Melting of Al by ultrafast laser pulses: dynamics at the melting threshold
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Melting of Al by ultrafast laser pulses: dynamics at the melting threshold Yudi Rosandi · Herbert M. Urbassek
Received: 22 September 2011 / Accepted: 3 August 2012 / Published online: 4 September 2012 © Springer-Verlag 2012
Abstract Using molecular-dynamics simulation, we investigate the melting of a thin Al slab by ultrafast laser irradiation. We employ a laser energy, which is just around the melting threshold. While the equilibrium electron–phonon coupling is well understood, we investigate the influence of the early (i.e., prior to electron thermalization) electronlattice energy transfer. To this end, as a model study, we vary the fraction of the laser energy, which is directly given to lattice atoms vs. that given to the electronic system. We find that the melting process depends sensitively on the early electron-lattice heating rate. The pressure build-up within the still solid parts of the slab is identified as the main agent which delays the melting transition. The changes in the simulated structure factor data suggest that X-ray measurements of thin films performed just around the melting transition— even if performed long after electron thermalization—may provide information on the early electron-lattice energy coupling process.
1 Introduction Laser-induced melting of metals induced by ultrashort laser pulses has been investigated repeatedly in the past [1–9]. The standard method of calculation employed consists in Y. Rosandi () · H.M. Urbassek Fachbereich Physik und Forschungszentrum OPTIMAS, Universität Kaiserslautern, Erwin-Schrödinger-Straße, 67663 Kaiserslautern, Germany e-mail: [email protected] url: http://www.physik.uni-kl.de/urbassek/ Y. Rosandi Department of Physics, Universitas Padjadjaran, Jatinangor, Sumedang 45363, Indonesia
the so-called two-temperature-model/molecular-dynamics (TTM/MD) scheme [3, 10]. In it, the atomic motion is described using molecular-dynamics simulation, while the electronic system is modeled using a continuum heat conduction equation [11]; energy transfer between the electronic and atomic system is taken into account in a consistent way both in the continuum and the molecular-dynamics model. The application of the TTM/MD model presupposes, however, that the electronic system has acquired in itself thermal equilibrium; otherwise the physical parameters entering the heat conduction equation—such as specific heat, electron–phonon coupling, and thermal conductivity— cannot be specified. The time needed for thermal equilibration between electron and atom system is larger; for the case of Al considered here, it is of the order of a few ps; see Eq. (10) below. However, after laser excitation, the electronic system needs time to acquire thermal equilibrium itself in the sense that it can be described by a thermal distribution and the thermal coefficients described above apply. Thermal equilibration within the electronic system of metals has been investigated and it has been found that thermalization may require a time of up to a few hundreds of fs [12]. In the class
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