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Chapter 3:
Ablation of Metals with Femtosecond Laser Pulses
Ultrafast laser processing of materials, especially metals and semiconductors, is a field of condensed-matter physics and materials science that has developed rapidly over the last few years. It has proved to be of considerable interest for both applied and fundamental research for a variety of reasons discussed extensively in the literature. Ultrafast lasers with a pulse duration of the order of 10–100 fs make it possible to limit the undesirable spread of the thermal process zone in a target. This capability is used for structural monitoring of thin metal films, laser synthesis and processing in thin film deposition, and laser micromachining and patterning. In addition to various applications, ultrashort laser pulses have opened up fresh opportunities for the study of electron-phonon interaction and hot electron phenomena in condensed matter. A large amount of the experimental and theoretical work on laser-matter interaction with ultrashort laser pulses has been done until recently (see, for example, Refs. 1–14). As first pointed out in Ref. 15, the absorption of laser radiation results in transient nonequilibrium of the electron gas with the lattice near a metal surface. Since the heat capacity of degenerate electron gas is small, the electron temperature follows, with practically no delay, the shape of the laser pulse. Heating of the lattice proceeds rather slowly due to the large difference between the electron and ion mass (or, similarly, due to the difference between the sound speed and the electron Fermi velocity). According to Refs. 16 and 17, the characteristic time of lattice heating for different metals lies in the range from several to several tens of picoseconds. This means that during a subpicosecond laser pulse, the laser energy absorbed is stored in the electron subsystem, whereas the lattice remains at a considerably lower temperature. Due to the low specific heat of degenerate electron gas, its temperature is much higher than in the case of electron-lattice equilibrium. The generation of hot electrons leads to anomalous emission of electrons and visible light from metals subjected to ultrashort laser pulses.
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