Ultra-Nonlinear\nSubcycle Photoemission of Few-Electron\nStates from Sharp Gold Nanotapers

The generation of ultrashort electron wavepackets is\ncrucial for\nthe development of ultrafast electron microscopes. Recent studies\non Coulomb-correlated few-electron number states, photoemitted from\nsharp metallic tapers, have shown emission nonlinearities in the multiphoton\nphotoemission regime which scale with the electron number. Here, we\nstudy few-electron photoemission from gold nanotapers triggered by\nfew-cycle near-infrared pulses, demonstrating extreme 20th-order nonlinearities\nfor electron triplets. We report interferometric autocorrelation traces\nof the electron yield that are quenched to a single emission peak\nwith subfemtosecond duration due to these high nonlinearities. The\nmodulation of the emission yield by the carrier-envelope phase suggests\nthat electron emission predominantly occurs during a single half cycle\nof the driving laser field. When applying a bias voltage to the tip,\nrecollisions in the electron trajectories are suppressed and coherent\nsubcycle electron beams are generated with promising prospects for\nultrafast electron microscopy with subcycle time resolution.