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Ultrafast electron microscopy and diffraction using nanoscale photoemitters

Mardi 30 août 2016 15:00 - Duree : 1 heure
Lieu : Salle "Remy Lemaire" K 223 (1er étage) bât. K de l’institut Néel/CNRS

Orateur : Claus ROPERS (University of Göttingen, Germany)

Novel methods in time-resolved electron microscopy, diffraction and spectroscopy promise unprecedented insight into the dynamics of structural, electronic and magnetic processes on the nanoscale. A key to the realization of such technologies is the generation of high-quality beams of ultrashort electron pulses. In this talk, our recent development of imaging and spectroscopy capabilities using localized electron sources will be discussed. Specifically, two approaches employing the enhanced spatial coherence in the photoemission from nanotips [1-4] will be presented, namely Ultrafast Low-Energy Electron Diffraction (ULEED) and Ultrafast Transmission Electron Microscopy (UTEM). ULEED allows for the study of structural dynamics with high temporal resolution and ultimate surface sensitivity. In a first application of this technique, we used a transmission geometry to study the order-to-disorder transition in an ultrathin polymer crystallite adsorbate layer on freestanding graphene [5,6]. Presently, we investigate the transformation between charge-density wave phases at the surface of the transition metal dichalcogenide 1T-TaS2. UTEM yields access to the dynamical structural and electronic response of matter with the imaging capabilities of electron microscopy. Besides materials science applications, the approach also provides for unique possibilities to harness the interaction of swift electrons with local optical modes. In particular, the spectroscopic study of electrons interacting with optical near-fields facilitates quantum-coherent manipulations of the momentum distribution in ultrashort electron pulses [7,8]. Further ongoing investigations with the UTEM involve spatio-temporal dynamics in nanostructured materials or the imaging of optically-induced magnetic structures [9].

References :

[1] B. Schröder, M. Sivis, R. Bormann, S. Schäfer, C. Ropers, "An ultrafast nanotip electron gun triggered by grating-coupled surface plasmons", Appl. Phys. Lett. 107, 231105 (2015).

[2] R. Bormann, S. Strauch, S. Schäfer, C. Ropers, "An ultrafast electron microscope gun driven by two-photon photoemission from a nanotip cathode", J. Appl. Phys. 118, 173105 (2015).

[3] A. Paarmann, M. Gulde, M. Müller, S. Schäfer, S. Schweda, M. Maiti, C. Xu, T. Hohage, F. Schenk, C. Ropers and R. Ernstorfer, "Coherent femtosecond low-energy single-electron pulses for time-resolved diffraction and imaging : A numerical study", J. Appl. Phys. 112, 113109 (2012).

[4] L. Wimmer, G. Herink, D. R. Solli, S. V. Yalunin, K. E. Echternkamp, and C. Ropers, "Terahertz control of nanotip photoemission", Nature Physics 10, 432 (2014).

[5] M. Gulde, S. Schweda, G. Storeck, M. Maiti, H. K. Yu, A. M. Wodtke, S. Schäfer, and C. Ropers, "Ultrafast low-energy electron diffraction in transmission resolves polymer/graphene superstructure dynamics", Science 345, 200 (2014).

[6] M. Gulde, A. Rissanou, V. Harmandaris, M. Müller, S. Schäfer, and C. Ropers, "Structure and dynamics of monolayer polymer crystallites on graphene", submitted (2015)

[7] A. Feist, K. E. Echternkamp, J. Schauss, S. V. Yalunin, S. Schäfer, and C. Ropers, "Ramsey-type phase control of free-electron beams", Nature Physics (2016, advance online publication).

[8] K. E. Echternkamp, A. Feist, S. Schäfer, and C. Ropers, "Quantum coherent optical phase modulation in an ultrafast transmission electron microscope", Nature 521, 200 (2015).

[9] T. Eggebrecht, M. Möller, J. G. Gatzmann, N. Rubiano da Silva, A. Feist, U. Martens, H. Ulrichs, M. Münzenberg, C. Ropers, and S. Schäfer, "Observation of an optically induced magnetic vortex glass in a continuous iron film", submitted (2016).

Contact : julien.renard@neel.cnrs.fr

Discipline évènement : (Physique)
Entité organisatrice : (Fondation Nanosciences)
Nature évènement : (Séminaire)
Evènement répétitif : (Séminaire Nanoélectronique Quantique)
Site de l'évènement : Polygone scientifique

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