Theory of High Harmonic Generation in Solids and Gases, at the Microscopic and the Macroscopic Level

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The use of ultrafast pulses of coherent light in the extreme ultraviolet (XUV) spectral region helps us understand the dynamics of atomic, molecular, and condensed-phase processes at the few-femtosecond and sub-femtosecond time scale. The natural time scale for electron dynamics is tens or hundreds of attoseconds, and these ultrafast pulses promise the capture and control of this "motion". The process of high harmonic generation (HHG) resulting from the interaction between an intense laser pulse and a nonlinear medium has proven to be a versatile source of such XUV light. In this project the PI and her group will study different aspects of the HHG process through calculations that consider both the microscopic and macroscopic laser-matter interactions. Some of these studies aim to characterize the properties of the XUV light itself, whereas other studies aim to probe the inherent dynamics of the strong-field interaction with the nonlinear medium. The majority of the work will be directly relevant to ongoing collaborations with experimental groups in the US. The project will serve the national interest through the progress of ultrafast science, as well as through development of human resources via the training of junior researchers at the undergraduate, graduate, and postgraduate levels.The PI and her group will study HHG in both gases and solids. For gas-phase HHG, the group will continue a collaboration with an experimental group with the goal of producing isolated attosecond pulses at MHz repetition rates in an enhancement cavity. For bulk-phase HHG one goal will be to investigate fundamental aspects of the generation process, in particular how the real-space picture of HHG in a bulk medium evolves from that in a gas medium. This will be done through a series of both model and ab-initio calculations. A second goal will be to build a flexible numerical tool for ab-initio calculations of HHG from a range of bulk solids. This tool will be based on inputs from a large-scale band structure code and coupled solutions of the semi-conductor Bloch equations and the Maxwell wave equation. This approach will facilitate the treatment of both linear and nonlinear processes that arise as the intense laser pulse propagates through the nonlinear crystal.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.