an Argon-Filled Cell

We demonstrate a pulse compression technique through filamentation in an argon-filled cell. By using a pair of chirped mirrors for dispersion compensation, we have successfully compressed the 53 fs pulse to 15 fs with good spatial qualities and good pulse stability. The total transmitted efficiency is more than 75%. The influence of the experiment parameters to the compressed pulses is also studied experhnentally.     

Phase Dependence of Few—Cycle Pulsed Laser Propagation in a Two—Level Atom Medium

The phase-dependent feature of few-cycle pulsed laser propagation in a resonant two-level atom medium in demonstrated by solving the full Maxwell-Bloch equations.Even in the perturbative region,the propagating carrier field and the corresponding spectra of the few-cycle pulsed laser are sensitive to the initial phase due to self-phase modulation.For the larger pulse area,the fact that the carrier-wave reshaping comes from the carrierwave Rabi flopping is also responsible for this sensitivity,and the phase-dependent feature is more evident.     

Observation of Two Quasi—Static Wings of Laser—Induced Collision Energy Transfer

We investigate experimentally laser-induced collision energy transfer for transitions both from Eu(6s6p)^8 P9/2 to Sr(5s10s)^1So and from Eu(6s6p)^s-P7/2 to Sr(5s8d)^1D2.Two quasi-static wings are observed in their excited function spectra.By using two dipole-dipole interactions,a spectral line from numerical calculation is consistent with the experimental line in two quasi-static wings.     

Optical Measuring Method for Quality Monitoring Purpose in Laser Welding of Tin－plates

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Generation of Helical and Axial Magnetic Fields by the Relativistic Laser Pulses in Under—dense Plasma：Three—Dimensional Particle—in—Cell Simulation

The quasi-static magnetic fields created in the interaction of relativistic laser pulses with under-dense plasmas have been investigated by three-dimensional particle-in-cell simulation.The relativistic ponderomotive force can drive and intense electron current in the laser propagation direction,which is responsible for the generation of a helical magnetic field.The axial magnetic field results from a difference bean of wave-wave,which drives a solenoidal current.In particular,the physical significance of the kinetic model for the generation of the axial magnetic field is discussed.     

Fraction and Temperature of Hot Electrons Generated by Laser—Plasma Interaction at the shenguang—Ⅱ Laser Facility

We have developed a three-dimensional electron-photon Monte Carlo transmission (MCT) code.The conversiontransmissivity coefficient of hot electron x-ray,ηct,is analysed theoretically and simulated numerically using the MCT code.Experiments of disc and cylindrical targets have been performed at the Shenguang-Ⅱ laser facility.A Multi-channel filter-fluorescence spectrometer was used to obtain the hard x-ray spectrum.The temperature and energy of hot electrons are induced from the hard x-ray spectrum and the ηct value.The average energy fraction of hot electrons is 14% of incident laser energy for the 1ωNd laser and the spherical target with an average temperature of hot electrons of 36keV,while it is 15% for the 1ωNd laser and the cavity target with an average temperature of 54keV.We have also obtained the results of 5.6% and 13.3keV for the 3ωNd laser and the disc target,4.9% and 17.9keV for the 3ωNd laser and the half cavity target with a thin wall,and 2.1% and 22.4keV for those with a thick wall.The experimental results agree with theory and simulation.     

Characteristics of Plasma Shock Waves Generated in the Pulsed Laser Ablation Process

We modify the Sedov theory to describe plasma shock waves generated in a pulsed laser ablating process,We also study the propagation characteristics of plasma shock during the preparation process of functional thin films deposited by a pulsed laser,In particular,we discuss in detail the temporal behaviour of energy causing the difference of the propagation characteristics between the plasma shock wave and the ideal shock wave in the point explosion model.Under the same experimental conditions,the theoretical results calculated with our modified Sedov theory are in good agreement with the existing experimental data.     

Laser—Surface Interaction of Compound Semiconductors—Desorption Induced by Electronic Excitation of Surface States

In this talk a review is given of currently carricd out studies on laser-induced desorption from surfaces of compound semiconductors.The reviewincludes only the non-thermal processes.which can be detected by eitherirradiation with laser of subgap photon energies or with extremely low-intensity pulsed lasers.It is emphasized also that laser induced non-thermaldesorption is a powerful technique for surface characterization and surfacecleaning.     

Development in Laser Induced Extrinsic Absorption Damage Mechanism of Dielectric Films

Absorption of host and the temperature-dependence of absorption coefficient have been considered in evaluating temperature distribution in films, when laser pulse irradiates on films. Absorption of dielectric materials experience three stages with the increase of temperature： multi-photon absorption; single photon absorption; metallic absorption. These different absorption mechanisms correspond to different band gap energies of materials, which will decrease when the temperature of materials increases. Evaluating results indicate that absorption of host increases rapidly when the laser pulse will be over. If absorption of host and the temperature-dependence of absorption are considered, the maximal temperatures in films will be increased by a factor of four.     

Effect of Incident Intensity on Films Growth in Pulsed Laser Deposition

Incident intensity, defined by the amount of particles deposited per pulse, is an important parameter in the film growth process of pulsed laser deposition （PLD）. Different from previous models, we investigate the irreversible and reversible growth processes by using a kinetic Monte Carlo method and find that island density and film morphology strongly depend on pulse intensity. At higher pulse intensities, lots of adatoms instantaneously diffuse on the substrate surface, and then nucleation easily occurs between the moving adatoms resulting in more smaller-size islands. In contrast, at the lower pulse intensities, nucleation event occurs preferentially between the single adatom and existing islands rather than forming new islands, and therefore the average island size becomes larger in this case. Additionally, our results show that substrate temperature plays an important role in film growth. In particular, it can determine the films shape and weaken the effect of pulse intensity on film growth at the lower temperatures by controlling the mobility rate of atoms. Our results can match the related theoretical and experimental results.     

Laser spectroscopy of jet－cooled CuF in visible region

The laser-induced fluorescence excitation spectra of jet-cooled CuF have been recorded in the range of 19000－21470cm^{-1}, in which the CuF radicals were produced by the reaction of SF_6 with copper atoms from a dc discharge-sputtering source under supersonic jet conditions. Eight observed vibronic transition bands have been assigned as the transition from the ground state X(^1Σ^+) to B(^1Σ^+), C(^1Π) and an unreported upper state. The rotational structure of all observed bands has been analysed at the estimated rotational temperature 80K. We determined the newly observed band to be the (^1Π, v=1)－X(^1Σ^+v=0) transition according to the character of the rotational structure and the isotopic shift analysis. In addition, the lifetimes of the states involved in these bands were measured.     

Numerical Simulation of Ultrafast Laser Pulse Propagation in Tenuous Plasmas： Envelope Evolving and Modulation

We propose an effective and useful numerical simulation scheme for the investigation of the ultra-fast laser pulses in tenuous plasmas. The accuracy of the method is tested by numerical examples. We check some special examples to investigate the laser envelope evolving and modulation in plasmas. Asymmetric two-peak modulation structure is found and its underlying physics is analyzed. The advantages and shortages of the method are also discussed.