Focused-ion-beam-milled GaAs nanotip: A semiconducting ultrafast photoemission electron source

We report fabrication and characterization of a GaAs nanotip that is carved out of a shard from a GaAs wafer using focused-ion-beam-milling. GaAs-based electron photoemission tips have been shown to produce spin-polarized electrons [1], and ultrashort nanoscale pulses of polarized free electrons from such tips are of interest in time-resolved electron microscopy as well as fundamental research on quantum mechanics [2]. In the present work, we demonstrate multiphoton photoemission from an ion-milled GaAs nanotip using an ultrafast near-IR Ti:Sa laser oscillator with a repetition rate of 90 MHz. Moreover, using a Cs dispenser installed in the proximity of the tip, we show that the photocathode work function can be systematically lowered and controlled at residual gas pressures of 10^-7 Torr by changing the Cs emission rate from the dispenser which enhances the electron emission while lowering the photon number needed for emission. About 8 photoelectrons per pulse was achieved this way with a moderate Cs deposition rate and the laser pulses focused to a free-space intensity of 10¹⁰ W/cm². [1] E. Brunkow, et al., Appl. Phys. Lett. 114, 073502 (2019). [2] S. Keramati, et al., Phys. Rev. Lett. 127, 180602 (2021).