|Title||Phase Matching and Coherence of High-Order Harmonic Generation in Hollow Waveguides|
|Year of Publication||2008|
Over the last twenty years, high-order harmonic generation has emerged as a scientifically useful, temporally and spatially coherent source of extreme ultraviolet light. Because of the nature of the generation process, the harmonics are emitted in a series of attosecond bursts, creating a spectroscopic and imaging tool with high temporal and spatial resolving power. The coherence and short time-duration, in combination with the accessibility of this table-top source, have made high harmonic generation a truly unique light source, opening possibilities for new science.
The current limitation to this source is its brightness, limited mainly through poor phase matching of the nonlinear conversion process. This thesis presents work performed toward better understanding of the physics and the engineering of high har- monic generation, specifically in a waveguide geometry. First is a survey of the relevant theory and review of previous work on high harmonic generation and phase matching techniques. Presented next are selected experimental results related to optimizing the harmonic photon yield in hollow waveguides, including a measurement of the photon yield at 45 eV, measurements of the density-dependent energy loss in the driving laser beam, and a study of the effects of modulation depth in the technique of quasi-phase matching using hollow waveguides with a modulated inner diameter.
Counterpropagating light is used for the first time as a tool for measuring the in- situ coherence length of the harmonic generation process. Through this measurement of the coherence length, several other quantities can be inferred. The ionization level at which different harmonic orders are generated can be determined, giving insight into the temporal dynamics of high harmonic generation. Intensity variations caused by energy loss in the driving laser beam due to ionization or refraction, or by interference between coupled modes of the waveguide, are measured. The dynamic nature of the coherence length of high harmonic generation makes an in-situ measurement of this kind crucial for any implementation of quasi-phase matching.
Finally, all-optical quasi-phase matching of high harmonic generation is demon- strated by using trains of counterpropagating pulses for periodic correction of the phase mismatch. Enhancements of more than two orders of magnitude are demonstrated at high photon energies, where conventional phase matching techniques are currently not possible. All-optical quasi-phase matching is also shown to be selective in terms of enhanced bandwidth, and can even isolate one of the two electronic trajectories con- tributing to harmonic emission. These advances have the potential for improving the harmonic flux at high photon energies that cannot currently be phase matched, and for permitting manipulation of the temporal and spectral structure of the harmonic emission.