Collective enhancement of above threshold ionization by resonantly excited helium nanodroplets

TL;DR

This paper demonstrates that resonantly excited helium nanodroplets significantly enhance above-threshold ionization electron emission, revealing a collective effect that boosts high-order nonlinear optical processes compared to isolated atoms.

Contribution

It reports the first observation of collective enhancement of ATI in helium nanodroplets excited by ultrashort XUV pulses, showing orders-of-magnitude increase over atomic responses.

Findings

Electron emission is enhanced by several orders of magnitude.

The enhancement depends on the number of excitations in the droplets.

Evidence suggests a local collective enhancement effect.

Abstract

Clusters and nanodroplets hold the promise of enhancing high-order nonlinear optical effects due to their high local density. However, only moderate enhancement has been demonstrated to date. Here, we report the observation of energetic electrons generated by above-threshold ionization (ATI) of helium (He) nanodroplets which are resonantly excited by ultrashort extreme ultraviolet (XUV) free-electron laser pulses and subsequently ionized by near-infrared (NIR) or near-ultraviolet (UV) pulses. The electron emission due to high-order ATI is enhanced by several orders of magnitude compared to He atoms. The crucial dependence of the ATI intensities with the number of excitations in the droplets suggests a local collective enhancement effect.