Laser-driven nanoplasmas in doped helium droplets: local ignition and anisotropic growth.

Doping a helium nanodroplet with only a tiny xenon cluster of a few atoms sparks complete ionization of the droplet at laser intensities below the ionization threshold of helium atoms. As a result, the intrinsically inert and transparent droplet turns into a fast and strong absorber of infrared light. Microscopic calculations reveal a two-step mechanism to be responsible for the dramatic change: Avalanchelike ionization of the helium atoms on a femtosecond time scale, driven by field ionization due to the quickly charged xenon core, is followed by resonant absorption enabled by an unusual cigar-shaped nanoplasma within the droplet.