Graphene Unit Cell Imaging by Holographic Coherent Diffraction

Longchamp, Jean-Nicolas; Latychevskaia, Tatiana; Escher, Conrad; Fink, Hans‐Werner

doi:10.1103/physrevlett.110.255501

Cited by 32 publications

(36 citation statements)

References 19 publications

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“…In general, fsPPM enables direct spatiotemporal probing of ultrafast processes on nanometre dimensions in the near-surface region of nanostructures, such as ultrafast carrier dynamics and currents, dynamics of interfacial fields, as well as ultrafast plasmonics. Ultimately, taking advantage of the high sensitivity of sub-keV femtosecond electron pulses combined with the magnification provided by PPM, our approach potentially allows the investigation of ultrafast phenomena on length scales down to the molecular level 35 . In addition to real space imaging, low-energy electron pulses are ideal probes for studying structural dynamics of 2D crystalline materials on the femtosecond time scale by time-resolved diffraction.…”

Section: Discussionmentioning

confidence: 99%

Femtosecond electrons probing currents and atomic structure in nanomaterials

Müller¹,

Paarmann²,

Ernstorfer³

2014

Nat Commun

123

158

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The investigation of ultrafast electronic and structural dynamics in low-dimensional systems such as nanowires and two-dimensional materials requires femtosecond probes providing high spatial resolution and strong interaction with small volume samples. Low-energy electrons exhibit large scattering cross-sections and high sensitivity to electric fields, but their pronounced dispersion during propagation in vacuum so far prevented their use as femtosecond probe pulses in time-resolved experiments. Here, employing a laser-triggered point-like source of either divergent or collimated electron wave packets, we developed a hybrid approach for femtosecond point projection microscopy and femtosecond low-energy electron diffraction. We investigate ultrafast electric currents in nanowires with sub-100 femtosecond temporal and few 10 nm spatial resolutions, and demonstrate the potential of our approach for studying structural dynamics in crystalline single-layer materials.

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Section: Discussionmentioning

confidence: 99%

Femtosecond electrons probing currents and atomic structure in nanomaterials

Müller¹,

Paarmann²,

Ernstorfer³

2014

Nat Commun

123

158

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“…1c (see Supplementary Material for details). The electrically grounded CNT is brought into the electron beam path at a typical distance of less than one micrometer from the tip, resembling a point projection microscopy configuration, that is also commonly used for electron holography [17,18]. CNT and the gold coated holey silicon nitride membrane act as a counter electrode for the biased tip.…”

mentioning

confidence: 99%

Highly Coherent Electron Beam from a Laser-Triggered Tungsten Needle Tip

et al. 2015

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We report on a quantitative measurement of the spatial coherence of electrons emitted from a sharp metal needle tip. We investigate the coherence in photoemission using near-ultraviolet laser triggering with a photon energy of 3.1 eV and compare it to DC-field emission. A carbon-nanotube is brought in close proximity to the emitter tip to act as an electrostatic biprism. From the resulting electron matter wave interference fringes we deduce an upper limit of the effective source radius both in laser-triggered and DC-field emission mode, which quantifies the spatial coherence of the emitted electron beam. We obtain (0.80±0.05) nm in laser-triggered and (0.55±0.02) nm in DC-field emission mode, revealing that the outstanding coherence properties of electron beams from needle tip field emitters are largely maintained in laser-induced emission. In addition, the relative coherence width of 0.36 of the photoemitted electron beam is the largest observed so far. The preservation of electronic coherence during emission as well as ramifications for time-resolved electron imaging techniques are discussed.Coherent electron sources are central to studying microscopic objects with highest spatial resolution. They provide electron beams with flat wavefronts that can be focused to the fundamental physical limit given by matter wave diffraction [1]. Currently, time-resolved electron based imaging is pursued with large efforts, both in realspace microscopy [2, 3] and in diffraction [4,5]. However, the spatial resolution in time-resolved electron microscopy is about two orders of magnitude worse than its DC counterpart [6], which reaches below 0.1Å [7]. Combining highest spatial resolution with time resolution in the picosecond to (sub-) femtosecond range requires spatially coherent electron sources driven by ultrashort laser pulses. Although laser-driven metal nanotips promise to provide coherent electron pulses with highest time resolution, a quantitative study of their spatial coherence has been elusive. Here we demonstrate that photoemitted electrons from a tungsten nanotip are highly coherent.So far no time-resolved electron based imaging instrument fully utilizes the coherence capabilities provided by nanotip electron sources. Meanwhile, nanotips operated in DC-field emission are known and employed in practical applications for almost half a century for their paramount spatial coherence properties [8]. Thence, highest resolution microscopy as well as coherent imaging, such as holography and interferometry, have long been demonstrated in DC-field emission [1, 9, 10]. Here we investigate whether these concepts can be inherited to laserdriven nanotip sources by comparing the spatial coherence of photoemitted electron beams to their DC counterparts. This would enable time-resolved high resolution imaging, but may also herald fundamental studies based on the generation of quantum degenerate electron beams [11].The spatial coherence of electron sources is commonly quantified by means of their effective source radius r eff . It eq...

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“…By reducing the tip-sample distance to the sub-µm range, the purely geometric projection transforms into a hologram and fsPPM merges into femtosecond low-energy electron in-line holography. In-line holographic imaging of individual biological specimen with 1 nm spatial resolution at the anode has been realized recently [20,52] by using graphene [19] as sample support, thus reducing the biprism effect which is detrimental if high spatial resolution is desired [53]. Beyond such sample restrictions, the spatial resolution of femtosecond in-line holography will ultimately be determined by the spatial coherence of the electron source, which is given by the effective source size r eff and the electron energy spread [54].…”

Section: Resultsmentioning

confidence: 99%

“…The achievable time resolution, however, is limited by the dispersive broadening of the single electron wave packets during propagation from tip to sample [16]. In addition, increased spatial resolution down to 1 nm or less can in principle be achieved by recording in-line holograms, requiring, however, tip-sample distances below 1 µm [18][19][20]. This strongly motivates the generation of femtosecond electron wave packets from the apex without direct far-field diffraction limited laser pulse illumination enabling further minimization of the tip-sample distance.…”

Section: Introductionmentioning

confidence: 99%

“…In view of its application for fsPPM, an ultrafast electron point source driven nonlocally by nanofocused SPPs lifts the constraint of restricted tip-sample distances and promises improved spatiotemporal resolution. It will further enable the implementation of in-line low-energy electron holography [18][19][20]35] with femtosecond temporal resolution and provides a new route towards ultrafast scanning tunneling microscopy and spectroscopy [36][37][38][39][40].…”

Section: Introductionmentioning

confidence: 99%

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Nanofocused Plasmon-Driven Sub-10 fs Electron Point Source

et al. 2016

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Progress in ultrafast electron microscopy relies on the development of efficient laser-driven electron sources delivering femtosecond electron pulses to the sample. In particular, recent advances employ photoemission from metal nanotips as coherent point-like femtosecond low-energy electron sources. We report the nonlinear emission of ultrashort electron wave packets from a gold nanotip generated by nonlocal excitation and nanofocusing of surface plasmon polaritons. We verify the nanoscale localization of plasmon-induced electron emission by its electrostatic collimation characteristics. With a plasmon polariton pulse duration below 8 fs at the apex, we identify multiphoton photoemission as the underlying emission process. The quantum efficiency of the plasmon-induced emission exceeds that of photoemission from direct apex illumination. We demonstrate the application for plasmon-triggered point-projection imaging of an individual semiconductor nanowire at 3 µm tip-sample distance. Based on numerical simulations we estimate an electron pulse duration at the sample below 10 fs for tip-sample distances up to few micrometers. Plasmon-driven nanolocalized electron emission thus enables femtosecond point-projection microscopy with unprecedented temporal and spatial resolution, femtosecond low-energy electron in-line holography, and opens a new route towards femtosecond scanning tunneling microscopy and spectroscopy.

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Graphene Unit Cell Imaging by Holographic Coherent Diffraction

Cited by 32 publications

References 19 publications

Femtosecond electrons probing currents and atomic structure in nanomaterials

Femtosecond electrons probing currents and atomic structure in nanomaterials

Highly Coherent Electron Beam from a Laser-Triggered Tungsten Needle Tip

Nanofocused Plasmon-Driven Sub-10 fs Electron Point Source

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