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Diffraction and microscopy with attosecond electron pulse trains

Nature Physics volume 14, pages 252–256 (2018)Cite this article

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Abstract

Attosecond spectroscopy1,2,3,4,5,6,7 can resolve electronic processes directly in time, but a movie-like space–time recording is impeded by the too long wavelength (~100 times larger than atomic distances) or the source–sample entanglement in re-collision techniques8,9,10,11. Here we advance attosecond metrology to picometre wavelength and sub-atomic resolution by using free-space electrons instead of higher-harmonic photons1,2,3,4,5,6,7 or re-colliding wavepackets8,9,10,11. A beam of 70-keV electrons at 4.5-pm de Broglie wavelength is modulated by the electric field of laser cycles into a sequence of electron pulses with sub-optical-cycle duration. Time-resolved diffraction from crystalline silicon reveals a < 10-as delay of Bragg emission and demonstrates the possibility of analytic attosecond–ångström diffraction. Real-space electron microscopy visualizes with sub-light-cycle resolution how an optical wave propagates in space and time. This unification of attosecond science with electron microscopy and diffraction enables space–time imaging of light-driven processes in the entire range of sample morphologies that electron microscopy can access.

In the experiment, we let the electron beam (0.48 × speed of light) and the laser beam (p-polarized, ~15 mW, peak field ~5 × 107 V m–1) hit the membrane under 35° and 60° from the surface normal, respectively. In this geometry (close to Brewster’s angle), there is predominantly a periodic acceleration and deceleration of the electrons that is dependent on the arrival time at the membrane with respect to the laser cycles. Calculations predict17,18,19 that such a periodic energy modulation19 can reshape the incoming electron packet after some free-space propagation into a train of attosecond pulses15,18,19,20 (see Fig. 1). A homogeneous compression of the entire electron beam (diameter 135 µm) is achieved by setting the optical focus larger (diameter ~300 µm) and the pulses longer in time (~1.7 ps). For temporal pulse characterization, we apply at 3.7 mm distance a second dielectric membrane (60 nm of silicon) and a second laser beam (‘excitation’ in Fig. 1). The optical peak field at the silicon membrane is several times higher (~2 × 108 V m–1) and the foil’s refractive index is greater (~3.5). Therefore, the previously negligible sideways forces are now substantial and create a time-dependent beam deflection; that is, a cathode ray oscilloscope at optical frequencies (petahertz). Further modulations in time are now irrelevant. As the sub-cycle electron pulse train generated at the first membrane is temporally synchronized to the characterization laser’s optical cycles17, each attosecond electron pulse in the pulse train sees the same kind of deflection dynamics, and a time-integrated measurement characterizes the average pulse duration and shape1.

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Fig. 1: Concept and experiment.
Fig. 2: Attosecond electron pulses.
Fig. 3: Atomic diffraction with attosecond electron pulses.
Fig. 4: Attosecond electron microscopy of electromagnetic waveform propagation.

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Acknowledgements

This work was supported by the European Research Council (grant DIVI) and the Munich-Centre for Advanced Photonics. Y.M. acknowledges support from a JSPS Postdoctoral Fellowship for Research Abroad. We thank B.-H. Chen and A. Ryabov for help with the laser, S. Stork for help with the foils and F. Krausz for awesome support and inspiring discussions.

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Authors and Affiliations

  1. Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748, Garching, Germany

    Yuya Morimoto & Peter Baum

  2. Max-Planck-Institute of Quantum Optics, Hans-Kopfermann-Str. 1, 85748, Garching, Germany

    Yuya Morimoto & Peter Baum

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  1. Yuya Morimoto
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  2. Peter Baum
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Y.M. and P.B. conceived the experiment, Y.M. measured the data, Y.M. and P.B. evaluated the data and Y.M. and P.B. wrote the manuscript.

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Correspondence to Peter Baum.

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Supplementary Information

Supplementary Figure 1

Supplementary Video 1

Attosecond electron microscopy of a traveling wave. The left panel shows the raw microscopic image of the silicon window in time. The right panel shows the change of the images with respect to the excitation delay. The scale bars represent 100 µm

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Morimoto, Y., Baum, P. Diffraction and microscopy with attosecond electron pulse trains. Nature Phys 14, 252–256 (2018). https://doi.org/10.1038/s41567-017-0007-6

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