All ultrafast electron microscopy techniques require a pulsed electron gun. Ideally, the electron pulses are as short as possible at the electron source and do not broaden during propagation towards the sample. In collaboration with Markus Raschke and Vasily Kravtsov (Nano-Optics Group, University of Colorado) and Alex Paarmann (FHI), we realized an electron point source based on a very sharp metallic tip providing electron pulses with a duration below 10 femtoseconds. A unique feature of this source is that the electron emission is not induced by illumination of the tip with a short laser pulse but rather by a pulsed plasmon. A surface plasmon polariton (SPP) is generated approximately 20 µm from the tip apex by illumination of a grating fabricated on the tip shaft, see figure. While the SPP propagates along the surface of the cone-shaped tip, its energy gets confined to a small volume. At the apex, the intensity of the plasmon is sufficiently high to induce nonlinear emission of an electron.
By replacing the diffraction-limited laser focus with a nano-confined near field to trigger electron emission, this allows us to significantly decrease the distance between the tip and a sample object down to the micrometer scale and below. This not only provides increased spatiotemporal resolution in point projection microscopy, but further promises the realization of in-line holographic imaging with low-energy electrons.
Melanie Müller, Vasily Kravtsov, Alexander Paarmann, Markus B. Raschke & Ralph Ernstorfer:
A nanofocused plasmon-driven sub-10 femtosecond electron point source.
Progress in ultrafast electron microscopy relies on the development of efficient laser-driven electron sources delivering femtosecond electron pulses to the sample. In particular, recent advances employ photoemission from metal nanotips as coherent point-like femtosecond low-energy electron sources. We report the nonlinear emission of ultrashort electron wave packets from a gold nanotip generated by nonlocal excitation and nanofocusing of surface plasmon polaritons. We verify the nanoscale localization of plasmon-induced electron emission by its electrostatic collimation characteristics. With a plasmon polariton pulse duration below 8 fs at the apex, we identify multiphoton photoemission as the underlying emission process. The quantum eciency of the plasmon-induced emission exceeds that of photoemission from direct apex illumination. We demonstrate the application for plasmon-triggered point-projection imaging of an individual semiconductor nanowire at 3 µm tip-sample distance. Based on numerical simulations we estimate an electron pulse duration
at the sample below 10 fs for tip-sample distances up to few micrometers. Plasmon-driven nanolocalized electron emission thus enables femtosecond point-projection microscopy with unprecedented temporal and spatial resolution, femtosecond low-energy electron in-line holography, and opens a new route towards femtosecond scanning tunneling microscopy and spectroscopy.