Two very efficient nonlinear laser absorption mechanisms in clusters.

Experiments show strongly enhanced absorption of ultrashort superintense laser beams in clustered matter in the so-called collisionless regime. Despite numerous particle in cell simulations confirming this behavior, the underlying physical processes are not sufficiently clear. The familiar linear resonance absorption does not apply as long as the plasma frequency exceeds that of the laser. However, we show here that with increasing laser intensity the oscillations become nonlinear and can enter into resonance with the laser frequency because of restoring force lowering in Coulomb systems. Excellent absorption already at moderate intensities is the consequence. The other absorption mechanism we analyze explicitly consists in the coherent superposition of electron-ion collisions in ionized clusters. Collisional absorption enhancement factors of several orders of magnitude are found.