High-Order Harmonic Generation Driven by Mid-Infrared Laser Light

<p>Light is a powerful tool for making observations of the physical world. In particular, light in the extreme ultraviolet (EUV) and X-ray regimes enable unique and higher resolution measurements than is possible using longer wavelengths. A relatively new technique called high-order harmonic\&nbsp; generation (HHG) provides a route for scientists to produce light in these useful spectral ranges, starting with lasers operating at more accessible wavelengths. HHG has been successfully applied to a number of applications including high resolution microscopy, spectroscopy, and measurements of magnetism, thermal transport, and molecular structure.</p> <p>This dissertation covers several illuminating studies of HHG in the temporal and spectral domains when the process is driven by long wavelength, mid-infrared light. Interestingly, the characteristics of the harmonic emission are highly dependent on the driving laser parameters and geometries. As the driving laser wavelength is increased, the harmonic cutoff and bandwidth naturally broaden, while the emitted pulse train reduces in length until a single isolated burst of phase-matched harmonics with sub-femtosecond duration is achieved. This trend is experimentally verified by performing an electric field autocorrelation of the harmonic emission. The resulting HHG supercontinuum has particular utility in X-ray absorption fine structure spectroscopies, where the nanoscale lattice structure can be probed. These spectroscopies have been performed on polymer, scandium, and iron samples using the broadest HHG bandwidths achieved to date, extending up to 1.6 keV. Pushing this harmonic cutoff further would conventionally require the use of longer wavelength drivers approaching the far-infrared regime. However, long driving wavelengths can also result in relativistic effects, resulting in longitudinal Lorentz drifts that could cause the HHG process to be inhibited. A theoretical accounting of all of the forces involved does not indicate HHG would be shut off entirely, however, and it is possible for HHG to occur even with driving wavelengths beyond 10 μm and harmonic cutoffs in the hard X-ray regime. The use of cylindrical vector beams or multi-beam geometries can also be used to compensate for relativistic effects, as well as to create new phase-matching conditions for sum and difference frequency processes. Through high-order difference frequency generation in a two-beam noncollinear geometry, it is predicted that the conventional phase-matching limitations could be signicantly exceeded, opening up the possibility to use visible drivers to reach the soft X-ray regime or further. Pushing the limits of the HHG spectral characteristics would inevitably enable new levels of capability for its applications.</p> <p>The investigations presented here will follow a progression from shorter to longer wavelengths as drivers for the HHG process, starting with experiments using the most commonly used Ti:sapphire wavelength of 800 nm, moving to 1.3 μm and 2.0 μm, then up to 3.9 μm, and ultimately arriving at theory for far-infrared drivers up to 20 μm. Furthermore, the conventional single-beam driving conguration will be primarily investigated, but new capabilities are predicted for multi-beam and multi-color geometries, which will be discussed.</p>
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University of Colorado Boulder
Boulder, Colorado
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