Probing Matter by Light

The success of ultrafast laser processing strategies to fabricate reproducible nanostructures below the diffraction limit was possible in part by understanding the complex material transformation pathways triggered by laser pulses. A continuously growing scientific community has focused on the development of experimental techniques, mostly based on probing matter by light, to unravel the underlying interaction mechanisms. Often supported by theoretical modelling, these techniques have greatly contributed to today’s understanding of laser-matter interaction and enabled the identification of processing conditions that would have hardly been identified with empirical parameter scans. In this chapter, we review these light-based probing techniques, which interrogate in-situ the transient state of matter throughout its transformation from the initial state to the final state. Each technique is introduced and discussed in terms of its suitability for measuring certain transient changes induced, its spatial and temporal resolution, and is accompanied by results that illustrate its potential. Finally, a few examples of the applicability of some of these techniques in an industrial environment are given.