LETTER

Applied Physics Express 15, 092006 (2022)
https://doi.org/10.35848/1882-0786/ac88b6

Phase-controllable multi-laser system with coaxially combined near-infrared andsubcycle mid-infrared and THz pulsed beams
Yusuke Arashida†, Naoki Umeda†, Hiroyuki Mogi, Masashi Ishikawa, Akira Hatanaka, Osamu Takeuchi, Shoji Yoshida,and Hidemi Shigekawa*
Faculty of pure and applied sciences, University of Tsukuba, 305-8573, Japan*

E-mail: hidemi@ims.tsukuba.ac.jp
Yusuke Arashida and Naoki Umeda equally contributed to this work.

†

Received July 8, 2022; accepted August 9, 2022; published online August 19, 2022
With the progress of infrared high-intensity pulse technology, the ultrafast control of electronic states has been attracting considerable attention.
Most elementary excitations of solids have resonance energy in the mid-infrared (MIR) frequency domain. Therefore, to understand these
dynamics in detail and open up new possibilities, new technological innovations are desired. In this study, we have developed a system thatcombines 8 fs near-infrared pulsed light with carrier envelope phase-controllable subcycle MIR and THz pulsed lights. This technology has
produced a new method that enables the analysis of phase-sensitive phenomena in the near-infrared to far-infrared region.
© 2022 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd
Supplementary material for this article is available onlineW

ith the innovation of femtosecond technology,
attempts to understand and control electronic statesin materials and devices in the ultrafast time
domain are being actively promoted.1–6) Among them, theattractive dynamics of physical properties exhibited byelementary excitations (e.g. excitons, phonons, superconducting quasiparticles, charge density waves) existing in theenergy region of 1–200 meV are of great interest.5,7–17) To
elucidate these physical characteristics, many THz and midinfrared (MIR) light sources have been developed,18,19) andrecently, electric ﬁeld-driven phenomena, as well as photondriven phenomena, have been reported.3,16) To understandthese phenomena in the ultrafast region more spectroscopically, in addition to the introduction of a wideband MIR
pulse, a new technological innovation is desired. One
attractive direction is to use a combination of carrier envelope
phase (CEP)-controlled subcycle electric ﬁelds, which haverecently become stable and usable.
In this study, we have developed a system that combines
8 fs near-infrared (NIR) pulsed light with CEP-controllable
subcycle MIR (1 cycle ∼50 fs, 30 THz bandwidth) and THz(1 cycle ∼1 ps, 1 THz bandwidth) pulsed lights. These three
beams can be used in a coaxial arrangement while maintaining their respective band conditions. The combination of
the monopolar subcycle electric ﬁelds enables the analysis ofultrafast quantum processes under well-controlled conditions
produced by an electric-ﬁeld-drive method, which opens up
new possibilities. For example, since the pulse width of the
MIR light is sufﬁciently smaller than the THz pulse width, aTHz pump and MIR probe method makes it possible toanalyze quasiparticle dynamics, which are sensitive to theTHz-drive phase condition.
Furthermore, the NIR pulse can be used to characterize the
MIR light waveform by electro-optic sampling. That is, sinceboth the real and imaginary parts of permittivity can befound, and thereby, the complex refractive index in the
wideband MIR region can be obtained, all the information
probed by the general optical measurement can be analyzed
with this system.20) In addition, the NIR pulse can be used asa probe to measure the carrier dynamics near the bandgap
edge using the pump–probe method based on absorption
bleaching.21)
In actual experiments, a pump–probe optical system thatuses MIR and THz lights generated by wavelength conversion is more complex than a system where only visible/NIR
lasers are used. The coaxial three-beam optical systemestablished in this study simpliﬁes the optical setup.
Therefore, combining this system with analytical techniques
performed under, for example, low-temperature and ultrahigh-vacuum conditions will be relatively straightforward,
further accelerating research on ultrafast physicalproperties.15,22–29)
Figure 1(a) shows a schematic diagram of the developed
system based on an optical parametric chirped pulse ampliﬁer(Venteon OPCPA).30) In this system, the wideband light
output (650–1050 nm) of Ti:S oscillator is ﬁrst divided into
two parts, one with a center wavelength of 1032 nm, andthe other with a wavelength range of 650–1020 nm.
Subsequently, the former light (center wavelength 1032 nm)
is ampliﬁed by ﬁber ampliﬁers to 90 W, which is divided into
two beams of 70 and 20 W. The 20 W light is used as the
NIR2 light beam shown in Fig. 1(a). On the other hand, NIR1
(4.0 W, 650 to 950 nm, pulse width 8.2 fs, repetition
frequency 4 MHz) shown in Fig. 1(a) is produced by passing
the latter light (650–1020 nm) through a non-collinear opticalparametric ampliﬁer twice using the second harmonic generation (SHG) of the 70 W light as the pump. The spectrumof NIR1 and its intensity waveform obtained by reconstructing it by the dispersion-scan method (D-scan, spherephotonics) are shown in Figs. 1(b) and 1(c), respectively.
The NIR1 pulse is divided into two beams, one for MIR
pulse generation [upper path in Fig. 1(a)] and the other forelectric-ﬁeld waveform sampling (lower path in the ﬁgure).
The subcycle MIR pulses are generated by injecting the NIR1
pulse into a GaSe crystal with a thickness of 30 μm throughlens f1 with a focal length of 150 mm.30) Using the incident
angle of 41° from the c-axis, a wideband MIR pulse is
obtained by the optical rectiﬁcation effect.19) The generated

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Appl. Phys. Express 15, 092006 (2022)

Y. Arashida et al.

(a)

(b)

(c)

(d)

(e)

Fig. 1. (Color online) (a) Schematic diagram of the developed system (see Figs. S1 to S3 for more detail, available online at stacks.iop.org/APEX/15/
092006/mmedia). f1, f2, f3: lenses with focal lengths of 150 mm, 200 mm, and 125 mm, respectively, DS: delay stage, ITO: fused silica plate(thickness = 1 mm) coated with indium tin oxide, and PCA: photoconductive antenna. (b) (c) NIR1 spectrum obtained by PCA and its intensity waveformreconstructed by D-scan (sphere photonics) method. (d) (e) NIR2 spectrum and its autocorrelation function using SHG.

MIR light is collimated by an off-axis parabolic OAP with a
focal length of 50 mm and transmitted into a Ge plate(undoped, 0.5 mm thickness) at an incident angle of ∼70°
to remove the fundamental wave. Since Ge shows a highabsorption coefﬁcient in the NIR/visible region with awavelength of less than 1.5 μm, the NIR pulse, which isthe fundamental wave of the laser, can be removed without
being transmitted.
On the other hand, the subcycle THz pulses are generated
by irradiating NIR2 into the LiNbO3 crystal by the tilt-pulsefront method.18) The primary diffracted light from a grating(1600 mm−1) is incident on the crystal using two lenses withfocal lengths of 200 mm ( f2) and 125 mm ( f3). The THz
pulses generated by the optical rectiﬁcation effect is collimated with a parabolic mirror with a focal length of 25 mm.
After that, as shown in Fig. 1(a), the MIR and THz lights are
combined in a coaxial arrangement by reﬂecting the spolarized THz light on the Ge substrate that transmits the
MIR light. At this time, the polarizations of the MIR and THz

lights are orthogonal. Next, the merged beams are reﬂectedby a quartz substrate with an ITO coating (IQP-501, UQG
Optics), and the NIR1 beam in the lower path is transmitted
from the opposite side to coaxially combine it with the THz
and MIR lights. For the substrate of ITO, quartz with athickness of 1 mm is used to reduce the dispersion of the
NIR1 pulse. Combining the three beams in this mannermakes it possible to simplify the optical system of pump–probe spectroscopy in the MIR region, which is effective inexperiments with analytical techniques performed under lowtemperature and ultrahigh-vacuum conditions as describedabove.
The combined beam was focused on a photoconductive
antenna (PCA: BATOP PCA-40-05-10-1060-0) using an offaxis parabolic mirror with a focal length of 50 mm, and theelectric ﬁeld waveform was evaluated. Sampling was performed by sweeping the delay time of NIR1 with respect tothe MIR and THz lights. The delay times of the NIR1, MIR,
and THz lights are controlled by independent delay stages,© 2022 The Author(s). Published on behalf of092006-2

The Japan Society of Applied Physics by IOP Publishing Ltd

Appl. Phys. Express 15, 092006 (2022)

and the delay time between the MIR and NIR1 lights and thatbetween the MIR and THz lights are expressed as tNM andtMT in the following part, respectively.
Figure 2 shows the electric ﬁeld waveforms measured
using the PCA. As shown in Fig. 2(a), the MIR waveform
was conﬁrmed to have a subcycle that oscillated with awavelength of about 50 fs. As shown in Fig. 2(b), its Fourier
spectrum has a center frequency of 20 THz and a bandwidthof 20 THz or more. The bandwidth of MIR is determined by
the phase matching of GaSe.19) The dip near 7 THz is
considered to be due to the absorption characteristics ofGaSe and Ge.19,31) Note that the actual bandwidth of the MIR
is broader than 20 THz if EO sampling or a different type of
PCA is used to measure.30) The electric ﬁeld of the THz
waveform is also a subcycle and close to monopolar, as
shown in Fig. 2(c). Its Fourier spectrum has a center
frequency of 1 THz and a bandwidth of about 1.5 THz, as
shown in Fig. 2(d). The THz band structure depends on the
NIR2 pulse structure.
By combining the MIR and THz electric ﬁelds, forexample, as shown in Fig. 3(a), it becomes possible toanalyze the quantum processes while they are dynamicallydriven and controlled. Figure 3(b) shows the waveform of the
combined beam of the MIR and THz electric ﬁelds. The
measurement was performed for tTM = 0, 275, and 500 fs bydirecting the measurement axis of the optical conduction
antenna to the middle of the polarization directions of the
MIR and THz electric ﬁelds. The high-speed modulation by
the MIR electric ﬁeld is manifested in the slow change inelectric ﬁeld with a period of 1 ps. tTM can be easily changedby adjusting the delay stage. That is, it is possible to irradiate
the MIR pulse at any desired phase position of the THz
electric ﬁeld during its irradiation of the sample and perform
a time-resolved measurement. Using this system, a newspectroscopic study of the ultrafast electric-ﬁeld-driven
phenomena produced by the attractive quasiparticles in solids
will be realized.
In this system, phase control of the MIR electric ﬁeld isalso important. The generation of MIR light using a GaSe
crystal and its phase control are realized by adjusting the(a)Y. Arashida et al.

superposition of the two types of phase matching conditions,Type 1 and Type 2.32) The effective nonlinear coefﬁcientd eff is d eff µ d 22 cosq cos3fGaSe for Type 1 andd eff µ d 22 cos2 q sin3fGaSe for Type 2,33,34) where d 22 is
the second order susceptibility tensor, q is the incident angleof the laser, and fGaSe is the azimuth angle. The ratio of Type
1 to Type 2 can be controlled by changing the in-plane crystal
axis direction. As an example, Figs. 3(c) and 3(d) show theresults of reversing the polarity of the MIR electric ﬁeld toshow the high controllability of the combined beams. Setting
fGaSe= 0° and 60, we can invert the polarity of the subcycle
MIR electric ﬁeld with the Type 2 component being
suppressed. Using the proposed system, it will be possible
to advance the understanding of and control of non-equilibrium electrons in solids.3,35) Further improvement of thecontrol of tunnel phenomena in metal nanostructures in theultrafast region is also expected.25,29,36,37)
Figure 4 shows the stability of the electric ﬁeld waveform,
which demonstrates its usefulness for measuring physicalproperties. Figure 4(a) shows the measurement results of the
combined waveform every 2 min for 3 h. The blue and green
parts are the negative and positive regions of the THz electric
ﬁeld, respectively. The sharp change near the delay time of
0 fs is the contribution of the MIR electric ﬁeld, which isstably located in the green part where the intensity of the THz
is positive.
To evaluate the relative time difference tTM between the
MIR and THz electric ﬁelds, tTM was extracted by ﬁtting the
waveform at each time with the function E (t, tMT ) = EMIR(t- tMT )-ETHz (t). Here, EMIR (t) and ETHz (t) are waveforms
obtained in advance by measuring the MIR and THz electricﬁelds independently. The results are shown in Fig. 4(b). The
initial value of tTM is about 6 fs, which changed only by
about −3 fs in 3 h. The changes were 16 fs for the peak-topeak value and 3.9 fs for the standard deviation. This meansthat the central time of the MIR electric ﬁeld is locked withan accuracy of 2% or better for one cycle of the THz electricﬁeld. Namely, a new system that enables the analysis ofphase-sensitive phenomena in the NIR to the far-infrared
region has been realized.
(b)

(c)
(d)

Fig. 2. (Color online) (a) Waveform of MIR electric ﬁeld and (b) its Fourier spectrum. (c) Waveform of THz electric ﬁeld and (d) its Fourier spectrum. Allwaveforms were measured using the PCA.
© 2022 The Author(s). Published on behalf of092006-3
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Appl. Phys. Express 15, 092006 (2022)(a)Y. Arashida et al.

(c)

(b)

(d)

Fig. 3. (Color online) (a) Schematic diagram of the electric ﬁeld waveform measurement of the three combined beams. (b) Waveform of the combined beamsof the MIR and THz electric ﬁelds obtained by changing the delay time tMT between them. (c) (d) Control of the MIR electric ﬁeld polarity at tMT = 0 fs bysetting the in-plane azimuths of the GaSe crystal to 0° and 60°, respectively.

Fig. 4. (Color online) Long-term stability of the combined electric ﬁeld waveform. (a) Electric ﬁeld waveform measured every 2 min. The color represents
the value of the electric ﬁeld (a.u.). (b) Difference in delay time between MIR and THz electric ﬁelds as a function of time.

In conclusion, as a new method that makes it possible tounderstand and control the dynamics in the ultrahigh-speed
region of a condensed system, we have realized a new system inwhich CEP-controlled wideband subcycle MIR (1 cycle: 50 fs)
and THz (1 cycle: 1 ps) electric ﬁelds are coaxially combined.
First, by using Brewster’s angle of the crystal, the MIR and
THz lights are combined while maintaining the band. These
two beams are also combined with NIR pulsed light (8 fs) using
an ITO-coated quartz beam splitter. This technology has createda new system that enables the analysis of phase-sensitive
phenomena in the NIR to far-infrared region. Since the pulse
width of the MIR light is sufﬁciently smaller than a quarter of
the THz pulse width, the combination of both electric ﬁelds
provides a new method of measuring the dynamics of quantum
processes, such as electric-ﬁeld-driven quasiparticle phenomena, while controlling them.33) Moreover, since both thereal and the imaginary parts of permittivity can be found, andthereby, the complex refractive index in the wideband MIR
region can be obtained, all the information produced by the
optical measurement in this region can be probed with thissystem. The simpliﬁed handling system with the coaxial beam
is expected to have a wide range of practical applications
because it is relatively easy to combine the optical system withultralow-temperature and ultrahigh-vacuum technologies.
Acknowledgments We acknowledge the ﬁnancial support of a Grant-inAid for Scientiﬁc Research (17H06088, 20H00341) from Japan Society for the
Promotion of Science.

1) T. Kampfrath, K. Tanaka, and K. A. Nelson, Nat. Photon. 7, 680 (2013).
2) D. N. Basov, R. D. Averitt, D. Marel, M. Dressel, and K. Haule, Rev. Mod.
Phys. 83, 471 (2011).
3) O. Schubert et al., Nat. Photon. 8, 119 (2014).
4) R. Matsunaga, N. Tsuji, H. Fujita, A. Sugioka, K. Makise, Y. Uzawa,
H. Terai, Z. Wang, H. Aoki, and R. Shimano, Science 345, 1145 (2014).

092006-4© 2022 The Author(s). Published on behalf of
The Japan Society of Applied Physics by IOP Publishing Ltd

Appl. Phys. Express 15, 092006 (2022)

Y. Arashida et al.

5) H. Hirori, K. Shinokita, M. Shirai, S. Tani, Y. Kadoya, and K. Tanaka, Nat.
Commun. 2, 594 (2011).
6) T. Oka and S. Kitamura, Annu. Rev. Condens. Matter Phys. 10, 387 (2019).
7) C. Giannetti, M. Capone, D. Fausti, M. Fabrizio, and D. Mihailovic, Adv.
Phys. 65, 58 (2016).
8) D. Fausti, R. I. Tobey, N. Dean, S. Kaiser, A. Dienst, M. C. Hoffmann,
S. Pyon, T. Takayama, H. Takagi, and A. Cavalleri, Science 331, 189(2011).
9) G. Günter et al., Nature 458, 178 (2009).
10) R. Mankowsky, A. von Hoegen, M. Först, and A. Cavalleri, Phys. Rev. Lett.
118, 1 (2017).
11) C. Vicario, C. Ruchert, F. Ardana-Lamas, P. M. Derlet, B. Tudu, J. Luning,
and C. P. Hauri, Nat. Photon. 7, 720 (2013).
12) B. Zaks, R. B. Liu, and M. S. Sherwin, Nature 483, 580 (2012).
13) S. R. Tauchert et al., Nature 602, 73 (2022).
14) P. Gaal, W. Kuehn, K. Reimann, M. Woerner, T. Elsaesser, and R. Hey,Nature 450, 1210 (2007).
15) Y. H. Wang, H. Steinberg, P. Jarillo-Herrero, and N. Gedik, Sience 342, 453(2013).
16) J. W. McIver, B. Schulte, F. U. Stein, T. Matsuyama, G. Jotzu, G. Meier,and A. Cavalleri, Nat. Phys. 16, 38 (2020).
17) T. Rohwer et al., Nature 471, 490 (2011).
18) H. Hirori, A. Doi, F. Blanchard, and K. Tanaka, Appl. Phys. Lett. 98,
091106 (2011).
19) C. Kübler, R. Huber, and A. Leitenstorfer, Semicond. Sci. Technol. 20, S128
(2005).
20) A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss, and W. Knox, Appl.
Phys. Lett. 74, 1516 (1999).
21) Z. Chi, H. Chen, Q. Zhao, and Y. X. Weng, J. Chem. Phys. 151, 114704(2019).

22) J. Reimann et al., Nature 562, 396 (2018).
23) G. T. Noe et al., Opt. Express 24, 30328 (2016).
24) C. Kealhofer, W. Schneider, D. Ehberger, A. Ryabov, F. Krausz, and
P. Baum, Science 352, 429 (2016).
25) G. Herink, D. R. Solli, M. Gulde, and C. Ropers, Nature 483, 190 (2012).
26) M. Hada, D. Yamaguchi, T. Ishikawa, T. Sawa, K. Tsuruta, K. Ishikawa,
S. Koshihara, Y. Hayashi, and T. Kato, Nat. Commun. 10, 4159
(2019).
27) J. G. Horstmann, H. Böckmann, B. Wit, F. Kurtz, G. Storeck, and C. Ropers,
Nature 583, 232 (2020).
28) M. Eisele, T. L. Cocker, M. A. Huber, M. Plankl, L. Viti, D. Ercolani,
L. Sorba, M. S. Vitiello, and R. Huber, Nat. Photon. 8, 841 (2014).
29) S. Yoshida, Y. Arashida, H. Hirori, T. Tachizaki, A. Taninaka, H. Ueno,
O. Takeuchi, and H. Shigekawa, ACS Photon. 8, 315 (2021).
30) K. Yoshioka, I. Igarashi, S. Yoshiada, Y. Arashida, I. Katayama, J. Takeda,
and H. Shigekawa, Opt. Lett. 44, 5350 (2019).
31) S. C. Shen, C. J. Fang, M. Cardona, and L. Genzel, Phys. Rev. B 22, 2913
(1980).
32) C. Gaida et al., Light: Sci. Appl. 7, 94 (2018).
33) K. R. Allakhverdiev, M. Ö. Yetis, S. Özbek, T. K. Baykara, andE. Y. Salaev, Laser Phys. 19, 1092 (2009).
34) J. Mei, K. Zhong, M. Wang, Y. Liu, D. Xu, W. Shi, Y. Wang, J. Yao,
R. A. Norwood, and N. Peyghambarian, Opt. Express 24, 23368 (2016).
35) R. Mankowsky, M. Först, and A. Cavalleri, Reports Prog. Phys. 79, 64503(2016).
36) T. L. Cocker, V. Jelic, M. Gupta, S. J. Molesky, J. A. J. Burgess, G. De Los
Reyes, L. V. Titova, Y. Y. Tsui, M. R. Freeman, and F. A. Hegmann, Nat.
Photon. 7, 620 (2013).
37) T. Rybka, M. Ludwig, M. F. Schmalz, V. Knittel, D. Brida, andA. Leitenstorfer, Nat. Photon. 10, 667 (2016).

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