Time-resolved photoemission electron microscopy

The excellent time structure of Synchrotron radiation and femtosecond lasers has opened the door to novel ways to time-resolved imaging of ultrafast processes using photoemission electron microscopy. Various kinds of periodic or repetitive processes can be studied by stroboscopic illumination with the pulsed photon beam. Although being a very young field of research, there is a rapid development in several groups that we briefly recall here. Two main applications have been established.

Stroboscopic imaging using Synchrotron radiation

Imaging of fast magnetisation processes by stroboscopic XMCD-PEEM was first demonstrated by Krasyuk et al. [1]. Using this method further information, precessional switching, incoherent magnetisation rotation, Gigahertz-Eigenmodes of ferromagnetic nanostructures or travelling spin waves in thin films have been imaged by several groups at BESSY Berlin, the ESRF Grenoble, the ALS Berkeley, the Swiss light source and in Japan [2-5].
In experiments using Synchrotron radiation the time resolution is limited by the photon pulse width which is typically 50-100 ps in standard operation down to a few ps in the low-alpha bunch-compression mode. The best resolution achieved so far is about 15ps further information [6]. 

Femtosecond-laser excited PEEM

Femtosecond lasers as excitation sources for time resolved PEEM imaging push the time resolution into the fs range. It started with the discovery of intense emission from localised surface plasmons LSPs in nanoparticles ("cluster plasmons") [7,8]. In a pioneering experiment Schmidt et al. [9] observed lifetime contrast in the 60 fs range in a semiconductor-metal heterostructure in cooperation with the Aeschlimann group, Essen/ Kaiserslautern. This experiment used "all optical pump-probe", being well established in spatially integrating experiments. Later, other groups obtained sub-femtosecond time precision in a phase-resolved Mach-Zehnder interferometric set-up [10]. Attosecond precision requires a highly sophisticated handling of ultrashort photon pulses. New magnetic contrast mechanisms have just been discovered that make the MCD-PEEM technique independent of Synchrotron radiation further information.

Time-resolved image detection

In the pump-probe modes the microscope is operated statically, i.e. time resolution is achieved by proper synchronisation of pump and probe pulses. Modification of the microscope opens up further possibilities: For time-resolved image detection a 3D (x,y,t)-resolving delayline detector has been developed [11]. Local microspectroscopy can be performed by implementing a low-energy drift space into the microscope column, establishing time-of-flight PEEM [12].  Additional active operation of the lens system paves the way for dynamic aberration correction [13,14] by circumventing one of the preconditions of Scherzer’s theorem.

[1] A. Krasyuk, A. Oelsner, S. A. Nepijko, A. Kuksov, C. M. Schneider, G. Schönhense;
Appl. Phys. A 76 (2003) 863
[2] S.-B. Choe et al., Science 304, 420 (2004).
[3] J. Raabe et al., Phys. Rev. Lett. 94, 217204 (2005).
[4] W. Kuch, Vogel, J., Camarero, J., Fukumoto, K., Pennec, Y., Pizzini, S.,
Bonfim, M., Kirschner, J. Appl. Phys. Lett. 85, . (2004)  440–442
 [5] C. M. Schneider, A. Kuksov, A. Krasyuk, A. Oelsner, D. Neeb, S. A. Nepijko, G. Schönhense, I. Mönch, R. Kaltofen, C. de Nadai, B. Brookes;
Appl. Phys. Lett. 85 (2004) 2562-2564
[6] A. Krasyuk, F. Wegelin, S. A. Nepijko, H. J. Elmers, G. Schönhense, M. Bolte, C. M. Schneider;Phys. Rev. Lett. 95 (2005) 207201
[7] O. Schmidt, G.H. Fecher, Y. Hwu, G. Schönhense, Surf. Sci. 482–485
(2001) 687–692.
[8] G.H. Fecher, O. Schmidt, Y. Hwu, G. Schönhense, J. Electr. Spectrosc.
Relat. Phenom. 126 (2002) 77–87.
[9] O. Schmidt, M. Bauer, C. Wiemann, R. Porath, M. Scharte, O. Andreyev, G. Schönhense, M. Aeschlimann, Appl. Phys. B 74 (2002) 223.
[10] A. Kubo, Onda K, Petek H, Sun Z, Jung YS, Kim HK. Nano Lett. 5 (2005)  1123
[11] A. Oelsner, O. Schmidt, M. Schicketanz, M. Klais, G. Schönhense,
V. Mergel, O. Jagutzki, H. Schmidt-Böcking, Rev. Sci. Instrum. 72
(2001) 3968.
[12] G. Schönhense, A. Oelsner, O. Schmidt, G.H. Fecher, V. Mergel, O.
Jagutzki, H. Schmidt-Böcking, Surf. Sci. 480 (2001) 180
[13] G. Schönhense, H. Spiecker, J. Vacuum Sci. Technol. B 20 (2002)
[14] Schönhense G, Spiecker H. Array for achromatic imaging of pulsed particle beams, US Patent 6,737,647, vol. B2, 2004.

The field is reviewed in:
G. Schönhense, H. J. Elmers, S. A. Nepijko, C. M. Schneider
Time-resolved photoemission electron microscopy
In: Advances in Imaging and Electron Physics(Ed. P. Hawkes) 142 (2006) 159-323

G. Schönhense, H. J. Elmers
PEEM With High Time Resolution – Imaging of Transient Processes and Novel Concepts of Chromatic and Spherical Aberration Correction,
Surf. and Interface Analysis 38 (2006) 1578-1587




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