【正文】 er for bothlong and ultrashort pulses with an optical penetration depth that is typicallyof the order of 10–20 nm, there is a large di?erence in the energy densitynear the surface. There are two quite di?erent causes. First, it is wellknownthat absorption increases slightly with temperature in metals for frequencies far away from interband transitions. For aluminium at wavelengths ofabout 800 nm, however, the greatest part of the absorption is governed by theinterband transition, which is decreasing with the phonon temperature dueto band broadening [5]. This e?ect, however, has to be taken into account forlong pulses only because, on the femtosecond scale, the phonon temperatureis changed little or not at all. On the other hand, the increase of the electrical resistivity caused by stronger electronelectron scattering for femtosecondpulses can even induce an increase in absorption [6, 7]. Experimentally, suchchanges can be seen in transient thermal re?ectivity pump and probe measurements in which the metal is heated with a strong pump laser and the changeof re?ectivity is detected。uttnerFig. Re?ectivity of gold, silver and aluminium between 1 and 2 eV. Datataken from [4].the absorption or the electronic thermal conductivity makes the problem noeasier.Today several laser systems are available for the generation of femtosecondlaser pulses. For example, Ti:sapphire, dye, excimer and free electron lasersare among those. In the following, however, attention will be restricted to theexamination of the Ti:sapphire laser because it is used in most experimentsbut also with the aim of making a parison between di?erent experimentsmore transparent. Furthermore, in many experiments the same pulse duration of τL= 100 fs was often used but not always the same wavelength. Inthe ground mode the Ti:sapphire laser possesses a working range of aboutλ = 650 nm ( eV) to λ = 1100 nm ( eV). Compared to the rareearthdoped gain materials like Nd:YAG or Nd:YLF this is very broad and resultsfrom the expanded gain bandwidth of the Ti3+ion. The maximum gainand laser e?ciency is achieved, however, around 800 nm. In noble metals theoptical properties are usually not very sensitive to a variation of the wavelength in this range. For aluminium, however, one has to be careful becauseit possesses an interband transition and therefore a drastic change of there?ectivity emerges around 820 nm as can be seen in Fig. .Another important aspect is that femtosecond lasers can deliver very highintensities in the focal spot of up to I = 1020W cm?2 at present. The light10 Femtosecond Laser Pulse Interactions with Metals 317pressure p = I/c at this intensity is p = 3 1015Pa = 30 Gbar. This is higherthan the typical value of yield stress for metals.Even at much lower intensities, say I = 1014W cm?2, interesting phenomena appear both for basic investigations and technological applications. Ina very small volume, calculated as the laser spot size times skin depth, theelectrons can reach temperatures of some tens of thousands Kelvin during thelaser pulse while the lattice basically remains thermally undisturbed. Photoemission of electrons starts if the electron kinetic energy bees larger thanthe work function. After the termination of the laser pulse the phonon temperature can rise rapidly close to or even above the critical temperature caused bythe phonon emission during the energy relaxation of the electrons. The metalis then in an unstable nonequilibrium state leading to ejection of materialdriven by the high critical pressure followed by a rapid quenching. Therefore,a great part of the absorbed laser energy can be removed from the bulk duringthe ablation process by the ejected material. This is the origin of one of thegreat advantages of femtosecond laser applications pared to nanosecondlasers, since the dissipation of energy due to thermal conduction plays a minorrole. The remaining heat a?ected zone therefore bees quite small and onlyvery little collateral damage may appear. Depending on the process conditions,this can be used for the processing of smooth and sharp structures in practically any material. Moreover, the optimum ablation e?ciency, de?ned as theratio of the volume of ablated matter to the laser pulse energy, was found forfemtosecond lasers [1]. What is Di?erent Compared to Longer Pulses? The ElectronElectron Scattering TimeThe phenomena of femtosecond laser interaction with metals are drasticallydi?erent from those related to laser pulses longer, say, than 100 ps. There is,however, in the following no sharp boundary between the two pulse rangesbecause it depends on the material and the property under investigation.Nevertheless, it will be assumed that long pulse behaviour is established if alocal thermal equilibrium exists between the electron and phonon subsystemsduring the laser pulse, equivalently, both systems have the same local temperature. This implies that for ultrashort laser pulses the electron temperaturesare higher than the phonon temperatures because the electromagnetic laser?elds couple only with the electrons. Indeed, the electron temperature canincrease to values far above the critical temperature of metals. In this way,the electronelectron scattering, usually of less importance than the electronphonon scattering time as a result of Pauli’s exclusion principle [2], beessigni?cant or even dominant. In the frame of the Fermi liquid theory, the318 Bernd H168。四、畢業(yè)設計進程安排及實習安排表一 進程安排及實習安排序 號設計（論文）各階段名稱日 期1資料收集，閱讀文獻，撰寫并完成開題報告。二、畢業(yè)設計的要求與數(shù)據(jù)針對激光實驗加工中得到的大量試驗數(shù)據(jù)進行歸門別類，借助HyperWorks/Hypersyudy中的實驗設計（DOE）模塊進行各種不同的激光加工工藝參數(shù)的優(yōu)化，在最終得到最優(yōu)的激光加工工藝參數(shù)。最后，謹向百忙之中審閱論文和參加答辯的每一位老師表示由衷的謝意！ 朱洪參考文獻[1] 閔亞能. 實驗設計（DOE）應用