Classical molecular-dynamics simulations of laser-irradiated clusters: Nonlinear electron dynamics and resonance-enhanced low-order harmonic generation

The nonlinear collective electron dynamics of a cluster irradiated by a strong near-infrared linearly polarized short laser pulse are studied by classical molecular-dynamics simulations for a small model cluster with similar to10(3) particles. The model brings forth almost all of the features of a cluster exposed to a strong laser pulse, such as inner and outer ionization, expansion of the ion core, etc. When the frequency of the incident radiation is near three- or five-photon resonance with the (time-dependent) frequency of the dipole Mie-plasmon excitation in the laser-ionized and expanding cluster, both the total electron acceleration and the local electric field acting on the ions inside the cluster exhibit a resonant enhancement at the odd harmonics of the fundamental frequency. The time evolution during the laser pulse of these odd harmonics is discussed for different parameters of the laser-cluster interaction. The presence of even low-order harmonics (in particular, of the second) in the local electric field at ion positions off the cluster center is also demonstrated. This indicates nonlinear laser excitation of the quadrupole surface plasmon.