|Title||High power ultrafast laser design and high-order harmonic generation in capillary discharge plasmas|
|Year of Publication||2007|
Chirped pulse amplification (CPA) has been used for over twenty years to generate high power, sub-picosecond laser pulses. In a chirped pulse amplifier, an ultrashort pulse is stretched, amplified, and compressed, usually using grating stretchers and compressors. Despite the low efficiency (50–60%) of grating compressors, this CPA scheme has remained virtually unchanged since the first demonstration [Strickland and Mourou, Opt. Commun. 56, 219 (1985)]. In the first half of this thesis, a novel amplifier design, based on downchirped pulse amplification (DPA), is presented. The DPA design utilizes a negative dispersion stretcher and a positive dispersion compressor, usually a block of transparent material. This design avoids many of the limitations, such as low efficiency and sensitivity to alignment, of traditional grating- and prism-based CPA systems. In addition to presenting the first demonstration of a DPA based amplifier system, the DPA scheme is combined with a simple, compact stretcher based on grisms to construct an ultrafast amplifier scalable to >40 kHz, a previously underdeveloped amplifier repetition frequency range.
CPA systems have also enabled the study of high-field physics, specifically highorder harmonic generation. High-order harmonic generation is a process which generates extreme-ultraviolet radiation through the interaction of an intense laser field with a medium, usually a noble gas. Depletion of the neutral atoms, plasma defocusing, and energy loss due to ionization limit the laser intensity and ultimately the highest possible photon energy that can be generated. Using ions in a waveguide as the generating medium can minimize these limitations, extending the maximum photon energy.
In the second half of this thesis, a method for generating high harmonic radiation from ions is demonstrated. A capillary discharge is used to preionize a low pressure xenon gas before a high intensity, ultrashort pulse is injected. The discharge generates ions and mitigates the plasma defocusing and ionization loss experienced by the ultrashort laser pulse, allowing for the generation of photon energies much higher than those previously observed in xenon. This method could be combined with the technique of quasi-phasematching to efficiently generate short wavelength radiation.