Ultrashort Pulsed Laser Ablation Of Metal Targets

Ultrashort, sub-picosecond pulsed laser ablation technique is currently attracting a great deal of attention both for fundamental physics and for technological applications. The first investigations on the interaction of Ti:sapphire femtosecond (fs) laser pulses with solid targets were mainly devoted to the study of the modifications of irradiated samples. These studies are still establishing fs laser ablation as the state-of-art technique for optimal control of material removal, due to its peculiarity to process virtually any material with high precision and minimal collateral damage. In a number of applications (micromachining, metal processing, surgical operation, and so on) advantages over nanosecond (ns) and picosecond (ps) laser pulses have been already demonstrated. A key benefit of ultrashort laser pulses lies in its ability to deposit energy into a material in a very short time period, before thermal diffusion can take place. Following linear or multiphoton absorption of the laser energy, electron temperatures can be quickly raised up to many thousands of degree Kelvin. With the subsequent energy transfer from the electron subsystem to the atomic lattice, material removal, ablation and plasma formation occur, as briefly discussed in chapter I. At laser pulse durations shorter than the typical electron to lattice relaxation times (about some ps), the system behaves roughly the same and the main properties of the plume are quite independent from material and laser parameters; then, we may say to be in the "ultrashort" regime.In this paper, and copper target as an example, the thermal behavior of target heated by ultrashort pulsed laser is studied. First of all, the effect of the electron temperature and lattice temperature distribution is discussed in different approximation electron thermal conductivity by two-temperature mode during the laser ablation process. Secondly, The two-temperature equation (DTE) was formatted simply, got the analytical solutions of the ultrafast laser ablation metallic materials. Subsequently, investigates the pulse train technology, simulate the thermal behavior of different pulse number per train. We confirm the distinctly different results on different number of pulses. The highest transient electron temperature is lowered and the thermolization time is prolonged by a pulse train, which preserves the advantages of ultrashort lasers. The pulse train technology can increase the photon efficiency in ablation and micromachining. Finally, The characteristics of thermionic emission of metal films during ultrashort pulse laser ablation are investigated by numerical simulation. The two-temperature model is used to calculate the electron and lattice temperatures while thermionic emission is incorporated into the model as a surface phenomenon. The Richardson-Dushman equation is employed to estimate the rate of thermionic electron emission. The influence of laser irradiance and the film thickness on the emission rate is examined.