Role of Electron-Hole Recollisions in High Harmonic Generation from Bulk Crystals

When intense laser pulses interact with an atomic or solid target, high order harmonics of the fundamental laser frequency are generated. In the case of atoms, this highly nonlinear optical process is initiated by ionization and terminated by the\ud energetic recollision and recombination of the ionized electron with its correlated\ud ion. In this thesis I demonstrate, both theoretically and experimentally, that high\ud harmonics from bulk crystals can originate from the recollision of electrons with\ud their associated holes, similarly to the atomic case, but where ionization is replaced\ud by excitation of electron-hole pairs that accelerate within the material. This model\ud is first derived from a quantum-mechanical theory of the solid-laser interaction, and\ud then confirmed experimentally in ZnO and Si crystals. Despite the link I establish\ud between high harmonic generation in solids and gases, there are notable dissimilarities. These include: a generalized motion of electrons and holes in their respective bands and its consequences, a more prominent role of dephasing and enhanced sensitivity to perturbing fields. These aspects are investigated throughout this thesis. Finally, I develop a method that exploits the recollision mechanism to reconstruct the momentum-dependent band structure of solids