Resolving the fundamental carrier dynamics induced in solids by strong electric fields is essential for future applications, ranging from nanoscale transistors1,2 to high-speed electro-optical switches3. How fast and at what rate can electrons be injected into the conduction band of a solid? Here, we investigate the sub-femtosecond response of GaAs induced by resonant intense near-infrared laser pulses using attosecond transient absorption spectroscopy. In particular, we unravel the distinct role of intra- versus interband transitions. Surprisingly, we found that despite the resonant driving laser, the optical response during the light–matter interaction is dominated by intraband motion. Furthermore, we observed that the coupling between the two mechanisms results in a significant enhancement of the carrier injection from the valence into the conduction band. This is especially unexpected as the intraband mechanism itself can accelerate carriers only within the same band. This physical phenomenon could be used to control ultrafast carrier excitation and boost injection rates in electronic switches in the petahertz regime.

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We thank M. C. Golling for growing the GaAs, and J. Leuthold and C. Bolognesi for helpful discussion. The authors acknowledge the support of the technology and cleanroom facility at Frontiers in Research: Space and Time (FIRST) of ETH Zurich for advanced micro- and nanotechnology. This work was supported by the National Center of Competence in Research Molecular Ultrafast Science and Technology (NCCR MUST) funded by the Swiss National Science Foundation, and by JSPS KAKENHI grant no. 26-1511.

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Author notes

    • M. Lucchini

    Present address: Department of Physics, Politecnico di Milano, Milano, Italy


  1. Department of Physics, ETH Zurich, Zurich, Switzerland

    • F. Schlaepfer
    • , M. Lucchini
    • , M. Volkov
    • , L. Kasmi
    • , N. Hartmann
    • , L. Gallmann
    •  & U. Keller
  2. Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany

    • S. A. Sato
    •  & A. Rubio


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F.S., M.L., L.G. and U.K. supervised the study. F.S., M.L., M.V., L.K. and N.H. conducted the experiments. M.V. also improved the experimental set-up and data acquisition system. F.S. fabricated the sample and analysed the experimental data. S.A.S. and A.R. developed the theoretical modelling. All authors were involved in the interpretation and contributed to the final manuscript.

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The authors declare no competing interests.

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Correspondence to F. Schlaepfer or U. Keller.

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