Enhancement of Above Threshold Ionization in Resonantly Excited Helium Nanodroplets

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 (MR) or near-ultraviolet (UV) pulses. The electron emission due to high-order ATI is enhanced by several orders of magnitude compared with He atoms. The crucial dependence of the ATI intensities with the number of excitations in the droplets suggests a local collective enhancement effect.

Automated summary

Clusters and nanodroplets have the potential to enhance high-order nonlinear optical effects due to their high local density, but only moderate enhancement has been achieved so far. By using ultrashort extreme ultraviolet (XUV) free-electron laser pulses to resonantly excite helium (He) nanodroplets, a significant increase in energetic electron emission compared to He atoms has been observed. This suggests a local collective enhancement effect dependent on the number of excitations in the droplets.