Gases of exploding clusters as a nonlinear optical medium

Summary form only given. Gases of clusters of noble atoms are ionized and explode when illuminated by intense laser pulses. The ensemble response of the time dependent polarizability of the clusters contributes to the effective dielectric constant of the gas and gives rise to interesting nonlinear phenomena. We explore these processes in a variety of ways. First, the heating of a single argon (Ar) cluster by a strong laser field is studied using an electrostatic particle-in-cell code (PIC) for a range of intensities and cluster sizes. Heating is dominated by a nonlinear resonant absorption process involving energetic electrons transiting through the cluster. This process gives rise to a threshold in field strength for strong absorption and controls the polarizability of the cluster. Calculations will be presented showing the dependence of the heating rate on cluster size, laser polarization, intensity and pulse duration. The results of the PIC code will also be compared with hydrodynamic calculations that are valid at lower intensities, below the threshold. Second, we simulate the observed self-guiding of a high-intensity (10/sup 14/-10/sup 16/ W/cm/sup 2/) laser pulse with femtosecond pulsewidths (70 fs to 1.5 ps) in the plasma formed by the interaction of the pulse with a gas of atomic clusters. Applications of this include generation of X-rays and energetic ions, and high harmonic generation among others. Results demonstrating the guiding effect are based on a Gaussian optics description of the laser pulse and a uniform density description of the exploding clusters modified to match the single cluster polarizability from the more realistic hydrocode runs. The equilibrium and stability of the guided states would be discussed in details in the paper. Also of interest is the modification of the pulse spectrum due to frequency selective absorption of the pulse and frequency shifts induced by the time dependence of the cluster polarizability.