Ab initio description of laser-induced ultrafast phenomena in the presence of surfaces and interfaces

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Ultrafast phenomena induced by femtosecond-laser pulses are believed to be, in most cases, nonthermal or to have at least a nonthermal origin. There is plenty of experimental evidence for nonthermal structural phenomena. Effects like excitation of coherent phonons, phonon squeezing, ultrafast melting, bond-softening and hardening were either directly measured or inferred from measurements. Our group has a vast experience in successfully explaining and predicting nonthermal structural phenomena induced by ultrashort laser and XUV pulses. We developed the code CHIVES, able to perform ab initio molecular dynamics simulations of laser excited materials, and which has been shown to be about two hundred times faster than other ab initio codes with the same accuracy. Since many experiments, particularly those based on ultrafast electron crystallography, are performed on thin films and since most applications of laser-solid interactions are aimed at the processing (e.g., nanostructuring) of surfaces, it is a challenge for theory to simulate laser induced ultrafast phenomena on surfaces and in thin films. No ab initio theory of nonthermal effects on surfaces and in thin films could be performed so far. In this project we plan to simulate ultrafast non thermal phenomena (1) in silicon thin films, (2) in a Si/SiC superlattice, and (3) in a Si/SiC thin film. Due to technical reasons, for the atomistic description of these systems convergence is poor, which would make the simulations forbiddingly slow. Therefore, it is necessary to further speed up CHIVES by improving the convergence of the underlying self-consistent calculations. From the proposed simulations we expect to predict novel nonthermal effects, like, suppression of surface melting (preempting) or anomalous diffusion of C atoms at the Si/C interface, which might open a new field of research in nonequilibrium physics and lead to new applications.In addition, the treatment of a Si/SiC thin film is a first step towards a realistic description of oxidized surfaces.