Ionisationsdynamik von Clustern in intensiven Laserfeldern
Final Report Abstract
We studied the interaction of intense laser pulses with atomic clusters using threedimensional particle-in-cell simulations and analytical models. The physics addressed included: (i) collisionless absorption, (ii) optimization of laser energy conversion, and (iii) harmonic generation in clusters. We showed that in the short-pulse regime efficient laser absorption and outer ionization occurs due to nonlinear resonance (NLR) absorption. At the time of NLR the instantaneous frequency of the electron motion in a time-dependent, anharmonic, effective potential transiently meets the laser frequency. NLR is the only possible absorption mechanism if the laser pulse is too short for the linear resonance to occur (or during the early cluster dynamics in longer pulses) and if electronion collisions are negligible. Maximum ion energies are mainly carried by the ions of highest charge state which appear close to the cluster boundary along the laser polarization. For deuterium clusters, although ions are only singly charged, high energy ions are also emitted predominantly along the laser polarization because of the asymmetric electron dynamics in a linearly polarized laser field. An optimization of laser energy conversion in a given cluster at a given laser intensity was envisaged with respect to the laser wavelength. It was shown that there exists an optimum laser wavelength which typically lies in the ultraviolet regime and may allow for a linear resonance during the early, rising part of a few-cycle pulse when the cluster expansion is inappreciable. The linear resonance for this optimal wavelength led to particularly efficient energy absorption and optimized average ion charge states much higher than for the conventional linear resonance (LR) occurring in the expanding cluster in the long-pulse and long wavelength regime. We investigated the harmonic emission from laser-driven cluster nanoplasmas. The main contribution to the harmonic signal was due to the deeply bound coherently oscillating electrons inside the cluster potential. The electrons leaving the cluster through the NLR emit incoherent radiation and therefore higher order harmonics are absent. We proposed a pump-probe experiment to measure the cluster charge density by detecting the dipole radiation at different time delays.
Publications
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D. Bauer and P. Mulser, Irreversible energy gain by linear and nonlinear oscillators, J. Phys.: Conf. Ser. 11, 169 (2005).
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D. Bauer and P. Mulser, Vacuum heating vs skin layer absorption of intense fs laser pulses Phys. Plasmas 14, 023301 (2007).
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M. Kundu and D. Bauer Nonlinear resonance absorption in the laser-cluster interaction Phys. Rev. Lett. 96, 123401 (2006).
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M. Kundu and D. Bauer, Collisionless energy absorption in the short-pulse intense laser-cluster interaction Phys. Rev. A 74, 063202 (2006).
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M. Kundu and D. Bauer, Optimizing the ionization and energy absorption of laser-irradiated clusters Phys. Plasmas 15, 033303 (2008).
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M. Kundu, S.V. Popruzhenko, and D. Bauer, Harmonic generation from laser-irradiated clusters Phys. Rev. A 76, 033201 (2007).
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S.V. Fomichev, D. F. Zaretsky, D. Bauer, and W. Becker, Classical molecular-dynamics simulations of laser-irradiated clusters: nonlinear electron dynamics and resonance-enhanced low-order harmonic generation, Phys. Rev. A 71, 013201 (2005).
