Flexible bimodal photoemission electron source based on Au-coated fiber optic nanotips

Ultrashort laser-driven coherent electron sources based on nanotip needles have been studied motivated by their desired properties for ultrafast electron diffraction and microscopy, electron point projection microscopy, etc. While the conventional schemes require focusing the fs laser beam on or near the apex of a nanotip, which is prone to misalignment and unwanted scattering, we demonstrate and characterize a nanotip electron source based on tapered optical fibers coated with Au on one end. Our theoretical analysis followed by detailed experiments at a range of visible wavelengths confirm that the observed nonlinear electron emission is assisted by surface plasmon excitation in the metallized fiber taper. Fiber tips leverage over the conventional tips in two major ways: 1) They are alignment-free by end-fire coupling, 2) They emit not only using ultrashort pulsed lasers but also by low-power CW diode lasers in the long wavelength regime. The fiber tips are useful where back-illumination or raster scanning an electron source is needed as in electron nanolithography using a single fiber or a bundle for parallel processing.