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Ever since the birth of the first laser, the frontiers of light-matter interaction have continued to expand. Starting with the second harmonic conversion, essentially a two-photon process in eV energy scales, the state-of-art Petawatt (PW) lasers have demonstrated MeV energy scale phenomena such as inverse-Compton scattering. These PW lasers offer a platform to create very hot and dense plasmas, facilitating laboratory simulation of astrophysical phenomena, particle acceleration on a tabletop, real-time x-ray diffraction of condensed matter, biological imaging and medical therapies for cancer. As the peak laser intensity can be several orders of magnitude stronger than the atomic field intensity, infrared laser light (~eV photon energy) can create relativistic (> MeV) electrons in a solid via collective plasma processes. The dynamics of the interaction are very crucial and it is therefore important to track the ultrafast spatio-temporal evolution of the plasma.
We will explore the fundamental ideas of how such ultrashort, relativistic laser pulses interact with solids. We shall look at various experimental techniques we use to understand different plasma properties and their dynamics. Towards the end, we will focus on an important parameter called "laser contrast" and how that affects the fundamental interaction mechanisms. In particular, we will discuss how to generate "extreme-contrast" pulses, characterize them and use them in our experiments. |