Physical mechanisms of electron emission from fibre optic nanotips, namely, tunnelling, multi-photon, and thermionic emission, either prevent fast switching or require intense laser fields. Time-resolved electron emission from nano-sized sources finds applications ranging from material characterisation to fundamental studies of quantum coherence. We present a nano-sized electron source capable of fast-switching (⩽1 ns) that can be driven with low-power femtosecond lasers. The physical mechanism that can explain emission at low laser power is surface plasmon enhanced above-threshold photoemission. An electron emission peak is observed and provides support for resonant plasmonic excitation. The electron source is a metal-coated optical fibre tapered into a nano-sized tip. The fibre is flexible and back illuminated facilitating ease of positioning. The source operates with a few nJ per laser pulse, making this a versatile emitter that enables nanometrology, multisource electron-lithography and scanning probe microscopy.

No abstract

We demonstrate a nanotip electron source based on a graded index multimode silica optical fiber, tapered at one end to a radius of curvature r ~ 50 nm and coated with a thin film of gold. We report observation of laserinduced electron photoemission at tip bias potentials below the onset of dark field emission (FE). Single-photon photofield emission (PFE) is identified as the emission mechanism, which exhibits fast switching times with an upper limit on the order of 1 μs. The explored fiber optic nanotips are flexible back-illuminated emitters, which can be operated in CW and pulsed modes using lasers with photon energies in the visible range or higher. The mechanical flexibility of the source can facilitate externally controlled positioning. Multiple, individually addressable, nanotips may be assembled into a bundle for applications such as computational electron ghost imaging.Nanotip electron photocathodes have been widely studied in recent years as point-like emitters. This is incentivized by the advances in laser technology, which has enabled joint time-resolved and spatially coherent operation [1][2][3][4][5] . Commonly, a pulsed laser beam is sent into the vacuum chamber hosting the needle source where it is tightly focused onto the nanotip apex 6-8 . This makes the beam alignment on the tip challenging and prone to mechanical vibrations. A few back-illuminated tip sources, for which some strict optical alignment requirements are circumvented, have been realized. For example, a 100-μm-long diamond nanotip needle, back-illuminated with a low-repetition-rate ns laser in the UV range, was shown to photoemit in the short-wavelength (i.e. single-photon absorption) domain 9 . Other works include the demonstration of an optical-fiber-based tungsten (W) (10 nm) nanotip source, in which the employed lowpower continuous-wave (CW) diode laser triggered electron emission, and the significant difference in current rise time (~ 0.01 s) and the fast optical switching time is indicative of thermionic emission 10 . The reported emission was observed in a scanning near-field optical microscopy (SNOM) multimode fiber which constituted a sub-wavelength aperture at its tapered apex. Similarly, a second fiber optic electron source was also studied in which the flat core end-surface of a wide-area fiber was coated with gold (Au)