Ultrafast laser excitation as a pathway to liquid-like transport rates in the solid state

This work presents an ultrafast laser-based mechanism for achieving liquid-like mass transport rates in solids. Transport at these rates was achieved by irradiating GaAs with a femtosecond laser (repetition rate = 1 kHz, tau = 150 fs, and lambda = 780 nm). Following irradiation, the material exists in a highly excited state in which the atomic bonds are softened. The probability of hopping between lattice/interstitial sites for atoms within the softened interatomic potentials is greatly increased. Evidence of the enhanced diffusion is observable in the rate at which islands and corrugated structures form on the GaAs surface. Quantification of the mass transport rate necessary to form the structures via Fick's law produces a diffusion coefficient of 1.18x10(-6) cm(2)/s, which is over 10 orders of magnitude higher than that for near-melt vacancy self-diffusion in crystalline GaAs. In fact, the calculated diffusion coefficient is on the order of magnitude of liquid GaAs, while estimates of temperature during mass transport place the crystal well below its melting point. These results suggest that ultrafast laser irradiation can be used to enhance the diffusion rate in materials without inducing a phase transition.