Walter Scott, Jr. College of Engineering

Graduate Exam Abstract

Brendan Reagan
M.S. Final
Feb 29, 2008, 3:30
Forestry 107
Enhanced High Harmonic Generation in a Capillary Discharge
Abstract: High harmonic generation is a very useful method for the generation of ultra-short pulses of coherent, short wavelength light using compact setups. In this process a laser-like beam of short wavelength light is created through the nonlinear interaction of an intense visible or infrared laser with a medium, usually a low pressure gas. The highest photon energy generated through this process is proportional to the intensity of the driving laser. Therefore, the generation of extremely short wavelength requires very high laser intensities. The high laser intensities lead to ionization of the generation medium, and this presents several problems. Most importantly, this ionization defocuses the laser and limits the peak obtainable laser intensity, and therefore the highest harmonic (shortest wavelength) generated. This thesis discusses the demonstration of a new technique for the generation of high-order harmonics in a capillary discharge plasma column. A capillary discharge can create a preformed, ionized medium for high harmonic generation that has several advantages. First, it negates the effect of ionization-induced defocusing, giving rise to a significant extension of the highest harmonics generated. Second, the capillary discharge plasma constitutes an index waveguide that can channel intense laser pulses allowing a long interaction length between the driving laser and the medium. Finally, the discharge produces the ionized medium using “cheap” electrical energy instead of expensive laser energy. Using this technique, harmonic emission up to photon energies of 160 eV, 170 eV, and 275 eV were observed from Xe, Kr, and Ar ions respectively. In each gas, the observed photon energy was higher than any previously reported measurement. Additionally, a new method for tuning the wavelength of the generated harmonics by varying the discharge current was demonstrated. A detailed analysis of these results is also presented that includes a hydrodynamic-atomic physics model of the capillary discharge plasma, laser ionization rates calculations, and plasma waveguide measurements and simulations. The combination of a capillary discharge created medium with new phase-matching techniques promises to extend efficient high harmonic generation to very high photon energies, potentially greater than 1 keV.
Adviser: Prof. Jorge Rocca
Co-Adviser: NA
Non-ECE Member: Prof. Chiao-Yao She (Physics)
Member 3: Prof. Mario C. Marconi
Addional Members: NA
Program of Study: