Femtosecond continua produced in gases

Self-focusing and continuum generation are explored in gases using ultrashort 625-nm pulses. Observations of spectral broadening and beam propagation in gases were made both below and above the self-focusing threshold. It is shown that the nonlinearity responsible for self-focusing and self-phase modulation saturates at an intensity of approximately 10¹³ W/cm² in xenon. It was found that a diffraction-limited beam can be almost totally reconstructed spatially after passing through the focal region, even though self-focusing has been initiated. The spectrum, however, is catastrophically changed by the process. For intensities significantly above the self-focusing threshold, conical emission was observed. Most of the observations described herein were made using an ~90-fs, 625-nm pulse with a maximum energy of ~500 μJ