| A comprehensive laser material ablation model, based on classical and semi-classical two-temperature models (TTM) together with a temperature-dependent optical model, was developed to describe energy transport, ultrafast phase changes, and material ablation of metal films irradiated by ultrashort laser pulses.;The extended Drude and the critical point model were proposed in this work to describe temperature-dependent optical reflectivity and absorption coefficient for gold and copper, respectively. After validated with experimental data, the two optical models were incorporated into the TTM to simulate laser energy deposition and the resulting thermal response, ultrafast phase changes from solid to liquid and from liquid to vapor, and phase explosion of metal films irradiated by ultrashort laser pulses. It was found that dynamic optical properties could play a very important role in modeling ultrashort-pulsed laser interactions with metal materials. Those constant reflectivity and absorption coefficient at room temperature (RT) which have been widely used in the TTM modeling are only suited for very low laser fluences.;A study for the classical and semi-classical TTM was performed for gold films. In the semi-classical TTM, due to the effect of electron drifting, slightly lower electron and lattice temperatures were obtained compared to those calculated by the classical TTM under the same laser irradiation conditions. Higher laser fluence and longer pulse duration could result in more distinct difference between the two models. The simulated melting velocity and depth were also compared, respectively; so were those of the evaporation. It was found that the semi-classical TTM results in less severe melting and vaporization than the classical TTM.;For multi-pulse irradiations, two mechanisms, laser energy absorption and heat conduction loss, are competing with each other for the thermal response. Thus, the effects of pulse number and separation time between two pulses in a femtosecond pulse burst were investigated. The results showed that with the same total energy in a laser burst, more pulses with a shorter separation time, e.g., 1 ps, or fewer pulses with a longer separation time, e.g., 100 ps, could achieve higher lattice temperature. For the dual (nanosecond + femtosecond) laser beams, lattice temperature could be increased by setting the pulse separation time in a femtosecond laser burst as short as possible, e.g., 1 ps.;Laser material ablations were investigated by including phase explosion to the above thermal model. It was shown that for high laser fluences phase explosion is a dominating mechanism in material ablation, while vaporization for low laser fluences. The simulated ablation depths correlated very well with existing experimental data over a broad range of fluences, 0.6 – 30 J/cm2. For laser ablations by laser bursts, it was also found that with very short separation times, e.g., 1 ps and 10 ps, the advantage of the burst mode is not evident in laser material ablation. However, with longer separation time, e.g., longer than 50 ps, a burst mode can ablate much more material than a single pulse. |
