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I NVITED P RESENTATION

SPring-8 as an IR-light source
T. Nanba1 , S. Kimura1,3 , H. Okamura2 , M. Sakurai2 , M. Matsunami1 , H. Kimura3 ,T. Moriwaki4 , Y. Ikemoto4 , T. Hirono4 , T. Takahashi5 , K. Shinoda6 , K. Fukui7 ,M. Teragami7 and Y. Kondo8
1 Graduate School of Science and Technology, Kobe University, Kobe 657-8501, Japan
2 Faculty of science, Kobe University, Kobe 657-8501, Japan

3 PRESTO, Japan Science and Technology Corporation, Japan
4 SPring-8/JASRI, Mikazuki, Hyogo 679-5198, Japan5 Research Reactor Institute, Kyoto University, Osaka 590-0494, Japan

6 Department of geosciences, Osaka City University, Osaka 558-8585, Japan
7 Faculty of Engineering , Fukui University , Fukui 910-8705, Japan

8 Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan

Abstract. SPring-8 at Harima in Japan is the third generation x-ray light source and a largest
storage ring for synchrotron radiation (SR) in the world. The construction of infrared (IR) beamline,
BL43IR, at SPring-8 has been completed in the fiscal year of 2000 for IR spectroscopic study andhas been utilized by many scientists. The beamline covers a very wide wavelength region from 500
nm (20,000 cm−1 , 2.5eV) to 0.1mm (100cm−1 , 12.5 meV) and offers very high brilliant IR photon
beam to the four experimental end stations. At BL43IR, a newly designed optical system including
so-called “magic mirror” are adopted in order to extract of an intense IR light from SPring-8. The
design of the optical system and the performance of SPring-8 as IR light sources are reported as wellas the recently obtained results

Introduction
Up to now, synchrotron radiation (SR) has been recognized as a powerful light source becauseof its many excellent properties such as high brilliance, a small divergence angle of the lightbeam in the direction perpendicular to the plane of the ring, a continuous spectral distributionof the intensity over the entire energy range in addition to a well defined pulsed structure, andso on. These prominent characteristics of SR have been proved to be held also in the infrared(IR)-far infrared (FIR) wavelength regions by several groups at different places [1-8]. Particularly, infrared synchrotron (IRSR) is naturally a polarized light beam with a quite different
polarization features compared to black body radiation sources, as its electric vector contains
mainly the component parallel to the plane of the ring and a small perpendicular component outof the plane. So we can get an almost linear polarization in the plane close to the ψ=0 plane ofthe ring. Alternately, one can obtain an IRSR light beam with a circular polarization without
any polarizer by extracting the radiation out of the plane of the ring. Such circularly polarized SR has been already performed in the measurement of a magnetic circular dichroism onmagnetic materials in the X ray to VUV region. Kimura has developed such circular dichroism
experiments in the infrared by IRSR [9].
cLNCMP
2003, ISBN 2-87649-035-8–

www.lncmp.org/fichiers/IRMMW 2001.pdf, www.irmmw.org/fichiers/IRMMW 2001.pdf

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Figure 1. Computed spectral distribution
curves of SR , (a) as a function of different bending radii , and (b) computed verticalangle distribution curves of photon numbers
at 100 µm as a function of different bending radii with the same acceleration energy(E) of 8 GeV. Solid Curves correspond to aparallel component and dotted curves to aperpendicular one

Therefore, SR can make possible experiments, for example, on IR spectroscopy under extreme experimental conditions such as high magnetic field and high pressure as well as IR
microscopic analysis on very small domains in synthetic specimens, etc.
SPring-8 at Harima in Japan can supply with higher brilliant IR photon beam to the experimental end stations than other small rings in the world. The utilization of the high brilliant
IR photon beam is the aim of the construction of the BL43IR at SPring-8. The BL43IR wasdesigned and constructed in FY1999 and dedicated for materials research from FY2000. Since
the storage ring is largest in the world, a so-called magic mirror was newly designed and installed as a new concept of a collection mirror of the light emitted from the long emission arc
length of the bending magnet [10]. In this paper, the design of the optical system and the recent
experimental results are reported.
Why SPring-8 as IR light source?
Fig.1(a) shows computed spectral distribution curves as a function of different bending radii (ρ)
with the same acceleration energy (E) of 8 GeV. Fig.1(b) shows the computed beam spread of
the IRSR with the wavelength of 100 µm emitted into an angle (ψ) perpendicular to the ringwith a unit horizontal angle (θ=1 mrad) as function of ρ with E= 8 GeV. These two figures
clearly show that the ring with a larger radius of the electron orbit can supply with more intense
IR photon beam to the end station. The brilliance of the IRSR goes up roughly with the factor ofρ1/2 . SPring-8 possesses the largest radius of the electron beam orbit of 39.8 meters among thestorage ring facilities in the world and this is the reason why we are concentrated on SPring-8.
Performance of BL43IR as an IR light source
The BL optics from bending magnet to interferometer is schematically drawn in Fig.2. The BL

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IRMMW 2001 – Special techniques

Figure 2. Schematic drawing of opticalsystem of infrared beamline, BL43IR, ofSPring-8. M1 represents a magic mirror.
W indicates optical window which separates ultrahigh-vacuum of upstream andlow-vacuum of downstream

was designed to use IR synchrotron radiation (IRSR) from a bending magnet with the acceptance angle of 36.5(horizontal) × 12.5(vertical) mrad2 . Since the radius of the electron beam
orbit is 39.3 m at the bending magnet and the horizontal acceptance angle is extremely large,
the emission length amounts to 1.43 m. To focus the light from the long emission length, weemployed a perfect focusing mirror to the circular orbit, so-called ”magic mirror” initially proposed by Lopez-Delgado and Szwarc [11]. However, they gave only the formation on emission
plane. Then we expanded the formation also to the vertical direction by assuming a sphericalcurve. Such a magic mirror was installed as M1 mirror in the figure.
The result of the ray trace at the focusing point is shown in Fig.3 (a). In order to investigate
the performance of the newly designed magic mirror, the two-dimensional cross-sectional sizeof IRSR was measured at the focusing point. The IRSR between 1,000-9,000 cm−1 was detectedby a HgCdTe detector by moving the detector stage against the incident photon beam. Fig.3 (b)shows the observed spot size, where “horizontal” means the component parallel to the ring. Theroot-mean-square spread of the Gaussian line shape (σx,y ) fitted to the horizontal and verticalcomponents are respectively 0.63 and 0.37 mm. The horizontal size is in good agreement withthe calculation of the ray trace, while the vertical one is an order of magnitude larger than thecalculation. The reason of discrepancy is considered to be the lack of surface accuracy of themagic mirror. The time structure at the focusing point was also observed and was recognized
to be almost equal to the longitudinal electron bunch length (σ ∼ 100 psec). The spectral
intensity distribution curve in 20,000 -100 cm−1 region was also measured and the almost halfof the theoretically expected photon flux was found to be observed at the sample position in theinterferometer besides the FIR region where the diffraction loss in intensity occurs [10].
Fig.4 shows the drawing of the measured spatial two-dimensional intensity distribution at
the focal position of the microscope with Schwartzshild mirrors (x8, NA=0.5) . The IR-beamwith FWHM of almost 15 (x) and 10 µm(y) was observed.
Experimental
At BL43IR, four kinds of experimental end stations has been installed at the downward of theinterferometer in order to cover interesting spectroscopic studies in the various kinds of fields
from physics, chemistry, earth science to a life science and so on. They are the stations for(1) microscope [12], (2) absorption-reflection experiment [13], (3) surface science [14], and (4)IRMMW 2001 – Special techniques

Intensity (Arb.Units)9-1332130-3000(a)X (µm)

30 -30Y (µm)

Figure 4. (top) Measurement of Integrated intensity profile of the microscope in the IR region between 1,000-9,000 cm−1 through 2µm pinhole on
the sample position with no other aperture(b)

Figure 3. (left) Ray trace result (a) and measured
two-dimensional beam size (b) at the focusing
point in Fig. 2

magneto-optical experiment with microscope. The IR photon beam can be guided to eachend station by switching a mirror. In this session, we show, as a typical example, the experimental result which was obtained at the microscope station. Fig.5 shows the spectral change
in the IR transmission spectra of (001) platelet single crystal of natural brucite using a diamond anvil cell (DAC) under the conditions at high temperature and high pressure. Brucite
is a prototype of hydrous magnesium silicate minerals and the behavior of hydroxyls (OH-)
under high pressure and high temperature is an interesting subject from stand point of earthscience. The crystal structure is a type of CdI2 one and simply shown chemically by (Mg(OH)2 ). The observed main peak at 3700 cm−1 in the absorption spectrum is due to the stretching motion of the dipole moment of OH− in a lower pressure phase. A new peak grows at3650 cm −1 with pressure in addition to the main peak in the lower pressure phase. This newpeak is assigned to be the absorption due to the induced dipole moment in the high pressure
phase by the partial rearrangement of the dipole OH− by the transfer of proton of the original
dipole to neighboring oxygen atoms [15]. These results suggest the existence of two kinds ofdipole moments, original dipole and pressure-induced dipole one in the high-pressure phase.

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Figure 5. IR absorption spectra of (001)platelet single crystal of natural brucite
Mg(OH)2 from Zimbabwe up to 16.5
GPa and 360 ◦ CConclusions
At SPring-8, the beamline (BL43IR) for the utilization of brilliant IRSR has been constructed
and experiments at BL43IR has proved SPring-8 to be a very powerful and useful source as IRand FIR-light source. SPring-8 will provide us with new experimental fields.
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