An ultrafast electron microscope gun driven by two-photon photoemissionfrom a nanotip cathode

Reiner Bormann,1 Stefanie Strauch,1 Sascha Schafer,1, a) and Claus Ropers1, b)

4th Physical Institute, Solids and Nanostructures, University of Gottingen, Gottingen 37077,Germany

(Dated: 12 October 2015)

We experimentally and numerically investigate the performance of an advanced ultrafast electron source,based on two-photon photoemission from a tungsten needle cathode incorporated in an electron microscopegun geometry. Emission properties are characterized as a function of the electrostatic gun settings, andoperating conditions leading to laser-triggered electron beams of very low emittance (below 20 nm·mrad) areidentified. The results highlight the excellent suitability of optically driven nano-cathodes for the furtherdevelopment of ultrafast transmission electron microscopy.

I. INTRODUCTION

Stroboscopic pump-probe experiments applying ultra-short electron pulses have started to provide a micro-scopic picture of optically-induced ultrafast structuralprocesses in complex materials1–12. The possibility tomanipulate electron pulses by static and temporallyvarying13–16 electromagnetic fields enabled the develop-ment of a variety of time-resolved electron diffraction andimaging schemes, specifically tailored to the investigationof ultrafast dynamics in bulk media1,17, at surfaces4,18

and in inhomogeneous systems6,19,20. In particular, em-ploying ultrashort electron pulses in the advanced op-tics environment of a transmission electron microscope17

allows for the investigation of sub-picosecond processeson few nanometer length scales20. However, in time-resolved nanoscale electron probing and high-resolutionimaging, spatial resolution may be strongly limited bythe achievable transverse beam properties. Specifically,continuous electron sources employed in electron mi-croscopy are characterized by effective source sizes be-low one nanometer21. In contrast, pulsed sources basedon planar photocathodes typically exhibit three orders ofmagnitude larger source sizes, governed by the photoe-mission focal spot22–24. In recent years, advanced pho-tocathodes were developed, in which nonlinear photoe-mission is confined to the nanoscale apex of sharp metaltips25–28. In addition to their advantageous transversebeam properties, at the apex of such structures, high elec-trostatic extraction fields can be sustained, yielding shortelectron pulses due to a strong suppression of electrondispersion4,6,19,29,30. First applications of laser-drivenneedle cathodes for ultrafast electron imaging and diffrac-tion were demonstrated4,31,32, and, in the case ultrafasttransmission electron microscopy (UTEM), femtosecondnanoscale electron probes were obtained20. Further im-provements of UTEM capabilities in terms of spatial reso-lution and average current require an optimized coupling

a)Electronic mail: schaefer@ph4.physik.uni-goettingen.deb)Electronic mail: cropers@gwdg.de

of photoemitted electron bunches into the lens system ofa UTEM.

Here, we experimentally and numerically investigatethe emission characteristics of a laser-driven tungstenneedle cathode within a Schottky-type electron gun. Weidentify contributions to the emitted electron beam orig-inating both from the tip apex and adjacent micrometer-scale shaft regions. Apex electrons can be efficiently se-lected by proper electron optics settings, yielding pulseswith predicted emittance values as low as a few nm·mrad,ideally suited for high-quality time-resolved electronimaging.

II. EXPERIMENTAL SETUP

In the experiments, nonlinear photoelectron emis-sion from needle emitters is induced by femtosecondlaser pulses (400 nm wavelength, 50 fs pulse width,250 kHz repetition-rate) employing a frequency-doubledTi:Sapphire laser (see experimental scheme in Fig. 1a).An electrochemically etched tungsten tip33 (Fig. 1d,e)is incorporated into an electron gun assembly compris-ing several electrodes that allow for a precise controlof the electrostatic environment of the tip (see below).The laser beam is focused onto the tip using a plano-convex lens (30 cm focal length, 20 µm full-width-at-half-maximum focal diameter) mounted on a three-dimensional translation stage, which allows for rasterscanning the excitation across the tip. Emitted elec-trons are detected using an imaging micro-channel plate(MCP) detector placed at a distance of 75 mm from thetip, in combination with a charge-coupled device (CCD)camera. In Fig. 1c, the detected number of electrons perpulse is plotted on a double-logarithmic scale as a func-tion of the incident pulse energy, exhibiting the character-istics of a two-photon photoemission process (quadraticintensity scaling) and reaching up to 10 electrons perpulse for pulse energies of a few nJ.

Applying independent voltages to the tip and suppres-sor/extractor electrodes (see Fig. 1a) yields control overallowed electron emission sites and beam characteristics.The extractor-suppressor unit resembles a parallel plate

2

ExtractorSuppressor

Tip emitter

Electron beam

Laser pulse

MCP

(a)

Utip

Extractor Suppressor

Tip

e-

dtip-sup

dext-sup

Uext Usup

(b)

(d)

100 µm

(e)D = 21 nm

100 nm

(h)

Γ > 1

(f )

Γ < 1

(g)

Γ ~ 1

101

100

10-1

Ele

ctro

ns

pe

r p

uls

e

4.00.4 1.0Pulse energy (nJ)

Γ = 0.99

(c)

FIG. 1. (a) Experimental setup showing the field emitterassembly and the detector (phosphor-screen/MCP) (b) Sup-pressor and extractor electrodes around an illuminated tip.Distances indicated by arrows. (c) Pulse energy dependenceof the photoelectron yield, indicating a two-photon photoe-mission process (red line: quadratic dependence). (d,e) Scan-ning electron micrograph of the tip protruding the suppressor(d) and a closeup of the apex region (e). D indicates apexdiameter (f-h) Calculated equipotential lines in the vicinity ofthe tip for various voltage settings. Blue lines correspond toa potential U higher than tip potential Utip, whereas red linesbelong to potentials U < Utip (dotted line: tip potential con-tour). The applied voltages at the tip and extractor are keptconstant, while a variation of the suppressor voltages leads todifferent values of Γ.

capacitor geometry (distance dext−sup = 700 µm), result-ing in a nearly homogeneous electrical field far away fromthe tip axis. The tip protrudes the suppressor in such away that the apex is at a distance of about dtip−sup =250 µm from the suppressor plane (see Fig. 1b). Varia-tion of a tip bias voltage shapes the field around the tip,

and, in particular, allows for a continuous tuning of thegradient and curvature of the potential near the apex.

In Figs. 1f-h, numerically calculated equipotential linesin the vicinity of the tip are plotted for different voltagesapplied to the electrodes. Red lines correspond to elec-tric potentials lower (more negative) than the tip poten-tial, whereas blue lines depict those with a higher value.For a description of the electrostatic field distribution,it is illustrative to relate the tip potential to that in theextractor-suppressor gap at a distance dtip−sup above thesuppressor, and far away from the tip axis. If the tip po-tential is substantially more negative than this referencepotential, the equipotential lines will be compressed nearthe apex, resulting in a strong surface electric field anda large curvature of the potential (Fig. 1f)34. In this sce-nario, the surface electric field draws electrons away fromthe tip, both for the apex and, with decreasing field mag-nitude, for large portions of the shaft. An attractive forceto the shaft is found closer to the tip base, so that pho-toemission is expected to be suppressed beyond a field-reversal point (position of contact between dashed tippotential line and tip surface). An increase of the tip po-tential (towards zero bias) shifts this field-reversal pointacross the shaft, reducing the surface electric field andconfining emission to areas close to the apex (Fig. 1g).Ultimately, emission is suppressed even from the apex(Fig. 1h). In order to quantitatively characterize theseregimes, we can introduce a dimensionless parameter Γcontaining the tip (Utip), suppressor (Usup) and extrac-tor (Uext) voltages, as well as the distances between theelectrodes:

Γ =Utip − Usup

Uext − Usup· dext−sup

dtip−sup. (1)

In typical geometries and depending on the specificelectrode shape and aperture diameters, suppression ofphotoemission at the apex occurs for Γ-values close tounity. In particular, for the setup used, we find the tran-sition point at Γc ≈ 1.2, both experimentally and in sim-ulations. Using this definition, conditions with Γ < Γc

correspond to the (static) field-enhanced regime with al-lowed emission from the tip apex and shaft.

III. RESULTS AND DISCUSSION

In the following, we demonstrate the Γ-dependent elec-tron emission in our experimental geometry. For se-lected values of Γ, Fig. 2a depicts the integrated electroncount rate on the detector with the laser spot positionraster-scanned across the tip. Two dominant featuresare evident for all Γ values: First, the largest photoelec-tron current is found at the tip shaft, for a focal posi-tion that linearly shifts towards the apex for increasingΓ. This feature exhibits a sharp drop towards the sup-pressor (right) and a tail in the direction of the apex

3

10010-2 10-1

Intensity (arb.u.)

(a)

50 µm

xz

0 50 100Tip axis z (µm)

0.9

1

1.1

1.2

0.8

Γ

100

10-2

10-4

Inte

nsi

ty (

arb

.u.)

(b)

0 50 100Tip axis z (µm)

0.9

1

1.1

1.2

1.3

Γ

100

10-4

10-2

10-5

10-3

10-1

Inte

nsi

ty (

arb

.u.)

(c)

0 50 100Tip axis z (µm)

0.9

1

1.1

1.2

0.8

Γ

(d)100

10-4

10-2

10-5

10-3

10-1

Inte

nsi

ty (

arb

.u.)

Γ = 0.80

Γ = 0.88

Γ = 0.96

Γ = 1.04

Γ = 1.11

Γ = 1.19

FIG. 2. (a) Total electron current recorded upon raster-scanning the optical focus across the emitter for different val-ues of the voltage ratio Γ. Top: Scanning electron micrographof the tungsten emitter (scale bar applies to all panels in (a)).(b-d) Electron signal as a function of Γ and laser spot positionalong the tip axis z ((b,c) experimental data for two differentemitter tips, (d) corresponding simulation results).

(left). Second, a substantial enhancement of the photoe-mission is observed upon apex excitation, irrespective ofthe applied voltages (0.8 < Γ ≤ 1.2; for Γ < 0.8 andUext − Utip > 690 V, static field emission was observed).Horizontal linescans on the tip axis as a function of Γare displayed in Figs. 2b,c for two different emitter tips,demonstrating the reproducibility of the findings. Bothmeasurements exhibit the two signatures from apex- andshaft-emitted electrons as vertical and diagonal lines, re-spectively. In addition, an enhanced electron current isfound at the merging of the lines near Γc and z = 0.The relative contributions from apex and shaft emis-sion varies for different tips, influenced by apex shapeand diameter, crystallographic orientation and surfaceroughness on the shaft (cf. Figs. 1b,c). We note thatthe two-photon process studied here leads to a weakerapex confinement of the emission than for higher optical

nonlinearities25,35–37. However, two-photon photoemis-sion offers the advantage of particularly narrow kineticenergy distributions and comparatively high quantum ef-ficiencies at moderate light intensities.

Photoemission from different tip regions results incharacteristic emission patterns on the MCP detector(Fig. 3). Figure 3a displays a series of detector images forincreasing Γ under apex excitation. Substantially belowthe cutoff point, apex-near electrons result in a largelyhomogeneous illumination of the detector screen. Forthe smallest Γ-values, only those electrons emitted fromthe central (front) region of the apex contribute to thesignal. An increase of Γ results in the additional col-lection of electrons from areas close to the apex. Theassociated emission forms a ring-shaped pattern whichis increasingly focused on the center of the detector forlarger Γ. The closed ring and the structured angularintensity distribution likely originates from diffractionaround the tip and facet-dependent work functions, aspreviously reported38, with some contribution from az-imuthal motion in the gun. In contrast, shaft illumina-tion (z = 30 µm, Γ = 1.089) leads to an arc-shapeddistribution with maximum intensity on the illuminatedside (cf. Fig. 3b).

In order to link the different photoemission features toparticular classes of electron trajectories, we perform nu-merical simulations of the electron paths within the elec-tron gun geometry for a set of initial parameters (emis-sion angle and position, initial kinetic energy). Owingto the cylindrical symmetry of the structure, trajectorycomputations are performed in the rz-plane, and we ne-glect contributions from azimuthal motions. The em-ployed tip radius is 25 nm, and the tip shape is based onthe contour of an etched tungsten tip used in the experi-ments. More information about the numerical procedureis found in Ref. 29. Voltage settings corresponding to theexperimental conditions are applied. Direct comparisonwith the experimental photocurrents are facilitated bypopulating the different electron trajectories according toa simplified model of the local photoemission probability.Specifically, we consider an emission probability scalingquadratically with the local optical intensity (two-photonphotoemission) for a given focus position. In the apex re-gion, a phenomenological field enhancement factor of 5 isemployed, which accounts for the geometrically increasedpolarizability39 and for the more favorable polarizationconditions for photoemission at the apex (larger normalcomponent of the field)26,40. We model the distributionof initial kinetic energies with a Gaussian centered at0 eV with a half-width-at-half-maximum of 0.75 eV, cor-responding to recently measured two-photon photoemis-sion spectra from tungsten tips20. Regarding the distri-bution of emission directions, an angle-independent prob-ability proved sufficient to account for the experimentalobservations, although a more elaborate model could ex-plicitly include surface orientations, the materials bandstructure, as well as photoemission transition matrix ele-ments. Generally, we find that the emission patterns and

4

(a)

Shaft

10 0.5Intenstiy (arb.u.)

Γ = 1.172 Γ = 1.177Γ = 1.089 Γ = 1.182

Γ = 1.187 Γ = 1.193 Γ = 1.198 (b)

3∙I3∙I

FIG. 3. (a) Electron distributions recorded on the detectorscreen for apex illumination and different values of Γ (b) Ex-emplary pattern for excitation on the tip shaft.

detected electron currents are more strongly influencedby the emission site than by the initial momenta.Using the simulations, we are able to reproduce the es-

sential qualitative observations made in the experiments.This includes the shrinking of the ring feature in theemission patterns upon approaching the cutoff (comparecomputed radial distribution functions in Fig. 4c), andthe Γ- and z-dependent spatial scans (Fig. 2d). A moredetailed picture is obtained by studying the electron tra-jectories originating at different emission sites. For anexemplary value of Γ = 0.96, Figs. 4a,b depict electrontrajectories starting (i) at the apex and being transmittedthrough the extractor (orange), (ii) near the apex and be-ing blocked by the extractor aperture (blue), (iii) on theshaft further from the apex and being bent through theextractor (green). In particular, at this Γ-value, trajec-tories of the classes (i) and (iii) correspond to a homoge-neous emission pattern on the detector and the arc-shapein Fig. 3b, respectively. The diagonal features in Figs. 2b-d, on the other hand, reflect the optimum conditions forfunneling shaft trajectories along a caustic through theextractor aperture.The simulations allow for a quantitative analysis of

the properties of the electron beam generated as a func-tion of Γ and for different photoelectron distributions.In particular, we are interested in the near-axis beamproperties relevant for applications in ultrafast imple-mentations of electron microscopy. The beam qualitycan be characterized by the occupied transverse phasespace area, quantified by the normalized transverse emit-tance εn,x = 1

mec

√⟨x2⟩⟨p2x⟩ − ⟨xpx⟩2, where ⟨x2⟩ and

⟨p2x⟩ are expectation values of the spatial and momentumdistributions22. For the central part of the beam (4 mmdiameter in the detector plane), Fig. 4d depicts the com-puted emittance and current as a function of Γ. Through-out a large range of Γ-values, the emittance remains be-low 20 nm·mrad, which is an exceedingly small value forlaser-triggered electron sources and highlights the utility

(a)

(b)

SuppressorExtractor

Tip

100 µm

25 µm

0.1 0.60.50.40.30.2σE (eV)

10

1

Em

itta

nce

(n

m∙m

rad

)(b)

1.20.6 0.8 1.0 Em

itta

nce

(n

m∙m

rad

)

Γ

Inte

nsi

ty (

arb

. u.)

/104

103

102

101

100

(d)

Γ = 0.59Γ = 1.01

µE = 0.75 eV µE = 0 eV

(e)

1

0.5

00

r (mm)10

Γ=1.064Γ=1.162Γ=1.176Γ=1.190In

ten

sity

(a

rb. u

.)

5

(c)

x100

FIG. 4. (a,b) Simulated electron trajectories for Γ = 0.96with emission sites at the apex (orange) and on the shaft(blue, green). (c) Radial distribution function for differentvalues of Γ and apex illumination. (d) Transverse beam emit-tance (red) and electron yield (black) as a function of Γ. (e)Emittance as a function of the standard deviation (σE) andthe maximum (µE) of the initial Gaussian-shaped kinetic en-ergy distributions.

of nanotip cathodes for time-resolved electron imaging.Near the cutoff (Γ ≈ 1.2), the static field-enhancementat the apex is suppressed, resulting in a strong emittanceincrease to values of several µm·mrad, as typically ob-tained for flat-photocathode geometries22,23. This sharpdrop of the beam quality follows from the ring-feature en-tering the central beam part, which is also evident in thepeaked current near the cutoff. For nanoscale imagingapplications, the normalized beam brightness, defined asBn = I/8π2ε2n,x (I: beam current)41, is the quantity de-termining image quality and contrast, and it is most com-mon to characterize continuous electron sources in termsof the brightness21. In the case of pulsed sources ap-plied in time-resolved imaging, the temporally averagedbeam brightness can be greatly varied by experimentalconditions (such as a choice of pulse duration and repeti-tion rate), which, however, may be severely constrainedfor a given material system investigated. For example,while operation at high-repetition rates in the MHz rangemay be tolerated by a specific photoelectron source, suchhigh pulse rates will be incompatible with the struc-tural relaxation times in most sample geometries. Forthe present experimental conditions (for single-electronpulses at 250 kHz pulse rate), a typical time-averaged

normalized brightness of Bn = 1 · 107 A/m2sr at Γ = 0.8

is obtained (corresponding to a reduced brightness21 of

5

Br = 40 A/m2srV). We note that closer to the cutoff

a brightness reduction of three orders of magnitude ispredicted. Finally, in order to study the influence of thevariation of initial electron velocities on the beam quality,Fig. 4e plots the resulting emittance as a function of theparameters of the energy distributions. Similarly to pla-nar cathodes42, we find that a substantial improvementof beam quality may be obtained by proper selection ofa photon energy that minimizes the initial kinetic energyspread.

IV. CONCLUSION

We have experimentally and numerically studied theemission characteristics of a nanotip photoelectron sourcedriven by two-photon photoemission. Integration of thetip into the electron gun environment of a transmissionelectron microscope allowed for systematic variation ofthe extraction field around the emitter, yielding controlover photoemission sites and the transverse beam prop-erties. Suitable low-emittance operating conditions wereidentified in a parameter region bounded at high and lowextraction fields by static field emission and nonlocalizedcontributions from shaft electrons, respectively. We be-lieve that the present findings will be instrumental in en-hancing the performance and local probing capabilities ofultrafast transmission electron microscopy. Further im-provements are envisaged by combining the electrostaticfield control implemented here with more spatially selec-tive and directional emission mechanisms from physically(shape, crystal orientation) and chemically (adsorbate-modified work functions) tailored nanotips.

ACKNOWLEDGMENTS

We gratefully acknowledge funding by the DeutscheForschungsgemeinschaft (DFG) (SFB-1073, project A5)and the European Research Commission (ERC-StG”ULEED”). We thank Philipp Kloth for help in tip fabri-cation, and Stefan Rost, Nora Bach, Benjamin Schroderand Armin Feist for useful discussions.

1R. J. D. Miller, “Femtosecond Crystallography with UltrabrightElectrons and X-rays: Capturing Chemistry in Action,” Science343, 1108–1116 (2014).

2D. J. Flannigan and A. H. Zewail, “4D Electron Microscopy:Principles and Applications,” Acc. Chem. Res. 45, 1828–1839(2012).

3M. Gao, C. Lu, H. Jean-Ruel, L. C. Liu, A. Marx, K. Onda,S.-y. Koshihara, Y. Nakano, X. Shao, T. Hiramatsu, G. Saito,H. Yamochi, R. R. Cooney, G. Moriena, G. Sciaini, and R. J. D.Miller, “Mapping molecular motions leading to charge delocal-ization with ultrabright electrons,” Nature 496, 343–346 (2013).

4M. Gulde, S. Schweda, G. Storeck, M. Maiti, H. K. Yu, A. M.Wodtke, S. Schafer, and C. Ropers, “Ultrafast low-energy elec-tron diffraction in transmission resolves polymer/graphene su-perstructure dynamics.” Science 345, 200–204 (2014).

5L. Piazza, C. Ma, H. X. Yang, A. Mann, Y. Zhu, J. Q. Li, andF. Carbone, “Ultrafast structural and electronic dynamics of the

metallic phase in a layered manganite,” Struct. Dyn. 1, 014501(2014).

6M. Muller, A. Paarmann, and R. Ernstorfer, “Femtosecond elec-trons probing currents and atomic structure in nanomaterials,”Nat. Commun. 5, 5292 (2014).

7M. Eichberger, H. Schafer, M. Krumova, M. Beyer, J. Demsar,H. Berger, G. Moriena, G. Sciaini, and R. J. D. Miller, “Snap-shots of cooperative atomic motions in the optical suppression ofcharge density waves.” Nature 468, 799–802 (2010).

8S. Schafer, W. Liang, and A. H. Zewail, “Primary structuraldynamics in graphite,” New J. Phys. 13, 063030 (2011).

9S. Wall, B. Krenzer, S. Wippermann, S. Sanna, F. Klasing,A. Hanisch-Blicharski, M. Kammler, W. G. Schmidt, andM. Horn-von Hoegen, “Atomistic Picture of Charge DensityWave Formation at Surfaces,” Phys. Rev. Lett. 109, 186101(2012).

10R. M. van der Veen, O.-H. Kwon, A. Tissot, A. Hauser, andA. H. Zewail, “Single-nanoparticle phase transitions visualizedby four-dimensional electron microscopy,” Nat. Chem. 5, 395–402 (2013).

11U. J. Lorenz and A. H. Zewail, “Observing liquid flow in nan-otubes by 4D electron microscopy,” Science 344, 1496–1500(2014).

12A. Yurtsever and A. H. Zewail, “Kikuchi ultrafast nanodiffractionin four-dimensional electron microscopy,” Proc. Natl. Acad. Sci.108, 3152–3156 (2011).

13P. Musumeci, J. T. Moody, R. J. England, J. B. Rosenzweig,and T. Tran, “Experimental Generation and Characterization ofUniformly Filled Ellipsoidal Electron-Beam Distributions,” Phys.Rev. Lett. 100, 244801 (2008).

14T. Van Oudheusden, P. L. E. M. Pasmans, S. B. Van Der Geer,M. J. De Loos, M. J. Van Der Wiel, and O. J. Luiten, “Compres-sion of subrelativistic space-charge-dominated electron bunchesfor single-shot femtosecond electron diffraction,” Phys. Rev. Lett.105, 1–4 (2010).

15R. P. Chatelain, V. R. Morrison, C. Godbout, and B. J. Siwick,“Ultrafast electron diffraction with radio-frequency compressedelectron pulses,” Appl. Phys. Lett. 101, 081901 (2012).

16S. P. Weathersby, G. Brown, M. Centurion, T. F. Chase, R. Cof-fee, J. Corbett, J. P. Eichner, J. C. Frisch, A. R. Fry, M. Guhr,N. Hartmann, C. Hast, R. Hettel, R. K. Jobe, E. N. Jongewaard,J. R. Lewandowski, R. K. Li, A. M. Lindenberg, I. Makasyuk,J. E. May, D. McCormick, M. N. Nguyen, A. H. Reid, X. Shen,K. Sokolowski-Tinten, T. Vecchione, S. L. Vetter, J. Wu, J. Yang,H. A. Durr, and X. J. Wang, “Mega-electron-volt ultrafast elec-tron diffraction at SLAC National Accelerator Laboratory,” Rev.Sci. Instrum. 86, 073702 (2015).

17A. H. Zewail, “Four-dimensional electron microscopy,” Science328, 187–193 (2010).

18A. Hanisch-Blicharski, A. Janzen, B. Krenzer, S. Wall, F. Klas-ing, A. Kalus, T. Frigge, M. Kammler, and M. Horn-von Hoegen,“Ultra-fast electron diffraction at surfaces: From nanoscale heattransport to driven phase transitions,” Ultramicroscopy 127, 2–8(2013).

19E. Quinonez, J. Handali, and B. Barwick, “Femtosecond pho-toelectron point projection microscope,” Rev. Sci. Instrum. 84,103710 (2013).

20A. Feist, K. E. Echternkamp, J. Schauss, S. V. Yalunin,S. Schafer, and C. Ropers, “Quantum coherent optical phasemodulation in an ultrafast transmission electron microscope,”Nature 521, 200–203 (2015).

21L. Reimer and H. Kohl, Transmission Electron Microscopy, 5thed. (Springer, 2008).

22D. H. Dowell and J. F. Schmerge, “Quantum efficiency and ther-mal emittance of metal photocathodes,” Phys. Rev. Spec. Top. -Accel. Beams 12, 074201 (2009).

23T. Li, B. L. Rickman, andW. A. Schroeder, “Emission propertiesof body-centered cubic elemental metal photocathodes,” J. Appl.Phys. 117, 134901 (2015).

6

24C. Gerbig, A. Senftleben, S. Morgenstern, C. Sarpe, andT. Baumert, “Spatio-temporal resolution studies on a highlycompact ultrafast electron diffractometer,” New J. Phys. 17,043050 (2015).

25C. Ropers, D. R. Solli, C. P. Schulz, C. Lienau, and T. El-saesser, “Localized Multiphoton Emission of Femtosecond Elec-tron Pulses from Metal Nanotips,” Phys. Rev. Lett. 98, 043907(2007).

26P. Hommelhoff, Y. Sortais, A. Aghajani-Talesh, and M. A. Kase-vich, “Field Emission Tip as a Nanometer Source of Free ElectronFemtosecond Pulses,” Phys. Rev. Lett. 96, 077401 (2006).

27H. Yanagisawa, “Site-selective field emission source by femtosec-ond laser pulses and its emission mechanism,” Ann. Phys. 525,126–134 (2013).

28B. Barwick, C. Corder, J. Strohaber, N. Chandler-Smith,C. Uiterwaal, and H. Batelaan, “Laser-induced ultrafast elec-tron emission from a field emission tip,” New J. Phys. 9, 142(2007).

29A. Paarmann, M. Gulde, M. Muller, S. Schafer, S. Schweda,M. Maiti, C. Xu, T. Hohage, F. Schenk, C. Ropers, and R. Ern-storfer, “Coherent femtosecond low-energy single-electron pulsesfor time-resolved diffraction and imaging: A numerical study,”J. Appl. Phys. 112, 113109 (2012).

30J. Hoffrogge, J. Paul Stein, M. Kruger, M. Forster, J. Hammer,D. Ehberger, P. Baum, and P. Hommelhoff, “Tip-based sourceof femtosecond electron pulses at 30 keV,” J. Appl. Phys. 115,094506 (2014).

31J. Vogelsang, J. Robin, B. J. Nagy, P. Dombi, D. Rosenkranz,M. Schiek, P. Groß, and C. Lienau, “Ultrafast Electron Emissionfrom a Sharp Metal Nanotaper Driven by Adiabatic Nanofocus-ing of Surface Plasmons,” Nano Lett. 15, 4685–4691 (2015).

32D.-S. Yang, O. F. Mohammed, and A. H. Zewail, “Scanningultrafast electron microscopy.” Proc. Natl. Acad. Sci. USA 107,

14993–14998 (2010).33P. J. Bryant, H. S. Kim, Y. C. Zheng, and R. Yang, “Tech-nique for shaping scanning tunneling microscope tips,” Rev. Sci.Instrum. 58, 1115 (1987).

34R. Gomer, Field Emission and Field Ionization (Harvard Uni-versity Press, 1961).

35G. Herink, D. R. Solli, M. Gulde, and C. Ropers, “Field-drivenphotoemission from nanostructures quenches the quiver motion.”Nature 483, 190–3 (2012).

36L. Wimmer, G. Herink, D. R. Solli, S. V. Yalunin, K. E.Echternkamp, and C. Ropers, “Terahertz control of nanotip pho-toemission,” Nat. Phys. 10, 432–436 (2014).

37P. Hommelhoff, C. Kealhofer, and M. A. Kasevich, “UltrafastElectron Pulses from a Tungsten Tip Triggered by Low-PowerFemtosecond Laser Pulses,” Phys. Rev. Lett. 97, 247402 (2006).

38H. Yanagisawa, C. Hafner, P. Dona, M. Klockner, D. Leuen-berger, T. Greber, M. Hengsberger, and J. Osterwalder, “OpticalControl of Field-Emission Sites by Femtosecond Laser Pulses,”Phys. Rev. Lett. 103, 257603 (2009).

39C. C. Neacsu, G. A. Reider, and M. B. Raschke, “Second-harmonic generation from nanoscopic metal tips: Symmetry se-lection rules for single asymmetric nanostructures,” Phys. Rev.B 71, 201402 (2005).

40C. Ropers, T. Elsaesser, G. Cerullo, M. Zavelani-Rossi, andC. Lienau, “Ultrafast optical excitations of metallic nanostruc-tures: From light confinement to a novel electron source,” NewJ. Phys. 9 (2007), 10.1088/1367-2630/9/10/397.

41M. Reiser, Theory and Design of Charged Particle Beams (Wiley,2008).

42M. Aidelsburger, F. O. Kirchner, F. Krausz, and P. Baum,“Single-electron pulses for ultrafast diffraction,” Proc. Natl.Acad. Sci. 107, 19714–19719 (2010).