Ab initio evidence for nonthermal characteristics in ultrafast laser melting

Laser melting of semiconductors has been observed for almost 40 years; surprisingly, it is not well understood where most theoretical simulations show a laser-induced thermal process. Ab initio nonadiabatic simulations based on real-time time-dependent density functional theory reveal intrinsic nonthermal melting of silicon, at a temperature far below the thermal melting temperature of 1680 K. Both excitation threshold and time evolution of diffraction intensity agree well with experiment. Nonthermal melting is attributed to excitation-induced drastic changes in bonding electron density, and the subsequent decrease in the melting barrier, rather than lattice heating as previously assumed in the two-temperature models.

Figure 1

Three models for ultrafast laser melting: (a) thermal melting, (b) plasma annealing (PA), and (c) thermal accelerated plasma annealing (TAPA).

Figure 2

Optical absorption spectrum (transition probability) of Si (solid line). The shadowed zones denote transition probabilities of selected excitation modes. The dash-dotted line is the experimental spectra redshifted by a scissor correction of 0.7 eV, adopted from Ref. [43]. Inset: Corresponding electronic bands of selected excitation modes. The band indexes are relative to the highest occupied state (set as $-1$).

Figure 3

(a),(b) The atomic structures during laser melting. (c)–(e) Evolution of RDF.

Figure 4

(a) RMSD as a function of time. (b) Simulated and experimental electron diffraction intensity of $\left(220\right)$ reflection as a function of time. Experimental data (time rescaled by a factor of 0.33) are taken from Refs. [3, 4]. (c) Ionic temperature as a function of time.

Figure 5

The (a) RMSD and (b) $\xi$ as a function of time under different laser intensities.

Figure 6

(a) Charge density difference between the ground state and the superposition of atomic charge densities. (b)–(d) Charge density difference induced by laser excitation in TTM-BOMD and TDDFT. The red (green) region represents the density increase (decrease) in charge density.

Figure 7

Phonon dispersion spectra of Si under (a) different laser intensities and (b) different electronic temperature.

Figure 8

(a) Ionic temperature as a function of time. (b) The RMSD as a function of time.