Self-guiding and red shifts of intense pulses propagating in clustered gases

Summary form only given. Intense laser interaction with clusters, Van der Waals-bonded aggregates of up to /spl sim/10/sup 7/ atoms, is of great interest owing to applications which include the generation of X-rays, energetic ions and electrons, and nuclear particles. In recent work, the time-resolved explosion of intense-laser-heated clusters has been observed, and this explosion scenario leads to intense laser self-focusing in a clustered gas. Here, we report measurements of the frequency shifts of intense laser pulses propagating in clustered gases. This measurement is consistent with our previous observations and model. We use a pump-probe setup for the frequency shift measurement. For a series of time-delayed 2/spl omega/ probe (sub-80 fs, 399 nm) spectra modulated by an intense pump (1.5 mJ, 80 fs, 798 nm) pulse co-propagating in a gas of argon clusters, we find that the probe increasingly red-shifts during the phase where the pump self-focuses. At later times, the shift goes blue and then relaxes back to zero. The red-shifts at early delays indicate the positive transients of the refractive index of pump-heated clusters, in agreement with our recent observation of self-focusing. This red-shift strongly contrasts with the blue-shift commonly observed in the ionization of non-clustered gases. At later times, however, the probe frequency shift becomes blue and then relaxes to zero owing to the dominant response of the underdense portion of the cluster plasmas at later times, with that response becoming progressively weaker.