Direct observation of electron propagation and dielectric screening on the atomic length scale

• Published in: Nature (London) Vol. 517; no. 7534; pp. 342 - 346
• Main Authors: Neppl, S., Ernstorfer, R., Cavalieri, A. L., Lemell, C., Wachter, G., Magerl, E., Bothschafter, E. M., Jobst, M., Hofstetter, M., Kleineberg, U., Barth, J. V., Menzel, D., Burgdörfer, J., Feulner, P., Krausz, F., Kienberger, R.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 15-01-2015; Nature Publishing Group
• Subjects: 639/301/357/995; Atomic properties; Crystals; Dielectrics; Electric properties; Electrodiffusion; Electrons; Humanities and Social Sciences; Lasers; letter; Light; Magnesium; multidisciplinary; Observations; Optics; Propagation; Properties; Science; Spectrum analysis; Structure; Studies; Tungsten; Ultraviolet radiation; Wave propagation; Germany
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature14094

Attosecond light pulses are now available experimentally, enabling ultrafast processes on the atomic scale to be probed; here the free-electron-like propagation of electrons through ultrathin layers of magnesium is observed in real time.
Attosecond electron transport chronoscopy in nanostructures
The recent availability of attosecond light pulses means that it is now possible to observe ultrafast processes at an atomic scale. So far, such measurements have been carried out in gases, but now Reinhard Kienberger and colleagues use attosecond pulses to probe a fundamental process in the solid state, namely the transport of electrons through a crystal. They use attosecond pulses to launch photoelectron wavepackets inside a tungsten crystal that is covered by a controllable number of magnesium layers. Measuring the time of arrival of the wavepackets at the surface as a function of the number of layers reveals free-electron-like propagation of electrons inside the magnesium layers. The study demonstrates that real-time access to atomic-scale electron transport on the surface is possible.
The propagation and transport of electrons in crystals is a fundamental process pertaining to the functioning of most electronic devices. Microscopic theories describe this phenomenon as being based on the motion of Bloch wave packets
1
. These wave packets are superpositions of individual Bloch states with the group velocity determined by the dispersion of the electronic band structure near the central wavevector in momentum space
1
. This concept has been verified experimentally in artificial superlattices by the observation of Bloch oscillations
2
—periodic oscillations of electrons in real and momentum space. Here we present a direct observation of electron wave packet motion in a real-space and real-time experiment, on length and time scales shorter than the Bloch oscillation amplitude and period. We show that attosecond metrology
3
(1 as = 10
−18
seconds) now enables quantitative insight into weakly disturbed electron wave packet propagation on the atomic length scale without being hampered by scattering effects, which inevitably occur over macroscopic propagation length scales. We use sub-femtosecond (less than 10
−15
 seconds) extreme-ultraviolet light pulses
3
to launch photoelectron wave packets inside a tungsten crystal that is covered by magnesium films of varied, well-defined thicknesses of a few ångströms
4
. Probing the moment of arrival of the wave packets at the surface with attosecond precision reveals free-electron-like, ballistic propagation behaviour inside the magnesium adlayer—constituting the semi-classical limit of Bloch wave packet motion. Real-time access to electron transport through atomic layers and interfaces promises unprecedented insight into phenomena that may enable the scaling of electronic and photonic circuits to atomic dimensions. In addition, this experiment allows us to determine the penetration depth of electrical fields at optical frequencies at solid interfaces on the atomic scale.