When a crystalline solid such as silicon is exposed to a laser beam of very high intensity, what happens to the crystal? It melts, even when the laser beam is an extremely short pulse of femtosecond duration, but not in the same way as it would if it were heated up slowly. Such laser-induced nonthermal melting processes have opened up not only a new window to the electronic and atomic dynamics in solid crystals that take place on unprecedented short time scales of femtoseconds but also new possibilities of laser-controlled material processing. In this theoretical paper, we present the first accurate quantum-mechanical simulation of the motions of hundreds of atoms in silicon crystal after femtosecond-laser excitation and reveal how the atomic nuclei vibrate under the influence of laser-excited electrons as the laser intensity is tuned across the threshold for the nonthermal melting of silicon.

In a crystal in thermal equilibrium with its surroundings, the constituent atomic nuclei vibrate—in concerted fashions—around their average positions that form the crystal lattice. Both the lattice constants and the mean-square amplitudes of the atomic vibrations (or, equivalently, the numbers of frequency-distinct phonons) depend not only on the temperature but also on the interactions that hold the nuclei together, to which the electrons contribute. When a femtosecond-laser pulse that can strongly excite some of the electrons hits the crystal, a substantial fraction of the electrons becomes heated almost instantaneously. This nonequilibrium heating alters the forces between the atomic nuclei. If the laser intensity is above a threshold, the altered forces can no longer hold the atoms together and the crystal eventually melts via an ultrafast process. If the laser intensity is below the threshold, the altered forces define new average atomic positions and new phonon modes. In either case, however, the positions and momenta of the nuclei cannot adjust to the new values immediately. What, then, happens to the nuclei immediately after the ultrafast laser excitation?

To answer this question accurately, we have taken into account the effect of the hot-electron plasma generated by the laser pulse in our quantum-mechanical description of the atomic motion. A newly developed computer algorithm has allowed us to simulate the motions of hundreds of silicon atoms. What we have discovered is the “squeezing” of the thermal-equilibrium phonons at below-the-threshold laser intensities: The motions of the nuclei switch synchronously between vibrations of amplitudes that are either smaller or bigger than the equilibrium amplitudes, corresponding to either squeezing in the real space or squeezing in the momentum space, respectively. Moreover, we have determined a correlation between the appearance of the squeezed thermal phonons below the melting threshold and the nonthermal melting: The former necessarily and sufficiently heralds the occurrence of the latter.

The conclusions we have drawn about silicon should apply generally to more atomic solids and add an important contribution to our understanding of the responses of solid materials to strong and ultrafast electronic excitations.