Photofield electron emission from an optical fiber nanotip

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)

Spatial Focusing of Surface Polaritons Based on Cross-Phase Modulation

“…Spatial focusing of surface plasmon polaritons (SPPs), especially at nanoscale, recently has been one of the hot spots in the field of micro-nano optics due to its huge application potentials [1]. It not only provides a powerful technical basis for the development of nano optical devices, but also extends the research realm of strong field micro-nano optics [2], such as near-field and super-resolution imaging [3][4][5][6][7][8][9][10][11][12][13], biological sensing [14,15], enhanced Raman spectroscopy [16][17][18][19][20][21][22], nonlinear spectroscopy [23][24][25] and photofield electron emission [26], etc.…”

Near-Monochromatic Tuneable Cryogenic Niobium Electron Field Emitter

“…√ 2mφ are the energy scale parameter for field emission with the work function φ and the field strength at the emission site F = |E| [37]. The field strength can be determined by F = βV tip , where β = (kR 0 ) −1 with R 0 being the base radius and a geometry factor typically taken as k = 5.7 for a metallic tips [38]. The FN emission current density goes as J ∝ F 2 exp(−Bφ 3/2 /F ), where B ≈ 7 V/eV 3/2 nm.…”