Femtosecond single-electron diffraction

Lahme, Stefan; Kealhofer, Catherine; Krausz, Ferenc; Baum, Peter

doi:10.1063/1.4884937

Cited by 67 publications

(60 citation statements)

References 64 publications

(49 reference statements)

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“…We expect ~10 electrons per individual attosecond pulse, or roughly 10 7 -10 8 electrons per second at 50-500-kHz repetition rate. This current is enough for pump-probe diffraction or microscopy of reasonably good-quality samples 35,36,38 . If a single isolated attosecond electron pulse is generated by single-opticalcycle filtering (see main text), we still can expect 10 5 -10 7 electrons per second.…”

Section: Foilsmentioning

confidence: 99%

Diffraction and microscopy with attosecond electron pulse trains

2017

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Attosecond spectroscopy1-7 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 techniques [8][9][10][11] . Here we advance attosecond metrology to picometre wavelength and sub-atomic resolution by using free-space electrons instead of higher-harmonic photons 1-7 or re-colliding wavepackets [8][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.Our concepts for generating attosecond electron pulses, direct streaking-oscilloscope characterization, atomic diffraction and proof-of-principle attosecond electron microscopy of electromagnetic fields are depicted in Fig. 1. A femtosecond laser 12 triggers electron emission from a photocathode (yellow) and produces electron pulses of ~1 ps duration at a central energy of E el = 70 keV (ref.13 ). The de Broglie wavelength is λ el ≈ 4.5 pm and spacecharge effects are avoided by using fewer than one electron per pulse at the sample. A first laser beam ('modulation') is used to compress the electron pulse into a sub-cycle pulse train. For this purpose, we let the laser beam and the electron beam intersect at a 50-nm-thick silicon nitride membrane that lets the electrons pass through. The membrane is also transparent to the laser beam (1,030 nm wavelength), but the refractive index of ~2 in combination with thin-layer interferences generates a phase shift between the incoming and outgoing electromagnetic waves. Therefore, the periodic electromagnetic acceleration and deceleration of the propagating electrons in the optical field cycles before and after the membrane do not cancel out after passage through the laser focus, as would happen in free space 13 . In the end, there remains a time-dependent overall momentum kick that is proportional to the change of vector potential at the membrane and therefore dependent on the optical phase. In contrast to metal foils 13,14 , graphite 15 or nanostructures 16 , a dielectric membrane has negligible linear or nonlinear optical absorption and can therefore sustain an extreme level of power and fields, enabling strong/effective compression and metrology.In the experiment, we let the electron beam (0.48 × speed of light) and the laser beam (p-polarized, ~15 mW, peak field ~5 × 10 7 V m -1 ) hit the membrane under 35° and 60° from the surface...

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Section: Foilsmentioning

confidence: 99%

Diffraction and microscopy with attosecond electron pulse trains

2017

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“…The pump-probe technique allows to do so: an ultrashort laser pulse (a pump pulse) excites different dynamics inside a material and a second ultrashort pulse (a probe pulse) is applied to obtain structural information at a sequence of time delays. In ultrafast electron microscopy (UEM) and ultrafast electron diffraction (UED) techniques, the second pulse, which is usually generated by photoelectric emission with femtosecond lasers, contains electrons [50][51][52]. In case of multi-electron bunches, such pulses get broadened during the propagation due to Coulomb repulsion between the particles [35].…”

Section: Chaptermentioning

confidence: 99%

Electron microscopy of electromagnetic waveforms

Ryabov

Baum

2016

Science

Self Cite

118

111

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Imaging electromagnetic waveforms Understanding the dynamics of electrons and the spatial and temporal evolution of electromagnetic fields within a material or device is often the key to optimizing performance. Ryabov and Baum show that electron microscopy can be used to measure collective carrier motion and electromagnetic fields with subcycle and subwavelength resolution. As an example, they used a train of compressed electron pulses to produce movies of the electromagnetic excitation in an optical excited metamaterial component. Science , this issue p. 374

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“…Recently, ultrafast electron microscopy has been built by modifying a conventional TEM from thermionic or field emission electron sources to ultrafast laser-excited photoemission electron sources [14][15] , allowing timeresolved optical pump -electron probe experiments. Dedicated kilo-electron-volt (keV) ultrafast electron diffraction machines have also been built and optimized [16][17] . Due to the strong space-charge forces with the moderate accelerating voltages of these instruments (200-300 keV), the beam charge density has to be reduced, sometimes to as little as a single electron per pulse, to reach a sub-ps temporal resolution while maintaining transverse emittance.…”

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confidence: 99%

Femtosecond mega-electron-volt electron microdiffraction

Shen

Lundström

et al. 2018

Ultramicroscopy

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Instruments to visualize transient structural changes of inhomogeneous materials on the nanometer scale with atomic spatial and temporal resolution are demanded to advance materials science, bioscience, and fusion sciences. One such technique is femtosecond electron microdiffraction, in which a short pulse of electrons with femtosecondscale duration is focused into a micron-scale spot and used to obtain diffraction images to resolve ultrafast structural dynamics over localized crystalline domain. In this letter, we report the experimental demonstration of time-resolved mega-electron-volt electron microdiffraction which achieves a 5 μm root-mean-square (rms) beam size on the sample and a 100 fs rms temporal resolution. Using pulses of 10k electrons at 4.2 MeV energy with a normalized emittance 3 nm-rad, we obtained high quality diffraction from a single 10 μm paraffin ( ) crystal. The phonon softening mode in optical-pumped polycrystalline Bi was also time-resolved, demonstrating the temporal resolution limits of our instrument design. This new characterization capability will open many research opportunities in material and biological sciences.Time-resolved x-ray 1-3 and electron microbeams 4-6 are emerging tools with broad applications in science and technology. Achieving high temporal resolving power provides insight into structural dynamics of materials in non-equilibrium states for understanding and controlling of energy and matter 7 . Delivering a focused, micron-scale beam to a sample under study is of paramount importance, as it enables the study of samples that cannot be prepared at the macroscale, or are intrinsically inhomogeneous. For example, the lateral size of a "large" protein crystal is typically less than 100 μm. Further, small protein crystals almost always exhibit greater perfection and less impurity than large crystals 8 . Matching the beam size to that of the small protein crystal sample provides the best signal-to-noise (SNR) ratio for crystallography. The inhomogeneous nature of natural and nanoscale materials also requires micron-scale probes to determine local composition, chemistry, and crystalline structure. With the advent of high brightness x-ray free-electron lasers and efficient x-ray focusing optics, x-ray microbeams with femtosecond level temporal resolving power have been achieved [9][10][11][12] . Extensive research and development efforts are underway to push the frontier of electron microbeams towards the femtosecond time scales temporal resolution in the Ultrafast Electron Diffraction (UED).Electron microdiffraction with continuous wave beams is a well-established technique in conventional transmission electron microscopy (TEM) 13 . Recently, ultrafast electron microscopy has been built by modifying a conventional TEM from thermionic or field emission electron sources to ultrafast laser-excited photoemission electron sources [14][15] , allowing timeresolved optical pump -electron probe experiments. Dedicated kilo-electron-volt (keV) ultrafast electron diffraction machines ha...

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Femtosecond single-electron diffraction

Cited by 67 publications

References 64 publications

Diffraction and microscopy with attosecond electron pulse trains

Diffraction and microscopy with attosecond electron pulse trains

Electron microscopy of electromagnetic waveforms

Femtosecond mega-electron-volt electron microdiffraction

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