Optimal control of the strong-field ionization of silver clusters in helium droplets

Abstract

Optimal control techniques combined with femtosecond laser pulse shaping are applied to steer and enhance the strong-field induced emission of highly charged atomic ions from silver clusters embedded in helium nanodroplets. With light fields shaped in amplitude and phase we observe a substantial increase of the Ag q+ yield for q > 10 when compared to bandwidth-limited and optimally stretched pulses. A remarkably simple double-pulse structure, containing a low-intensity prepulse and a stronger main pulse, turns out to produce the highest atomic charge states up to Ag 20+ . A negative chirp during the main pulse hints at dynamic frequency locking to the cluster plasmon. A numerical optimal control study on pure silver clusters with a nanoplasma model converges to a similar pulse structure and corroborates that the optimal light field adapts to the resonant excitation of cluster surface plasmons for efficient ionization.

Resonant charging of Xe clusters in helium nanodroplets under intense laser fields

“…Control of the absorption [17] and the emission characteristics by the pulse structure has been demonstrated with various schemes, e.g. stretched pulses [4,6,8,15], pumpprobe techniques [17,18,19,20], and multi-parameter pulse shaping [5,21]. Typically, resonant plasmon excitations lead to the emission of laser aligned energetic electrons [6,7,9] and highly charged atomic ions with high kinetic energy [8,20].…”

Control of Ionization in the Interaction of Strong Laser Fields with Dense Nanoplasmas

“…Experimentally, optimal control techniques combined with femtosecond laser pulse shaping were applied to steer and enhance the strong-field induced emission of highly charged atomic ions from silver clusters embedded in helium nanodroplets [8]. A Ti:sapphire laser system, consisting of an oscillator, a pulse shaper, and a pulse amplifier, delivered pulses with energies of up to 2.5 mJ and pulse durations down to 35 fs full width at half maximum (FWHM).…”

“…In this case, a multiparameter genetic algorithm procedure has been used for tailoring the pulses in order to maximize the abundance of highly charged ion species [8,9]. The following ansatz for the intensity www.cpp-journal.org envelope of the pulse, I(t), was made…”

“…In optimal control experiments, the yield of highly charged ions has been maximized by applying an evolutionary algorithm scheme within a feedback loop. Experiments as well as calculations were performed, e.g., for silver clusters [8,9] in which highly charged silver ions could be produced very efficiently with a pulse structure consisting of a weak pre-pulse followed by a main pulse with larger intensity. In the free-optimization experiment, the evolutionary algorithm optimizes the relative intensity ratio, the optimal delay and, eventually, the chirp of the trailing pulse.…”

Ionization enhancement and suppression by phase-locked ultrafast pulse pairs

“…Pulse trains have been used for coherent control of a diverse array of processes, such as photoelectron angular distributions [16,17], magnetization [18], and molecular vibration and rotation [19]. At the same time, they have appeared in optimal-pulse solutions in control experiments * dbfoote@umd.edu † wth@umd.edu [15,[20][21][22][23]. Deconstruction of these pulses, to determine how they achieve their goals, has proven to be a difficult and arduous task.…”