A carbon-nanotube-based electron source with a 0.3-eV energy spread and an unconventional time delay.

Conventional metal-tip-based laser-driven electron sources are normally constrained by a trade-off between energy spread and pulse width due to optical-field-induced free electron acceleration. This makes it challenging to surpass the current state-of-the-art, which exhibits energy spreads exceeding 1 eV and pulse durations of hundreds of femtoseconds. Here we report an unconventional delayed emission from a one-dimensional carbon-nanotube-based electron source. By utilizing a special pump-probe approach, we apply 7-fs laser pulses to the carbon-nanotube emitters and observe free electron emission tens of femtoseconds after the pulse. This delayed emission results in a substantially reduced energy spread of approximately 0.3 eV and an electron pulse width of about 13 fs. Through time-dependent density functional theory calculations, we find that the delayed emission is driven by the interplay of collective oscillations and electron-electron interactions. Our results may provide a promising technology for developing cutting-edge ultrafast electron sources.