Liang Wen-Xi

Generation of surface electrons in femtosecond laser-solid interactions

The characteristics of hot electrons produced by p-polarized femtosecond laser-solid interactions are studied. The experimental results show that the outgoing electrons are mainly emitted in three directions: along the target surface, the normal direction and the laser backward direction. The electrons flowing along the target surface are due to the confinement of the electrostatic field and the surface magnetic field, while the electrons in the normal direction due to the resonant absorption.

Ultrafast Electron Diffraction with Spatiotemporal Resolution of Atomic Motion

Ultrafast electron diffraction (UED) is a rapidly advancing technique capable of recording the atomic-detail structural dynamics in real time. We report the establishment of the first UED system in China. Employing this UED apparatus, both the coherent and the concurrent thermal lattice motions in an aluminium thin-film, trigged by ultrafast laser heating, have been observed. These results demonstrate its ability to directly measure a sub-milli-angstrom lattice spacing change on a sub-picosecond time scale.

Interaction of femtosecond laser pulses with metal photocathode

The features of interaction of femtosecond laser pulses with photocathode are studied theoretically in this paper. The surface temperature of the metal cathode film while femtosecond laser pulses irradiation is studied with two-temperature model. With a simple photoelectric model we obtain the optimum metal film thickness for the back-illuminated photocathode. The generated ultrashort photocurrent pulses are strongly dependent on the temperature of the electron gas and the lattice during the femtosecond laser pulse irradiation on the photocathode.

Triggering and guiding of high voltage discharge by using femtosecond laser filaments with different parameters

This paper demonstrates the triggering and guiding of the stationary high voltage (HV) discharges at 5–40 kV by using plasma filaments generated by femtosecond laser pulses in air. A significant reduction of the breakdown voltage threshold due to the pre-ionization of the air gap by laser filamentation is observed. The discharge experiments are performed by using laser pulses with different energy from 15–60 mJ. The electron density of filaments is detected by sonography method. The influence of the electron density of laser filaments on the triggering and guiding HV discharge is experimentally investigated. The results have shown that the behaviour of plasma filaments can strongly affect the efficiency of triggering and guiding HV discharge.

Designing a diffractive optical element for controlling the beam profile in a three-dimensional space using the simulated annealing algorithm

We have designed a spatially quantized diffractive optical element (DOE) for controlling the beam profile in a three-dimensional space with the help of the simulated annealing (SA) algorithm. In this paper, we investigate the annealing schedule and the neighbourhood which are the deterministic parameters of the process that warrant the quality of the SA algorithm. The algorithm is employed to solve the discrete stochastic optimization problem of the design of a DOE. The objective function which constrains the optimization is also studied. The computed results demonstrate that the procedure of the algorithm converges stably to an optimal solution close to the global optimum with an acceptable computing time. The results meet the design requirement well and are applicable.