Laser-induced tunnel ionization ‌and femtosecond nonthermal melting in MgO

H. Zhao1

1Jozef Stefan Institute, Ljubljana, Slovenia

Laser-induced melting plays a crucial role in advancing manufacturing technology and ultrafast science [1-3]; however, its atomic processes and microscopic mechanisms remain elusive due to complex interplays between many degrees of freedom within a timescale of ~100 femtoseconds. We employ MgO as an example to investigate the nonequilibrium mechanisms of laser-driven ultrafast nonthermal melting of wide-gap materials. Our study is based on real-time time-dependent density functional theory (rt-TDDFT) molecular dynamics (MD) simulations. We report here that laser melting is greatly accelerated by tunnel ionization processes. The tunneling processes generate a large number of photocarriers and results in intense energy absorption, instantaneously changing the potential energy surface of lattice configuration. The strong electron-phonon couplings and fast carrier relaxation enable the efficient energy transfer between electrons and the lattice. The modulation of melting thresholds and phase boundary demonstrate the possibility of manipulating phase transition on demand. A shock wave curve is also obtained at moderate conditions (P < 50 GPa), extending Hugoniot curve to new regimes. These results account well for the latest ultrafast melting experiments, and provide atomistic details and nonequilibrium mechanism of photoinduced ultrafast phase transitions in wide-gap materials.

1. S. K. Sundaram et al., Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses. Nat. Mater. 1, 217-224 (2002)
2. C. W. Siders et al., Detection of nonthermal melting by ultrafast X-ray diffraction. Science 286, 1340-1342 (1999)
3. E. H. Penilla et al., Ultrafast laser welding of ceramics. Science 365, 803-808 (2019)