• Open Access

Few-Femtosecond Dynamics of Free-Free Opacity in Optically Heated Metals

A. Niedermayr, M. Volkov, S. A. Sato, N. Hartmann, Z. Schumacher, S. Neb, A. Rubio, L. Gallmann, and U. Keller
Phys. Rev. X 12, 021045 – Published 25 May 2022

Abstract

Interaction of light with an excited free-electron gas is a fundamental process spanning a large variety of fields in physics. The advent of femtosecond laser pulses and extreme-ultraviolet sources allowed one to put theoretical models to the test. Recent experimental and theoretical investigations of nonequilibrium aluminum, which is considered to be a good real-world representation of an ideal free-electron metal, showed that, despite significant progress, the transient hot-electron/cold-ion state is not well understood. In particular, the role of plasmon broadening, screening, and electron degeneracy remains unclear. Here, we experimentally investigate the free-free opacity in aluminum on the few-femtosecond timescale at laser intensities close to the damage threshold. Few-femtosecond time resolution allows us to track the purely electronic contribution to nonequilibrium absorption and unambiguously separate it from the slower lattice contribution. We support the experiments with ab initio calculations and a nearly free electron model in the Sommerfeld expansion. We find that the simplest independent-particle model with a fixed band structure is sufficient to explain the experimental findings without the need to include changes in screening or electron scattering, contrasting previous observations in 3d transition metals. We further find that electronic heating of a free-electron gas shifts the spectral weight of the absorption to higher photon energies, and we are able to distinguish the influence of the population change and the chemical potential shift based on the comparison of ab initio calculations to a simplified free-electron model. Our findings provide a benchmark for further investigations and modeling of dense nonequilibrium plasma under even more extreme conditions.

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  • Received 1 June 2021
  • Accepted 15 March 2022

DOI:https://doi.org/10.1103/PhysRevX.12.021045

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Atomic, Molecular & OpticalCondensed Matter, Materials & Applied Physics

Authors & Affiliations

A. Niedermayr1,∗, M. Volkov1,2, S. A. Sato3,4, N. Hartmann1, Z. Schumacher1, S. Neb1, A. Rubio4,5, L. Gallmann1, and U. Keller1

  • 1Department of Physics, ETH Zurich, 8093 Zurich, Switzerland
  • 2University of Konstanz, 78457 Konstanz, Germany
  • 3Center for Computational Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan
  • 4Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany
  • 5Center for Computational Quantum Physics (CCQ), Flatiron Institute, New York 10010, New York, USA

  • To whom correspondence should be addressed; niedermayr@phys.ethz.ch

Popular Summary

Absorption of light by electrons in solids is a fundamental process in nature and the first step for a wide range of phenomena initiated by the addition of light energy to the system. Using ultrashort laser pulses, the dynamical interaction of electrons with light can be tracked on a timescale down to a few attoseconds. On these shortest timescales, the interaction of the freely moving electrons in metals with photons remains unclear. In this work, we present time-resolved measurements of the reaction of these free electrons to light absorption and identify the mechanisms participating in this process. We find a universal photon energy dependence of the underlying physical mechanisms that applies to all materials with a finite density of freely moving electrons.

We use two-pulse experiments where a short infrared pulse excites free electrons in aluminum and a delayed attosecond pulse probes the resulting dependence of their photoabsorption on time and energy. The high time resolution allows us to distinguish between the fast response of the electrons and the much slower reaction of the atomic lattice. Theoretical analysis enables us to further break down the observed effects into different contributing mechanisms and show the universality of our observations.

Understanding the details of photoabsorption by free electrons allows for studies of more complex materials with both free and bound electron states in the initial and final states of the photon transition. The observed effects must be taken into account whenever free electrons contribute to the photoabsorption.

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Vol. 12, Iss. 2 — April - June 2022

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