Ab initio simulation of intense short-pulse laser irradiation of metals and semi-conductors

The effect of intense ultra-laser irradiation on crystal stability is not completely elucidated. Ultrashort laser pulses heat electrons to a very high temperature and leave the lattice relatively cool since the heat capacity of electrons is much smaller than that of lattice. This non-equilibrium system can be described as two subequilibrium systems : the hot electrons and a cold lattice. We studied the effect of this intense electronic excitations on the interatomic forces and the possible melting of the underlying lattice for a semi-conductor (Si) and two metals (Al and Au). We used {\it ab initio} linear response to compute the phonon spectrum in the Density Functional Theory framework for several electronic temperatures ranging from 1 to 6 eV. We found that semi-conductors and metals behave in an opposite ways when increasing electronic temperature. Phonon instability appears in silicon at a electronic temperature of $1.5\;\rm{eV}$ inducing the melting of the lattice. Gold samples become more stable. The Debye temperature was deduced from the phonon spectrum and using the Linderman criterion, we showed that gold undergoes a sharp increase of its melting temperature under intense laser irradiation. The same effect is observed for aluminium.