Professor Justin Wark

Abstract:

High harmonics have been observed from a 0.8 TW, 350-fs KrF CPA laser focused to 8 x 10(17) W cm(-2) in gas jet targets. Estimates of the absolute conversion efficiencies have been made. The efficiency and relative strength of the harmonics was found to be largely independent of the position of the laser focus within the gas jet, and of the density, suggesting a coherence length limited by dephasing effects due to plasma electrons. The highest harmonics are attributed to ions rather than neutrals.

Abstract:

We report on multiphoton ionization experiments using picosecond (ps) and sub-ps W-laser radiation at focused intensities up to 10(18) W/cm(2). The experiments are concerned with determining the electron temperature of optically ionized gases produced by intense KrF lasers. Thomson scattering, stimulated Raman scattering (SRS), and X-ray emission measurements have been made and compared with modeling calculations of heating. A particular objective is to identify the respective roles of above-threshold ionization, nonlinear inverse bremsstrahlung absorption, and SRS in determining the temperature of the electrons. Results for 350-fs pulses are compared with previous measurements for 12-ps pubes (for which strikingly different behavior is observed). The importance of using sub-ps, short-wavelength lasers to minimize electron temperature is confirmed.