Blue-shifted third-harmonic generation and correlated self-guiding during ultrafast barrier suppression ionization of subatmospheric density noble gases

The generation of frequency upshifted, pressure-tunable third-harmonic pulses during ultrafast ionization of subatmospheric noble gases at peak intensity 10(16) W/cm(2) is reported. Pressure tuning of the power spectrum centroid over the range 10(-5) less than or similar to Delta omega/omega less than or similar to 10(-1), corresponding to a pressure range of 0.1 less than or similar to p less than or similar to 700 Torr, is demonstrated. We also demonstrate that the blue-shifted portions of both the 3 omega and the omega pulses are preferentially self-guided in a limited pressure range (10 less than or similar to p less than or similar to 500 Torr), which is key to separating the pressure-tunable component of the harmonic from nontunable harmonic generated by means of atomic nonlinearities. (C) 1996 Optical Society of America