Energy-Resolved Cross-Correlation of THz Field Emission of Electrons from a Metal Nanotip

Abstract

Intense THz pulses, combined with the lightning-rod field enhancement at metal nanotips, can drive field emission of keV-energy, femtosecond-duration electron bunches, yet the dynamics governing this emission remain largely unexplored. Here we probe these dynamics by cross-correlating the energy-resolved emission driven by two independently generated single-cycle THz pulses focused onto a tungsten nanotip, scanning their relative delay over 160 ps. At total bunch charges approaching $10^6$ e$^-$, the delay-energy emission map near zero delay reveals electrons accelerated up to 2.1 keV, with a cross-correlation width of ~210 fs full width at half maximum at the highest energies. Apart from replicas caused by optical reflections in the setup, no emission echoes are observed over the full delay range, ruling out long-lived antenna resonances such as Sommerfeld-wave reflections along the tip shaft. A 1D Fowler-Nordheim model incorporating a low-$Q$, sub-picosecond antenna response for the local field enhancement reproduces the peak electron energies and the asymmetry of the delay-energy map. Our results indicate that each THz pulse produces a single, isolated electron burst, and that energy filtering can select its shortest, highest-energy component.