No abstract

Many ultrafast structural phenomena in solids at high fluences are related to the hardening or softening of particular lattice vibrations at lower fluences. In this paper we relate femtosecond-laser-induced phonon frequency changes to changes in the electronic density of states, which need to be evaluated only in the electronic ground state, following phonon displacement patterns. We illustrate this relationship for a particular lattice vibration of magnesium, for which we-surprisingly-find that there is both softening and hardening as a function of the femtosecond-laser fluence. Using our theory, we explain these behaviours as arising from Van Hove singularities: We show that at low excitation densities Van Hove singularities near the Fermi level dominate the change of the phonon frequency while at higher excitations Van Hove singularities that are further away in energy also become important. We expect that our theory can as well shed light on the effects of laser excitation of other materials.

When a femtosecond-laser pulse excites a solid it may, among other ultrafast processes, induce coherent phonons, phonon frequency changes, thermal phonon squeezing, and nonthermal melting. Using our in-house code for highly excited valence electron systems, where laser-induced interatomic forces are computed "on the fly" from ab initio theory, we performed molecular dynamics simulations of supercells with up to 800 atoms. For Si we found that thermal phonon squeezing precurses nonthermal melting as a function of fluence. Furthermore, our molecular dynamics trajectories showed that nonthermal melting includes a stage during which the atoms move fractionally diffusive. We also simulated femtosecond-laser-excited Ge. In addition, we explain the electronic origin of laser-induced phonon frequency redshifts and blueshifts in Mg.

Ultrashort optical pulses can be used both to create fundamental quasiparticles in crystals and to change their properties. In noble metals, femtosecond lasers induce bond hardening, but little is known about its origin and consequences. Here we simulate ultrafast laser excitation of silver at high fluences. We compute laser-excited potential-energy surfaces by all-electron ab initio theory and analyze the resulting quantum lattice dynamics. We also consider incoherent lattice heating due to electron-phonon interactions using the generalized two-temperature model. We find phonon hardening, which we attribute to the excitation of s electrons. We demonstrate that this may result in phonon vacuum squeezed states with an optimal squeezing factor of ∼0.001 at the L-point longitudinal mode. This finding implies that ultrafast laser-induced bond hardening may be used as a tool to manipulate the quantum state of opaque materials, where, so far, the squeezing of phonons below the zero-point motion has only been realized in transparent crystals by a different mechanism. On the basis of our finding, we further propose a method for directly measuring bond hardening.