SPECTRAL SHIFTING BY OPTICAL AND COLLISIONAL IONIZATION OF AN INTENSELASER-PULSE IN DENSE NOBLE-GASES

We report the changes observed in intense (similar to 10(15) W cm(-2)) 1-mu m-wavelength laser pulses propagating through argon and krypton at a range of densities below and above atmospheric. The transmitted p ulses show blue shifting of the fundamental spectrum and defocusing of the spatial profile. Our interpretation is aided by simultaneous meas urements of the plasma luminescence and also by comparison with a two- dimensional numerical model. We infer from our results that laser-indu ced collisional ionization is important at the higher gas densities ev en when pulses with a duration as short as 2 ps are used. (C) 1996 Opt ical Society of America