Infrared and Terahertz Synchrotron Radiation:
Optics and Applications
Shin-ichi Kimura
UVSOR Facility, Institute for Molecular Science, and School of Physical Sciences,
The Graduate University for Advanced Studies, Okazaki 444-8585, Japankimura@ims.ac.jp

1 Introduction
One method for creating intense as well as bright terahertz (THz) and infrared (IR) light is by using radiation from relativistically accelerated electrons, such as synchrotron radiation (SR) and free-electron laser (FEL). SR
is the radiation emitted from bending-magnets and insertion devices (undulators/wigglers). Because SR consists of broadband electromagnetic waves fromthe X-ray to the THz range, light with the desired wavelength and intensity(photon ﬂux) can be obtained by using suitable optics. SR is a good source forspectroscopy, and over 20 SR facilities worldwide are equipped with IR/THz
beamlines. In contrast, FEL is monochromatic light with high coherence andrelatively high intensity. These properties of FEL make it worthwhile as an
excitation source and as monochromatic probing light.
The size of the SR light source is determined by the size of the electron
beam in the accelerator. The electron beam size is similar to the wavelengthof THz light (i.e. sub millimeter size). Because the emission angle of SR issmall, the electron beam emittance (i.e. the area occupied by the particlesof the beam in space and momentum phase space) is smaller than that of
the THz light. The SR light can therefore be focused down to spot size, with
an ideal diﬀraction limit. This property makes the light suitable for microscopic spectroscopy (micro-spectroscopy). In fact, the purpose of almost all of
the world’s IR/THz beamlines is micro-spectroscopy and micro-imaging withhigher spatial resolution than with conventional IR microscope.
In the case of SR light sources, the emission point is located in a vacuum
chamber with a bending magnet. Since the vertical space available for collecting the SR is narrow, all of the emitted light cannot be collected. If wehave a very wide horizontal acceptance angle, the emission point becomes anemission arc. Therefore, laboratory optics cannot be utilized with IR/THz SR.
In addition, the emission point in an electron accelerator is in an ultra-highvacuum (∼ 10−8 Pa), whereas THz light needs a pressure of less than 1 Pa for
excluding the absorption of water vapor. To separate the ultra-high vacuum of

2Shin-ichi Kimura

the accelerator and the low vacuum of the downstream optics, optical windows
are used. Because SR light is more broadband than that from conventional
THz/IR light sources, the use of optical windows restricts the wavelength
regions available. To optimize the use of the wide wavelength region of SR,
exchange of optical windows without breaking the vacuum is eﬀective.
In Japan, there are currently three IR/THz beamlines. The ﬁrst IR/THz
beamline, BL6A1, has been constructed at UVSOR in 1986. [1] In 2003,
UVSOR was upgraded to the lowest emittance ring (27 nm rad) among smallSRs, with an acceleration energy below 1 GeV and its name was changed
to UVSOR-II. [2] Simultaneously, BL6A1 was upgraded to the new IR/THz
beamline, BL6B, with high brilliance and high ﬂux [3]. To obtain higher performance, we employed a so-called “magic mirror” with a large acceptanceangle (215(H) × 80(V) mrad2 ) [4]. Upgrading the beamline made the photon ﬂux of BL6B four times larger, and the brilliance 102 times larger, thanthose of BL6A1. In 1999, another IR beamline was constructed at SPring-8(in Hyogo Prefecture, Japan) [5], which is the largest SR ring in the world.
The beamline was equipped with a three-dimensional magic mirror for theﬁrst time. At SPring-8, spectroscopy at high temperature and high pressure
are used to investigate conditions inside the Earth. Spectroscopy of correlatedmaterials at low temperature and high pressure is also performed. In 2009, a
new IR micro-spectroscopy beamline was constructed at the small SR ring,
AURORA, at Ritsumeikan University. [6] The beamline will be dedicated toindustrial use.
To perform IR/THz spectroscopy using SR, special optics that diﬀer fromthose used in laboratory systems are needed. In this paper, the IR/THz optics
developed for use with SR are introduced, as in an example of their application
using IR/THz SR.

2 Principle of three-dimensional magic mirror optics for
THz/IR SR
SR is characterized by the emission angle of light. When a relativistic electron
beam moves in an external ﬁeld, the radiation spectrum (∂ 2 P (ω)/∂ω∂Ω) froma minute orbital is [7]( ωρ )2∂ 2 P (ω)
e2=∂ω∂Ω
12π 3 ε0 c c

(

1
+ θ2
γ

)
2
× {K2/3(ξ) +γ 2 θ2K 2 (ξ)} (1)1 + γ 2 θ2 1/3Here, e is the elemental charge, ε0 the dielectric constant of the vacuum, cthe velocity of light, ρ the orbital radius, and γ the ratio of energy to rest mass.
K2/3 (ξ) and K1/3 (ξ) are modiﬁed Bessel functions, and ξ = ωR/3c(1/γ 2 +θ2 )3/2 , where R is the radius of the electron orbit. The function corresponds tothe vertical distribution of SR in the orbital plane. For instance, the vertical

IR/THz Synchrotron Radiation

3ｚ

Fig. 1. Vertical distribution of bending-magnet radiation of the UVSOR-II storage
ring at photon energies of 0.1 and 100 eV.

emission of the bending-magnet radiation at UVSOR-II is shown in Fig. 1. In
the ﬁgure, the emission at a photon energy of 100 eV is located around theorbital plane (θz = 0 mrad), but the emission of the 0.1-eV photon energy
expands above 10 mrad from the orbital plane. The intensity at 0.1 eV ismuch lower than that at 100 eV. These properties imply that the verticallywide acceptance angle at 0.1 eV is needed for high intensity.
On the horizontal axis, on the other hand, the emission from a minuteemission angle is the same as in the above function, but the circular orbit ofelectron beams made by the Lorentz force due to the presence of the dipole
magnet is important. This indicates that SR from the dipole magnet is not an
ideal point source, but is instead an arc source. The light cannot be focused toa point by using circular/toroidal mirrors. Thus these mirrors cannot maintainhigh brilliance.
The “magic mirror” for bending-magnet radiation focused to one point was
discovered by Lopez-Delgado and Szwarc in 1976. [8] In this paper, the shape
was deﬁned only in the orbital plane. In reality, however, SR is emitted not
only in the orbital plane but also to the horizontal axis. We therefore extended
the shape of the magic mirror to a three-dimensional one, creating a “threedimensional magic mirror (3D-MM),” and we installed this mirror in the IR
beamline (BL43IR) at SPring-8. [9] SPring-8 is the largest SR ring in theworld with an orbital radius of about 40 m. The acceptance angle of 36 mrad
corresponds to the emission length of 1.44 m (Fig. 2a). If a toroidal mirror isadded to collect the light, then the focal image, calculated by a ray tracing
method, is not good (Fig. 3a). This result indicates that the high brillianceof SR cannot be preserved by using a toroidal mirror. We therefore employed
the 3D-MM. The following calculations are used to form the 3D-MM:

4

Shin-ichi Kimura

Fig. 2. Schematic ﬁgure of the optics using a three-dimensional magic mirror (3DMM) at BL43IR of SPring-8 (a) and the curvature of the 3D-MM surface (b).

1{(d0 − ρθ)2 − ρ2 a2 } + aρ sin θAM (θ) = 2(2)
d0 − ρθ − a cos θ√
M I(θ) = ρ2 + AM (θ)2 + a2 − 2a(ρ sin θ + AM (θ) cos θ)(3)()
2AM )(θ)(d0 − ρθ − AM (θ)
1
AM (θ) − a cos θR(θ) =
× cos{ cos−1} (4)d0 − ρθ2
M I(θ)(θ) sin θ)(|R(θ)|{sin θ − ρ cos θ−AMν
M I(θ)√}x(θ, ν) = ρ cos θ−AM (θ) sin θ+{1−cos tan−1
R(θ)
2(AM (θ)−a cos θ)2+
M I(θ)(5)

IR/THz Synchrotron Radiation

5

Fig. 3. Ray tracing images at the ﬁrst focus using a toroidal mirror (a) and a threedimensional magic mirror (b) at BL43IR of SPring-8. (c) Time structure at the focal
point, using the three-dimensional magic mirror.

y(θ, ν) = ρ sin θ+AM (θ) cos θ+(θ) cos θ()|R(θ)|{− sin θ + a−ρ sin θ−AMν
M I(θ)√{1−cos tan−1}
R(θ)
2(AM (θ)−a cos θ)2+
M I(θ)(z(θ, ν) = R(θ) sin tan−1

ν
R(θ)

)

(6)
(7)

Here, ρ is the orbital radius of electron beams, a the distance between thecenter of the electron beam and the focal position, d0 the optical length fromthe standard position, θ the acceptance angle of SR, and ν the vertical sizeof the magic mirror. For instance, the parameters for BL43IR at SPring-8 are
as follows; ρ = 39271.8mm, a = 46025.4mm, d0 = 69807.4mm, θ =1.4804 to
1.5169 rad, ν =-22 to +22 mm. The meanings of AM , M I, and R are thesame as in ref. [8]. The vertical radius of the magic mirror is obtained for an
approximation of a spherical shape by using the distance from the emission
point to the focal point.
The shape of a magic mirror surface with the above parameters is shownin Fig. 2b. The surface is very complex because of the non-spherical and non-6Shin-ichi Kimura

Fig. 4. Overview of the UVSOR-II storage ring.

symmetric shape. To fabricate the surface, a recently developed numericalcutting method is used.
The focal image calculated by using the ray tracing method is shown in
Fig. 3b. The focal size is much smaller than that with the toroidal mirror (seeFig. 3a). The magic mirror cannot only focus SR from the circular electron
orbital; it can also reduce the time structure of the emission length to thelongitudinal width of the electron bunch (∼ 100 ps), because the magic mirror
is characterized by a constant distance from the emission point to the focal
point including the trajectory of the electron bunch. The calculated time
structure is shown in Fig. 3c. In the calculation, the beam length was assumed
to be 15 ps. The time structure at the focal point was evaluated to be 59 ps
about 4 times longer than the electron beam length.

3 IR/THz beamline optics at UVSOR-II
The 3D-MM was installed in the THz/IR beamline BL6B at UVSOR-II(Fig. 4) in 2004. [3] UVSOR-II is a relatively small SR ring with a circumference of about 53 m and an electron beam bending radius is 2.2 m. If a 3D-MM
is introduced into such a small SR ring, a large acceptance angle can be employed. For instance, at UVSOR-II, the acceptance angle became 215 mrad in
the horizontal plane and 80 mrad in the vertical plane. The combination of alarge acceptance angle and the shape of the 3D-MM gives high brilliance.
The layout and optics of the beam extraction part (front-end) are shown in
Figs. 5a and b, respectively. The ﬁrst mirror in the bending-magnet chamber isIR/THz Synchrotron Radiation

7

Fig. 5. Top view (a) and 3D view (b) of the front-end optics of BL6B, UVSOR-II.

a magic mirror with dimensions of 300 mm (horizontal) by 100 mm (vertical).
A copper pipe (diameter 5 mm) for water cooling is located in the orbital
plane in front of the magic mirror to reduce heat load due to the increases
photon energy of the SR.
The THz/IR SR extracted by the magic mirror is derived to the ﬁrst
focusing point (P1) by two plane mirrors (M1 and M2). The position and angleof the light can be controlled by M1 and M2 (Fig. 5b). The light emittance of
the THz/IR beamline is the product of the light size and the emission angle at
P1. The beam size at P1 calculated by the ray-racing method, and the actual
beam size obtained, are shown in Figs. 6a and b. The beam in the experimental
result is spherical and about 1.2 mm in diameter, in contrast with the ellipticalshape of 1×2 mm2 in the calculation. This is because the calculation was done
at a photon energy of 0.1 eV, whereas the experimental result was obtained ata photon energy range of 0.05 to 2 eV by using an HgCdTe (MCT) detector.
The result indicates that the SR light can be ideally focused by using the
3D-MM.

8

Shin-ichi Kimura

Fig. 6. Ray trace result at the ﬁrst focal point at a photon energy of 0.1 eV (a)and beam proﬁle measured by using an MCT detector (b) at the ﬁrst focusing point(P1) in Fig. 5.

The distance from the magic mirror to P1 is 2.5 m and the horizontal
solid angle from P1 to the magic mirror is about 86 mrad. By using theexperimental beam size at P1, the horizontal emittance at a photon energy of
0.1 eV (σx · σx′ ) becomes 52 µm·rad. In the vertical plane, on the other hand,the emission angle (σy′ ∼12 mrad) shown in Fig. 1 at 0.1 eV and the lengths
between the center of the emission arc and the magic mirror (about 1 m) andbetween the magic mirror and P1 (about 2.5 m) make the emittance (σy · σy′ )′5.4 µm·rad. The emittance of a conventional IR spectrometer is σx,y ·σx,y=200

IR/THz Synchrotron Radiation

9

Fig. 7. Calculated (dashed line) and experimentally obtained (solid line) brillianceof BL6B, UVSOR-II compared with those of a thermal (black body) light source at
T = 1400 K (dash-dotted line).

to 1000 µm·rad. This implies that SR light has a much lower emittance thanconventional laboratory sources.
This low emittance gives high brilliance. In fact, in terms of black bodyemission at 1400 K, the brilliance of BL6B at UVSOR-II is several orders ofmagnitude greater than that of a conventional source (Fig. 7). The diﬀerencein the THz photon energy region below 50 meV is much larger than thatin the IR region above 50 meV. This indicates that the light is suitable formicro-spectroscopy in the THz region.

4 IR/THz beamline optics at UVSOR-II
The BL6B IR/THz of UVSOR-II covers the range of 0.5 meV to 2 eV (4 to
16 000 cm−1 ) by using two Fourier transform IR interferometers (FTIRs).
One is a Martin-Puplett-type (MP) interferometer (FARIS-1, Jasco Inc.) thatcovers a photon energy range of 0.5 to 10 meV. The other is Michelson-type
interferometer (MI) (Vertex 70v, Bruker Inc.) covering 5 meV to 2 eV andwith four beam splitters (Mylar 23 µm, Mylar 6 µm with multilayer coating,10Shin-ichi Kimura

Fig. 8. Layout of the beam transport of BL6B, UVSOR-II.

Ge/KBr, and quartz). The beamline layout is shown in Fig. 8. The IR/THzSR focused by the 3D-MM is formed into a parallel beam by a collimator(M4) and is guided to the MP. When the parabolic mirror M3 is placed in the
optical path, the IR/THz SR is guided to the MI. The IR/THz SR passing
through the MP is guided to the reﬂection/absorption end-station (RAS) by
a plane mirror (M8) in the switchyard #1. The light after the MI is guided
to the micro-spectroscopy end-station (MSS) and the RAS by a plane mirror(M6) in the switchyard #2.
4.1 THz/IR reﬂection/absorption spectroscopy using SR
Figure 9 shows the optics of the RAS. Using the optics, both a reﬂection
spectroscopy and transmission spectroscopy are available. The two experiments can be changed by adding a parabolic mirror MRA 3 without breaking
the vacuum. Samples and all optical components are located in a high vacuum chamber (≤ 10−6 Pa) evacuated by a turbo molecular pump (300 liter/s).
There is no optical window in front of the samples. This has the advantagesof no limitation of the photon energy and no interference by optical windows.
An evaporator for gold ﬁlm on the sample surface is added for measurement of
absolute values. An automatic measurement system is also added to measure
the relative intensity between the sample and a reference mirror at anotherIR/THz Synchrotron Radiation

11

Fig. 9. Top-view of the optical path of the reﬂection/absorption end-station (RAS).
The reﬂection/transmission experimental setup can be changed by moving theparabolic mirror MRA 3.

sample space. [10] The spectral distribution of IR/THz SR using RAS in thewavenumber region below 1000 cm−1 is shown in Fig. 10. The spectrum iscovered down to ∼4 cm−1 (∼0.13 THz) by using three kinds of combinationsof light source (UVSOR-II), FTIR (FARIS-1 or Vertex 70v), and detector(InSb hot-electron bolometer or Si bolometer). The typical intensity in thisregion is about 100 times higher than thermal sources.
4.2 THz/IR micro-spectroscopy using SR
In the visible region, optical lenses are generally added for focusing. However,
optical lenses cannot be used in the THz/IR region, because astigmatism
appears: diﬀerent refractive indexes occur because if the presence of impurities
and many absorption lines from the lattice and molecular vibration modes.
In the case of THz/IR, one mirror system consisting of one concave and one
convex spherical mirror, namely Schwarzschild mirrors, is used (Fig. 11d).
This system is used because it eliminates the astigmatism and gives highreﬂectivity in the THz/IR region due to Drude reﬂection of metals.
IR/THz SR is a powerful tool for micro-spectroscopy and imaging not
only in the IR region but also in the THz region because of its high brilliance. Almost all of the world’s IR/THz SR beamlines provide commercial
IR microscopes that are easy to use. However, because the Schwarzschild mirrors in commercial IR microscopes are small (typically 2 inches in diameter),the microscopes are available only in the mid-IR region, i.e., they are not12Shin-ichi Kimura
Frequency (THz)Intensity (arb.units)

0.1

3.3

1

10

UVSOR-II+UVSOR-II+UVSOR-II+

FARIS+FARIS+

Vertex Mylar6 m+

InSb bolo.

Si bolo.

Si bolo.

10

100

30.3

1000
-1

W avenumbers (cm )Fig. 10. Spectral distribution of the combination of light source (UVSOR-II), FTIR(FARIS-1 or Vertex 70v), and detector (InSb hot-electron bolometer or Si bolometer)in the THz region of BL6B, UVSOR-II. The lowest accessible wavenumber is about
4 cm−1 (∼0.13 THz).

suitable for the THz region. However, we have installed a new microscope inorder to cover the IR region and down to the THz region (Fig. 11). [12] Someof the main features are: 1) a large working distance, because experimentswith speciﬁc requirements (at very low temperatures, under high pressures,for near-ﬁeld spectroscopy, etc.) need to be performed; and 2) availability of
the THz region, because the quasiparticle states of correlated materials andﬁnger print vibration modes of proteins appear only in this spectral range.
In order to achieve these goals, we use one set of large Schwarzschild mirrors(diameter = 140 mm, numerical aperture = 0.5, working distance = 106 mm,
magniﬁcation = ×8) to reduce the diﬀraction eﬀect in the THz region.
After installing of the THz microscope, we checked the spatial resolution in
the diﬀerent wavenumber ranges covered by the diﬀerent FTIR beam splitters(Fig. 12). The spatial resolution was recorded by using a Bruker Vertex 70v
interferometer and the transmission conﬁguration. In the wavenumber region
above 500 cm−1 , the globar lamp gives good contrast. However, the use of
UVSOR-II increases the intensity by 102 and decreases the beam size by about
10 times. This implies that the brilliance of the IR/THz SR is 103 times higher
than that of thermal sources. This is consistent with the brilliance measured at
the ﬁrst focusing point P1 (Fig. 8). If a pinhole 500 µm in diameter is installed
at a counter focus position of the Schwarzschild mirror, the beam size becomes
the diﬀraction limit. In the wavenumber region below 600 cm−1 , in which a

IR/THz Synchrotron Radiation

13

Fig. 11. 3D view of the THz micro-spectroscopy apparatus (a); top view of the
optics (b), and photo (c) of the THz/IR micro-spectroscopy end-station (MSS) of
the IR/THz beamline (BL6B) at UVSOR-II. (d) indicates the conﬁguration of a
Schwarzschild mirror.

Mylar 6-µm beam splitter is used, a good beam proﬁle was observed withUVSOR-II, in contrast with the very weak peak given by a globar lamp. In
addition, use of a Mylar 23-µm beam splitter covering the lower wavenumber
region below 200 cm−1 gave good contrast and intensity in the peak image.
The cut-oﬀ wavenumber for UVSOR-II is 40 cm−1 (150 cm−1 in the caseof the SPring-8 IR beamline [11]). This indicates that the THz microscopeusing UVSOR-II is a very eﬃcient tool for micro-spectroscopy in both the IR
and the THz region. This instrument is used for electrodynamics studies ofstrongly correlated materials at low temperatures and under high pressures,
and also for molecular vibration imaging of living cells.

5 Application of THz spectroscopy using SR:
Temperature-dependent THz spectra of SmB6
SmB6 , which is a typical Kondo semiconductor, has been studied over three
decades. In the previous studies, two diﬀerent energy gap sizes were proposed

14

Shin-ichi Kimura

Fig. 12. Spectral distribution and spatial resolution of THz microscope at several
wavenumber regions using UVSOR-II compared with a globar lamp. Labels in the
top ﬁgure indicate the combinations of beam splitter of the FTIR (Bruker), low-passﬁlter, and detector. Labels on the left side indicate the light sources and aperturesat other focuses of the Schwarzschild mirror. Fine structures in the spectral distributions are due to the absorption of water vapor and can be reduced by nitrogenpurge. See the text for details.

one about 5 meV and the other about 15 meV [13]. However, a recent study
using higher purity samples revealed that the lower energy absorption band
originates from impurities, because the lower energy absorption becomes small
with increasing purity [14, 15]. The higher energy gap is concluded to be intrinsic and to originate from hybridization between the localized Sm 4f state
and the Sm 5d conduction band, so-called c-f hybridization [16]. In the caseof c-f hybridization, a rigid band model suitably explains the physical properties. However, the speciﬁc heat curve cannot be explained by a rigid bandmodel [17]. This means that there are temperature-dependent parameters in
the electronic structure as well as in the charge dynamics.
On the other hand, the magnetic excitation at 14 meV increases withdecreasing temperature below 20 K, as observed by using inelastic neutron
scattering [18]. The magnetic excitation energy is similar to the energy gap
observed in optical spectra. Therefore, if the c-f hybridization bands exist at

IR/THz Synchrotron Radiation

15

SmB

(a)

6

1.0

T=

1.0

36K

1.0
32K
(b)

SmB

20K

1.0

18K

1.0

16K

1.0

14K

-12

1.0

3

-3

24K

(cm )

1.0

19

2

10

Neff

1.0

4

6

28K

)

20

10

1.0

(sec

1.0

1

12K

)

18

R

(

10K
5K

0.9

0

2

4

10

10

20

T

30

0
40

(K)

6

(meV)

Fig. 13. (a) Temperature dependence of reﬂectivity spectrum of SmB6 in the THz
region (thick lines) and ﬁtting functions by the combination of a Drude and two
Lorentz functions (thin lines). Successive curves are oﬀset by 0.03 for clarity. (b)Parameters obtained for the eﬀective electron number (Nef f ) and the relaxation
time (τ ) of the Drude function as a function of temperature.

the energy gap edge, the carriers should have the same temperature dependence. Optical reﬂection spectroscopy is good as a probe for investigating thecharacter of carriers. In the case of SmB6 , the signal from the thermally excited carriers appears in the THz region. The change in the ﬁtting parametersof the Drude function as a function of temperature must reﬂect the property
at the energy gap edge. We therefore measured the temperature dependenceof the reﬂectivity spectrum in the THz region.
The thick lines in Figure 13a show the temperature dependence of thereﬂectivity spectrum [R(ω)] of SmB6 . At T =5 K, the reﬂectivity does not
approach unity with decreasing photon energy. This shows an insulating character. With increasing temperature, the reﬂectivity at the lower energy portion
increases. The high reﬂectivity originates from the Drude component due tothe presence of thermally excited carriers. The Drude component indicates theelectronic structure of the thermally excited area near the energy gap edge.
The curves ﬁtted by using one Drude and two Lorentz functions are indicated by the thin solid lines in Fig. 13a. The ﬁtting function is as follows:ε̂(ω) = ε∞ −n∑
4πNef f e2
4πNi e2+22m(ω − iω/τ ) i=1 m{(ωi0 − ω 2 ) − iω/τi }16Shin-ichi Kimura¯¯¯ 1 − ε̂(ω)1/2 ¯2¯¯R(ω) = ¯
1 + ε̂(ω)1/2 ¯Here, ε̂(ω), n̂(ω) and R(ω) are a complex dielectric function, a complex refractive index and a reﬂectivity spectrum, respectively. ε∞ is the sum of ε1
above the measured energy region, Nef f and τ are the eﬀective number andthe relaxation time of the carriers, respectively, and m the rest mass of anelectron. Ni , ωi0 and τi are the intensity, resonance frequency and relaxation
time, respectively, of i-th bound state, including energy gaps in the electronic
structure and optical phonons. The two Lorentz functions were set to the main
gap at 15 meV and to the impurity state at 5 meV. The ﬁtting functions reproduced the experimental curves well. The obtained Nef f and τ , derived fromthe ﬁtting parameters, are shown as functions of temperature (Fig. 13b). If
an activation type behavior [Nef f ∝ exp(−2∆/kB T ), where ∆ is the energy
gap and kB is the Boltzmann constant] is expected, then log Nef f should be
proportional to 1/T . However, log Nef f is roughly proportional to T , as shownin Fig. 13b. This indicates that the energy gap cannot be explained by therigid band model, but it may also indicate that the energy gap shrinks withincreasing temperature. At two-state model could be assumed to explain thisbehavior [19]. On the other hand, τ rapidly increases on cooling below 20 K.
This temperature is coincident with that of the growth of the magnetic excitation at 14 meV [18]. This means that thermal excitation of the carriers isstrongly related to magnetic excitation at 14 meV.

6 Applications of THz micro-spectroscopy using SR
6.1 Experimental procedure of THz spectroscopy under highpressure
THz spectroscopy under high pressures has been performed by using the MSS,
as shown in Fig. 14. To detect the charge dynamics, the optical reﬂection spectroscopy [R(ω)] in the THz region must be performed, as mentioned in theprevious section. High pressure is produced by using a diamond anvil cell(DAC). The experimental setup for reﬂection spectroscopy at high pressure
and low temperature at MSS is shown in Fig. 14. Because the sample area inthe pressure cell is smaller than 1 mm in diameter, we have to use a microscopic technique. THz spectroscopy under high pressures is diﬃcult to performusing conventional FTIRs.
R OptDAC-LT, easyLab Technology Ltd.)A membrane-type DAC (Diacell⃝
was employed to produce high pressures on the samples. Figure 14c is a photoof the setup combined with the THz microscope. Pressure was applied to samples at low temperatures by using the He gas inlet to the membrane. Because
the culet plane areas of diamond and of the membrane are 1.13 mm2 and
1030 mm2 , respectively, the pressure on samples is about 103 times larger

IR/THz Synchrotron Radiation

17

Fig. 14. Schematic ﬁgure of a diamond anvil cell (DAC) with a gas membrane
combined with a liquid-helium-ﬂow-type cryostat for reﬂection spectroscopy at highpressure and low temperature (a); the sample setup in the DAC (b); and photo of
the experimental apparatus and the DAC (c).

than the helium gas pressure. The maximum pressure at the sample position
is about 8 GPa. A sample with a typical size of 0.4×0.4×0.05 mm3 was set
in a DAC with Apiezon-N grease as a pressure medium, with gold ﬁlm as areference and ruby tips for a pressure reference. The pressure was calibrated
by ruby ﬂuorescence measurement.
6.2 Electronic structure of SmS under high pressure
SmS is an insulator (the so-called “black phase”) with a gap size of 1000 K(∼80 meV) at ambient pressure [20]. Above about 0.7 GPa, the sample color
changes to gold (golden phase) and the Sm-ion changes from divalence tomixed valence [21]. To investigate the mechanism of this transition, we performed THz reﬂection spectroscopy under pressure.
The R(ω) obtained for SmS at 300 K as a function of pressure is shown by
the thick lines in Fig. 15a. At ambient pressure, the spectrum indicates an insulating one, because the low energy limit does not approach unity and a clear
large peak due to the TO-phonon between the Sm2+ and S2− ions appears.
When pressure was applied, the background intensity increased with increasing pressure. The background indicates the appearance of carriers. Therefore,
the carrier density increased with increasing pressure. The ﬁtting curves of
the combination of a Drude function and Lorentz functions are shown by thethin solid lines in the same ﬁgure. The Lorentz function was set to reproduce

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Shin-ichi Kimura

Fig. 15. (a) Pressure dependence of the reﬂectivity spectrum [R(ω)] of SmS (thicklines) in the black phase at 300 K. The ﬁtting curves from the combination of a
Drude and the Lorentz functions are plotted as thin solid lines. Successive curves
are oﬀset by 0.5 for clarity. (b) Pressure dependence of energy gap evaluated by
Drude and Lorentz ﬁtting of R(ω) spectra. See the text for details.

the TO-phonon. To ﬁt the spectra obtained, only the Nef f in the Drude function is changed; all of the other parameters (τ in the Drude function and allparameters in the Lorentz function) are ﬁxed. The Nef f obtained is plottedin Fig. 15b. The log Nef f is proportional to the pressure up to 0.65 GPa.
This pressure dependence indicates that the energy gap closes with increasingpressure. The energy gap size (2∆) at ambient pressure was evaluated to beabout 1000 K [20]. By evaluating the gap size from Nef f , the energy gap at
the black-golden phase boundary is 850 K; the gap then suddenly closes to
100 K in the golden phase by ﬁrst-order transition [22].

7 Conclusion and outlook
In this paper, we have pointed out the high brilliance of the infrared (IR)and terahertz (THz) synchrotron radiation (SR) and have introduced thecharacteristic optics by which “three-dimensional magic mirror (3D-MM)”
is used to focus the bending-magnet radiation. The 3D-MM is an ideal opticalcomponent for focusing bending-magnet radiation, because not only can theIR/THz Synchrotron Radiation

19

synchrotron light be focused down to a diameter close to the electron beam
size but also the time structure can be reduced to that of the electron beam
length. Using 3D-MM, some advanced optical experiments have so far been
performed (e.g. highly precise THz reﬂection spectroscopy, and THz spectroscopy at high pressure and low temperature). 3D-MM can also be used fortime-resolved spectroscopy because of the conservation of the time structureof electron beams.
Recently, THz coherent SR (THz-CSR) has been developed in storagerings. THz-CSR, which is emitted when the electron beam length or the longitudinal structure is shorter than the wave length, has intensity several ordersof magnitude higher than that of normal SR. The light has properties not onlyof high intensity but also full coherence. The light can be used for new experiments (e.g., THz-pump probe spectroscopy, and THz phase contrast imaging).
Such new experiments will open up a new frontier in spectroscopy.

Acknowledgments
We would like to thank UVSOR staﬀ members for their devoted support andMr. Takuya Iizuka for his providing Fig. 14. High purity samples of SmB6
and SmS were provided by Prof. Satoru Kunii and Prof. Noriaki K. Sato,respectively.

References
1. T. Nanba, Y. Urashima, M. Ikezawa, M. Watanabe, E. Nakamura, K. Fukui,and H. Inokuchi, Int. J. Infrared Millimeter Waves 7, 759 (1986).
2. M. Katoh, M. Hosaka, A. Mochihashi, J. Yamazami, K. Hayashi, Y. Hori, T.
Honda, K. Haga, Y. Takashima, T. Koseki, S. Koda, H. Kitamura, T. Hara, andT. Tanaka, AIP Conf. Proc. 705, 49 (2004).
3. S. Kimura, E. Nakamura, T. Nishi, Y. Sakurai, K. Hayashi, J. Yamazaki, and
M. Katoh, Infrared Phys. Tech. 49, 147 (2006).
4. S. Kimura, E. Nakamura, J. Yamazaki, M. Katoh, T. Nishi, H. Okamura, M.
Matsunami, L. Chen, and T. Nanba, AIP Conf. Proc. 705, 416 (2004).
5. H. Kimura, T. Moriwaki, N. Takahashi H. Aoyagi, T. Matsushita, Y. Ishizawa,
M. Masaki, S. Ohishi, H. Okuma, T. Nanba, M. Sakurai, S. Kimura, H. Okamura, N. Nakagawa, T. Takahashi, K. Fukui, K. Shinoda, Y. Kondo, T. Sata,
M. Okuno, M. Matsunami, R. Koyanagi, and Y. Yoshimatsu, Nucl. Instrum.
Meth. A 467-468, 441 (2001).
6. T. Yaji, Y. Yamamoto, T. Ohta, and S. Kimura, Infrared Phys. Tech. 51, 397(2008).
7. J. D. Jackson, Classical Electrodynamics, Third Edition (John Wiley & Sons,
Inc., 1999).
8. R. Lopez-Delgado and H. Szwarc, Opt. Commun. 19, 286 (1976).

20

Shin-ichi Kimura

9. S. Kimura, H. Kimura, T. Takahashi, K. Fukui, Y. Kondo, Y. Yoshimatsu, T.
Moriwaki, T. Nanba, and T. Ishikawa, Nucl. Instrum. Meth. A 467-468, 437(2001).
10. S. Kimura, JASCO Report 50, 6 (2008). [in Japanese]
11. Y. Ikemoto, T. Moriwaki, T. Hirono, S. Kimura, K. Shinoda, M. Matsunami,
N. Nagai, T. Nanba, K. Kobayashi, and H. Kimura, Infrared Phys. Tech. 45,369 (2004).
12. S. Kimura, Y. Sakurai, E. Nakamura, and T. Mizuno, AIP Conf. Proc. 879, 595(2006).
13. G. Travaglini and P. Wachter, Phys. Rev. B 29, 893 (1984).
14. T. Nanba, H. Ohta, M. Motokawa, S. Kimura, S. Kunii, and T. Kasuya, Physica
B 186-188, 440 (1993).
15. S. Kimura, T. Nanba, S. Kunii, and T. Kasuya, Phys. Rev. B 50, 1406 (1994).
16. G. Aeppli and Z. Fisk, Comments Condens. Matter Phys. 16, 155 (1992).
17. T. Kasuya, K. Takegahara, Y. Aoki, K. Hanzawa, M. Kasaya, S. Kunii, T. Fujita,
N. Sato, H. Kimura, T. Komatsubara, T. Furuno, and J. Rossat-Mignod, Valence
Fluctuation in Solids, p. 215, (North-Holland, 1981).
18. P.A. Alekseev, J.M. Mignot, J. Rossat-Mignod, V.N. Lazukov, I.P. Sadikov, E.S.
Konovalova, and Yu.B. Paderno, J. Phys.: Condens. Matter 7, 289 (1995).
19. B. Gorchunov, N. Sluchanko, A. Volkov, M. Dressel, G. Knebel, A. Loidl, andS. Kunii, Phys. Rev. B 59, 1808 (1999).
20. K. Matsubayashi, K. Imura, H.S. Suzuki, T. Mizuno, S. Kimura, T. Nishioka,
K. Kodama, and N.K. Sato, J. Phys. Soc. Jpn. 76, 064601 (2007).
21. J.L. Kirk, K. Vedam, V. Narayanamurti, A. Jayaraman, and E. Bucher, Phys.
Rev. B 6, 3023 (1972).
22. T. Mizuno, S. Kimura, K. Matsubayashi, K. Imura, H.S. Suzuki, and N.K. Sato,J. Phys. Soc. Jpn. 77, 113704 (2008).