Generation and Characterization of Attosecond Microbunched Electron Pulse Trains via Dielectric Laser Acceleration

Norbert Schönenberger, Anna Mittelbach, Peyman Yousefi, Joshua McNeur, Uwe Niedermayer, and Peter Hommelhoff

Phys. Rev. Lett. 123, 264803 – Published 26 December 2019

Abstract

Dielectric laser acceleration is a versatile scheme to accelerate and control electrons with the help of femtosecond laser pulses in nanophotonic structures. We demonstrate here the generation of a train of electron pulses with individual pulse durations as short as $270 \pm 80 attoseconds$ (FWHM), measured in an indirect fashion, based on two subsequent dielectric laser interaction regions connected by a free-space electron drift section, all on a single photonic chip. In the first interaction region (the modulator), an energy modulation is imprinted on the electron pulse. During free propagation, this energy modulation evolves into a charge density modulation, which we probe in the second interaction region (the analyzer). These results will lead to new ways of probing ultrafast dynamics in matter and are essential for future laser-based particle accelerators on a photonic chip.

Received 25 June 2019
Revised 28 October 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.264803

Physics Subject Headings (PhySH)

Laser applications Near-field optics Single-particle dynamics Ultrafast optics

Atomic, Molecular & OpticalAccelerators & Beams

Authors & Affiliations

Norbert Schönenberger^1,*, Anna Mittelbach¹, Peyman Yousefi ¹, Joshua McNeur¹, Uwe Niedermayer², and Peter Hommelhoff ^1,†

¹Department of Physics, Friedrich-Alexander Universität Erlangen-Nürnberg (FAU), Staudtstraße 1, 91058 Erlangen, Germany
²Technische Universität Darmstadt, Institut für Teilchenbeschleunigung und Elektromagnetische Felder (TEMF) Schlossgartenstraße 8, 64289 Darmstadt, Germany

^*norbert.schoenenberger@fau.de
^†peter.hommelhoff@fau.de

Net Acceleration and Direct Measurement of Attosecond Electron Pulses in a Silicon Dielectric Laser Accelerator

Dylan S. Black, Uwe Niedermayer, Yu Miao, Zhexin Zhao, Olav Solgaard, Robert L. Byer, and Kenneth J. Leedle

Phys. Rev. Lett. 123, 264802 (2019)

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Vol. 123, Iss. 26 — 31 December 2019

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Images

Figure 1
Sketch of the experimental setup with modulator and analyzer structure and sketches of the electron phase space behavior. (a) Laser-emitted electrons are focused into the center of the channel of the first dielectric laser acceleration structure, comprised of two rows of pillars, the modulator. An SEM image of modulator and analyzer structure can be seen in the background of this sketch. After the electrons have propagated through the analyzer structure, their energy is measured with a magnetic deflection spectrometer. (b) Sketch of the evolution of the electron pulse duration. At the source, the electron pulse duration resembles that of the triggering UV laser pulse ( $\sim 100 fs$ ). During propagation through the electron column, trajectory effects increase the electron pulse duration to roughly 400 fs at the modulator. The pulsed laser beam acting on each arriving electron pulse modulates the energy of the electrons. During subsequent propagation, the energy modulation leads to a density modulation. At the temporal focus, the minimum electron pulse duration of each bunchlet is reached. The position of the temporal focus depends on the amplitude of the energy modulation in the modulator. Here microbunching at the position of the analyzer is shown. (c) Sketch of the phase space evolution during the electron drift. The vertical axis denotes the energy of the electrons plotted over one cycle ( $- π \dots π \equiv 6.45 fs$ ). The faster higher energy electrons catch up with the slower electrons, forming the microbunched pulse train. (d) Example spectrogram of the electrons after interaction in the modulator only (laser intensity of $3 \times 10^{11} W {cm}^{- 2}$ ). The red curve shows the homogeneous broadening inside the red region. (e) Example spectrogram with modulator and analyzer structure illuminated ( $1.5 \times 10^{10} W {cm}^{- 2}$ in the modulator, $2.5 \times 10^{10} W {cm}^{- 2}$ in the analyzer). The periodicity with the optical period of 6.45 fs and suboptical cycle duration features are clearly visible.
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Figure 2
Spectrograms, electron density and phase space at the analyzer structure. The left column shows experimental data, next to it we show simulated spectrograms, and the third and forth column show the electron density distribution and phase space at the position of the analyzer, respectively. The color scale of the first two columns shows electron counts, normalized to the maximum count of electrons for each data set individually. Peak fields are given next to the spectra. Underbunching is clearly visible in the first two rows, where the temporal focus lies behind the analyzer. Shortest micropulses are shown in (c), with a pulse duration of $(270 \pm 80) as$ . Various degrees of over bunching are displayed in the last three rows. The forth column shows the phase space distribution of the microbunches: the vertical axes represent the electrons’ energy. When the maximum and minimum of the modulation coincide in time, the temporal focus is reached. After that the characteristic overbunching shape is formed, when the high energy electrons have passed the slow electrons in the analyzer and the temporal focus lies in front of it.
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Figure 3
Microbunch duration versus incident field strength on the modulator after a fixed drift length of $30 μ m$ . The blue datapoints show the measurements with their respective microbunch lengths, while the red curve shows the simulation results, with a minimum of 125 as.
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