Laser induced Coulomb explosion and stimulated Raman scattering in cluster plasmas

Abstract

Laser-driven Coulomb explosion can induce stimulated Raman scattering in cluster-embedded plasmas. The propagation and scattering of electromagnetic waves have been studied to show that Coulombic expansion of atomic clusters significantly modifies the scattering properties. When the cluster plasma collision is negligible, the cutoff frequency occurs due to the resonance, and this cutoff frequency is lowered as the cluster size increases. On the other hand, when there are collisions, the electron-ion collisions inhibit cluster electrons from absorbing laser energy resonantly, in which case the electromagnetic wave dispersion relation is not much affected by the presence of the cluster. Enhancement in the resonance absorption of laser energy is achieved when laser frequency becomes inversely proportional to the cubic root of the normalized cluster radius. The expansion of a dense cluster into the ambient plasma density is shown to accompany a swift decrease in the plasma wave frequency. The variation of the growth rate of the Raman backscattered wave with respect to the wave number, plasma density, and sizes of the clusters shows that, in a high-density cluster plasma, only long-wavelength modes survive until the end of the cluster expansion. The short wave scattering mode, which initially grows faster, quickly damps out in the early phase of the expansion.