Compact femtosecond electron diffractometer with 100 keV electron bunches approaching the single-electron pulse duration limit

Waldecker, Lutz; Bertoni, Roman; Ernstorfer, Ralph

doi:10.1063/1.4906786

Cited by 85 publications

(85 citation statements)

References 30 publications

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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%

“…The electron source applied in this study provided 1-ps pulses with a current of ~1 electron per pulse at 130 µ m beam diameter, which was enough to observe 1% Bragg intensity changes within 100-s integration time. State-of-the-art dense-pulse electron sources 35,36 provide ~1,000 electrons in a 100-µ m-diameter beam at ~100 fs pulse duration (that is, a peak current 10 4 times higher than in our experiment). Due to the velocitymatching geometry at the dielectric foils, such brightness will more or less directly convert to the attosecond pulse train; losses occur only from the temporal background and from foil absorptions.…”

Section: Foilsmentioning

confidence: 99%

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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%

Section: Foilsmentioning

confidence: 99%

Diffraction and microscopy with attosecond electron pulse trains

2017

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“…Specifically, we probe the in-plane structural dynamics following resonant electronic excitation with a short (50 fs) laser pulse by femtosecond electron diffraction, described in Ref. [22]. The spectrum of the excitation pulses is centered at a photon energy of 1.55 eV, with significant spectral weight at the A-exciton transition at 1.59 eV [23].…”

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confidence: 99%

Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe2

et al. 2017

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We investigate the interactions of photoexcited carriers with lattice vibrations in thin films of the layered transition metal dichalcogenide (TMDC) WSe 2 . Employing femtosecond electron diffraction with monocrystalline samples and first-principles density functional theory calculations, we obtain a momentumresolved picture of the energy transfer from excited electrons to phonons. The measured momentumdependent phonon population dynamics are compared to first-principles calculations of the phonon linewidth and can be rationalized in terms of electronic phase-space arguments. The relaxation of excited states in the conduction band is dominated by intervalley scattering between Σ valleys and the emission of zone boundary phonons. Transiently, the momentum-dependent electron-phonon coupling leads to a nonthermal phonon distribution, which, on longer time scales, relaxes to a thermal distribution via electron-phonon and phononphonon collisions. Our results constitute a basis for monitoring and predicting out of equilibrium electrical and thermal transport properties for nanoscale applications of TMDCs. DOI: 10.1103/PhysRevLett.119.036803 Semiconducting transition metal dichalcogenides combine crystal structures of chemically stable twodimensional layers with indirect band gaps in the visible and near infrared optical range. Their intrinsic stability down to monolayer thicknesses [1,2] in combination with the possibility to create artificial stacks [3,4] suggests them for electronic and optoelectronic applications like nanoscale transistors or photodetectors with atomically sharp p-n junctions [5][6][7]. In such devices, the electronic mobilities, electronic coupling, and heat conductivities within the layers and across interfaces are of central interest. Whereas macroscopic heat and electrical transport properties can be measured directly, the observation of the underlying microscopic processes, i.e., the scattering processes of carriers and of phonons, requires methods with time, momentum, and energy resolution to be understood in detail. Such information can be decisive in the correct determination of transport properties [8,9] or energy relaxation [10][11][12]. A momentum-resolved view of scattering processes will in addition be of uttermost importance in conceiving novel quantum technologies harnessing spin and valley degrees of freedom [13][14][15], as they utilize carrier populations at specific positions in momentum space. While time-and angle-resolved photoemission spectroscopy provides this level of detail for electron dynamics, see, for instance, Refs. [16,17], techniques for studying ultrafast structural dynamics have not yet reached the equivalent level of resolution. Recently, the investigation of the time-and momentum-resolved phonon population has been demonstrated with ultrafast x-ray and electron diffraction [18][19][20][21].This work reports a momentum-resolved study of scattering processes and the resulting energy transfer between photoexcited electrons and phonons in thin bulklike films of WS...

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“…10,11 In condensed matter ultrafast electron diffraction (UED), femtosecond dynamics have been observed using compact electron guns 12,13 where the charge per pulse is limited and the source-to-target distance is kept very short to reduce the Coulomb broadening of the electron pulses. An alternative approach has been the use of radio-frequency (RF) cavities to deliver temporally compressed electron pulses at the sample position.…”

Section: -9mentioning

confidence: 99%

Femtosecond gas phase electron diffraction with MeV electrons

et al. 2016

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We present results on ultrafast gas electron diffraction (UGED) experiments with femtosecond resolution using the MeV electron gun at SLAC National Accelerator Laboratory. UGED is a promising method to investigate molecular dynamics in the gas phase because electron pulses can probe the structure with a high spatial resolution.Until recently, however, it was not possible for UGED to reach the relevant timescale for the motion of the nuclei during a molecular reaction. Using MeV electron pulses has allowed us to overcome the main challenges in reaching femtosecond resolution, namely delivering short electron pulses on a gas target, overcoming the effect of velocity mismatch between pump laser pulses and the probe electron pulses, and maintaining a low timing jitter. At electron kinetic energies above 3 MeV, the velocity mismatch between laser and electron pulses becomes negligible. The relativistic electrons are also less susceptible to temporal broadening due to the Coulomb force.One of the challenges of diffraction with relativistic electrons is that the small de Broglie wavelength results in very small diffraction angles. In this paper we describe the new setup and its characterization, including capturing static diffraction patterns of molecules in the gas phase, finding time-zero with sub-picosecond accuracy and first time-resolved diffraction experiments. The new device can achieve a temporal resolution of 100 fs root-mean-square, and sub-angstrom spatial resolution. The collimation of the beam is sufficient to measure the diffraction pattern, and the transverse coherence is on the order of 2 nm. Currently, the temporal resolution is limited both by the pulse duration of the electron pulse on target and by the timing jitter, while the spatial resolution is limited by the average electron beam current and Faraday DiscussionsCite this: DOI: 10.1039/c6fd00071a PAPER View Article OnlineView Journal the signal-to-noise ratio of the detection system. We also discuss plans for improving both the temporal resolution and the spatial resolution.

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Compact femtosecond electron diffractometer with 100 keV electron bunches approaching the single-electron pulse duration limit

Cited by 85 publications

References 30 publications

Diffraction and microscopy with attosecond electron pulse trains

Diffraction and microscopy with attosecond electron pulse trains

Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe2

Femtosecond gas phase electron diffraction with MeV electrons

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