24F10
Femtosecond electron deflectometry to measure the emission time of thelaser-produced fast electrons from a solid target
フェムト秒電子偏向法によるレーザー生成高速電子の固体ターゲットからの放出時間測定
Shunsuke Inoue, Shigeki Tokita, Kazuto Otani, Masaki Hashida and Shuji Sakabe
井上 峻介, 時田 茂樹，大谷 一人，橋田 昌樹，阪部 周二
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo, Kyoto 606-8502, Japan
京都大学 化学研究所 〒611-0011 京都府 宇治市 五カ庄
京都大学大学院 理学研究科 〒606-8502 京都府 京都市 左京区 北白川
The emission duration of fast electrons from the surface of solid target irradiated by a laser pulse, i.e., thepulse duration of fast electrons, was investigated by using femtosecond electron deflectometry, for the laser
pulse durations of 200fs, 400fs, 540fs, and 690fs. We have found that the emission duration depends on theduration of the incident laser pulse. It is indicated from numerical calculations of 3D charged particle
dynamics that the emission duration of fast electrons is almost equal to the duration of laser pulse.

1. Introduction
The generation and transport of fast electrons
during interaction between intense femtosecond
laser and solid target are key issues for thedevelopment of attractive applications such as fast
ignition for inertial confinement fusion [1], particle
acceleration [2], ultrashort electron pulse generation[3], and high-quality X-ray production [4]. Recently,
in order to illustrate the collective behavior of fastelectrons immediately after femtosecond laser isirradiated, the dynamics of the fast electrons havebeen measured directly by using a laser accelerated
proton beam with high temporal resolution of a few
picoseconds [5]. This technique, however, cannot
achieve the temporal resolution of femtosecond
order because of the duration of proton beams, sothat it is not sufficient to observe the motion of fastelectrons that is emitted from laser plasma. The
emission time of fast electrons has not been
observed during and immediately after laser pulse
irradiation.
In this paper, we present the behavior of fastelectrons emitted immediately after laser pulse,
measured by using femtosecond electron
deflectometry. By changing the durations of theincident laser pulses from 200 fs to 690fs, theemission duration of electrons was varied. By
comparing experimental results and the numericalcalculations, it is indicated that the emission time
was almost equal to the duration of laser pulse.
2. Experimental Setup
The the experimental setup for femtosecond
electron deflectometry is described in Ref. [6]. A

laser pulse from a Ti:sapphire chirped-pulse
ampliﬁcation system is divided into two half pulses(in Fig. 1 in Ref. [6], an upper pulse and a lowerpulse). The time delay between two pulses wascontrolled with an actuator. These divided twopulses were focused on to a solid target (Al foil of12µm thickness) at an incident angle of 45° byusing an F/3 off-axis parabolic mirror. The distance
between two focal positions was 27±1 µm. The
position of the target surface was carefullymeasured with a laser displacement sensor andprecisely adjusted, so that the position
displacements of the target surface from the laser
focal positions were less than ±3 µm. The laserpulse duration was varied between 200 fs and 690
fs by controlling the distance of the two gratings ofthe compressor. The laser pulse energy wascontrolled in order that the intensity of each pulseon the target was 1 × 1016 W/cm2.
The two laser pulses produced two electron
pulses emitted omnidirectionally. The electrons
emitted in specular direction were collected andfocused onto a fluorescent screen by an electron
lens. The sensitivity of this imaging system washigh enough for the electron source image to beobtained by only one laser shot [7]. The energy ofelectrons imaged on the screen was selected by theelectron lens, and it was 120 keV.
When the time delay between the upper andlower laser pulses is small, the two electron pulses
will be deflected by electromagnetic force (mainlyCoulomb repulsive force) from each electron source,
immediately after they are emitted from the target.
This deflection is detected corresponding to theFig. 1. The dependence of the deflection on the timedelay between two laser pulses.

displacement of the distance of two electron sources
imaged on the screen.
3. Results and Discussions
Figure 1 shows the dependence of the deflection
on the time delay between the two laser pulses.
Each shows the deflection at the laser pulse
duration of 200 fs (circle), 410 fs (square), 540 fs(diamond), or 690 fs (cross). The deflection was
normalized by each maximum value. Each point
was obtained by averaging over of 30-50 laser shots.
The solid lines in the Fig. 1 are fitted withexponential functions to evaluate the interaction
times of two electron pulses. It was observed thatthe deflections decrease when the time delay
between two laser pulses increases for any laser
pulse durations. When the time delay was over ±2
ps, deflections were not observed. Figure 2 shows
the dependence of the interaction time of the twoelectron pulses on the laser pulse duration. Theinteraction time is determined as the temporalinterval given by half width of e-1 maximum of thefunction shown in Fig. 1. As shown in Fig. 2, as thelaser pulse duration increases, the interaction timeof the two electron pulses increases. This indicatesthat the emission duration of the electron pulses
from the target surface (or laser plasma) correlate
closely with laser pulse duration.
To support our results, we have calculated thestrength and duration of the electromagnetic fields

Fig. 2. The dependence of the electron pulseinteraction duration of the laser pulse duration.

Fig. 3. Comparison between the deflections
measured by femtosecond deflectometry (circle) andcalculation (cross).

produced by electron pulses with GENERALPARTICLE TRACER (GPT) code [8]. It is assumed
that initial electron pulses emit isotropically, thetemperature is 60 keV, and the total charge of one
electron pulse is 0.3 nC. These initial states ofelectron pulses base on experimental results. The
electron pulse duration is assumed as 200 fs, 410 fs,540 fs, or 690 fs, corresponding to incident laser
pulse durations respectively. Figures 3 shows thedeflections obtained by experiment (circle) andcalculation (cross) at each pulse duration. The error
bars show the standard deviations of theexperiments. The experimental results well agreewith the numerical results, when the laser pulse
duration is assumed to be equal to the electron pulse
duration.
3. Summary
We have measured the emission times of fastelectrons produced under femtosecond laser pulse
irradiation by employing femtosecond electron
deflectometry. With numerical calculations, it isindicated that the emission duration of fast electrons
is equal to the duration of incident laser pulse.
References[1] Max Tabak et al., Phys. Plasmas 1, (1994) 1626.
[2] S. C. Wilks et al., Phys. Plasmas 8, (2001) 542.
[3] S. Tokita et al., Phys. Rev. Lett. 105 (2010) 215004.
[4] Ch. Reich et al., Phys. Rev. Lett. 84, (2000) 4846.
[5] L. Romagnani et al., Phys. Rev. Lett. 95 (2005)
195001[6] S. Inoue et al., Appl. Phys. Lett. 99 (2011) 031501.
[7] S. Inoue et al., Rev. Sci. Intrum. 81 (2010) 123302.
[8] General Particle Tracer, http://www.pulsar.nl/gpt