Ultrafast Dynamics of Laser-Excited Solids
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Ultrafast Dynamics of Laser-Excited Solids D.A. Reis, K.J. Gaffney, G.H. Gilmer, and B. Torralva Abstract We discuss recent experimental and theoretical results on ultrafast materials dynamics. Intense, femtosecond lasers can deposit energy in a time that is short compared with relaxation processes and can generate extremely large carrier densities that drive bond softening, nonthermal melting, and ablation. In particular, we present optical experiments on electronic softening of coherent phonons in bismuth and x-ray experiments on ultrafast disordering in indium antimonide that probe the bonding of the lattice under successively higher carrier concentrations. We review a number of molecular dynamics simulations and their assumptions, which address nonthermal melting. Large-scale molecular dynamics simulations elucidate the role of void formation in laser ablation. Keywords: laser ablation, simulation.
Introduction
Experiments
The very high carrier densities generated with intense, ultrafast lasers have important implications for materials processing.1 The excitation of carriers by linear and nonlinear absorption of femtosecond pulses occurs for the duration of the pulse. For most materials, this excitation is fast compared with carrier diffusion and carrier–phonon scattering but can be comparable to carrier–carrier scattering. Because energy deposition occurs on a time scale that is fast compared with relaxation, the structural response of materials to femtosecond laser excitation is fundamentally different than with longer pulses. At relatively low densities of excited carriers—less than 1% of the valence electrons—the lattice response to laser excitation can be dominated by impulsive excitation of coherent optical phonons2 and high-frequency acoustic phonon pulses (strain).3 At intermediate densities, ⬃1%, the dense electron–hole plasma can lead to significant bond softening4,5 as well as density-dependent diffusion,6 while at high densities, ⬃10%, carrier excitation significantly softens and even destabilizes the lattice, leading to nonthermal melting 7 and ablation.8 We report on recent experiments and simulations that have dramatically increased our understanding of key aspects of ultrafast dynamics.
Two recent experiments probed the bonding of the lattice under two successively higher levels of excitation: strong electronic softening9 and nonthermal melting.10,11 In the first experiment, Murray et al.9 investigated the structural dynamics of semimetallic bismuth through changes in the optical properties. In the second experiment, Lindenberg and Gaffney et al.10,11 used ultrafast x-ray diffraction12 to measure atomic displacements during the process of the ultrafast disordering and eventual melting of a solid. Bismuth exhibits large-amplitude, lowfrequency oscillations in optical reflectivity following short-pulse laser excitation. At ambient temperature and pressure, bismuth forms the rhombohedral (A7) structure, which is a Peierls distortion of a simple cubic lattice with two atoms per unit cell. At a s
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