Convection roll-driven generation of supra-wavelength periodic surface structures on dielectrics upon irradiation with femtosecond pulsed lasers

摘要

The significance of the magnitude of Prandtl number of a fluid in thepropagation direction of induced convection rolls is elucidated. Specifically,we report on the physical mechanism to account for the formation andorientation of previously unexplored supra-wavelength periodic surfacestructures in dielectrics, following melting and subsequent capillary effectsinduced upon irradiation with ultrashort laser pulses. Counterintuitively, itis found that such structures exhibit periodicities, which are markedly, evenmultiple times, higher than the laser excitation wavelength. It turns out thatthe extent to which the hydrothermal waves relax depends upon the laser beamenergy, produced electron densities upon excitation with femtosecond pulsedlasers, magnitude of the induced initial local roll disturbances and themagnitude of the Prandtl number with direct consequences on the orientation andsize of the induced structures. It is envisaged that this elucidation may beuseful for the interpretation of similar, albeit large-scale periodic orquasi-periodic structures formed in other natural systems due to thermalgradients, while it can also be of great importance for potential applicationsin biomimetics.