Optimal Control of the Strong-Field Ionization of Silver Clusters in Helium Droplets

TL;DR

This paper demonstrates how shaped femtosecond laser pulses, optimized via control techniques, significantly enhance the ionization of silver clusters in helium droplets, revealing a simple double-pulse structure that maximizes charge states.

Contribution

It introduces a combined experimental and numerical approach using optimal control to identify pulse shapes that maximize ionization of silver clusters in helium droplets, highlighting the role of plasmon resonance.

Findings

Enhanced Ag$^{q+}$ yield with shaped pulses

Double-pulse structure with prepulse and main pulse

Optimal pulses resonate with cluster surface plasmons

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.