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Compact THz-radiation source consisting of a bulk
semiconductor, a mode-locked fiber laser, and a 2 T permanent
magnet
Ono, Shingo; Tsukamoto, Takeyo; Sakai, Masahiro; Liu,
Zhenlin; Otake, Hideyuki; Sarukura, Nobuhiko; Nishizawa,
Seizi; Nakanishi, Akio; Yoshida, Makoto
REVIEW OF SCIENTIFIC INSTRUMENTS, 71(2): 554-556
2000-02
http://repo.lib.nitech.ac.jp/handle/123456789/4377
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REVIEW OF SCIENTIFIC INSTRUMENTS
VOLUME 71, NUMBER 2
FEBRUARY 2000
Compact THz-radiation source consisting of a bulk semiconductor,
a mode-locked fiber laser, and a 2 T permanent magnet
Shingo Onoa) and Takeyo Tsukamoto
Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan
Masahiro Sakai, Zhenlin Liu, Hideyuki Ohtake, and Nobuhiko Sarukura
Institute for Molecular Science (IMS), Myodaiji, Okazaki 444-8585, Japan
Seizi Nishizawa
Jasco Corporation, Ishikawa, Hachioji, Tokyo 192-8537, Japan
Akio Nakanishi
Sumitomo Special Metals Co., Ltd, Egawa, Shimamoto-cho, Mishima-gun, Osaka 618, Japan
Makoto Yoshida
Imra America, Inc. JLO, Kojiritsuki, Hitotsugi-cho, Kariya 448, Japan
共Received 6 July 1999; accepted for publication 20 October 1999兲
A compact THz-radiation source using an InAs semiconductor is demonstrated with a turn-key
femtosecond fiber laser as an excitation source and a newly designed 2 T permanent magnet. By
using a newly designed magnetic circuit, a compact permanent magnet which can exceed the
remanence magnetic field was obtained. This compact source is sufficiently intense for absorption
spectroscopy. © 2000 American Institute of Physics. 关S0034-6748共00兲02302-9兴
The 2 T permanent magnet unit consisted of eight Nd–Fe–B
magnet pieces. Each of them was magnetized in different
ways as shown in Fig. 2. The remanence magnetic field of
the Nd–Fe–B material itself was 1.3 T 共NEOMAX-44H兲.
Owing to the new magnetic circuit design,9 the magnetic
field in the center exceeded the remanence magnetic field of
the material. The permanent magnet only weighs about 5 kg.
The 2 T permanent magnet unit is cylindrical in shape, 128
mm diameter and 56 mm thick, which makes it smaller and
much lighter than an electromagnet. In the far field, the vector sum of magnetic moments is close to zero. Therefore, the
magnet has a very small leak magnetic field owing to the
new magnetic circuit. This is an advantage of this radiation
source for further system integration.
The experimental setups are shown in Figs. 3共a兲 and
3共b兲. The THz radiation is generated from an InAs emitter
irradiated with femtosecond pulses in a magnetic field. The
magnetic field direction can be reversed by rotating the magnet. In Figs. 3共a兲 and 3共b兲, the magnetic field was parallel to
the sample surface, and the excitation laser was vertical to
To date, various THz-radiation sources have been intensively studied including photoconductive switches irradiated
with ultrashort optical pulses.1–3 The average power of THzradiation source excited by short optical pulses has been microwatt level at most, mostly limited by the electric breakdown of the photoconductive antennas. An intense, compact,
and simple light source is required for applications in sensing
or imaging. Zhang et al. reported a compact THz-radiation
source consisting of a biased photo conductive antenna and a
mode-locked fiber laser as an excitation source.4 They have
also reported the quadratic dependence of the laser induced
THz radiation on the magnetic field.5 We have also demonstrated the strong enhancement of THz-radiation power with
a magnetic field by using an InAs semiconductor instead of
GaAs.6 This remarkable enhancement was also confirmed by
many groups, such as Zhang et al.7 and Beigang et al.8 The
advantage of the magnetic-field-enhanced scheme is flexibility of the geometry. In this article, we report on a compact
THz-radiation source consisting of a fiber femtosecond laser
and a newly designed 2 T permanent magnet.
The photograph of the compact THz-radiation source is
shown in Fig. 1. A mode-locked frequency-doubled Erdoped fiber laser delivered 170 fs pulses at 780 nm with a
48.5 MHz repetition rate 共IMRA model FA7850/10SA兲 with
30 mW average power and 4.1 kW peak power. The modelocked fiber laser is a completely turn-key system. It is much
smaller than a mode-locked Ti:sapphire laser that requires
daily alignment. The used semiconductor sample was undoped bulk InAs with a 共100兲 surface. The InAs sample itself
is highly reflective, unlike transparent GaAs, and the THz
radiation was totally generated in the reflection direction.
FIG. 1. Photograph of a compact THz-radiation source with a bulk semiconductor, a fiber femtosecond laser, and a 2 T permanent magnet. Including the laser, the size is less than 40⫻30⫻15 cm.
a兲
Electronic mail: shingo@ims.ac.jp
0034-6748/2000/71(2)/554/3/$17.00
554
© 2000 American Institute of Physics
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Rev. Sci. Instrum., Vol. 71, No. 2, February 2000
THz-radiation source
555
FIG. 4. Comparison of intensity of THz radiation in four geometrical layouts.
FIG. 2. Experimental setup of the compact THz-radiation source. This illustration indicates the geometrical layout with an InAs sample placed 45°
to the excitation laser.
the magnetic field. The InAs sample was placed 45° and 0°
to the excitation laser in Figs. 3共a兲 and 3共b兲, respectively.
The excitation laser irradiated the sample with a 2-mm-diam
spot. The intensity of the THz radiation obtained by a liquidhelium-cooled InSb bolometer 共QMC model QFI/2BI兲 is
shown in Fig. 4. The highest power was obtained by the
setup shown in Fig. 3共a兲. The significant difference of the
radiation power by inverting the magnetic field is partly attributed to asymmetric enhancement of THz radiation for
magnetic direction that was observed in high field case.10 At
present the intensity is about a thousand times smaller than
the intensity which is generated by using a 1.5 W femtosecond mode-locked Ti:sapphire laser and a huge 1.7 T
electromagnet.6 Believing the calibration of manufacturer,
the average power is estimated to submicrowatt level. The
spectra of the THz radiation were obtained by a Polarizing
Michelson interferometer with the bolometer as shown in
Fig. 5. Many water vapor absorption lines were clearly observed. Therefore, the THz-radiation source is already usable
for spectroscopy. The center frequency of the radiation was
shifted by inverting of the magnetic field direction due to the
FIG. 3. Geometrical layout. The InAs sample surface was parallel to the
magnetic field, and the excitation laser was vertical to the magnetic field. 共a兲
The InAs sample was placed 45° to the excitation laser. 共b兲 The InAs sample
was placed 0° to the excitation laser 共Fig. 2兲.
difference of the electron acceleration direction.10
In conclusion, a compact THz-radiation source using an
InAs semiconductor was demonstrated with a genuine turnkey fiber femtosecond laser as an excitation source and a
newly designed 2 T permanent magnet. Such a simple and
compact source will open up new application for THz radiation. After further optimization, a higher power can easily be
obtained because of the quadratic magnetic field and excitation power dependence of THz-radiation intensity.6 In the
near future, a 3 T magnetic field will be realized by having a
smaller center gap in the magnet. A 3 T magnetic field is
sufficient for the high power THz-radiation source because
of the saturation of THz-radiation power around a 3 T magnetic field.10 The power of mode-locked fiber lasers will be
dramatically increased. Therefore, the compact THzradiation source will be much improved.
FIG. 5. Spectrum of THz radiation from the compact source for different
geometry.
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556
This work is partially supported by the Sasakawa Scientific Grant, the Sumitomo Foundation, Research Foundation
for Opto-science and Technology, and the Grant-In-Aid for
Science Research from the Japanese Ministry of Education,
Science and Culture.
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