Microscopic and Macroscopic Aspects of High Harmonic Generation

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Short pulses of coherent light in the extreme ultraviolet (XUV) spectral region can be used to investigate the ultrafast dynamics of atomic, molecular, and condensed-phase processes, which typically occur on the few-femtosecond time scale, and can be important in a wide range of scientific applications. A process known as 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 pulses. In this project, mechanisms underlying HHG will be studied through theoretical modeling and calculations that consider both the microscopic and macroscopic laser-matter interactions. Some of these studies will characterize the properties of the XUV light generated by HHG, whereas other studies will 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 several experimental groups in the US and abroad.In the source development studies associated with this project, the LSU group will concentrate on characterizing and optimizing the HHG process from transparent crystals. It is only recently that such crystals have been shown to produce harmonics, and this has generated a lot of excitement due to the potential for increased yield and ease of manipulation of the generated light. However the mechanism by which harmonics are generated in condensed phase materials is still not well understood. In this part of the project, the group will expand on their current microscopic, quantum mechanical model for the laser-crystal interaction, which links the emitted light to the band structure of the crystal, as well as build a large-scale model describing the process of generation, propagation, and absorption of the XUV light in the macroscopic medium. In a separate part of this project, the LSU group will probe the dynamics of the strong-field laser-matter interaction itself, primarily through the investigation of HHG in the vicinity of excited states in atomic gases. In particular, a search will be conducted for signatures of these excited states in the spatial, temporal, and spectral properties of the emitted XUV light.