Abstract

Understanding the interaction of an intense ultrashort laser pulse with an optical-field-ionized plasma is of fundamental interest for recently proposed x-ray laser schemes [1] and the guiding of optical pulses in plasma channels [2]. Because field ionization rates depend strongly on the laser intensity which varies in both time and space, both the spatial and temporal properties of field-ionized plasmas must be considered when characterizing such plasmas by ionization induced spectral blueshifting. Spectral blueshifting occurs when an intense laser pulse propagates through a gas and field ionization causes a rapid increase in the electron density along the propagation path. Temporal variation in the electron density causes plasma-induced self-phase modulation, and the spectrum of the laser pulse consists of blueshifted components [3]. In previous work, we have investigated the spectral characteristics of spectral blueshifting using a femtosecond KrF excimer laser pulse focused to peak intensities of 10¹⁴ Wcm^-2 in high pressure rare gases [4]. Here, we present the results of two-color pump probe experiments to investigate the temporal and spatial characteristics of spectral blueshifting, and demonstrate new techniques for visible and VUV ultrashort pulse characterization based on plasma-induced cross-phase modulation.

© 1994 Optical Society of America