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Attosecond Electron Processes in Solids

Final Report Summary - ATTOTRON (Attosecond Electron Processes in Solids)

Final Report

Grant Agreement number: 302135
Project acronym: ATTOTRON
Project title: Attosecond Electron Processes in Solids
Funding Scheme: FP7-MC-IOF
Period covered - start date: 01/10/2012
Period covered - end date: 30/04/2014
Name, title and organisation of the scientist in charge of the project's coordinator:
Project Website:

In the solid-state physics community there is a microscopic understanding developed to explain band structure and carrier dynamics therein. These predictions have never been put under test with the temporal resolution that are achieved within this project. The application of attosecond transient absorption technology represents the initiation of a new field in condensed matter physics (and the interdisciplinary link to ultrafast laser science).
The objective of this proposal was to apply the toolbox of attosecond science to the investigation of electronic processes inside matter via transient absorption spectroscopy.
During the outgoing phase of the FP7-MC-IOF project ATTOTRON the applicant successfully performed the proposed experiment to explore the ultrafast electron dynamics in Silicon dioxide. After recording the experimental data a a high-level data analysis and interpretation was performed in an international collaboration with theoreticians from Georgia State University (M. Stockman) and a team of experimentalists at the Max-Planck-Institute of Quantum Optics. These efforts resulted in the publication of the findings in:

[1] Controlling dielectrics with the electric field of light
Nature 493, 75–78 (03 January 2013) doi:10.1038/nature11720

Martin Schultze, Elisabeth M. Bothschafter, Annkatrin Sommer, Simon Holzner, Wolfgang Schweinberger, Markus Fiess, Michael Hofstetter, Reinhard Kienberger, Vadym Apalkov, Vladislav S. Yakovlev, Mark I. Stockman & Ferenc Krausz

By now, the paper has been cited 96 times and the findings presented in this groundbreaking manuscript supported the validity of the main objective of the proposal: Attosecond transient absorption spectroscopy as a novel tool to investigate charge dynamics in solid samples. Based on these findings in the outgoing host institution a novel setup was build and optimized by the applicant for this type of experiments in parallel to data analysis and manuscript preparation of [1].
In the course of the preparation of the manuscript a number of new aspects determining the performance of these measurements have been unearthed. Particularly it turned out that the unparalleled temporal resolution of the recorded data provides insight into specifics of light induced transfer of electronic population that yet have not been accounted for in stae-of-the-art theories. Only recently, high-level theoretical modelling of these processes provide temporal resolutions as achieved in the experiments. In order to better connect to these recent efforts, the project coordinator, the host institution and the applicant have decided to refocus the part of the proposal that initially targeted at the investigation of semiconductor nanoparticles (project months 13-18) on the investigation of silicon as the most prototypical and technological relevant semiconductor.
The applicant led a collaboration with K. Yabana, Prof. at Univ. of Tsukuba, Japan and D. Prendergast, Director, Lawrence Berkeley National Lab, USA to support the analysis of the experiment that he performed during the outgoing phase targeted at exploring ultrafast charge transfer. This led to yet another publication in one of the leading scientific journals:

[2] Attosecond band gap dynamics in Silicon
Science Vol. 346 no. 6215 pp. 1348-1352 (2014)

Martin Schultze, Krupa Ramasesha, C.D. Pemmaraju, S.A. Sato, D. Whitmore, A. Gandman, James S. Prell, L. J. Borja, D. Prendergast, K. Yabana, Daniel M. Neumark, Stephen R. Leone

The ability to transfer electrons from valence to conduction band states in a semiconductor is the basis of modern electronics. In this work, attosecond XUV spectroscopy was used to resolve this process in silicon in real-time. Electrons injected into the conduction band by few-cycle laser pulses were found to alter the silicon XUV absorption spectrum in sharp steps synchronized with the laser electric field oscillations. The observed ~450 attosecond step rise time provides an upper limit for the carrier-induced band gap reduction and the electron-electron scattering time in the conduction band. This electronic response is separated from the subsequent band gap modifications due to the lattice motion, which have a time scale of 60±10 fs, characteristic of the fastest optical phonon. Quantum dynamical simulations interpret the carrier injection step as light-field induced electron tunneling, while core-level absorption theory validates the methodology as a universal tool to follow band gap dynamics with sub-femtosecond resolution. A schematic of the novel experimental concept is attached as figure.

Ongoing studies in small-band gap semiconductors promise an in-depth understanding of the dynamics of visible excitations in solids and the ultrafast coupling of electronic and nuclear kinetics in solids. A theoretical framework for the analysis of data recorded in the newly developed experimental scheme is currently under development in collaboration with theorists.