Nanocrystalline structure of nanobump generated by localized photoexcitation of metal film

The extreme cooling rates in material processing can be achieved in a number of current and emerging femtosecond laser techniques capable of highly localized energy deposition. The mechanisms of rapid solidification of a nanoscale region of a metal film transiently melted by a localized photoexcitation are investigated in a large-scale atomistic simulation. The small size of the melted region, steep temperature gradients, and fast two-dimensional electron heat conduction result in the cooling rate exceeding $1013K/s$ and create conditions for deep undercooling of the melt. The velocity of the liquid/crystal interface rises up to the maximum value of $∼80m/s$ during the initial stage of the cooling process and stays approximately constant as the temperature of the melted region continues to decrease. When the temperature drops down to the level of $∼0.6Tm$⁠, a massive homogeneous nucleation of the crystal phase inside the undercooled liquid region takes place and prevents the undercooled liquid from reaching the glass transition temperature. The prediction of the nanocrystalline structure of the surface features generated in laser nanoprocessing has implications for practical applications of nanostructured surfaces and calls for experimental verification of the computational results.