Ultrafast transmission electron microscopy (UTEM) is a laser pump/electron probe technique which enables the investigation of ultrafast processes on nanometer length scales [1]. Here, the dynamics of an inhomogeneous system after ultrashort laser excitation are probed by stroboscopic illumination with sub-picosecond electron pulses. However, current implementations to create short electron pulses, employing a flat photocathode, are intrinsically limited by their low emittance.
We present the implementation of a pulsed electron source, based on localized laser-triggered emission from a needle-shaped tungsten emitter [2], which we employ in a commercial Schottky field emitter TEM. Within this setup, we experimentally characterize the minimum spot size, overall brightness and intrinsic emittance of the electron beam. To further study the emission properties of the electron gun, numerical finite element calculations are carried out. In addition, photon induced near-field electron microscopy (PINEM) [3] of metallic nanostructures is utilized to investigate the temporal structure of the electron pulses, currently yielding pulse durations of 700 fs.
These electron bunches will allow us to study structural dynamics of heterogeneous systems at and near interfaces, defects and structural inhomogeneities with a sub-ps temporal and nanometer spatial resolution.
[1] A.H. Zewail, Science, 328, 187 (2010).
[2] C. Ropers, D. R. Solli, C. P. Schulz, C. Lienau, T. Elsaesser, Phys. Rev. Lett. 98, 043907 (2007).
[3] B. Barwick, D. J. Flannigan, A. H. Zewail, Nature, 462, 902 (2009).