High harmonic generation in gas phase and condensed matter

Abstract

The thesis presented here is comprised of two major investigations into strong field processes resulting from interaction between femtosecond laser pulses and atoms, either in isolation or assembled into a crystalline solid form. Our theoretical approach throughout these investigations is based on the numerical solution of the time-dependent Schrodinger equation (TDSE) using massively parallelised, high-performance computational tools. In particular, Chapter 2 explores high harmonic generation (HHG) in gas targets, a non-linear process in which an electron escapes an atomic system via tunnelling only to recombine with its parent system due to the oscillation of the interacting laser pulse and release a harmonic photon multiple of the driving laser frequency. In our gas HHG investigations we simulate high harmonic spectra using an efficient single-active-electron (SAE) TDSE treatment for noble gases and transitional metals, and introduce a novel efficient approach to modelling electron correlation processes in HHG, a multiplicative correlation enhancement factor (CEF) constructed as a ratio of photo-ionisation cross-sections (PICS) under the Hartree-Fock Approximation (HFA), without correlations, and the Random Phase Approximation with Exchange (RPAE), with correlations. We demonstrate the success of the adopted TDSE treatment for HHG with krypton and xenon targets where we reproduce the Cooper minimum at ~ 88 eV for the former and demonstrate the giant autoionizing resonance (GAR) at 100 eV using our CEF approach for the latter. We also consider xenon in a two-colour setup and demonstrate the success of the CEF approach in more complex laser pulse situations, reproducing the observed enhancement across varying relative phase. Our results across our HHG simulations for gas targets provide us with a solid foundation for further investigation into HHG in transition metals and solids. In Section 2.2.3 we demonstrate the application of the correlation enhancement approach to transition metal Mn and its ionic species Mn+, successfully reproducing the giant autoionizing resonance in both at 50 eV as reported in experiment and demonstrating the significant difference between ionic Mn+ and Mn in the contributions of the 4s and 3d_m=0 initial states for 400 nm calculations. Extending our investigation of HHG to solid targets, Chapter 3 explores a new simulation technique using time-dependent density functional theory (TDDFT) to model the richer dynamics of inter- and intra-band harmonic generation. We adopt an ab initio approach to model high harmonic generation, through the SALMON-TDDFT program and explore thin film semiconductors in line with recent work by collaborators. In particular, we review literature for diamond and establish clear harmonic structure where previously propagation and dephasing techniques had been required to resolve theory and experiment. We consider long 200 fs full duration pulses at 800 nm wavelengths and review recent literature regarding bulk silicon at 3000 nm and demonstrate clear harmonics at 2000 and 3000 nm, failing to observe any joint density of states (JDOS) effect and subsequent noisy-to-clean harmonic transition. We also demonstrate the effect of electromagnetic propagation through Maxwell+TDDFT calculations for thick samples of silicon, finding a pronounced effect as noted by Floss et al. previously. Finally we demonstrate the importance of the dephasing effect through a first-of-its-kind molecular dynamics simulations for silicon, without requiring phenomenological relaxation parameter T_2, and suggest helium or liquid nitrogen cooling of solid targets could improve harmonic returns. Finally we summarise and consider the future research based on the body of work presented here in Chapter 4.