Plasmonic-Nanofocusing-Based Electron Holography - ACS

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Plasmonic-nanofocusing-based electron holography Jan Vogelsang, Nahid Talebi, Germann Hergert, Andreas Wöste, Petra Groß, Achim Hartschuh, and Christoph Lienau ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00418 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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Plasmonic-nanofocusing-based electron holography Authors: Jan Vogelsang1,§,†, Nahid Talebi2, Germann Hergert1, Andreas Wöste1, Petra Groß1, Achim Hartschuh3, Christoph Lienau1,†

1

Institut für Physik and Center of Interface Science, Carl von Ossietzky Universität Oldenburg,

26111 Oldenburg, Germany 2

Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany

3

Department of Chemistry and Center for NanoScience, Ludwig-Maximilians-Universität Mün-

chen, 81377 München, Germany §

Present address: Department of Physics, Lund University, Box 118, 22100 Lund, Sweden

ABSTRACT: Point-projection microscopy (PPM) with low energy electrons is developing into a powerful tool for holographic reconstruction of individual proteins and solid state nanostructures. In combination with laser-based photoemission schemes it offers the exciting prospect of ultrafast coherent electron holography of single nanostructures. Such experiments would greatly benefit from a freestanding electron source with femtosecond time resolution, few-nm emitter size and good coherence properties. Here, we use plasmonic nanofocusing on a conical gold taper and multiphoton photoemission from the taper apex to create such a source. It is implemented in a PPM setup and used to record inline holograms of thin bundles of single-walled carbon nanotubes, demonstrating an effective emitter radius of less than 5 nm. We show that the same concept can also be transferred to tungsten tips, offering further improvements in emitter size and brightness. Numerical simulations show that such an ultrafast, low-energy electron source present a highly interesting tool for probing optical fields at surfaces with nanometer spatial and femtosecond time resolution.

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KEYWORDS: surface plasmon polaritons, ultrafast electron holography, adiabatic nanofocusing, coherent electron pulses, point-projection electron microscopy, metal tips


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In low-energy point-projection electron microscopy (PPM), a spherical electron matter wave is emitted from a nanoscopic source and diffracted off a small object in its close vicinity. The resulting interferogram, an in-line hologram, between this reference wave and the scattered wave is recorded on a distant screen. Numerical methods for reconstructing the object shape from such holograms are highly developed 1-2 and have recently been used, e.g., for imaging of single proteins with subnanometer resolution3, of individual tobacco mosaic viruses4, gold nanorods5 or single charges in graphene6. Even though electron interferometry in general7-8 and PPM in particular9-11 have been introduced several decades ago, many of the exciting applications mentioned above have been reported only recently. This is partly due to the difficulty in preparing thin substrates that are sufficiently transparent for low energy electrons while providing sufficient mechanical stability for depositing nanoparticles or biomolecules. Recently, freestanding graphene substrates have been fabricated that meet these requirements12. Low energy holography is particularly attractive for the analysis of single proteins or nanoparticles3 because of its sensitivity to weak local electric fields and since the use of low energy electrons drastically reduces the radiation damage of the nanoparticles that is unavoidable when using high energy electrons.13 In light of the recent exciting developments in time-resolved electron microscopy14-22 and in ultrafast photoemission from metal nanotip emitters23-28, it seems interesting to also consider the use of ultrafast electron sources for time-resolved electron holography23. This could open up a new approach towards probing the dynamics of local electric fields and - more general - of nonequilibrium optical excitations in nanostructures with a spatial resolution in the nanometer regime. The close proximity between emitter and sample avoids temporal spreading of the electron wavepacket on its path towards the sample and could bring the time resolution of electron microscopy into the few femtosecond regime or even below29 without the need for additional gating concepts18, 30. Consequently, the combination of PPM with ultrafast laser-driven nanotip emitters has achieved considerable attention during the last years. The technique has been introduced in Ref. 29 and Müller


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et al. have used it in an important proof-of-concept experiment to study photogenerated currents in nanowires with ~ 100 fs time and several tens of nm spatial resolution.20 Charge distributions near nanotips have been imaged31 and a conceptually similar scheme has been used for time-resolved electron diffraction of graphene layers17. Further improvements in spatial resolution are expected by analyzing interference patterns recorded with laser-triggered photoemission. The excellent coherence properties of metal nanotip emitters have already been demonstrated by recording carbon nanotube (CNT) interferograms using one-photon photoemission32. So far, time-resolved electron holography has not yet been demonstrated with nanotip photoemitters. This may partly be due to strong laser fields that are needed to trigger photoemission by directly illuminating the apex of these tips. This requires large tip-sample distances of several microns to prevent undesired sample illumination, limiting both the spatial and temporal resolution. Recently, plasmonic nanofocusing (PN)33-35 has been introduced as an elegant concept to overcome these challenges. In PN, plasmon waves are optically launched at the shaft of a conical nanotaper. They propagate towards the taper apex where they are strongly spatially and temporally confined and can drive multiphoton photoemission. This creates a bright and spatially isolated nanometer-sized electron source with pulse duration as short as 10 fs23, 27-28, 36. The absence of direct apex excitation facilitates sub-micron emitter-sample distances, promising further substantial improvements of the spatiotemporal resolution of ultrafast PPM (UPPM). In first implementations of PN in PPM, signatures of photoinduced charge separation in nanowires have been seen23 and - most recently - the motion of electrons photoemitted from a plasmonic nanoantenna has been probed with ~25 fs time and 20 nm spatial resolution36. So far, however, the spatial coherence properties of this source have not yet been explored. In particular, it is not yet known how the strong transient heating of the laserdriven electron gas in the sharp metal tip affects the coherence of the emitted electron beam, crucial for applications in time-resolved electron interferometry or holography. Here, we combine plasmonic nanofocusing and multiphoton photoemission on a conical gold taper to create a freestanding, ultrafast and coherent electron source for time-resolved holography. In-line


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holograms of thin bundles of single-walled carbon nanotubes demonstrate an effective emitter radius of less than 5 nm. We show that the same concept can also be transferred to tungsten tips, offering further improvements in emitter size and brightness and introducing an ultrafast, low-energy electron source for probing optical fields at surfaces with nanometer spatial and femtosecond time resolution. RESULTS AND DISCUSSION a) Experimental setup Ultrashort laser pulses with a pulse duration of less than 20 fs are generated at 5-kHz repetition rate in a home-built noncollinear optical parametric amplifier system spectrally tunable from 1200 nm to more than 2800 nm37. The laser pulses are split into two parts using a 50/50 beam splitter and are focused onto opposite sides of a sharply etched metallic taper via two identical beam paths using two parabolic mirrors with effective focal lengths of 75 mm. For all measurements presented here except one, only one of the two beams is used. This is shown schematically in Figure 1a. Such longwavelength pulses are used to obtain a high-order nonlinearity in the photoemission from the taper apex25-26 and to confine the photoemission to the region of highest local field enhancement, i.e., the taper apex. This effectively suppresses photoemission from all other regions of the taper. The longwavelength pulses also ensure long surface plasmon propagation of several tens of microns in PN, even for lossy tungsten tapers.


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Figure 1. (a) Schematic of the experimental setup. The grating coupler of a gold taper is illuminated with ultrashort laser pulses. The laser pulses are coupled to surface plasmon polaritons, which propagate towards the taper apex where they induce multiphoton photoelectron emission. The electrons form a diverging beam and move through the sample plane towards a distant detector. Electron diffraction off the sample results in a holographic image on the detector screen. (b) Low magnification scanning electron microscope (SEM) image of a sharply etched gold taper showing the taper including the grating coupler in a distance of 50 µm from the taper apex. The grating coupler consists of three lines width a width and depth of 200 nm each. The grating constant is chosen such that it can efficiently couple laser pulses with a central wavelength of 1800 nm to SPPs. (c) High magnification SEM image of the same taper as in (b) revealing an apex radius of 9 nm. (d) Spectrum of the laser pulses used to photoemit electrons from the gold taper. (e) Cross-correlation measurement between the surface plasmon at the taper apex and a second laser pulse directly illuminating the apex (blue circles). As a solid line, a simulation assuming a nonlinear order of N=5 of the photoemission process is shown. A plasmon pulse duration of 18 fs is deduced.

Two scanning electron microscope (SEM) images of the taper are shown in Figure 1b,c revealing an apex radius of 9 nm. The taper is equipped with a grating coupler milled into the metal using a foACS Paragon Plus Environment

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cused beam of gallium ions and is placed in an ultrahigh vacuum chamber with a base pressure of 5 ⋅10−10 hPa. The laser beam (spectrum shown in Figure 1d) is steered onto the grating coupler

launching surface plasmon polariton (SPP) pulses on the metal taper. The fraction of the SPPs that are coupled into the lowest order angular momentum eigenmode of the taper38 experience an increasing surface plasmon refractive index and are adiabatically compressed as they approach the taper apex33, 38-39. Here, they transform into localized surface plasmons with a strong electric field component along the taper axis34, 40-41. At the apex, this field is strong enough to induce multiphoton emission of a diverging electron wavepacket 23, 27-28. The incident laser power is chosen sufficiently low to remain in the regime of multiphoton photoemission26, requiring 6-7 photons to overcome the work function of gold. The emitted electron wavepacket is accelerated by a static bias field towards an MCP/phosphor screen two-dimensional electron detector. In order to determine the plasmon duration at the taper apex, we perform a nonlinear crosscorrelation measurement between the surface plasmon reaching the apex and a second laser pulse focused directly at the apex. The corresponding electric fields interfere and result in multiphoton electron emission. The electron count rate is plotted in Figure 1e versus the temporal delay of the two pulses. The experimental results are well reproduced by assuming a fifth-order photoemission process and a plasmon duration of 18 fs (full width at half maximum of the intensity profile) at the taper apex, only marginally larger than the laser pulse duration of 16 fs. This proves that this adiabatic nanofocusing technique is inherently broadband and the electrons are indeed emitted from the taper apex in only two to three cycles of the plasmonic field.


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Figure 2. (a) Electrostatic field electron emission from a sharp gold taper is used to image a carbon nanotube at a distance of 2.5 µm from the taper apex. A hologram of the nanotube forms on the detector in distance of 75 mm. (b) Cross section along the white dashed line shown in (a). Several interference fringes (black dots) are clearly visible. (c) Cross section from an identical measurement, but with plasmon-driven electron generation. The noise level is larger due to the reduced total number of electrons. Interference fringes can also be clearly observed.

b) In-line electron holography of carbon nanotube bundles using an ultrafast, plasmonicnanofocusing electron source To study the imaging properties of this freestanding, ultrafast electron source, a thin carbon nanotube bundle, suspended on a TEM grid, is placed at a distance of 2.5 µm in front of the taper. For this, we first operate the source in DC-field emission mode, applying a small static bias voltage between taper and sample is used to trigger electron emission. The DC bias is set to 55 V (-30 V on the taper, 25 V on the sample, detector on ground potential), such that a few hundred electrons are tunneling out of the taper apex per second. The electron wave diverges and is partially scattered by the carbon nanotube. Transmitted and scattered electron waves overlap on the detector screen at a distance of d=75 mm and form an interference pattern that is aligned along the nanotube axis10, 32, 42.


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This is shown in Figure 2a, recorded with an emission current of approximately 0.5 fA. A cross section perpendicular to the nanotube along the white dashed line is shown in Figure 2b, revealing 11 interference maxima in total, covering a distance of s=2.6 mm on the screen. Stable electrostatic electron emission from a sharp gold taper is achieved by keeping the pressure in the vacuum chamber below 10 −9 hPa and cleaning the taper apex gently by illuminating it with the laser pulses described before for two minutes. This sets a stable emission rate of a few hundred electrons per second. The same experiment is now repeated using plasmon-driven electron emission. To suppress electrostatic electron tunneling out of the taper, the voltage is reduced to 40 V (-60 V on the taper, -20 V on the sample). The laser spectrum is centered around a wavelength of 1800 nm (Figure 1d). The energy of the laser pulse exciting the SPP is set to 440 pJ to emit approximately one electron per laser shot on average. The field strength on the grating coupler is 0.55 V/nm and is enhanced by nanofocusing to 6 V/nm at the apex. For direct apex illumination, our tips typically show a field enhancement of about 1026, 43. Consequently, a field strength of 0.6 V/nm is needed to reach the same photoemission count rate as for plasmonic nanofocusing photoemission. This value of 0.6 V/nm is well confirmed by our current measurements. A cross section through the CNT interferogram created with apex illumination is shown in Figure 2c. Unlike in Figure 2b, now seven interference fringes are now observed, extending over s=1.6 mm on the detector.

The transverse coherence of the electron wave is directly related to the effective source size of the electron emitter, i.e., the radius of a virtual emitter of spherical electron waves located inside the nanotip32, 44-45. Consequently, the angle under which constructive (destructive) interference is seen can be used to probe the transverse coherence of scattered and unscattered waves and thus to deduce an upper limit of the effective electron source size re . The equation re =

2λ d πs


(1)

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relates the apparent extension of the interference fringes s on the detector screen to the emitter size

re 44-45. Here, re represents an upper limit of the actual emitter size since any type of experimental imperfections, being it mechanical vibrations, a finite extent of the diffracting object or an energetic spread of the electron beam will reduce the extent s of the fringes. The de Broglie wavelength λ of the electrons on the detector is 0.22 nm in the static case and 0.16 nm in the case of plasmondriven emission. The distance s refers to the total width of the screen region that is covered by fringes on both sides of the nanotube on the screen at distance d. Using Equation (1), an effective source size of 4.1 nm in the electrostatic case compared to 4.7 nm in the case of plasmon-driven electron emission is deduced. Both values are quite similar, with the electrostatic source size being a bit smaller. Both values are substantially higher than the record sub-nm values that are obtained in state-of-the-art continuous wave PPMs with single-crystalline tungsten tips3, 6. The spot size for DC emission may be limited both by the mechanical stability of our setup and the shape of the soft gold tips. Since the source size for plasmon-driven electron emission is only slightly larger, it is difficult to clearly identify the cause for this increase. We expect that it may be related to a nonequilibrium excitation of the laser-driven electron gas in the tip. Based on our earlier results we expect that the excitation of such a tip with pulse inducing multiphoton photoemission results in a broad, most likely athermal distribution function of the electrons46. This hot electron distribution may likely spread out the effective emitter size. Also, the spatially inhomogeneous optical near field near the taper apex will accelerate the emitted electrons in different directions, which is likely to make the effective source size appear larger25, 47. Currently, however, the difference in deduced emitter sizes is too small to make a definite assignment. More work is needed to pinpoint the physical causes that limit the experimentally observed emitter size. Earlier ultrafast PPM measurements revealed spatial resolutions in the range of 100 nm20 or 25 nm36. The demonstrated interference patterns in Figure 2 and the deduced emitter size of less than 5 nm definitely shows that a much higher spatial resolution may be achieved when probing sufficiently thin nanostructures such as CNT or single, suspended nanoparticles in general. Most likely, this spatial resolution can be improved much further by


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choosing other nanotip materials. Much smaller emitter sizes are routinely achieved with monocrystalline tungsten tips in DC-driven PPM and have also been seen for laser-driven one-photon photoemission from such tips32. So far, however, these laser-driven experiments have been performed by directly illuminating the taper apex. Such a direct tip-illuminating scheme complicates timeresolved UPPM and holography experiments and this certainly warrants efforts in implementing plasmonic nanofocusing photoemission for taper materials other than gold.

c) Plasmonic nanofocusing photoemission from tungsten tapers In an attempt to further improve the emitter size, we have replaced the gold by a sharply etched tungsten taper and have implemented a plasmonic nanofocusing to induce multiphoton photoemission from the taper apex. Scanning electron microscope images of the taper are depicted in Figure 3a. A grating coupler was milled with the same parameters as for the gold taper except for the grating constant. The grating constant has been increased to 6 µm to allow for grating coupling of longer wavelength pulses centered around 2200 nm. In the lower part of Figure 3a, the shape of the taper is shown as it appeared during inspection with the SEM. Here, the sharp tungsten taper is still covered with a thin oxide layer, not distinguishable from bare tungsten in the SEM. Inside the PPM, this layer is removed by resistive heating immediately before using the taper for electron emission.


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Figure 3. (a) Scanning electron microscope images with different magnifications of a sharply etched tungsten taper equipped with a grating coupler. The grating coupler consists of three lines with a width and depth of 200 nm each. The grating constant is chosen such that it can efficiently couple laser pulses with a central wavelength of 2200 nm to SPPs. The apex radius is smaller than 6 nm. (b) Nanotubes are positioned at a distance of 1200 nm from the taper apex and imaged with a diverging electron wave emitted from the apex of a tungsten taper by electrostatic tunneling. A magnification of 40,000 is reached. (c) Cross section along the black dashed line in (b). Several interference fringes can be clearly observed.

The tungsten taper is first used for electrostatic imaging of a bundle of carbon nanotubes, inducing DC field emission by applying a bias voltage of -50V to the taper. The nanotube bundles are deposited on a thin carbon mesh, placed at 1200 nm distance from the taper apex. The resulting hologram is shown in Figure 3b. Compared to the image recorded with a sharp gold taper shown in Figure 2a, shot noise is much reduced. This is due to the high currents supported by tungsten tapers of more than 10 nA6 compared to less than 1 fA observed before for the gold taper. We experimentally find that tungsten tapers withstand currents in the nA-range without visible degradation, while gold tapers are immediately destroyed using currents above 1 pA. Consequently, using electrostatic elecACS Paragon Plus Environment

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tron emission from a tungsten taper, images can be recorded at least thousand times faster than using electrons from gold tapers. The image shown in Figure 3b was recorded within 400 ms compared to several minutes recording time for the image shown in Figure 2a.

A cross section along the black line in Figure 3b is shown in Figure 3c. As much as 25 interference fringes are counted covering a distance of s = 6.0 mm on the detector. Both, sample and detector were kept on ground potential, such that the hologram forms at a kinetic energy of 50 eV and hence a de Broglie wavelength of 0.15 nm. This gives an effective source radius of 1.4 nm, which is three times less than that of the gold taper. We expected a substantial improvement in source size already from the comparison of the SEM images in Figures 1b,c and 3a, but a thin oxide layer on the tungsten taper hindered an exact determination of the apex radius from these images. A quantitative analysis of the image contrast in Figure 3b requires an accurate modelling of the holographic pattern that results from the interference between the incident, quasi-monochromatic electron wave and its diffraction by the CNT bundle. To first order, the amplitude of the diffracted wave can be expressed using a Fresnel-Kirchhoff type diffraction integral, assuming a complex-valued transmission function of the CNT bundle5, 45. We have performed such a modelling assuming a step-like transmission function and find that the resulting holograms accounts well for the interference outside the shadow image of the nanotube sample. Such a simplified modelling, however, fails to resolve the interference inside then CNT region. Here, the correct phase profile of the transmission function needs to be considered to fully reproduce the experimental data, confirming that the measurements in Figures 3 should indeed be interpreted as an in-line hologram recorded with quasimonochromatic, spherical wave excitation. A full account of this modelling is outside the scope of this paper and will be given in a forthcoming publication. The significantly improved source size and hence spatial resolution of the tungsten emitters is evidently very interesting for ultrahigh time resolution electron microscopy, especially if such experiments can be performed at small emitter-sample distances that are crucial for achieving high spatio-


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temporal resolution. PN offers a viable strategy for reducing the emitter-sample distance but has, so far, not yet been demonstrated. The substantial dielectric losses of tungsten restrict the SPP propagation lengths to prohibitively short distances in the commonly used near-infrared spectral range. Desired grating-apex distances of at least 30 µm can only be reached when working with laser wavelengths above 1800 nm. As a compromise between output power of the NOPA-DFG laser system, which decreases for longer wavelengths, and efficient SPP propagation, we have tuned our laser system to a central wavelength of 2200 nm to demonstrate PN on tungsten. The corresponding broad spectrum is shown in Figure 4a. The spectral part at the central wavelength will propagate for 80 µm before it has decayed to 1/e of its original intensity, which greatly exceeds the desired propagation length of 30 µm. At this wavelength, approximately eight photons are needed to induce multiphoton photoemission.

Figure 4. (a) Spectrum of the laser pulses used to photoemit electrons from the tungsten taper. (b) Point-projection image of a carbon grid recorded by DC field emission. (c) Image recorded with laser-driven multiphoton emission from the tungsten taper induced by plasmonic nanofocusing. The image contrast is similar to that in (b) demonstrating that electrons are emitted from the taper apex. Compared to (b), the electrons are distributed more homogeneously on the detector screen. This is due to a reduced DC bias voltage, resulting in a broader photoelectron emission angle

To test PN on conical tungsten tapers, 20-fs pulses with a pulse energy of 2.6 nJ are used at 5 kHz repetition rate. They are focused to a spot diameter of 15 µm on the grating coupler, resulting in a peak electric field strength of 0.7 V/nm. The focus size is much smaller than the apex-grating distance, preventing simultaneous apex illumination when steering the beam to the grating coupler.


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The pulse energy is set to emit approximately one electron per laser shot from the tungsten taper and these electrons are used for laser-triggered PPM. As a reference, we first recorded a pointprojection image of a transparent carbon grid by using electrostatic field emission with a bias voltage of 120 V (Figure 4b). When suppressing DC emission by reducing the bias voltage to 80 V, and triggering photoemission by grating coupling, essentially the same image of the carbon grid is recorded. Except for the higher number of electrons in the reference image, both images show almost identical contrast and resolution. This proofs that, in both cases, the electrons are clearly emitted from the taper apex, also when the grating coupler is illuminated. Hence, efficient plasmonic nanofocusing of an SPP can be achieved on tungsten tapers. This requires increasing the laser wavelength above 2 µm, where the SPP propagation length becomes sufficiently long.

Our experiments show that so far, plasmonic nanofocusing on tungsten tapers is slightly less efficient than on gold tapers. We expect that a further increase in efficiency of plasmonically-driven photoemission from tungsten tapers may be achieved by increasing the excitation laser wavelength beyond 3 µm. Few-cycle pulses in this range are readily available from NOPA sources and their use might be instrumental using tungsten tapers for ultrafast electron holography. Also, the use of a thin dielectric coating or a molecule reducing the work function at the very apex might improve the performance of these freestanding ultrafast electron sources.

We finally note that in the presented experimental configuration, some electrons are emitted also from the grating coupler on the taper shaft. Due to the large apex-sample distance, these electrons do not reach the detector such that a high contrast image forms in Figure 4c. For shorter apexsample distances, electrons emitted from the grating coupler are accelerated more strongly in forward direction and hence form a constant background signal on the detector. This background may


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be removed, e.g., by using a delay-line electron detector to record the time of flight of each electron on its path from photoemission to detector.

Figure 5. (a) Schematic of a proposed ultrafast electron holography experiment. Probe electrons emitted by PN are diffracted off a nanostructured sample which is optically excited by the evanescent field of an ultrashort pump pulse. Arrival position and kinetic energy of the transmitted electrons are recorded as a function of time delay between optical pump and electron probe. (b) Simulated kinetic energy spectrum (red solid line) of an ultrashort electron pulse after interaction with the near-field of an optically-pumped carbon nanotube. For comparison, the spectrum of the incident pulse is given in blue. Clear PINEM sidebands at the photon energy of 0.7eV can be observed. The spectrum shows additional modulations at the work function of the sample of 5 eV. (c) Snapshots of the wavefunction amplitude of an ultrashort electron pulse with 60 eV kinetic energy and 10-fs duration interacting with a light-driven dielectric cylinder of 50-nm diameter (circle in the ACS Paragon Plus Environment

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center). The peak amplitude of the driving laser is 5x109 V/m. Reshaping of the angular spread and kinetic energy distribution of the electron wavepacket is visible. Such a detector may thus be used to record kinetic energy spectra of the photoemitted electrons while conserving the spatial resolution of the PPM measurement.

d) Numerical modelling of ultrafast electron holography with low-energy electrons. Given the high time resolution of our electron beam, it will be interesting to explore the effects of strong, transient optical near-fields on these spectra in what may be termed an ultrafast low-energy near-field electron microscope (Figure 5a). As first observed in Ref. 22, the motion of a swift electron through the intense optical near field of a nanostructured sample leads to the formation of a comb of photon sidebands in the electron energy distribution, allowing to probe these near fields in photon-induced near-field electron microscopy (PINEM). When using strong quasi-monochromatic or bichromatic light fields to drive the nanostructure, the shape of the free electron wavepacket can be coherently controlled16, 48, opening the way to attosecond phase-resolved electron microscopy18, 30

. Here, the role of the nanostructure is, in greatly simplified words, to provide the momentum that

is necessary to bridge the momentum mismatch between the swift electron (moving at roughly 2/3 of the speed of light c0 for a 200 keV electron) and the far-field photon that excites the nanostructure. The nanostructure induces a coupling between far-field light at frequency ωL and a free electron, moving at speed ve , that can be characterized by a generalized coupling constant16. g=

 ω  e F ( z ) exp  −i L z  dz . ∫ 2hωL  ve 

(2)

Here, F ( z ) denotes the longitudinal amplitude component of the optical near field along the trajectory of the electron, propagating along the z-axis. The momentum mismatch between electron and light, ∆p / h = ωL / v e , induces an oscillatory term in the integrand in Eq. (1). For swift electrons, usually considered in (PINEM)22, the period of the oscillation is in the order of the photon wavelength. For low-energy electrons, however, moving at


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less than 0.06 c0 for energies up to 1000 V, oscillations of the integrand with few tens of nm period result. Hence, for nanoobjects with spatially extended near-fields, with dimensions approaching the photon wavelength, the light-electron coupling is quite weak. Therefore, PINEM effects have so far not yet been studied with low energy electrons. For small nanostructures, with dimensions in the ten nm range, and spatially highly confined near-fields, however, quite sizeable coupling constants are obtained. Also, angular deflections by confined near-fields, scaling inversely proportional to the kinetic energy, are more pronounced. Ultrafast low-energy electrons might therefore emerge as sensitive probes of highly confined optical near-fields. To explore this sensitivity, we performed semi-classical simulations using a combined MaxwellSchrödinger numerical solver.49 In these simulations, we study how the electromagnetic near fields created by the material polarization and current distribution act back on the wavepacket motion of the passing electron. For calculating the electromagnetic fields, a finite-difference time-domain method is used, whereas the dynamics of the electron wave function are simulated using a pseudospectral method. We consider an electron pulse with carrier kinetic energy of 60 eV in the form of a diverging Gaussian beam, with the source point located at 1µm distance from the sample. Near the sample plane, its pulse duration is approximately 10 fs. As the sample, we take a dielectric cylinder (a greatly simplified model of a CNT) with a radius of 50 nm. For simplicity, the optical response of the sample is modeled by the dispersive dielectric function of carbon and embedded in a step cylindrical potential with a height of 5 eV, the work function of carbon. A 20-fs laser pulse centered at 1250 nm (peak field amplitude E0 = 5 × 109 V/m, polarized along the propagation direction of the probe electrons) excites the sample at delay of τ = 0 , relative to the arrival time of the electron pulse. We find marked changes in the kinetic energy spectra of the electron wavepacket, shown in Figure 5b. The spectrum shows characteristic PINEM sidebands16, 22 at the photon energy of 0.7 eV, reflecting the absorption and emission of multiple quanta of light as a result of the interaction of the


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probe electrons with the optical near field of the sample. Weaker, higher energy modulation at the energy of the work function of the cylinder are superimposed. The effect of the electron-sample interaction is nicely captured by time-dependent simulations of the local electron density (Figure 5c). The interaction of the electron probe with the sample causes pronounced interferences both in transverse and longitudinal direction. Inelastic electron-light scattering results in a periodic modulation of the phase of the electron wave function. This leads to a breakup of the electron wavepacket into a train of sub-cycle pulses, with a spatial periodicity given by the momentum matching condition ∆k = ωL / v e (see Eq. (1)).16, 18, 48 Evidence for this attosecond pulse train has been seen in recent experiments with swift electrons.16, 18, 48 Here, we see the emergence of this pulse in the light-driven scattering of slow electrons from a single nanostructure. Evidently, the low-energy electron pulse is scattered into a wide range of deflection angles. In forward direction, the interaction with the optical near field causes significant acceleration and deceleration of the transmitted pulse and - thus - the appearance of PINEM sidebands in the kinetic energy spectrum50. Additionally, also the angular distribution of the electron wavepacket is manifestly altered by the interaction with the local optical near-field of the nanostructure. This angular spreading is weak and usually neglected - for swift electrons, yet dominates the response of slow electrons due to their longer interaction time with the local optical near field. Hence our simulations demonstrate the sensitivity of both the kinetic energy distribution in forward direction and the angular spreading of the wavepacket to the inelastic electron-light interaction. Altogether, ultrafast low-energy microscopy emerges as an excellent tool for probing the optical near-fields of resonantly and nonresonantly excited dielectric, metallic and hybrid nanostructures with high spatiotemporal resolution. The results in Fig. 5 also show that the inelastic interaction between probe electrons and optical near fields is confined to a narrow seam around the nanotube sample. This confinement is a direct consequence of the slow velocity of the probe electrons. Momentum matching between near field and passing electrons (Eq. (2)) requires a k-vector component k z = ω v e along the propagation di-


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rection. This corresponds to near fields with a wave vector component in normal direction of

= /v − or, in other words, a decay length of = | | ≃

. For v=0.06 c0 and a

wavelength of 1200 nm, this amounts to a decay length of the optical near field of only 12 nm. As such, the inelastic interactions is spatially highly confined to the direct vicinity of the wire boundaries and the Coulomb delocalization for slow electrons is less significant than for fast electrons.

SUMMARY AND CONCLUSIONS In summary, we have reported steps towards studying the diffraction of ultrafast, low-energy electron pulses by single nanostructures. Adiabatic nanofocusing of ultrashort surface plasmon polariton pulses on conical gold tapers, combined with multiphoton emission from the taper apex has been used to create a freestanding, ultrafast source of low-energy electron pulses. The source has been implemented in a point-projection microscope and first in-line holograms from a carbon nanotube bundle have been recorded, indicating an emitter size of better than 5 nm and demonstrating the excellent spatial coherence properties of this source. For sharp, conical tungsten tips, offering even smaller emitter sizes, we have demonstrated plasmonic nanofocusing by using a broadband laser source with a center wavelength of 2200 nm. Here, the SPP propagation length becomes sufficiently long to achieve efficient plasmon localization and multiphoton photoemission from the taper apex. A nanofocusing efficiency comparable to gold tapers is found for tungsten. Such tungsten tapers are expected to further increase the spatial resolution to the nanometer level and potentially below.32 Also, tungsten tapers will help to greatly extend the lifetime of these freestanding ultrafast electron emitters due to their enhanced mechanical stability and the possibility to sharpen the tapers in situ. We consider these important steps towards the development of ultrafast point-projection electron holography.

Our results suggest that such freestanding, ultrafast electron sources provide an important tool for probing the dynamics of local electric fields near the surface of individual nanostructures. SimulaACS Paragon Plus Environment

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tions of the interaction of ultrafast low-energy electron pulses with optical near fields of single nanostructure highlight the sensitivity of the slow electrons to the amplitude, direction and phase of the optical near field. In contrast to inelastic electron-light scattering of swift electrons, angular deflections of the low energy electrons are much more pronounced, providing new contrast mechanisms for the streaking of optical near fields of single nanostructures by ultrafast low-energy electron pulses. The proposed energy-resolved electron holography with ultrahigh temporal resolution opens up the possibility to investigate local optical field changes in a variety of heterogeneous systems, including, e.g., single nanoantennas, random nanostructures, interfaces, or two-dimensional materials coupled to different types of antennas. The sensitivity to local electric fields makes it an ideal tool to characterize, tailor and optimize local optical near fields for various types of applications. A first time-resolved point-projection microscopy experiment using plasmonic nanofocusing has been reported recently36 probing the effects of electrons that are photoemitted from the hotspot of a plasmonic antenna on ultrafast electron pulses emitted by a plasmonic nanofocusing tip. These measurements show that at least a few million electrons are needed to reach a sufficient signal-tonoise ratio in a single point-projection image. A series of approximately 100 point-projection images has been recorded over the course of 8 hours in Ref. 36. The employed, soft gold tip showed no degradation over the course of this measurement and we found that the same tip can be used for several such measurement cycles provided that they are performed at a sufficiently background pressure in the range of 10-10 - 10-9 mbar. These demonstrate the feasibility of our approach and shows that the lifetime of the tip is not a critical limiting factor. These measurements have been performed at 5-kHz repetition rate, resulting in acquisition times of several minutes for a single image. Recently, NOPA systems delivering tunable few-cycle pulse at repetition rates between 100 kHz and a few MHz have been developed. Their use can reduce the image acquisition time to a few seconds or even less. This would great enhance the sensitivity of the time-resolved electron microscopy images. Such a laser system is currently under development in our group.


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The highest time resolution in ultrafast electron holography is reached when emitting not more than a single electron from the nanotip per laser shot. This necessarily implies that a few millions of excitation pulses are needed to record a time-resolved electron microscope image. For solid state structures such a metallic, semiconducting or hybrid nanoparticles, this appears to be no significant limitation since these materials can easily withstand these excitation cycles. For softer nanomaterials, organic semiconductors, proteins or molecules, however, such a large number of excitation pulses may prove prohibitive and this may limit the applicability of ultrafast electron holography to these classes of materials. This limitation may be overcome by extracting more than a single electron per shot from the tip. The use of such a multi-electron emission regime can reduce the number of optical excitation pulse needed to record a time-resolved image by two orders of magnitude or even more, albeit at the expense of a possible reduction in time resolution. This multi-electron emission regime opens up an interesting avenue towards ultrafast electron holography of soft nanomaterials.

METHODS CNT bundles. Purified single-walled carbon nanotubes with a diameter distribution of 1.0–1.4 nm, produced by chemical vapor deposition (CVD) were used as purchased from Cheap Tubes USA. The material was dispersed in dichloromethane, sonicated for 1 hour and drop-cast on lacey carbon TEM grids.

Gold and Tungsten tips. Single-crystalline gold nanotips were fabricated from polycrystalline gold wires (99.99%) with a diameter of 125 µm (Advent Research Materials). After cleaning in ethanol, the wires were annealed at 800 °C for 8 h and then slowly cooled down over another 8 h to room temperature. These annealed wires were then electrochemically etched in HCl (aq. 37%). For etching, rectangular voltage pulses with a frequency of 3 kHz and a duty cycle of 10% were applied between the wire and a platinum ring serving as the counter electrode. The tips were inspected by scanning electron microscopy and tips with a diameter of less than 20 nm were selected. Using fo-


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cussed ion beam milling, a grating coupler consisting of three equidistant lines was written into the gold tip, in a distance of 50 µm from the taper apex. Tungsten tips were fabricated from monocrystalline tungsten wire with a diameter of 125 µm (oriented in [111] direction, metal impurities less than 30 parts per million by weight, Kore Technology). The wire was electrochemically etched in a sodium hydroxide solution (3 mol/l) forming a lamella inside a gold wire ring. One end of the tungsten wire was immersed into a bath of sodium hydroxide solution. A dc voltage of 12 V was applied between the ring and the bath until a sharp tip was formed. A grating coupler was milled into the tungsten surface as in the case of gold, but at a shorter distance of 30 µm from the taper apex.

Lasers and UPPM. Few-cycle femtosecond pulses in the near infrared spectral region and with a duration of less than 20 fs are supplied at a repetition rate of 5 kHz by a home-built noncollinear optical parametric amplifier system37. A white light supercontinuum generated with a pump pulse at 800 nm is amplified separately both in the visible spectral range and in a narrow region between 830 nm and 900 nm. A good spectral tunability in the infrared between 1200 nm and more than 2800 nm is reached by spectrally tuning the narrow spectral part above 800 nm followed by difference frequency generation with the broad visible part. The sample is placed at a variable distance from the taper apex. The distance between taper apex and sample is controlled by a slip-stick stage with a travel range of 20 mm and an accuracy of 1 nm (Attocube ECS3030). The section that is imaged can be selected by laterally shifting the sample using two slip-stick stages that are identical to the first. The probe electrons are recorded with a detector that is placed 75 mm behind the sample. It consists of a microchannel plate (MCP) of 45-mm diameter, followed by a 40-mm diameter P43 phosphor screen. The emission pattern is recorded by a CCD camera (PCO Pixelfly USB) with 1392 x 1040 pixels.


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Numerical simulations. In our analysis, we consider the interaction of an incident electron wavepacket with the optical near fields and electronic excitations of a small CNT bundle, taken as an infinitely-long cylinder. For this, coupled Maxwell-Schrödinger equations are numerically solved in the time domain using a toolbox described earlier.49,52 Conceptually, the material polarization and current distribution obtained by solving the Schrödinger equation act as source terms in Maxwell’s equations, creating electromagnetic fields which then act back on the probe electron and the charges inside the nanostructure. For this, the simulations are performed in two connected sub-domains, in which the dynamics of the electromagnetic fields (Maxwell-domain) and electron wave packets (Schrödinger-domain) are simulated. For solving Maxwell’s equation, a FDTD method which is generalized for self-consistently simulating nonlinear optical effects is used51, whereas the timedependent Schrödinger equation (TDSE) is solved using a Fourier method. In the latter method the spatial differentiation is performed iteratively in the spatial-frequency domain, and then inversely transformed into the real-space51. The time-propagator is approximated by a second-order differencing scheme. The connection between the subdomains is provided by a direct mapping of the potentials from the Schrödinger domain to the Maxwell domain. In order to terminate both domains, an absorbing boundary condition in addition to a perfectly matched layer is incorporated. To describe the elastic scattering of the probe electrons by the CNT, the CNT is modelled as a steplike binding potential with a binding energy given by the work function of the CNT. This scattering gives rise to a diffraction of the incident electron wave by the CNT. The interference of the scattered electron wavepacket with the incident electron wavepacket leads to the holographic interference fringes that are seen, e.g., in Fig. 3b. Optical near-fields of the CNT sample are calculated by homogeneously filling the cylinder with a dielectric with the frequency-dependent dielectric function of carbon. Near fields are obtained by coupling an off-resonant optical pulse (20-fs pulse centered at 1250 nm) to this dielectric. The interaction of these fields with the probe electron is described by the TDSE.


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To study the effects of nonlinear electronic excitations of the CNT sample, specifically laser-driven photoemission, on the probe electron dynamics, the bound and unbound electronic states of the CNT are taken as eigenstates of the cylindrical step potential. The interaction of the driving laser with this electronic system is calculated by solving the TDSE.

AUTHOR INFORMATION Corresponding Authors †

E-mail: [email protected].

†

E-mail: [email protected].

ORCID Jan Vogelsang: 0000-0002-9664-6265 Petra Groß: 0000-0002-7692-4184 Christoph Lienau: 0000-0003-3854-5025

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge financial support by the Deutsche Forschungsgemeinschaft [SPP1391, SPP1839 (Tailored Disorder) and SPP 1840 (QUTIF)], the Korea Foundation for the International Cooperation of Science and Technology (Global Research Laboratory Project, K20815000003), and the German-Israeli Foundation (GIF Grant No. 1256). J.V. acknowledges a personal grant from the Studienstiftung des Deutschen Volkes. We thank one of the Reviewers of this manuscript for valuable discussions.

REFERENCES


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23. Muller, M.; Kravtsov, V.; Paarmann, A.; Raschke, M. B.; Ernstorfer, R., Nanofocused Plasmon-Driven Sub-10 fs Electron Point Source. Acs Photonics 2016, 3, 611-619. 24. Krueger, M.; Schenk, M.; Hommelhoff, P., Attosecond control of electrons emitted from a nanoscale metal tip. Nature 2011, 475, 78-81. 25. Herink, G.; Solli, D. R.; Gulde, M.; Ropers, C., Field-driven photoemission from nanostructures quenches the quiver motion. Nature 2012, 483, 190-193. 26. Piglosiewicz, B.; Schmidt, S.; Park, D. J.; Vogelsang, J.; Groß, P.; Manzoni, C.; Farinello, P.; Cerullo, G.; Lienau, C., Carrier-envelope phase effects on the strong-field photoemission of electrons from metallic nanostructures. Nat Photonics 2014, 8, 37-42. 27. Vogelsang, J.; Robin, J.; Nagy, B. J.; Dombi, P.; Rosenkranz, D.; Schiek, M.; Groß, P.; Lienau, C., Ultrafast Electron Emission from a Sharp Metal Nanotaper Driven by Adiabatic Nanofocusing of Surface Plasmons. Nano Letters 2015, 15, 4685-4691. 28. Schroder, B.; Sivis, M.; Bormann, R.; Schafer, S.; Ropers, C., An ultrafast nanotip electron gun triggered by grating-coupled surface plasmons. Applied Physics Letters 2015, 107, 231105. 29. Quinonez, E.; Handali, J.; Barwick, B., Femtosecond photoelectron point projection microscope. Rev Sci Instrum 2013, 84, 7. 30. Morimoto, Y.; Baum, P., Diffraction and microscopy with attosecond electron pulse trains. Nat. Phys. 2018, 14, 252-256. 31. Bainbridge, A. R.; Myers, C. W. B.; Bryan, W. A., Femtosecond few- to single-electron point-projection microscopy for nanoscale dynamic imaging. Struct. Dyn.-US 2016, 3, 023612. 32. Ehberger, D.; Hammer, J.; Eisele, M.; Kruger, M.; Noe, J.; Hogele, A.; Hommelhoff, P., Highly Coherent Electron Beam from a Laser-Triggered Tungsten Needle Tip. Physical Review Letters 2015, 114, 227601. 33. Stockman, M. I., Nanofocusing of Optical Energy in Tapered Plasmonic Waveguides. Physical Review Letters 2004, 93, 137404. 34. Ropers, C.; Neacsu, C. C.; Elsaesser, T.; Albrecht, M.; Raschke, M. B.; Lienau, C., Gratingcoupling of surface plasmons onto metallic tips: A nanoconfined light source. Nano Letters 2007, 7, 2784-2788. 35. Gross, P.; Esmann, M.; Becker, S. F.; Vogelsang, J.; Talebi, N.; Lienau, C., Plasmonic nanofocusing - grey holes for light. Advances in Physics-X 2016, 1, 297-330. 36. Vogelsang, J.; Hergert, G.; Wang, D.; Groß, P.; Lienau, C., Watching charge separation in nanoantennas by ultrafast point-projection electron microscopy. Light-Sci. Appl. 2018, in press. 37. Vogelsang, J.; Robin, J.; Piglosiewicz, B.; Manzoni, C.; Farinello, P.; Melzer, S.; Feru, P.; Cerullo, G.; Lienau, C.; Gross, P., High passive CEP stability from a few-cycle, tunable NOPADFG system for observation of CEP-effects in photoemission. Opt Express 2014, 22, 25295-25306. 38. Esmann, M.; Becker, S. F.; da Cunha, B. B.; Brauer, J. H.; Vogelgesang, R.; Gross, P.; Lienau, C., k-space imaging of the eigenmodes of sharp gold tapers for scanning near-field optical microscopy. Beilstein J Nanotech 2013, 4, 603-610. 39. Babadjanyan, A. J.; Margaryan, N. L.; Nerkararyan, K. V., Superfocusing of surface polaritons in the conical structure. Journal of Applied Physics 2000, 87, 3785-3788. 40. Becker, S. F.; Esmann, M.; Yoo, K.; Gross, P.; Vogelgesang, R.; Park, N.; Lienau, C., GapPlasmon-Enhanced Nanofocusing Near-Field Microscopy. Acs Photonics 2016, 3, 223-232. 41. Guo, K.; Verschuuren, M. A.; Koenderink, A. F., Superresolution imaging of the local density of states in plasmon lattices. Optica 2016, 3, 289-298. 42. Hwang, I. S.; Chang, C. C.; Lu, C. H.; Liu, S. C.; Chang, Y. C.; Lee, T. K.; Jeng, H. T.; Kuo, H. S.; Lin, C. Y.; Chang, C. S.; Tsong, T. T., Investigation of single-walled carbon nanotubes with a low-energy electron point projection microscope. New Journal of Physics 2013, 15, 14. 43. Park, D. J.; Piglosiewicz, B.; Schmidt, S.; Kollmann, H.; Mascheck, M.; Gross, P.; Lienau, C., Characterizing the optical near-field in the vicinity of a sharp metallic nanoprobe by angleresolved electron kinetic energy spectroscopy. Annalen Der Physik 2013, 525, 135-142. 44. Latychevskaia, T., Spatial coherence of electron beams from field emitters and its effect on the resolution of imaged objects. Ultramicroscopy 2017, 175, 121-129. ACS Paragon Plus Environment

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For Table of Contents Use Only

Title: Plasmonic-nanofocusing-based electron holography

Authors: Jan Vogelsang, Nahid Talebi, Germann Hergert, Andreas Wöste, Petra Groß, Achim Hartschuh, Christoph Lienau

The TOC graphic visualizes the working principle of the microscope presented in the manuscript: Laser pulses couple to surface plasmons bound to a sharp metal tip. When the plasmons reach the taper apex, they induce the emission of short electron bursts. The electrons are subsequently used to probe carbon nanotube bundles placed in close vicinity of the taper apex. In a distance, a hologram of the carbon nanotubes can be observed by detecting the interference of scattered and unscattered parts of the electron wave originating from the taper apex.


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Plasmonic-Nanofocusing-Based Electron Holography - ACS

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