Applications of Ultrafast Laser Ablation to Material Colouring and Physical Unclonable Functions

Abstract

As lasers become increasingly common in both scientific and industrial settings, the list of applications for which they are utilised has grown at an incredible rate. Lasers allow the direct alteration of a near limitless number of materials with applications in nearly every current field of scientific inquiry we know. However, the nature of the interactions between these increasingly complex systems and the materials they are applied to is not always fully understood. These laser-matter interactions form an integral part of a great many laser applications including plasmonic colouring, photochemistry, cryptography, laser engraving, microfluidics, and wettability control amongst many more. Lasers offer several advantages over some processing techniques due to their flexibility to rapidly pattern areas ranging from very small, even submicron sized features, all the way up to large areas greater than tens of centimetres. However, such applications require an in depth understanding of the nature of the interactions between the irradiating laser light and the underlying substrates involved. To that end, I have presented here several novel developments in the understanding of the nature of laser generated structures on a variety of material systems including laser coloured copper, laser generated nanoparticle based Physical Unclonable Functions (PUFs) and finally laser generated debris formation on polycarbonate. In the first publication, the interactions of plasmonic nanoparticles, oxide species and diffractive effects originating from laser-induced periodic surface structures (LIPSS) were identified as working together to yield complex far field responses which have previously been largely attributed to a single driving mechanism. The work on PUFs served to create a simplified one step process to generate nanoparticle based optical ‘fingerprints’ which combined with a simple open-source image processing algorithm serve as a simple and industrially compatible security feature. Finally, a detailed exploration of the mechanisms driving the far field colourimetric response as well as the chemical features of laser irradiated polycarbonate was developed highlighting the importance of laser repetition rate on the chemical, morphological and optical changes of the material. These results highlight the complexity and multi-faceted nature of ultrafast laser-matter interactions and indicate that the interpretation of the optical and chemical properties often requires an interplay of a variety of effects that culminate in the creation of unique and useful structures. This work also highlights that lasers can often be used to quickly generate randomized features with potential cryptographic applications that in other works often require complex or prohibitively difficult manufacturing methods in order to accomplish similar results.