We study in detail a time-dependent light-matter interaction process that appears highly non-linear: high harmonic generation (HHG). In this process an electron is tunnel ionised from the valence shell of an atom, accelerated in an oscillating electric field due to the interacting laser pulse, and brought back to recombine with the parent ion, releasing a photon multiple of the laser wavelength. Due to the non-linearity HHG has great potential as a cheap and tuneable tabletop source of extreme-UV light. To model and improve the efficiency of high energy returns we implement and explore the time-dependent Schrödinger equation (TDSE) and time-dependent density functional theory (TDDFT) approaches to theoretically simulate HHG for noble gases/laser-ablated plumes and bulk solids respectively.

In this presentation we will explore Kr, Xe and laser-ablated Mn in addition to bulk semiconductors Si and Diamond at mid-IR and near-IR laser wavelengths as quasi-1D HHG systems and demonstrate the underlying correlation dynamics and generated enhancement factors for Xe and Mn leading to improved HHG efficiencies promoted by resonance, as observed in experiment. Similarly we will explore the recent push towards bulk solids for HHG due to their increased densities and analyse the difference in spectral features for both gas and solid targets, in particular demonstrating good baseline theoretical simulations of HHG accurate to experiment without applying ultrafast dephasing times or treatment of light-propagation effects for solids. In extension we will present a first-of-its-kind molecular dynamics calculation to model dephasing and utilise coupled Maxwell equations in TDDFT to explore these effects and demonstrate their importance in achieving even greater agreement with experiment.