Investigation of nanostructures physics requires atomic spatial resolution, meV spectral resolution and femto to nanosecond time-resolution. Accessing all these informations simultaneously would be a breakthrough in nanophysics.

Ultrafast Transmission Electron Microscopes (UTEM) combining subpicosecond temporal resolution and nanometer spatial resolution have recently emerged as unique tools for investigations at both ultimate spatial and temporal resolutions [1]. However, the performances of state-of-the-art UTEM are, in practice, brightness limited by their ultrafast electron source. These sources are commonly based either on photocathode excited by an ultrafast laser beam [2] or, very recently, on Schottky type assembly [3].

The FemtoTEM project aims at developing an alternative Ultrafast Transmission Electron Microscope based on a high brightness laser-driven cold field emission electron source working under 200kV acceleration voltage. The latter consists of a metallic nanotip in tungsten illuminated by femtosecond laser pulses [4]. This development has been achieved by bringing together a commercial femtosecond laser source and a customized 200kV cold-field emission Transmission Electron Microscope Hitachi HF2000.

The main difficulty was to develop the ultrafast cold field emission source modifying the old HF2000 design, well known in CEMES [5]. Indeed, to perform a proper laser assisted field emission, the femtosecond laser beam need to be injected in the ultra high vacuum and high voltage area of the gun, where the tip is located, then focused and aligned in three dimensions on the fine apex of the FE W tip. The new design has been thought to allow such complex operation while keeping the possibility of cold field emitting electrons in continuous operation, as for the standard source. This new design has been finally produced and patented [6].

To build this new source, deep modifications, compared to the original Hitachi design, have been implemented, from the high voltage gun housing and cable to the inner structure of the gun assembly. With the help of finite element modeling, and ray tracing software, the influence of the new design on the electric field, and electrons trajectories, brightness, … has been investigated and compared to experimental results (see Figure). Last results will be also presented highlighting the potentiality of this new source for ultrafast electron holography application, for which a good brightness is mandatory. 

[1] Zewail, A. H., Science, 2010, 328, 187-193

[2] Zewail, A. H., USPTO n°US7,154,091 of December 26. 2006

[3] Bormann, R. et al, Journal of Applied Physics, 2015, 118, 173105

[4] Hommelhoff, P. et al, Phys. Rev. Lett., 2006, 96, 077401

[5] Houdellier, F. et al, Carbon 2012, 50 (5), 2037–2044

[6] Arbouet A. & Houdellier F., USPTO No.US9,263,229 B2 of February 16.2016

 Acknowledgments

This work was funded through the support of the « Institut National de Physique du CNRS »- INP-CNRS and the ANR FemtoTEM n°ANR-14-CE26-0013-01. The authors acknowledge the European Union under the Seventh Framework Programme under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2.

Figures:

Figure: a- FemtoTEM microscope overview b,c- Absolute value of the Electric field at the tip apex before and after modification of the CFEG to inject, focus and align the ultrafast laser beam, calculated by FEM simulation. d,e- SIMION ray tracing simulation of the electrons trajectories before and after modification of the CFEG.

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