Development of a high brightness ultrafast Transmission Electron Microscope based on a laser-driven cold field emission source

Abstract

We report on the development of an ultrafast Transmission Electron Microscope based on a cold field emission source which can operate in either DC or ultrafast mode. Electron emission from a tungsten nanotip is triggered by femtosecond laser pulses which are tightly focused by optical components integrated inside a cold field emission source close to the cathode. The properties of the electron probe (brightness, angular current density, stability) are quantitatively determined. The measured brightness is the largest reported so far for UTEMs. Examples of imaging, diffraction and spectroscopy using ultrashort electron pulses are given. Finally, the potential of this instrument is illustrated by performing electron holography in the off-axis configuration using ultrashort electron pulses.

High brightness ultrafast transmission electron microscope based on a laser-driven cold-field emission source: principle and applications

“…temporal profile) computed for an excess energy density of 1 eV/nm 3 . We have performed systematic measurements of the emitted current as a function of the optical excitation and obtained consistently a nonlinearity in the ∼ 3 − 3.5 range [35]. According to our simulations, this value would correspond to a pulse duration in the 262-393 fs range and an energy spread between 0.7 and 0.78 eV.…”

“…The latter has been estimated using electrodynamical simulations [43]. The apex of the nanotips used in our electron source has a peculiar shape which can be approximated by a nanosphere (see Figure 2-d in [35]). In our experiments, the average laser power sent inside the electron gun is in the range 4-8 mW at a laser repetition rate 2 MHz.…”

“…The signal-to-noise ratio of phase measurement will directly influence the precision of the measured quantity. In a recent publication, we reported on the necessary optimization of electron holography experiments acquired with femtosecond electron pulses due to the low electron probe current available in our laser driven nanoemitter technology [35,62]. We showed that working using such an ultrafast electron source is ultimately equivalent to work in severe low-dose imaging conditions with a dose lying between 5-20 electrons per square Angström.…”

“…The ultrafast TEM developed in our laboratory is based on a laser-driven coldfield emission source. The architecture of this electron source and its potential for ultrafast electron microscopy and holography has been detailed in earlier publications [11,34,35].…”

Field emission microscopy pattern of a single-crystal diamond needle under ultrafast laser illumination

“…During the past decades, the ultrashort electron pulses have attracted significant interest for a wide range of applications particularly in the field of electron microscopy [1][2][3][4] and portable x-ray sources [5,6]. For instance, the metallic field-emitters (in particular tungsten tips) triggered by ultrashort (femtosecond) laser pulses have already been integrated into electron guns of ultrafast electron microscopes for a decade now [7,8]. In addition, the carbon-based materials (such as carbon nanotubes (CNT), carbon or diamond nano-cones, diamond nanocrystals, etc) have been intensively studied as cold cathodes [9][10][11].…”