Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Margaret M. Murnane

Second Advisor

Henry C. Kapteyn

Third Advisor

Steven Cundiff

Abstract

Recently, ultrafast, coherent X-ray science and technology has received a lot of attentions, especially with respect to tabletop X-rays produced by high harmonic generation (HHG). The femtosecond-to-attosecond X-ray pulse enables the dynamics of chemical reactions, nano-materials and bio-molecular systems to be studied with unprecedented temporal and spatial resolution. However, the bright HHG light source is limited to < 150 eV spectral region due to phase-mismatch issue and the lack of the right driving laser wavelength. Development of phase-matching scheme and different-wavelength lasers for extending bright HHG to shorter wavelengths becomes a challenge. Over the past five years, we essentially solve the high-harmonic phase matching problem using longer driving wavelengths. Our experimental results have shown that full phase matching of HHG scales very strongly with wavelength of the driving laser, making it possible for the first time to obtain bright phase-matched emission to the 0.5 keV using a 2 mum laser and the keV using a 3.9 mum laser with a conversion efficiency >1000 times that previously reported. Their supercontinuum bandwidths are capable of generation extremely short pulses down to single digit attoseconds ( 10--18 sec). This timescale is remarkable in that it approaches a new regime where light transit times approach atomic dimensions. On the other hand, by using a shorter wavelength of driving laser (0.4 um), we also optimized the HHG flux ˜ 10 times brighter than before in the EUV spectral region between 45 and 60 eV. We found that the most substantial HHG enhancement arises when the right combination of the laser wavelengths, gas species and gas pressures. A high flux tabletop coherent X-ray beam line is feasible to implement, for applications in biological and materials imaging, or as a seeding source for a free-electron laser amplifier.

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