Controlling Voronoi partitions on femtosecond-laser-superheated metal surfaces

Abstract

Ultrafast and high intensity laser pulses can drive materials into highly non-equilibrium conditions. At a sufficiently high intensity, femtosecond laser pulses can turn a solid material into a superheated solid or a superheated liquid state. For metals with low electron–phonon coupling strength, e.g., gold (Au), a superheated liquid state can lead to the formation of a family of unique surface structures, the so-called Voronoi partitions (VPs). Although chaotic in nature, we demonstrate here a control of VP formation through varying the laser fluence and pulse number, resulting in a rich variety of VPs. To understand the formation of complex surface geometries under laser irradiation, we utilizes a numerical approach that integrates the finite difference time domain of electromagnetic field method with the two-temperature model. By studying the spatio-temporal distribution of electromagnetic and temperature fields and by analyzing the geometric properties of the VPs, we provide a framework for understanding the emerging surface patterns. With VPs, novel structures are generated with promising potential applications, namely, Gecko-skin-like nano-spikes and nano-cauliflower-like surface structures.