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ArticleStreaking of a Picosecond Electron Pulse with a Weak Terahertz
Pulse
Wataru Yajima, Yusuke Arashida,* Ryota Nishimori, Yuga Emoto, Yuki Yamamoto, Kohei Kawasaki,
Yuri Saida, Samuel Jeong, Keishi Akada, Kou Takubo, Hidemi Shigekawa, Jun-ichi Fujita,
Shin-ya Koshihara, Shoji Yoshida, and Masaki Hada*

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ABSTRACT: Optoelectronic devices driven by terahertz (THz)waves have been used to control electron pulses, such as throughacceleration, compression, and characterization of ultrashortpulsed electrons. In this study, we developed a THz generation
setup with a lithium niobate crystal via the tilted-pump-pulse-front
approach. Furthermore, a THz electro-optical sampling setup, anultrashort pulsed electron generation system, and a THz streaking
system with a simple butterfly-shaped THz optoelectronicresonator were also developed to determine the lower limits of
THz intensities that can be used for characterization of picosecond
pulsed electrons. Multiple deflections of the electron pulse occur
when the duration is longer than the half cycle of the electric fieldat the THz optoelectronic resonator, which requires a higher THz intensity and makes the analyses difficult. The results obtainedwith appropriate analyses show that a weak THz field of a few kV/cm resonated with the THz optoelectronic device is sufficient tocharacterize picosecond pulsed electrons.
KEYWORDS: terahertz wave, electron beam, streaking method, pulse duration characterization, ultrafast time-resolved electron diffraction,
ultrafast phenomena■

INTRODUCTION
Ultrafast time-resolved electron diffraction measurements havedirectly provided the atomic-level structural dynamics ofmolecules or materials rearranged by photoexcitation on the
femto-to-picosecond time scale.1−14 The time resolution ofultrafast time-resolved electron diffraction measurements relieson the pulse duration of the probe electron pulse since theoptical pulse used for excitation (approximately 100 fs) is
usually shorter than the electron pulse. Thus, the characterization of the probe electron pulse is essential. Theinteractions of electric fields and electrons have been used
to measure the duration of electron pulses, that is, theinteractions of laser-plasma with electron pulses (plasma
method),15−19 the interactions of ponderomotive forces withelectron pulses (ponderomotive method),20−24 the interactions of the instantaneous high voltage electric fields andelectron pulses (photoswitch streaking method),25−27 and theinteractions of electric fields generated by terahertz (THz)waves and electron pulses (THz streaking method).28−33 The
fast response form sample is also used to estimate the pulse
duration.34−36 The plasma method constructs a relatively
simple setup compared with other methods; however, theaccuracy of the pulse duration is limited since the laser plasma
occurs on the picosecond time scale. The ponderomotive© 2022 American Chemical Society

method requires a specific optical setup inside the vacuum
chamber. The photoswitch streaking method involves
fabricating an ultrafast photoswitching device based on lowtemperature grown GaAs substrates. THz generation, andcharacterization systems outside the vacuum chamber are
required for the THz streaking method. Among thesetechniques, the THz streaking method would be one of thesuitable options for the already existing ultrafast time-resolved
electron diffraction setups because very large reconstruction
inside the vacuum chamber is not required for thismethod.29−33 THz streaking techniques that are applied forelectron pulses or X-ray free electron lasers have beenreported in several studies.37−41 THz waves can also be used
to control the acceleration, deflection, and compression ofelectron pulses.29,32 However, most studies on the THz
streaking of electron pulses used experimental setups that
Received: August 22, 2022
Published: December 13, 2022

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Articlevery close to the THz resonating device, leading to beam
diameter enlargement at the THz optoelectronic resonating
device. In this study, we developed a THz generation setup
with a lithium niobate (LiNbO3) crystal, a THz electro-optical(EO) sampling setup, an ultrashort pulsed electron generation
system, and a THz streaking system with a simple butterfly
shaped optoelectronic device to determine the lower limits of
THz intensities that can be used for characterization ofpicosecond pulsed electrons. The fundamental principle of thebutterfly shaped THz resonator is similar to the fieldenhancement effects of ultraviolet, visible, or infrared light at
the metal nanostructures used in photon-induced near-field
electron microscopy experiments.42 Another challenge involves the characterization of picosecond pulsed electrons
with the THz streaking method. When the electron pulse isshorter than the half period of the main component of theelectric field generated at the THz resonator, we cannot
directly measure the duration of the electron pulse by thewidth broadening of the electron spot (streaking length)
because such an electron beam is deflected more than once,i.e., undergoes multiple deflections. Due to the multiple
deflections, the THz streaking signals with high frequency andhigh intensity are overlapped at the same position on thescreen. Therefore, the streaking signals with low frequency
and low intensity are obtained. Thus, it requires a relatively
higher THz intensity when multiple deflections occur. In thissituation, the electron pulse duration cannot be determined
simply by the streaking length on the screen. We analyzed theobtained THz streaking signals with low frequency and low
intensity and showed that a weak (a few kV/cm) THz fieldresonated with the optoelectronic device is sufficient tocharacterize femto-to-picosecond pulsed electron.

Figure 1. (a) Schematic of the electron beam pulse duration
measurement setup by the THz streaking method. The electron
beam shape at the CMOS camera is inset in the figure. The blackarrow in the inset indicates the deflection direction of the electron
beam by the THz pulse. The symbols THG, MCP, WP, and BD
correspond to the third harmonic generation, microchannel plate,
Wollaston prism, and balanced photodetector, respectively. Front
view (b) and top view (c) of the orientation of the butterfly shape,
the direction of electron beam propagation (blue arrows), thedirection of THz wave propagation (red arrows), and thepolarization axis of the THz wave (black arrows).

■

EXPERIMENTAL METHODSA THz generation setup, EO sampling setup, ultrashort pulsed
electron generation system, and THz streaking system areshown in Figure 1a. An optical pulse from a Ti:sapphire
regenerative amplifier with a wavelength of 800 nm, a pulse
duration of 100 fs, and a repetition rate of 1 kHz was split into
two arms (electron generation arm and THz generation arms)
with a beam splitter. An optical pulse in the electron
generation arm was converted into an ultraviolet (UV) pulse
at a wavelength of 267 nm with nonlinear optical crystals via
third harmonic generation. The UV pulse was focused onto

were customized to maximize the interaction of THz waves
and electron pulses through THz streaking. Therefore,
whether very weak THz can be utilized for the THz streaking
method is worth determining. This method also has
considerable merit for the already existing ultrafast timeresolved electron diffraction setups because of space
limitations. In an existing setup, the THz beam may beinterfered with by the sample holder or magnetic lens. In
addition, the THz focusing optics may not always be placed

Figure 2. (a) Waveform of the THz pulse obtained by EO sampling and (b) its Fourier transform (intensity spectrum). The black line indicatesthe reproducible signal.
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ArticleFigure 3. (a) Photograph of the THz optoelectronic resonator. The THz pulse has a polarization angle at which the electric field oscillates, as
indicated by the red arrow. (b) Two-dimensional map of the calculated electric field distribution generated by the THz pulse at the THz
optoelectronic resonator shown in a vectorial manner. The simulations are performed to set the electric field component of the THz
optoelectronic resonator parallel to the polarization angle of the incident THz pulse. (c) Simulated waveform of the absolute value of the electricfield generated in the central part of the THz optoelectronic resonator and (d) its Fourier transform (intensity spectrum). (e) Local fieldenhancement as a function of the frequency.

the Au photocathode to generate an electron pulse in a
vacuum chamber of the ultrashort pulsed electron generation
system. A 35 kV electrostatic field accelerated an electron
pulse generated by the photoelectric effect. A pinhole resized
the electron beam to 100 μm in diameter, which is slightly
smaller than the clear aperture of the THz optoelectronicresonator. The typical duration of an electron pulse generated
from a direct-current electron gun is a few ps. After passing
through the THz optoelectronic resonator, the electron pulse
was focused with a magnetic lens, hit on a microchannel plate,and detected by a complementary metal oxide semiconductor(CMOS) camera. The electron beam on the CMOS camera isshown in the inset of Figure 1a. The geometry of theorientation of the butterfly shape, the direction of electron
beam propagation, the direction of THz wave propagation,and the polarization axis of the THz wave are shown in Figure
1b,c.
We chose a tilted-pump-pulse-front scheme with a LiNbO3
crystal to generate the THz pulse.43−45 Since the initial
diameter of the optical pulse was 12 mm, which is large forthe following optical systems, the optical pulse, after passing
through a delay stage, was reduced in diameter to 6 mm using
a telescope. The pulse front of the optical pulse was tiltedusing a transmission grating to match the THz generation
angle at the LiNbO3 crystal, with a focusing diameter of ∼1
mm. A THz optical pulse with a source diameter of ∼1 mm
was generated from a LiNbO3 crystal. The first lens with a
focal length of 30 mm parallelized the diverged THz wave
generated from a LiNbO3 crystal. The size of the parallel THz
beam was measured to be ∼15 mm. The parallel THz wave
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ArticleFigure 4. (a) Zoomed-out schematic illustration depicting the THz resonator with the electron beam and THz wave. An xyz-axis indicator is theinset in the figure. Zoomed-in schematic illustrations of the multiple deflections of short (b) and long (c) electron pulses. The yellow ellipsoidshows the electron pulse. The solid white and black arrows are the electric field and the direction of the electron pulse, respectively.

was focused on the THz optoelectronic resonator by thesecond lens with a focal length of 200 mm. The beam size on
the THz resonator was ∼7 mm based on the ratio of the focal
lengths of the two lenses. The electron pulse streaked by theTHz pulse was observed by changing the time delay between
the electron pulse and THz pulse reaching the resonator withthe delay stage. According to the calculation of the camera
length, the one-pixel shift of the electron pulse on the CMOS
camera corresponds to an electron deflection of 75 μrad. To
obtain a temporal profile of the THz electric field, wedeveloped an EO sampling setup with a ZnTe crystal. For EO
sampling, we used a fundamental optical pulse for the probe.
A flipping mirror can switch the THz pulse to introduce it toeither the electron streaking system or EO sampling setup.

calculated to be 6.2 kV/cm at the EO sampling setup using
the following equation:46ETc
In r41d I0
3(1)

where ET, c, ω, n, d, ΔI, and I0 are the electric field of the THz
wave, speed of light, angular frequency of the probe light,
refractive index of the nonlinear optical crystal (ZnTe) for theprobe light, thickness of the nonlinear optical crystal (ZnTe),
modulated intensity of the probe light, and initial intensity ofthe probe light, respectively. A pyroelectric detector alsomeasured the intensity of the THz pulse. The generated THz
pulse at the LiNbO3 crystal was estimated to be approximately
100 kV/cm; however, beam defocusing by lenses, reflection
loss by mirrors, and absorption by air decrease the intensity of
the THz pulse at the position of the EO sampling setup andthe THz optoelectronic resonator. The THz pulse was
generated from a point source (diameter of ∼1 mm) as aspherical wave; therefore, we placed a lens with a short focal
length (f = 30 mm) just behind the LiNbO3 crystal totransform the wave into a parallel wave. In the EO sampling
setup, another lens (f = 100 mm) was set to obtain a large
incident fluence at the surface of the ZnTe crystal. The
diameter of the THz optical beam was defined by the ratio of
the focal lengths of the two lenses because the diameter of theTHz beam at the LiNbO3 crystal had a finite size. The focal■

RESULTS AND DISCUSSION
The THz optical pulse was characterized by EO samplingmeasurements with a near-infrared probe light. Figure 2a,b
show a temporal profile of the THz wave measured from EO
sampling measurements and its Fourier transform spectrum,
respectively. The results show that the THz pulse has afrequency from 0.3−1.0 THz. The typical THz frequency
generated from the LiNbO3 crystal is 0−3 THz,44 and wedetermined that the oscillation with a frequency from 0−3
THz was a reproducible signal. The THz electric field was119

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ArticleFigure 5. (a) Degree of deflection of the electron beam (1 fC/pulse) by the electrical field at the THz optoelectronic resonator as a function of
the delay time and (b) its Fourier transform (intensity spectrum). (c) Deflection of the electron beam (2 fC/pulse) and (d) its Fourier transform(intensity spectrum). The following discussion shows that higher frequencies (>0.5 THz) are considered noise because the convolution with theelectron pulse should attenuate the higher frequency signal. Lower frequencies (<0.1 THz) are also considered noise because of the stability ofbeam pointing. The oscillation with a frequency from 0.1−0.5 THz is determined to be a reproducible signal. The black lines indicate thereproducible signals in (a) and (c).

present case.47 A temporal profile of the THz wave at thecenter of the THz optoelectronic resonator obtained by thecalculation and its Fourier transform spectrum are shown in
Figure 3c and d, respectively. According to the simulation, theelectric field at the center of the resonator contains frequency
components of 0.2−1.1 THz. Figure 3e shows the local fieldenhancement (Fourier spectrum of Eout/Ein) as a function of
the frequency. The maximum local field enhancement isapproximately 13 at the frequency of ∼0.75 THz. For thepresent THz streaking measurements, the local field enhancement at frequencies of 0.27−0.41 THz is more critical, andthe value at frequencies of 0.27−0.41 THz is 1.7−3.1. The
electric field enhancement by the THz resonator is welldescribed by the simple RLC model, as shown in the
Supporting Information, Figure S2.
The duration of the electron pulse can be measured by thestreak length on the screen when the electron pulse is shorter
than the half cycle of the electric field generated by the THz
wave (Figure 4a), where the half cycle of the electric fieldcorresponds to ∼0.6 ps. However, as shown in Figure 4b,
multiple deflections occur for an electron pulse that is longer
than the half cycle of the electric field generated by the THz
wave; that is, when the electron pulse duration is longer thanthe half cycle of the THz streaking field, the electron pulse islength of the lens for focusing on the THz optoelectronicresonator was 200 mm due to space limitations. Since thelarge beam (∼15 mm) was gradually focused on the THz
resonator (∼7 mm), half of the THz beam was interfered withby the anode in this setup. The THz electric field at the THz
optoelectronic resonator was estimated to be 2.6 kV/cm at the
maximum.
An aluminum film thickness of 30 μm was used as a THz
optoelectronic resonator. Figure 3a shows the resonatormechanically processed in a butterfly shape. The directionof the THz electric field is depicted as arrows inset in the
picture. Using the results of EO sampling, an electric field in
the THz optoelectronic resonator induced by the THz opticalpulse was calculated by the finite difference time domain
method in CST Studio Suite (Dassault Systèmes). Figure 3b
shows the two-dimensional distribution of the electric field
intensity at t = 0 from the calculation. The electric field wasamplified at the center of the butterfly shape. The threedimensional electric field of the profile of the THz resonator
at t = 0 is shown in Figure S1. Since we used an s-polarizedTHz wave, the effects of the x- and z-axes are negligibly small,as shown in the three-dimensional simulated electric field at
the resonator. Thus, the phase modulation effect of the THz
wave interfered with by the resonator would be small in the120

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ArticleFigure 6. (a) Simulated waveform of the electric field generated in the central part of the THz optoelectronic resonator and the Gaussianfunctions (half-widths of 0.3 and 2.6 ps) representing the electron beam pulse distribution. The inset is an enlarged view of the black dashed area.
(b) Fourier transform (intensity spectra) of the waveform of the electric field and the Gaussian functions shown in (a). (c) Calculated waveformof the electric field convoluted with the extended Gaussian function (2.6 ps) and (d) its Fourier transform (intensity spectrum). (e) Calculated
waveform of the electric field convoluted with the short Gaussian function (0.3 ps) and (f) its Fourier transform (intensity spectrum).

deflected several times at the resonator. Therefore, theduration of the electron pulse cannot be estimated only by
the streak length. In this case, the signal from the fast
deflection is deformed, and the slow components are recordedon the screen.
Figure 5a,b shows the time-dependent electron deflection,
derived from the center position of the electron, by the THz
wave and its Fourier transform spectrum, where the electron
pulse contains a charge of 1 fC. Similarly, those results with anelectron pulse containing a charge of 2 fC are shown in Figure
5c,d. As shown in Figure 5a, the degree of electron deflection
vibrated depending on the time delay. The Fourier transform
spectrum of the electron deflection (Figure 5b) is different

from that of the electric field at the THz optoelectronicresonator (Figure 3d). This discrepancy occurs because theelectron pulse duration is longer than the rise time of theelectric field generated at the resonator and because of theaforementioned multiple-deflection effect.
The temporal profile of the electron deflection should
correspond to the convolution of the electron pulse andelectric field to consider the multiple deflections (Figure 6a)
as follows:h(t ) =121f (t)g ( )d(2)
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CONCLUSION
In summary, we developed a THz generation setup with a
LiNbO3 crystal, a THz EO sampling setup, an ultrashortpulsed electron generation system, and a THz streaking
system with a simple butterfly shaped optoelectronic device.
The electron pulse duration was estimated to be 2.6 ps using
an extremely weak THz pulse (2.6 kV/cm) on a simple THz
optoelectronic resonator. Because multiple deflections would
not occur in a shorter electron pulse (<0.6 ps), the required
THz intensity would be even smaller than in the present case.
We cannot generate a shorter electron pulse in this setup dueto the geometry of the electron source. However, we havedeveloped ultrafast time-resolved electron diffraction setups at
7514,48,49 and 100 keV35 accelerations with a radio frequency
compression cavity. These two setups provide an electron
pulse duration of ∼1 ps or <100 fs. For future experiments,
measuring shorter and longer durations of electron pulses byapplying the THz streaking setup to these time-resolved
electron diffractometers would be important. Comparing theconvolution of a temporal profile of the electron pulse andelectric field in a THz optoelectronic resonator with theFourier components of a deflection time profile is an effective
method for characterizing an electron pulse with a duration
longer than a THz wave period. In the present study, thestreaking signal overlaps because the electron pulse duration islonger than the half period of the main component of theTHz wave. However, we can also address that, for future
experiments, measurements with a higher time resolution (lessthan 10 fs) can be realized using the THz streaking method
for time-resolved electron diffraction measurements, i.e., three
beams (an optical pulse for the pump line, an electron pulse
for the probe line, and a THz pulse for the streaking line) canbe used for the pump−probe experiment using a shortelectron pulse (<0.6 ps) without multiple deflections.50

where h(t), f(t), and g(t) are the temporal profiles of theelectron deflection, the electric field at the THz optoelectronicresonator, and the electron pulse (Gaussian function),
respectively. In the Fourier space, this convolution is reduced
to simple multiplication, as shown in the following equation:
H( ) = F( )G( )Article(3)

where H(ω), F(ω), and G(ω) are the Fourier transforms ofh(t), f(t), and g(t), respectively. A temporal profile of theelectron pulse and its Fourier components used for the
convolution are shown as the blue (longer electron pulse) andblack (shorter electron pulse) lines in Figure 6a,b. An electron
pulse was set as a Gaussian function with a pulse duration of
2.6 ps. The calculated time profile of the convolution of theelectron pulse and electric field at the THz optoelectronicresonator and its Fourier transform spectrum are shown in
Figure 6c and d, respectively. Lower frequency components
were emphasized, and higher frequency components were
suppressed. The convolution with a Gaussian function works
as a low-pass filter, as shown in Figure 6b. The time profiles ofthe electron pulse deflection from both the experiment andthe simulation had peaks at 0.27 and 0.42 THz. Comparedwith the experimental signal for the calculation, the half-widthof the electron pulse was estimated to be 2.6 ps. We define t =0 as the peak of the envelope of the electron deflection. The
Hilbert transform provides the electron deflection envelope, as
shown in the Supporting Information (Figure S3). According
to the Hilbert transform, the accuracy of t = 0 is estimated tobe ∼90 fs (rms), which is comparable to the value (∼50 fsrms) obtained for the XFEL with timing jitter correction.36
The Fourier transform spectra obtained by the experimentand calculation agree well with each other (Figure 5b andFigure 6d). However, the experimental waveform (Figure 5a)
is not reproduced in the calculation (Figure 6c). Note that theelectron pulse is larger (100 μm in diameter) than theaperture of the THz optoelectronic resonator (50 μm × 50μm), and thus, the electric field applied to the electron beamin the resonator is not uniform in the experiment. We
calculated the electron deflection at several points in theresonator (Supporting Information), and the average of theseelectron deflections (Figure S4) is almost the same as the
waveform at the center part shown in Figure 6c. The
Supporting Information shows that the difference between theexperimental and calculated waveforms is caused by thedeflection of the electron pulse by the original THz wave
(Figure S5). From the 2 fC/pulse results, the peak of 0.42
THz was decreased compared with 1 fC/pulse. This suggeststhat the increase in the electron pulse duration, that is, theduration of the electron pulse with 2 fC/pulse, would be morethan 3 ps. Thus, the extremely weak THz field of less than a
few kV/cm resonated with the optoelectronic device issufficient to characterize the picosecond electron pulse
duration. Because multiple deflections would not occur in ashorter electron pulse (<0.6 ps), the required THz intensity
would be estimated to be even smaller than kV/cm. Owing to
the suppression of the multiple deflections, the intensity of the
convoluted signal of the electric field generated at the THz
resonator and shorter Gaussian function is much stronger thanthat with the more extended Gaussian function (Figure 6e,f).
Thus, the electric field at the THz resonator will more
effectively work for the shorter electron pulse. The duration ofan electron pulse that is shorter than 0.6 ps will be more easily
detected using this THz streaking system.

■ASSOCIATED CONTENT* Supporting Information
sı

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsphotonics.2c01304.
Detailed description of the three-dimensional electricfield at the THz resonator, RLC model of the resonator,
Hilbert transform of the deflection of the electron
beam, electron beam deflection in detail, and additional
characterization of electron deflection by a THz wave
(PDF)

■

AUTHOR INFORMATION

Corresponding Authors

Yusuke Arashida − Graduate School of Science andTechnology, University of Tsukuba, Tsukuba 305-8573,
Japan; orcid.org/0000-0001-6366-3934;
Email: arashida.yusuke.kb@u.tsukuba.ac.jp
Masaki Hada − Graduate School of Science and Technology
and Tsukuba Research Center for Energy Materials Science(TREMS), University of Tsukuba, Tsukuba 305-8573,
Japan; orcid.org/0000-0001-8148-0971;
Email: hada.masaki.fm@u.tsukuba.ac.jp
Authors

Wataru Yajima − Graduate School of Science and Technology,
University of Tsukuba, Tsukuba 305-8573, Japan;
orcid.org/0000-0002-8581-768X
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Ryota Nishimori − College of Engineering Sciences, Universityof Tsukuba, Tsukuba 305-8573, Japan
Yuga Emoto − Graduate School of Science and Technology,
University of Tsukuba, Tsukuba 305-8573, Japan
Yuki Yamamoto − Graduate School of Science andTechnology, University of Tsukuba, Tsukuba 305-8573,
Japan; orcid.org/0000-0002-4132-1236
Kohei Kawasaki − Graduate School of Science andTechnology, University of Tsukuba, Tsukuba 305-8573,
Japan
Yuri Saida − Graduate School of Science and Technology,
University of Tsukuba, Tsukuba 305-8573, Japan;
orcid.org/0000-0003-2489-1701
Samuel Jeong − Graduate School of Science and Technology,
University of Tsukuba, Tsukuba 305-8573, Japan
Keishi Akada − Graduate School of Science and Technology,
University of Tsukuba, Tsukuba 305-8573, Japan
Kou Takubo − School of Science, Tokyo Institute ofTechnology, Tokyo 152-8551, Japan; orcid.org/00000002-2591-1244
Hidemi Shigekawa − Graduate School of Science andTechnology, University of Tsukuba, Tsukuba 305-8573,
Japan; orcid.org/0000-0001-9550-5148
Jun-ichi Fujita − Graduate School of Science and Technology,
University of Tsukuba, Tsukuba 305-8573, Japan
Shin-ya Koshihara − School of Science, Tokyo Institute ofTechnology, Tokyo 152-8551, Japan
Shoji Yoshida − Graduate School of Science and Technology,
University of Tsukuba, Tsukuba 305-8573, Japan
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Funding

This work was supported by Kakenhi Grants-in-Aid (Nos.
JP18H05208, JP19H00847, and JP20H01832) and the
Leading Initiative for Excellent Young Researchers of the
Ministry of Education, Culture, Sports, Science andTechnology (MEXT), Japan. This work was also supported
by JST FOREST Program, Grant No. JPMJFR211V. A part ofthis work was supported by “Advanced Research Infrastructure for Materials and Nanotechnology in Japan(ARIM)” of the Ministry of Education, Culture, Sports,
Science and Technology (MEXT); Grant No.
JPMXP1222BA0009.
Notes

The authors declare no competing financial interest.

■

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