Abstract
Intense ultrashort laser pulses can melt crystals in less than a picosecond but, in spite of over thirty years of active research, for many materials it is not known to what extent thermal and nonthermal microscopic processes cause this ultrafast phenomenon. Here, we perform ab-initio molecular-dynamics simulations of silicon on a laser-excited potential-energy surface, exclusively revealing nonthermal signatures of laser-induced melting. From our simulated atomic trajectories, we compute the decay of five structure factors and the time-dependent structure function. We demonstrate how these quantities provide criteria to distinguish predominantly nonthermal from thermal melting.
Highlights
Intense ultrashort laser pulses can melt crystals in less than a picosecond but, in spite of over thirty years of active research, for many materials it is not known to what extent thermal and nonthermal microscopic processes cause this ultrafast phenomenon
The so-induced atomic disordering, which results in a liquid state,6 is called nonthermal melting, because it is caused by the interaction of hot electrons with room-temperature atomic nuclei, which are out of thermal equilibrium with each other
The past thirty-five years have seen increasingly sophisticated theoretical models of ultrafast melting of Si, which have developed from microscopic considerations19 through moleculardynamics simulations based on tight-binding theory4,17,20 to ab-initio molecular-dynamics simulations, at first for modest system sizes with N 1⁄4 64 silicon atoms per supercell21 and recently for supercells with N 1⁄4 640 and 800.22,23 Size matters, because for the calculation of certain quantities, for example, the time-dependent structure function, it is necessary to sample a fine mesh of scattering vectors q between the main diffraction peaks, which can only be achieved by simulating a large supercell [see Eq (2) below]
Summary
When an intense femtosecond-laser pulse excites silicon with a photon energy larger than the electronic band gap, most of the deposited energy is absorbed directly by the electrons. The often used extrapolation of the result of Ref. 12 to an excitation density of 14.1%, which we applied in our ab-initio molecular-dynamics simulations below, gives a thermalization time of 530 ps, suggesting that ultrafast melting of silicon may be a purely nonthermal process. The extrapolation to such high densities is not supported by experiment or theory, leaving the question about the character of the femtosecond-laserinduced solid-liquid transition in silicon, i.e., predominantly nonthermal or thermal open. Its timescale is indicative for the incoherent electron-phonon coupling time in liquid silicon, which appears to be roughly two orders of magnitude faster than the above-mentioned extrapolation of Ref. 12 for solid silicon. In order to find the signatures of nonthermal melting, we performed density-functional-theory molecular-dynamics simulations of silicon after intense femtosecond-laser excitation. It is important to stress that our conclusions are not specific for silicon but can be applied to other materials as well
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