Investigation Of Ultrafast Electronic Behavior In Metal Irradiated By Femtosecond Laser

The interaction between ultrashort pulse laser and solid surface is the hot issue which experts and scholars have pay close attention for a long time. With the rapid development of laser technique, laser irradiate material is no longer only belongs to the laboratory which has been applied to many practical applications such as:biolo-gy, medical treatment, national defense, archaeology, production and manufacturing, etc. Corresponding to the increasing requirements of practical technique, comprehen-sive and thorough understanding of the interaction process between laser and material is the key subject which researchers are facing to. During the interaction between laser and material, the energy transfer process is extremely complicated. Among the whole process, there exist many phenomena such as:electron emission, avalanche ionization, melt, vaporization, phase explosion and Coulomb explosion. The mentioned phenom-ena display in different ways depend on the parameters of incident laser pulse and the characteristics of the irradiation sample. Investigating the interaction between laser and material to explain the physical process and mechanism can provide a theoretical prin-ciple for the practical applications.The present investigation status and the existing problems of the interaction be-tween laser and solid materials have been reviewed. The investigation sample includes metal, semiconductor and dielectric medium. And the laser pulse width ranges from nanosecond to femtosecond scale. By concluding the previous researches, we focus our investigation on the interaction between cooper and femtosecond pulse laser, and the mainly works are listed as follow:1. In this paper, we theoretically investigate the electron emission during femtosec-ond laser ablation of single-layer metal (Cu) and double-layer structures. The double-layer structure is composed of a surface layer (Cu) and a substrate layer (Au or Cr). The calculated results indicate that the double-layer structure brings a change to the electron emission from Cu surface. Compared with the ablation of single-layer, double-layer structure may be helpful to decrease the relaxation time of electron temperature, and optimize the electron emission by diminishing the tailing phenomenon under the same absorbed laser fluence. With the increase of the absorbed laser fluence, the effect of optimization becomes significant. The study provides a way to optimize the electron emission which can be beneficial to generate laser induced ultrafast electron pulse sources.2. Ultrafast thermionic emission from gold film irradiated with a femtosecond laser pulse in the presence of an additional electric field is analyzed using a two tem-perature equation combined with a modified Richardson equation. The calculated results show that the duration of the emission is below one picosecond. Supplying an additional electric field is found to change the emission from the metal surface. Given the same laser fluence, this additional field reduces the work function of the metal, and thus improves the efficiency of thermionic emission. These results help to understand the mechanism and suggest ways to improve emissions in the context of ultrafast thermalized electron systems.3. We use a computational model to study the ablation mechanism of metal target irradiated by femtosecond pulse laser. It is confirmed that the Coulomb explosion can occur during femtosecond laser ablation of metal. The influence of thermal ablation and Coulomb explosion on the ablation depth is respectively investigat-ed. Comparing the calculated results with the experimental ones, we find that the theoretical results which consider the thermal ablation only agree well with the experimental ones at high laser fluence, and those which take the Coulomb explosion into account fit well with the experimental ones at lower laser fluence, which exactly explains the ablation mechanism. In contrast with the previous theoretical results which only consider the thermal ablation, our theoretical sim-ulation describes the ablation mechanism straightforward by making comparison of ablation depth, and provides a more reasonable explanation that fits with the actual ablation process.4. We present a study of femtosecond laser induced breakdown spectroscopy (LIBS) of Cu which is preheated to different temperatures. We detect the intensity of plasma emission spectroscopy, and calculate the plasma temperature and electron density by means of Boltzmann plot and Stark broadening methods respectively. We also simulate the processes of femtosecond laser irradiating metal through the two-temperature model. From both experiment and simulation we obtain a same result:as the sample temperature increases from 295 K to 393 K, the plasma and lattice temperature increase about 100 K in both experiment and simulation. Through our work it can be found that the method of preheating the ablation target in femtosecond LIBS can help to strengthen the intensity of the spectroscopy, and enhance the detective sensitivity of LIBS.