Final Report Summary - NLL (Nonlinear Laser Lithography) Since its invention, the laser has been idealised as the perfect tool that can work with ease and surgical precision on any material. Lasers are indeed ubiquitously used to structure all kinds of materials both by removing or adding together pieces with sub-microscopic precision. In the overwhelming majority all of these, the laser processes the material point by point, like pixels on a screen. This project has set out to achieve unprecedented control over matter through laser light by exploiting self-organisation, meaning the complex patterns and structures emerge spontaneously despite using a simple and unstructured laser beam. No evident aspect of the final pattern is explicitly included in the laser beam, but that pattern emerges following its interactions with the material. Self-organised laser-patterning has a history that spans more than five decades. Nevertheless, such self-organised structures notoriously suffered from a lack of controllability, uniformity and repeatability. Laser-material processes have traditionally been idealised as a one-way interaction, whereby only the laser beam is meant to modify the material. The possibility of laser-induced changes in the material modifying the laser beam, in return, and that this intrinsic feedback could open new doors was rarely considered. The radical concept behind this project has been to arrange for strong feedback processes: The beam causes changes in material properties, which, in turn, affect and alter the laser beam as it interacts once again with the same material. While this is often the case, normally, the effect is weak. In all cases we explored, this feedback is strong enough that its presence changes the nature of the interaction qualitatively. We have shown that by controlling these feedback processes, we can exert a previously unimaginable level of control over the self-organisation process. We demonstrated extremely uniform, highly repeatable, self-replicating and self-healing structures. Throughout the project, we developed several different implementations of our approach of nonlinear laser lithography (NLL), including a 3D version that allows the creation of self-organised structures deep inside silicon (Si). By building analogies between these similar, yet different implementations and the phenomenon of mode-locking of lasers, we could identify essential physical concepts that transcend the details among these systems of self-organisation. For example, before our work, laser-based self-organised patterns were limited to parallel stripes. We have shown the possibility to create infinitely many different periodic self-organised patterns and formulated a theoretical method to predict which patterns are possible and how to steer the system to a desired one. We achieved nearly complete independence from material type, size and shape, demonstrating the first steps towards a universal self-organisation methodology. In addition to self-organisation based on nonlinear-feedback-driven laser-material interactions, the same core concept has yielded striking advances in other areas. These include the invention of ablation-cooled laser-material removal, a method to generate arbitrarily complex 3D holograms and a novel form of optical tweezers based on nonlinear feedback forces.
