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

TL;DR

This paper explains how ultrashort laser pulses induce convection-driven, supra-wavelength periodic structures on dielectric surfaces, highlighting the roles of Prandtl number, laser energy, and electron density in pattern formation.

Contribution

It introduces a physical mechanism linking convection roll dynamics and supra-wavelength structure formation on dielectrics after femtosecond laser irradiation, a previously unexplored phenomenon.

Findings

Structures have periodicities larger than the laser wavelength.

The size and orientation depend on laser energy and fluid properties.

Prandtl number influences the relaxation of hydrothermal waves.

Abstract

The significance of the magnitude of Prandtl number of a fluid in the propagation direction of induced convection rolls is elucidated. Specifically, we report on the physical mechanism to account for the formation and orientation of previously unexplored supra-wavelength periodic surface structures in dielectrics, following melting and subsequent capillary effects induced upon irradiation with ultrashort laser pulses. Counterintuitively, it is found that such structures exhibit periodicities, which are markedly, even multiple times, higher than the laser excitation wavelength. It turns out that the extent to which the hydrothermal waves relax depends upon the laser beam energy, produced electron densities upon excitation with femtosecond pulsed lasers, magnitude of the induced initial local roll disturbances and the magnitude of the Prandtl number with direct consequences on the orientation and size of the induced structures. It is envisaged that this elucidation may be useful for the interpretation of similar, albeit large-scale periodic or quasi-periodic structures formed in other natural systems due to thermal gradients, while it can also be of great importance for potential applications in biomimetics.