Optical gating and streaking of free electrons with sub-optical cycle precision

Abstract

The temporal resolution of ultrafast electron diffraction and microscopy experiments is currently limited by the available experimental techniques for the generation and characterization of electron bunches with single femtosecond or attosecond durations. Here, we present proof of principle experiments of an optical gating concept for free electrons via direct time-domain visualization of the sub-optical cycle energy and transverse momentum structure imprinted on the electron beam. We demonstrate a temporal resolution of 1.2±0.3 fs. The scheme is based on the synchronous interaction between electrons and the near-field mode of a dielectric nano-grating excited by a femtosecond laser pulse with an optical period duration of 6.5 fs. The sub-optical cycle resolution demonstrated here is promising for use in laser-driven streak cameras for attosecond temporal characterization of bunched particle beams as well as time-resolved experiments with free-electron beams.

ARTICLE

Received 5 Oct 2016 | Accepted 19 Dec 2016 | Published 25 Jan 2017

DOI: 10.1038/ncomms14342 OPEN

Optical gating and streaking of free electrons with sub-optical cycle precision

M. Kozk1, J. McNeur1, K.J. Leedle2, H. Deng2, N. Schnenberger1, A. Ruehl3, I. Hartl3, J.S. Harris2,4, R.L. Byer4

& P. Hommelhoff1,5

The temporal resolution of ultrafast electron diffraction and microscopy experiments is currently limited by the available experimental techniques for the generation and characterization of electron bunches with single femtosecond or attosecond durations. Here, we present proof of principle experiments of an optical gating concept for free electrons via direct time-domain visualization of the sub-optical cycle energy and transverse momentum structure imprinted on the electron beam. We demonstrate a temporal resolution of1.20.3 fs. The scheme is based on the synchronous interaction between electrons and the near-eld mode of a dielectric nano-grating excited by a femtosecond laser pulse with an optical period duration of 6.5 fs. The sub-optical cycle resolution demonstrated here is promising for use in laser-driven streak cameras for attosecond temporal characterization of bunched particle beams as well as time-resolved experiments with free-electron beams.

1 Department of Physics, Friedrich-Alexander-Universitat Erlangen-Nrnberg (FAU), Staudtstrasse 1, 91058 Erlangen, Germany. 2 Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA. 3 Deutsches Elektronen-Synchrotron DESY, D-22607 Hamburg, Germany. 4 Department of Applied Physics, Stanford University, Stanford, California 94305, USA. 5 Max-Planck-Institute for the Science of Light, Staudtstrasse 2, 91058 Erlangen, Germany. Correspondence and requests for materials should be addressed to M.K. (email: mailto:[email protected]

Web End [email protected] ).

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Contemporary physics and chemistry aim to record phenomena occurring on the time and length scales of electronic dynamics in atomic systems14. Recent

progress in the generation of attosecond extreme ultraviolet (XUV) pulses57 allows for direct insight into ultrafast processes in the sub-femtosecond regime. Advanced experimental techniques such as attosecond optical spectroscopy5,8,9, high-harmonic orbital tomography1,10 or laser-induced electron self-diffraction11 have enabled the direct observation of electron dynamics in atoms. Ultrafast electron diffraction and microscopy2,12,13 further enable atomic spatial resolution by employing free electron bunches at energies of 30200 keV with de-Broglie wavelengths on the order of several picometers, but with temporal resolution so far limited to tens-hundreds of femtoseconds1315. The temporal resolution of these techniques can be pushed toward the atomic unit of time (24 as) by the experimental technique presented here.

A natural way to manipulate free electrons on the attosecond timescale would be by using the electromagnetic eld of a laser pulse oscillating with a period of several femtoseconds. Similar to optical streaking and bunching schemes1521, attosecond temporal resolution may be obtained by mapping time to electron energy. When nonlinear forces can be neglected, however, no net energy gain can be imparted to a charged particle by a light wave in vacuum because of energy and momentum conservation. For efcient energy exchange, the symmetry of the eld must be broken via the introduction of a boundary surface as shown in refs 15, 17, 20, 21. Metallic thin lms15, nanowires21, carbon nanotubes17, metallic nanotips20 and microstructures22 have all been used as spatial boundaries for the laser eld (for comparison of advantages/disadvantages of different schemes see Supplementary discussion, Experimental techniques for electron streaking by optical elds).

The laser-driven gating and streaking of electrons presented in this paper is based on the energy and momentum exchange between a laser eld and an electron in the proximity of a periodic nanostructure utilizing the inverse SmithPurcell effect2330. A silicon nano-grating serves as a phase mask for incoming laser pulses, leading to the excitation of a travelling evanescent mode propagating parallel to the electron beam with a phase velocity equal to the electron velocity ve bc (refs 2630),

where c is speed of light. The amplitude of the synchronous mode decays exponentially with the distance from the grating surface (for details see Methods section). The direction and strength of the resulting electromagnetic force depends on the optical phase of the excited mode with respect to the arrival time of electrons30 (see Fig. 1a). Spectral ltering of the electrons after interaction with the laser elds yields temporal gating of the electron bunches with sub-optical cycle temporal resolution.

ResultsTemporal gating of electron beam by optical elds. The experimental set-up for demonstration of laser-driven gating in the time domain is shown in Fig. 1b,c (for details see section Methods section). A sub-relativistic continuous electron beam with an initial energy of Ek0 28.1 keV (b 0.32) is focused close

to the surface of the grating, parallel to the grating vector. Two infrared femtosecond laser pulses (central wavelength of l 1.93 mm, pulse length of t 60050 fs and maximum inci

dent eld of E0 1.30.1 GV m 1) with an adjustable relative

time delay are focused onto the grating surface at different locations along the electron beam path. After interaction with the grating near elds, the electrons reach an energy high-pass lter transmitting only those with energies Ek4eUs, where e is the electron charge and Us is the applied retarding voltage.

1,000

a

Electron arrival time (fs)

2 4 6

200

1

0

1

Transverse distance y (nm)

Electric field component z(arb.u.)

100

0

Vacuum

Silicon

p

500

0 400 600

200

Longitudinal coordinate z (nm)

b

Energy high-pass

x

detector

MCP

70 nm

Ek0 = 28.1 keV

t=nT

c

Energy

detector

Femtosecond laser pulses

t=(n+1/2)T

Figure 1 | Experimental set-up used for demonstration of electron gating with sub-cycle resolution. (a) The electric eld componentEz (colour scale) of the accelerating mode as a function of the electron arrival position within one grating period lp (arrival time) and transverse distance from the silicon grating (grey structure) surface. Arrows indicate the direction and magnitude of the nal momentum change of the travelling electron. (b,c) Sketch of the experimental setup: a continuous electron beam (electrons indicated by spheres) interacts with two subsequent laser pulses (red curves) focused down to radii of w 71 mm in the vicinity

of a Si grating (indicated by grey background pattern). The modulation of electron energy DEk Ek-Ek0 during the interaction with the rst laser pulse

(visualized by the colour scale of the spheres representing the electrons) is further enhanced (Dt nT, a) or cancelled (Dt (n 1/2)T, b) by

interaction with the second laser pulse depending on the relative phase difference between the laser pulses. Slow electrons are ltered out by an energy high-pass lter so that only accelerated electrons (red) are detected by a micro-channel plate detector.

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Optical cycle (T)

(eUsEk0)(keV)

0

1

2

a

3

b

c

d

Electron current (counts per sec)

0.6

0.4

0.2

0.0

Peak

FWHM

(fs)

Peak

FWHM

(fs)

400

Oscillation amplitude (arb.u.)

2

>30 eV >1 keV

200

>1keV

0.0 0.5 1.0

2

>1.3 keV

0.0 0.5 1.0

1

0

180 0 600

400

200

200

185

190

Time delay (fs) Time delay (fs)

Figure 2 | Optical phase-dependent energy modulation of electrons on the 1 fs time scale. (a) Measured phase-dependent oscillations of the accelerated electron current for a minimum energy gain (eUs Ek0) of 1 keV (squares) compared with the theoretical curve (red), demonstrating phase controlled

energy modulation. The theory curve was calculated for the transverse distance of electron beam centre from the grating surface y 100 nm. The electron

current oscillation amplitude was used as a tting parameter. (b) FWHM of the individual peaks shown in (a) as a function of (eUs Ek0) compared to

theory (solid curve). (c) Phase-dependent oscillation amplitude over a longer time window at (eUs Ek0) 30 eV (squares), 1 keV (circles) and 1.3 keV

(triangles) tted by Gaussian functions (curves). The data is vertically shifted for clarity. (d) FWHM of Gaussian peaks shown in (c) (squares) as a function of (eUs Ek0) compared to theory (solid curve). Error bars were obtained from the statistical error of individual measurements of accelerated electron

current, pointing stability of electron beam and the timing stability of the experiment (for details see Methods section).

Depending on their arrival time at the grating with respect to the phase of the accelerating eld, electrons are either accelerated, decelerated or deected (see the electric eld and the nal momentum change of the travelling electron in Fig. 1a)30. In our experiment, we rst imprint a periodic energy modulation to the electron beam via interaction with the rst laser pulse. By adjusting the time delay Dt of the second laser pulse, this electron energy modulation can be enhanced for Dt nT

or suppressed for Dt (n 1/2)T, where T l/c is the period

of the driving laser eld and n 2 N. When only accelerated

electrons are detected, this leads to a current that oscillates as a function of Dt. This experiment is similar in nature to a Ramsey-type control of coherent energy exchange between electrons and two spatially separated optical near-elds demonstrated recently in ref. 22. The sub-cycle energy modulation of the electron beam is maintained due to the short propagation distance l 181 mm between the laser spots

(corresponding to a travel time of 19010 fs) and the small relative velocity change of the electrons Dve/ve0p1.2%.

The resulting phase-dependent oscillations of the accelerated electron current are shown in Fig. 2a for electrons with minimum energy gain of 1 keV compared with the theoretical model (see Methods section). The temporal duration of the electron bunch accelerated within one optical cycle of the accelerating mode is apparent from the width of the individual peaks in the phase oscillations. The peak width decreases with increasing spectrometer voltage Us from 3.20.1 fs (FWHM) at a minimum energy gain of (eUsEk0) 30 eV to 2.00.3 fs for (eUsEk0) 1

keV (Fig. 2b, detail in Supplementary Fig. 1). This is a consequence of the smaller phase-space volume occupied by electrons that gain the highest energy (see also ref. 15).

As the temporal prole of the electron energy modulation reects the eld of the driving laser pulse, it changes on two different time scales. It reects both the carrier frequency o (phase oscillations) and the envelope function of the laser pulse eld Bexp( 2ln2t2/t2). To observe the effect of laser pulse

envelope on the accelerated population of electrons, we monitor the amplitude of the sub-cycle current oscillations (c.f. Fig. 2a) as a function of the time delay between the laser pulses

(see Fig. 2c,d). Here the width of the resulting cross-correlation changes from 49030 fs at (eUs Ek0) 30 eV to 18040 fs

at (eUs Ek0) 1.3 keV. This demonstrates that apart from

sub-cycle temporal resolution, the gating scheme can provide an envelope temporal resolution more than three times ner than the duration of the driving laser pulse.

As can be seen in Fig. 2a, this technique generates a pulse train of sub-cycle femtosecond electron bunches. To generate a single bunch, that is, to demonstrate the feasibility of single-cycle gating operation, we simulate gating of a DC electron beam driven by ultrashort infrared (l 2 mm, t 10 fs corresponding to

1.6 optical cycles) laser pulses with a stable carrier-envelope phase (for details see Methods section).

The calculated accelerated electron current as a function of electron arrival time and minimum energy gain is shown in Fig. 3. Single-cycle gating can be achieved by the detection of electrons with energy gains above 100 eV. The physical principle used is analogous to the isolation of single attosecond XUV pulses via spectral ltering of the high-energy part of the high harmonics generated from ultrashort laser pulses6,31. We note that our scheme does not rely on the ballistic velocity bunching taking place during free propagation of the electrons after their interaction with the laser pulse, as discussed in refs 18,20 (for a detailed discussion see Supplementary discussion, High-pass energy gating versus attosecond microbunching and Supplementary Fig. 2).

Electron transverse streaking at optical frequencies. Not only sub-cycle temporal gating but also transverse streaking of free electrons is possible by a simple variation of the presented set-up (Fig. 4a), providing for deection-based applications and an improvement in temporal resolution. This laser-driven transverse streaking is achieved by using the time-dependent near elds of a nano-grating tilted with an angle of a 37 relative to the

axis of electron beam propagation32. Electrons thus gain transverse momentum, the magnitude and direction of which depends on the electron arrival time with respect to the optical phase of the laser eld and distance from the grating surface32.

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200

a

e detector MCP

x

Minimum energy gain (eV)

150

e beam

100

t=nT

Femtosecond laser pulses filter

Adjustable knife-edge

50

b

0

10

0 10

Electron arrival time (fs)

Figure 3 | Single-cycle optical gating of an electron beam. Simulated dependence of the accelerated electron current (colour scale) on the electron arrival time with respect to the laser pulse and the minimum electron energy gain (eUs Ek0) for a few-cycle incident laser pulse

(see Methods section). Single-cycle gating operation in temporal window of duration ts-c 1.2 fs is reached for ltering of electrons with

(eUs Ek0)4100 eV. The laser pulse parameters used for this simulation

are E0 1.3 GVm 1, t 10 fs, l 2 mm.

c

186

Min.deflection angle (mrad)

Electron current (counts per sec)

6

4

Bunches shorter than 1/4 of the laser optical cycle can be streaked by this scheme with a close-to-linear dependence of the deection angle on the electron arrival time.

For a demonstration of the dependence of the electron deection angle on the optical phase of the deecting elds, we again use two spatially separated laser pulses with an adjustable time delay. The polarizations of both laser pulses are rotated to be parallel to the grating vector. Spatial ltering of the deected electrons is performed with a metallic knife edge (see Methods section for details).

In Fig. 4c, we show the measured electron current as a function of the minimum deection angle and of the phase difference between the two laser pulses (the deection angle of the beam centre is shown in Supplementary Fig. 3). The maximum observed deection angle of y 6.5 mrad is six-times higher

than the undeected electron beam divergence angle y0

2

0

Relative phase (rad)

Figure 4 | Optical phase-dependent deection of electrons on the 1 fs time scale. (a) Sketch of the experimental set-up used for demonstration of sub-cycle transverse streaking of electrons. Electron beam (spheres) interacts with the near elds (red regions) of silicon grating (grey) tilted by the angle a 37. Spatially (knife edge) and spectrally (energy high-pass

lter) ltered electrons are detected by the MCP. (b) Measured current of spatially (deection angle y46 mrad, represented by the dashed line in (c) and spectrally (DEk4400 eV) ltered electrons as a function of time delay Dt between laser pulses. Peaks are tted by Gaussian functions (red)

with tFWHM 1.30.3 fs. (c) Dependence of the measured electron current (colour scale) on the minimum deection angle and the relative phase between the pair of driving laser pulses for electrons with energy gain

DEk430 eV. The full line indicates the electron beam divergence angle. Error bars were obtained from the statistical error of individual measurements of accelerated electron current, pointing stability of electron beam and the timing stability of the experiment (for details see Methods section).

1.1 mrad.

Because of the geometry of the tilted grating, the deecting and accelerating/decelerating forces are coupled. This is advantageous in terms of temporal resolution due to the possibility of simultaneous spatial and spectral ltering of the detected electrons. In Fig. 4b, we show the phase-dependent electron current for electrons with a deection angle y46 mrad and an energy gain DEk4400 eV. Double ltering (energy and spatial) further decreases the phase-space region occupied by detected electrons and improves the temporal resolution of sub-cycle gating to 1.30.3 fs.

DiscussionWe have thus shown that by utilizing the interaction of electrons with the laser-induced near elds of a nano-grating, we can control the energy and deected spatial prole of the electron beam with sub-optical cycle precision. These results will lead to interesting developments in ultrafast electron diffraction and microscopy2,4,8,13,33 experiments, an example set-up of which we show in Fig. 5a. Here an electron bunch rst interacts with the accelerating eld excited by a few-cycle laser

pulse leading to modulation of its kinetic energy Ek. The gated electrons are then transmitted and/or diffracted by a sample. Here the dynamics excited by, for example, a foregoing attosecond XUV pulse (generated via high-harmonic generation from a split off fraction of the accelerating laser pulse) can be probed as a function of time delay. Using the detection of electrons with the highest energy gain ltered by an imaging spectrometer, single femtosecond temporal resolution will be achieved. Detection of high-energy electrons will also maintain the spatial mode of the electron beam after the gating interaction as they

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a b

~100 fs

<1fs

Imaging spectrometer

Electron bunch

Electron

Sample

Electron detector

Figure 5 | Single-cycle optical gating and streaking experiments with electron beams. (a) Electrons (spheres) coming from the left obtain an energy modulation DEk by the interaction with an ultrashort laser pulse (red) with optical period T in the vicinity of a single ridge nanostructure (grey). Afterwards they interact with the sample and probe electron dynamics excited by an attosecond XUV pulse (violet). Electrons with the highest energy gain (red)

accelerated within one optical cycle are ltered by the spectrometer and detected by a micro-channel plate detector. With nowadays available parameters, a time resolution below 1 fs is possible. (b) An electron bunch (spheres) is transversely streaked by the phase-dependent deecting elds generated by an ultrashort laser pulse (red) and its transverse spatial distribution is detected by a spatially resolved electron detector. Whereas in (a) timing information is mapped onto energy, here it is mapped to the spatial domain like in a classical streak camera, offering a temporal resolution down to 10 as(see section Simulations).

T t

Laser pulse

undergo minimal deection27,30 (for details see Supplementary discussion, Electron beam emittance after interaction with optical near-elds). When combined with attosecond XUV pulses generated by the same laser, the optical gating of an electron beam allows for all-optical timing control of the experiment, avoiding the electronic timing jitter usually present in state-ofthe-art experiments employing microwave electron bunch compression34.

Additionally, the angular phase-dependent streaking of electrons demonstrated here can have direct application as a laser-driven analogy of a classical streak camera (Fig. 5b) in which the temporal prole of an electron bunch is translated into a transverse spatial distribution. Here, a temporal resolution of B10 as could be directly accessible with available laser and electron beam technology (for details see Methods section).

In summary, we have experimentally demonstrated laser-driven temporal gating as well as transverse streaking of free electrons at dielectric nanostructures with single femtosecond temporal resolution. This technique can be directly applied to ultrafast electron diffraction and microscopy. Moreover, the presented physics is not limited to sub-relativistic electrons26. It also enables precise bunch length measurements of relativistic charged particle beams (not only electrons) such as those generated in modern free-electron-laser-based light sources.

Methods

Laser and electron beams. An overview of the optical set-up is shown in Supplementary Fig. 4. A thulium-doped infrared femtosecond bre laser is used in the experiments. The laser parameters are: full-width at half-maximum (FWHM) pulse length t 60050 fs, central wavelength l 1930 nm, repetition rate of

1 MHz and a maximum pulse energy of 500 nJ. The beam is divided by a polarizing beamsplitter into two arms. The polarization angle of the beam in each armis adjusted by half-wave plates to be parallel to the grating vector K in all experiments. The temporal delay between pulses is controlled by an optical delay stage and two wedge prisms with an opening angle of 0.5 for ne tuning, allowing for a precision of B200 as. Both laser beams are focused on the silicon grating by a single f 32 mm aspherical lens leading to two spots spatially separated about

l 181 mm with the 1/e2 radii of w 71 mm. For the phase-dependent

measurements, the temporal delay between laser pulses is rst set to 19010 fs corresponding to the travel time of electrons between the two laser spots and then phase controlled by the wedge prisms. The electron beam is generatedby a scanning electron microscope column (Hitachi S-series) and focused to a nal 1/e2 radius of we 7020 nm with a divergence angle y0 1.10.1 mrad and

a probe current Ip 31 pA. The electron beam energy was adjusted in the range

of Ek 2530 keV to match the synchronicity condition in each experiment29.

The width of the electron beam energy distribution is specied to be dEkB3 eV.

The distance de 10030 nm between the centre of the electron beam and the

grating surface was chosen as a compromise between the transverse decay length of B100 nm (see below) of the accelerating mode and the clipping of the electron beam on the grating edge.

Accelerating and streaking element. Silicon gratings are fabricated from phosphorus-doped silicon substrates with a conductivity of 510 ohm cm. Semiconductor material is used to suppress charging of the grating. However, in the spectral region of our laser operation (l 1930 nm), silicon optical properties

can be treated as dielectric. Nanopatterning is performed by electron beam lithography (JEOL JBX-6300) and reactive ion etching. The grating is prepared on a mesa structure 50 mm above the substrate surface to minimize electron beam clipping on the substrate edges. In the gating experiment, the grating period is lp

620 nm, the trench depth td 400 nm and the trench width tw 45% of

lp leading to synchronicity of the fundamental spatial harmonic with b 0.32

(28 keV) electrons. In the streaking experiment, a grating with a smaller period (lp 465 nm, td 400 nm, tw 45% of lp) is used and tilted with respect

to the electron propagation direction at an angle of a 37 in the x-z plane

(see Supplementary Fig. 4). In this geometry, the fundamental harmonic has a phase velocity synchronous with an electron velocity of b lp= l cos a

0:3

corresponding to an electron energy of 25 keV. The eld amplitudes of the accelerating mode and the streaking mode decay exponentially with the distance from the grating surface as Ed / exp d=G, where d is the distance from the

sample surface and G bgl/(2p)E100 nm is the decay constant, with b ve/c and

g 1 b2 1=2 (refs 27,30).

Detection set-up. Accelerated electrons are transmitted through a retarding eld spectrometer that serves as an energy high-pass lter with applied DC retardation voltage Us. They are then detected by a Chevron-type micro-channel plate detector (MCP) and individual electron counts are temporally correlated with a fast photodiode signal detecting the laser pulses in a time-to-digital converter35.

The accelerated electron signal forms a peak in the resulting histogram plot of MCP counts versus temporal delay between each count and the following laser pulse (Supplementary Fig. 5). The accelerated electron current is limited bythe continuous thermo-emission electron beam used in the experiments. For laser-triggered eld-emission electron microscopes, fA-pA current can be expected in optical gated operation36. For spatial ltering of the deected electrons,a metallic knife edge with precise transverse in-situ motorized position control (motion in x-direction with precision of 1 mm) is placed in the electron beam 140.5 mm downstream of the silicon grating.

Data processing. The count rate of accelerated electrons is obtained from each measurement by integrating the counts in the signal peak and subtracting the background count level by the relationship:

S

Rt

0 Stdttint ; 1

where S(t) is the measured time-dependent signal, t1 and t2 represent boundaries of

t Stdt t tt R

t

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the integration time window which contains the acceleration signal, and tint is the

time over which data is collected (see Supplementary Fig. 5).

Experimental error of accelerated electron current (error bars shown in Figs 2 and 4 and Supplementary Fig. 1b) is due primarily to two causes. The statistical error is given by the ratio between the signal count rate S and the statistical noise

level

Nbgt2 t1 t

int

q , where Nbg is the average background count level per bin detected out of the signal time window. The second contribution arises from the pointing instability of the electron beam. The focus position instability of approximately 10 nm causes a signal instability of 10% due to the transverse decay of acceleration and streaking elds.

Simulations. Electromagnetic elds of the accelerating mode (z-component shown in Fig. 1a) are calculated by a nite-difference time-domain technique. In the simulation, the grating has an innite periodicity in the z direction so that Floquet periodic boundary conditions can be applied. The incident laser pulse is treated as a Gaussian pulse with electric eld Et A exp 2 ln 2t2=t2int expiot, where

A is the eld amplitude, o is the laser circular frequency and the interaction duration tint t1 bct=w

2 1=2 describes the temporal and spatial overlap

of the electromagnetic eld with electrons moving perpendicularly to the laser propagation with velocity bc (ref. 30). The electron beam is described by a classical particle model with Gaussian prole of electron density ne exp 2 ln 2y2 w2e

with we 70 nm and Coulomb repulsion neglected. We note that Coulomb

repulsion effects are insignicant in our results due to the low electron current (pA) leading to less than 1 interacting electron per laser pulse. Single electron trajectories are calculated by numerical integration of the relativistic equation of motion with the Lorentz force ddt gmeve

q E ve B

using elds obtained by nite-

difference time-domain calculations, with g the Lorentz factor, me the electron rest mass, v the electron velocity, E the electric eld and B the magnetic eld. To obtain the accelerated electron count rates shown in Figs 2a,b and 3 we numerically integrate the number of electrons with an energy gain DEk exceeding the difference between the initial electron energy and the spectrometer potential barrier (eUs-Ek0). For the calculation of the phase-dependent electron count rate in the gating experiment (Fig. 2a), two consecutive Gaussian laser pulses with temporal separation Dt are used as the incident eld. Extensive simulation results are shown in Supplementary Fig. 1 compared with measured data.

The theoretical curve shown in Fig. 2d, depicting the FWHM of the curves in Fig. 2c, is obtained by the following analysis. The accelerating eld of both laser pulses is analytically approximated in the electros rest frame as:

Ea Ea0 exp y=G

cos ot0

exp ve t t0

2

64

=w

f g2

exp 2 ln 2 t t0

=t

f g2

exp ve t t0 Dt

Dz

=w

f g2

exp 2 ln 2 t t0 Dt

cos o t0 Dt

f g

=t

f g2

3

75;

2

where Ea0 is the eld amplitude of the synchronous mode, t0 is electron arrival time at the rst spot, Dt is time delay between laser pulses and Dz is spatial separation distance of the two laser spots. The energy gain of individual electrons is obtained by integration of the accelerating eld over time in linear approximation. Here the small velocity change of electrons (p1.2%) during the interaction with therst spot was neglected. Electrons with energy gain above a given threshold are numerically integrated leading to the oscillatory current as function of Dt.

The results of single-cycle gating of electrons shown in Fig. 3 are calculated for a structure containing only one grating tooth (prole shown in Supplementary Fig. 6). Assuming the incident laser eld is a plane wave and the grating is innitely periodic in the z direction implies that the accelerating eld is innitely periodic. The energy gain of electrons is then periodic in their arrival time t0 and it is not possible to resolve electrons accelerated by only a single optical cycle of the driving pulse. Hence a grating with at most a few grating teeth is required. The gated electron current depends on the initial electron bunch properties. Assuming an initial bunch length of te 300 fs and a waist radius of the electron beam of

we 70 nm, the single-cycle gated electron current represents 0.3% of the initial

electron current, which is highly attractive for time-resolved experiments requiring very high temporal resolution. Note that because of the small transverse beam dimensions we consider 1 electron per pulse or less as an ideal bunch charge37. We note that the small degradation of transverse electron beam properties does not cause the loss of spatial resolution for short electron propagation lengths after interaction with the laser eld (for the behaviour of the transverse electron beam properties see the discussion of the beam emittance change in Supplementary discussion, Electron beam emittance after interaction with optical near-elds).

The intrinsic limit for the achievable temporal resolution of our proposed attosecond streak camera based on phase-dependent electron deection is calculated as follows (laser power and phase uctuations are not included in the calculations). The transverse angular streaking velocity is dened as vs djt dt,

where j(t0) is the deection angle and t0 the electron arrival time. Angular resolution can be written as dj arctandr=D, where dr 10 mm is spatial

resolution of electron detector and D 20 cm is the distance between

detector and streaking element. The nal temporal resolution is then given by

dt

dj=vs

q , where y is transverse distance from the grating. For the laser parameters used in the experimental data presented here

(l 1,930 nm, t 600 fs and a peak eld of 1.3 GV m 1) and a transmission

electron microscope with typical electron beam parameters (Ek 100 keV,

we 1 nm), the temporal resolution equals dt 10 as, well below the atomic unit

of time (24 as). We note that using double-sided structures, the spatial proleof acceleration mode can be changed to cosh(y/G) leading to constant acceleration eld in the centre of the structure38. Such a mode will then even offer 10 as temporal resolution for larger electron beam diameters (1020 nm). Operation with this temporal resolution will require intensity- and phase-stabilized laser source39.

Data availability. The data that support the ndings of this study are available from the corresponding author upon request.

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Acknowledgements

The authors acknowledge funding from ERC grant Near Field Atto. This publication is funded in part by the Gordon and Betty Moore Foundation through Grant GBMF4744 Accelerator on a chip International ProgrammeACHIP.

Author contributions

M.K. and J.McN. carried out the experiments and designed the silicon gratings. K.J.L. and H.D. prepared the silicon gratings. N.S., A.R. and I.H. developed the laser pulse compression and provided the laser. M.K. performed the simulations and created the gures. M.K., J.McN. and P.H. interpreted the results and wrote the manuscript. J.S.H. and R.L.B. supervised the experiments at Stanford, P.H. in Erlangen. M.K. and J.McN. are co-rst authors.

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How to cite this article: Kozk, M. et al. Optical gating and streaking of free electrons with suboptical cycle precision. Nat. Commun. 8, 14342 doi: 10.1038/ncomms14342 (2017).

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