High-temporal-resolution electron microscopy for imaging ultrafast electron dynamics

Journal name:
Nature Photonics
Volume:
11,
Pages:
425–430
Year published:
DOI:
doi:10.1038/nphoton.2017.79
Received
Accepted
Published online

Abstract

Ultrafast electron microscopy (UEM) has been demonstrated as an effective table-top technique for imaging the temporally evolving dynamics of matter with a subparticle spatial resolution on the timescale of atomic motion. However, imaging the faster motion of electron dynamics in real time has remained beyond reach. Here we demonstrate more than an order of magnitude (16 times) enhancement in the typical temporal resolution of UEM by generating isolated ∼30 fs electron pulses, accelerated at 200 keV, via the optical-gating approach, with sufficient intensity to probe efficiently the electronic dynamics of matter. Moreover, we investigate the feasibility of attosecond optical gating to generate isolated subfemtosecond electron pulses and attain the desired temporal resolution in electron microscopy to establish ‘attomicroscopy’ to allow the imaging of electron motion in the act.

At a glance

Figures

  1. Experimental set-up of the optical gating in the UEM.
    Figure 1: Experimental set-up of the optical gating in the UEM.

    A portion of infrared laser pulses generates ultraviolet laser pulses by two sequential second-harmonic generation processes. These pulses are directed to the photoemissive cathode to generate ultrafast electron pulses. The remaining infrared pulses are divided into two equal beams sent to two NOPA systems to generate visible optical gating pulses (∼30 fs) and NIR laser pulses (∼33 fs). The delays between these pulses are controlled by linear delay stages. These pulses are recombined and focused onto the specimen in the microscope and the electron-energy spectrum of the electron pulses is acquired by an electron-energy spectrometer. An energy spectrum for electron–visible photon coupling is shown in the inset.

  2. Temporal characterization of an ultrafast electron pulse.
    Figure 2: Temporal characterization of an ultrafast electron pulse.

    a, The electron-energy spectrum of the original ultrafast electron pulse (ZLP spectrum) is shown in the shaded pink curve, and the electron-energy spectrum with the coupling between the electron and the gating visible (ћω = 2.25 eV) laser pulse (τvis = 0 fs) is plotted in blue. b, Measured electron-energy spectrogram of the electron-energy spectra as a function of the gating visible laser pulse delay τvis, where the gated electrons appear on both sides of the ZLP (energy-gain and energy-loss sides). The residual ZLP at τvis = 0 fs is subtracted for better visualization. c, The cross-correlation temporal profile, calculated by integrating the coupling spectral peaks of the energy spectrum at each instant of the gating pulse arrival time (τvis), is shown in open black circles and its fitting is shown as a red curve over a blue shaded area. The white shaded area and white dotted line show the time of the gating window (the pulse duration of the gating visible pulse).

  3. Temporal characterization of a gated electron pulse.
    Figure 3: Temporal characterization of a gated electron pulse.

    a, Illustration of the cross-correlation measurement principle to characterize the temporal profile of the gated electron pulse. b, Electron-energy spectra of the optimum electron–photon coupling between the original electron pulse and the gating visible pulse (30 fs) (black line), the original electron pulse with both visible and NIR (ћω = 1.675 eV) laser pulses (red line) and the original electron-pulse spectrum (ZLP) (orange curve). The right inset shows the image of the induced near-field using the gated electrons in silver nanowire. For a description of the left inset, see d. c, Cross-correlation electron energy spectrogram of the electron–photon coupling between the NIR laser pulse and both the original and gated electron pulses. The ZLP is suppressed for a clear illustration of the gating effect. d, Cross-correlation temporal profile retrieved from the measured spectrogram in c carries the signature of the gated electron pulse through the coupling of the original electron pulse with the NIR laser pulse, shown by the connected open circles. This curve and its expanded view (left inset in b) show clearly the dip in the electron counts caused by the gated electron pulse. The orange curve represents the fitting of the measured cross-correlation temporal profile of the NIR pulse coupling with the original electron pulse in the absence of a gating visible pulse. e, Cross-correlation of the gated electron and NIR pulses, obtained by subtraction of the temporal profiles in d, is plotted in black dots along with its fitting (red line). A fit of a measured temporal profile of the gating visible pulse is shown in blue. f, The same as in e, but for the case of a longer visible gating pulse (∼110 fs).

  4. Attosecond optical gating of the electron pulse.
    Figure 4: Attosecond optical gating of the electron pulse.

    a, The optical attosecond gating pulse. b, The simulated optical attosecond gating spectrogram of a 75 fs electron pulse by the optical attosecond pulse at an intensity below the saturation level. c, The electron energy spectrum at τvis = 0 fs. ZLP in b and c is subtracted for a better visualization. d, The attosecond optical-gating efficiency as a function of the pulse duration of the original electron pulse.

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Author information

  1. Present address: Department of Physics, University of Arizona, Tucson, Arizona 85721, USA

    • M. Th. Hassan
  2. Deceased.

    • A. H. Zewail

Affiliations

  1. Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, USA

    • M. Th. Hassan,
    • J. S. Baskin,
    • B. Liao &
    • A. H. Zewail

Contributions

M.Th.H. conceived the idea; M.Th.H., J.S.B. and A.H.Z. designed the experiment; M.Th.H. and J.S.B. conducted the experiments; M.Th.H., J.S.B. and A.H.Z. conducted the analysis of the first set of results. M.Th.H. and B.L. conducted the simulations; B.L., J.S.B. and M.Th.H. interpreted the data and contributed to the preparation of the manuscript.

Competing financial interests

The authors declare no competing financial interests.

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