Harmonic generation from laser-irradiated clusters

Abstract

The harmonic emission from cluster nanoplasmas subject to short, intense infrared laser pulses is analyzed by means of particle-in-cell simulations. A pronounced resonant enhancement of the low-order harmonic yields is found when the Mie plasma frequency of the ionizing and expanding cluster resonates with the respective harmonic frequency. We show that a strong, nonlinear resonant coupling of the cluster electrons with the laser field inhibits coherent electron motion, suppressing the emitted radiation and restricting the spectrum to only low-order harmonics. A pump-probe scheme is suggested to monitor the ionization dynamics of the expanding clusters. PACS numbers: 36.40.Gk, 52.25.Os, 52.50.Jm During the last decade, the study of rare gas and metal clusters interacting with intense infrared, optical and ultraviolet laser pulses has emerged as a promising research field (see [1] for a state of the art-review). Generation of fast electrons and ions, production of high charge states, generation of x-rays and fusion in deuterium clusters have been observed in laser-cluster experiments with laser intensities up to 10 20 W/cm 2 . The hot and dense nonstationary, nanometer-sized plasmas far from equilibrium whose dynamics occur on the femtosecond time scale-so-called nanoplasmas-are new physical objects with interesting properties very different from plasmas where finite-size effects can be neglected. The probably most important feature of these nanoplasmas is the unsurpassed efficient energy transfer from the laser light to the charged particles [2].While the energy absorption by cluster nanoplasmas has been widely studied both in experiments and theory, much less attention has been paid to the laser harmonic emission from such systems. Meanwhile, it is known from the physics of intense laser-atom interaction that the effects of multiphoton absorption, leading to so-called above threshold ionization and high order harmonic generation, are intimately related and can be described as different channels of the highly nonlinear laser-atom coupling. In intense fields, the laser-cluster coupling is also known to be highly nonlinear. In fact, nonlinear resonance [3] has been shown (both in simulations and simple analytical models [4,5,6,7]) to be of particular importance for the energy transfer from the laser pulse to the electrons of the nanoplasma and the subsequent outer ionization. Most naturally the question arises whether laser-driven clusters can be an efficient source of highorder harmonics as well. Up to now, very few experimental results on harmonic generation (HG) from cluster targets have been published. In Refs. [8,9,10] HG from rare-gas clusters irradiated by infrared pulses of moderate intensity (≃ 10 13 − 10 14 W/cm 2 ) was measured. It was shown that under such conditions harmonics can be generated up to higher orders and with higher saturation intensities than in a gas jet. However, the applied intensities were definitely not high enough to create a dense nanoplasma inside clusters so that the effects ob...

Light wave driven electron dynamics in clusters

“…The nonlinearity of this process arises from the fact that the laser displaces the electron cloud relative to the ions. The electrons that are driven beyond the cluster surface feel a nonlinear restoring force, resulting in the creation of odd harmonic radiation [56,57]. For the cluster sizes investigated here, propagation effects do not play a major role in odd harmonic generation, as they are well described by electrostatic MD (see Fig.…”

Application of nanoparticle-containing laser plasmas for optical harmonic generation

“…7,9,10,14 It may be noted that a pronounced resonant enhancement for NPs is found only for yields of the low-order harmonics, while the high-order harmonics completely disappear from the harmonic spectra in the simulations. 14 The controlled enhancement of low-order harmonics from 6 ϫ 10 5 -noble-gas clusters was discussed in Ref. 4.…”

“…In the meantime, previous studies of harmonic generation in such objects were limited to exotic nanoclusters ͑Ar, Xe͒, which are formed in high-pressure gas jets due to rapid cooling by the adiabatic expansion. The physical origin of this process in the gas clusters [5][6][7][8][9][10][11][12][13][14][15] is mostly attributed to standard atomic harmonic generation, modified by the fact that in clusters the atoms are disposed to each other.…”

Characterization of cluster/monomer ratio in pulsed supersonic gas jets

“…Shim et al [29] observed this resonantly enhanced third harmonic generation from expanding clusters. Molecular dynamic (MD) and particle-in-cell (PIC) simulations [30,31] confirm the resonant enhancement of low-order harmonics, but show that high-order harmonics are suppressed because of the dynamical stochasticity of the electron motion caused by the nonlinear resonance.…”