Controlled surface structuring with nonlinear laser lithography

Abstract

Self-organisation has always fascinated researchers from all branches of sciences and engineering. Despite its ubiquity, our present understanding of its core principles and in particular how to control self-organised phenomena is at its infancy. A particularly rich case of self-organisation arises from the interactions of intensely powerful laser beams with material surfaces. As this phenomenon leads to formation of sub-wavelength, thereby, nanoscale periodic structures through a simple, one-step process performed in ambient atmosphere, there has been tremendous interest in its use in applications, ranging from tribology to data storage. However, there remains much to be desired in terms of our ability to control, regulate, dynamically modify the resulting structures. A technique recently demonstrated in our group, Nonlinear Laser Lithography (NLL), has made possible the creation of extremely uniform, virtually perfectly periodic self-organised nanostructures, which are in the form of parallel nanoscale lines. These nanostructures can be used to cover or tile indefinitely large areas without any apparent loss in quality or uniformity. Armed with this advance, we are now in a position to look beyond getting simply periodic structures and to develop conceptual tools and practical techniques for advanced control of the self-organisation process and to create a vast array of self-organised structures. In this thesis, we first develop a rigorous theoretical model for NLL, which we then show to possess excellent predictive power and can efficiently guide the experiments. We first reveal an interesting, self-organised effect, namely that the nanostructures respond to a tilting of the laser beam's wavefront in a manner that is strongly analogous to the well-known Doppler effect. Further, building on the rigorous model developed in this thesis, we propose and experimentally demonstrate that noise or modulations in the laser beam or defects on the surface can each steer the self-organised process. We further show that by deliberately introducing noise or defects, we can achieve patterns that are impossible to achieve otherwise. As an ultimate demonstration of this capability, we report on the creation of all the Bravais lattices possible for a surface. While the main results to be reported concern the NLL technique, the conceptual tools developed in this thesis rely on general properties of selforganisation through an interplay of positive and negative nonlinear feedback mechanisms. This defines a broad class of self-organising systems. As such, it is likely that the techniques we introduce can be appropriately adapted to achieve similar control over self-organised patterns forming in entirely different physical systems.