Pulse-Length Dependence of the Anisotropy of Laser-Driven Cluster Explosions: Transition to the Impulsive Regime for Pulses Approaching the Few-Cycle Limit

We have experimentally observed anisotropic ion emission from Xe and Ar clusters under intense ultrashort (∼30 fs) laser irradiation, with up to 1.8 times more energetic ions emitted in the direction perpendicular to the laser polarization than in the parallel direction. As the pulse length was varied in the range of 8-160 fs, we found this anisotropy to first grow and then diminish. Treating electrons inside the unexpanded cluster as a harmonic oscillator qualitatively demonstrates how intracluster electric field can result in an ion emission anisotropy of this kind. Our observations give direct access to the initial charging dynamics present in the first few cycles of an intense laser field interacting with any nanoscale dielectric.