Femtosecond electron pulses are typically created by illuminating a flat photocathode  with femtosecond laser pulses. [1] However, flat photocathodes have a low reduced brightness,  2 orders of magnitude lower than a Schottky electron source. A higher brightness can be achieved using a cold field emitter illuminated with femtosecond laser pulses. [2] Using a cold field emitter illuminated with UV pulses the group of Zewail has realized an ultrafast SEM. [3] However, such an USEM cannot easily be switched back to continuous beam operation. In addition, the pulse has to be accelerated from the tip onwards which leads to a broadened pulse at the sample.

 Here, we propose a beam blanker for use in regular EMs that allows switching between continuous-beam and ultrafast modes of operation. Previous approaches to ultrafast beam blanking were based on beam blankers using GHz magnetic or electric fields. [4,5] These GHz cavities are still relatively large and can’t be inserted directly in a standard commercial SEM.

 We use a miniaturized beam blanker controlled by a photoconductive switch, illuminated with femtosecond laser pulses, as schematically depicted in Figure 1. Hence, the blanker is locked jitter-free to the laser. We show that such a beam blanker needs to have micrometer scale dimensions for ultrafast operation. COMSOL simulation results, including the full 3D blanker design, are used to evaluate the time response of the system.

 We fabricated and integrated the deflector plates and the photoconductive switch in a one micrometer-scale device, see Figure 2. We will show fabrication results of the ultrafast blanker and its incorporation on an insert for a FEI Quanta FEG 200 SEM. We will also show alignment of both laser and electron beam on the ultrafast beam blanker. Also results will be presented showing laser triggered deflection of the electron beam.

References:

[1] A. H. Zewail, “Four-dimensional electron microscopy.” Science 328, 5975,  187–93 (2010).

[2] P. Hommelhoff, Y. Sortais, A. Aghajani-Talesh, and M. a. Kasevich, “Field Emission Tip as a Nanometer Source of Free Electron Femtosecond Pulses,” Phys. Rev. Lett., 96 (7), 077401 (2006).

[3] D.-S. Yang, O. F. Mohammed, and A. H. Zewail, “Scanning ultrafast electron microscopy.,” Proc. Natl. Acad. Sci. U. S. A., 107 (34), 14993–8, (2010).

[4] K. Ura, H. Fujioka, and T. Hosokawa, “Picosecond Pulse Stroboscopic Scanning Electron Microscope,” J. Electron Microsc., 27 (4), 247–252 (1978).

[5] A. Lassise, P. H. A. Mutsaers, and O. J. Luiten, “Compact, low power radio frequency cavity for femtosecond electron microscopy.,” Rev. Sci. Instrum., 83 (4), 043705 (2012).

Figures:

Figure 1: Schematic of the ultrafast beam blanker: A photoconductive switch drives a beam blanker, and this blanker sweeps the electron beam over an aperture to create an ultrafast electron pulse.

Figure 2: Fabrication result of the ultrafast beam blanker. The blue arrow indicates the trajectory of the electron beam. The red laser pulse indicates the position and illumination of the photoconductive switch. View (a) along the optical axis of the laser pulse and (b) view along the axis of the electron beam.

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EMC Abstracts - https://emc-proceedings.com/abstract/laser-triggered-microfabricated-ultrafast-electron-beam-blanker/