Application of Terahertz Synchrotron Radiation atUVSOR-IIShin-ichi Kimura∗,† and Takafumi Mizuno††∗UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585,
Japan

Abstract. The infrared-terahertz beamline at UVSOR-II has been upgraded in 2004 for covering very low energy region
below 0.3 THz (= 10 cm−1 ). At the beamline, several characteristic experiments in the terahertz region are performed. Twoexamples of such experiments, reflectivity measurements at low temperatures and at high pressures are reported.
Keywords: Synchrotron radiation, Terahertz spectroscopy, Infrared
PACS: 41.60.Ap, 42.72.Ai, 78.30.-j, 78.68.+m

INTRODUCTION
Recently, electromagnetic waves in the terahertz region(THz) attract attention because it will become a new
probing light source for non-destructive examinations inplace of the x-ray. The THz light can also reveal thelow energy excitation of materials, for instance, vibration
modes of large molecules such as proteins, weak bonding between surfaces and adsorbed molecules, and theelectrodynamics of quasiparticles of solids and so on. To
study such basic sciences, the firstly dedicated infraredsynchrotron radiation beamline (BL6A1) has been constructed at UVSOR about two decades ago [1] and somany scientific activities have been produced from thebeamline. In 2003, UVSOR was upgraded to the lowest emittance ring (27 nm rad) among small SR’s withthe acceleration energy below 1 GeV and the name wasalso changed to UVSOR-II [2]. Simultaneously, BL6A1
was also upgraded to the new IR-THz beamline (BL6B)
with high brilliance and high flux as shown in Fig. 1 [3].
To obtain the higher performance, we employed a socalled “magic mirror” with a large acceptance angle(215(H) × 80(V) mrad2 ), which has been successfully
installed at the infrared beamline of SPring-8 [4], for the
first mirror [5]. The upgrading of the beamline gives usthat the photon flux and brilliance of BL6B become fourand 102 times larger than BL6A1, respectively. The spectral intensity in the THz region becomes very high as aSi bolometer saturates.
At the present, two permanent end stations areequipped at BL6B. One is the apparatus for reflectionabsorption spectroscopy of solid samples with mm-sizeconstructed two decades ago [1]. The apparatus can beremoved and replaced to apparatuses brought by users,
for instance, a magneto-optical apparatus [6] and an
infrared reflection-absorption spectroscopy apparatus

for adsorbed molecules [7]. The other is a THz microscope for the reflection and transmission spectroscopy ofsmall samples with the diffraction-limit resolution [7].
The microscope covers down to 30 cm−1 (= 3.7 meV)that is lower wavenumber than that of a commercial
infrared microscope because large Schwarzschild mirrors are employed. At an infrared beamline BL43IR ofSPring-8, there is also a microscope which covers down
to 100 cm−1 (= 12 meV) [8]. Since the brilliance in theTHz region of BL6B of UVSOR-II is higher than that ofBL43IR of SPring-8, the accessible lowest wavenumberof BL6B is lower. This is the advantage to investigate
the electrodynamics of solids under pressures and otherfunctionalities. These two permanent end stations areopened for users.
In this paper, we present two experimental results obtained by using the two permanent end stations at BL6Bof UVSOR-II. One is the electromagnetic dynamics ofstrongly correlated insulator, SmB6 , at low temperature.
The other is the insulator-metal transition of SmS underpressures using the THz microscope.

TERAHERTZ SPECTROSCOPY OF
SMB6 AT LOW TEMPERATURES
SmB6 , which is one of typical Kondo semiconductors,
has been studied over two decades. In the previous studies, two different energy gap size were proposed, one isabout 5 meV and the other about 15 meV [9]. However,a recent study using higher purity samples reveals thatthe lower energy absorption band originates from impurities because the lower energy absorption becomes
small [10, 11]. The higher energy gap is concluded to be
intrinsic and to originate from the hybridization between
the localized Sm 4 f state and the Sm 5d conduction

Bending #6
Magic mirror

To BL6U
Diamond /z-cut Quartz
windowJASCOFARIS-1

THz microscope

Bruker
IFS66v/S
Refection-absorption
spectroscopy station0FIGURE 1.

500

1000

The top view (left) and the picture (right) of the infrared-terahertz beamline (BL6B) at UVSOR-II.

SmB(a)

61.0

T=

1.036K

1.032K(b)

SmB

1.0

18K

1.0

16K

1.0

14K

3-12-320K

(cm )

24K

1.0

19

2

10Neff

1.0

4

628K

1.0)

20

101.0(sec

1.0

112K)

18R

(10K
5K

0.902

4

101020T

30040(K)

6(meV)FIGURE 2. (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)Obtained parameters of the effective electron number (Ne f f )and the relaxation time (τ ) of the Drude function as a functionof temperature.

band, so-called c f –hybridization [12]. In the case of thec f –hybridization, a regid band model is suitable to explain the physical properties. However, the specific heat
curve cannot be explained by a rigid band model [13].
This means that there is the temperature dependent parameters in the electronic structure as well as in thecharge dynamics.
On the other hand, the magnetic excitation at 14 meVgrows up with decreasing temperature below 20 K
observed by a neutron inelastic scattering [14]. Themagnetic excitation energy is similar to the energy
gap observed in optical spectra. Therefore, if the c f –hybridization bands exist at the energy gap edge, thecharacter of carriers should have the same temperaturedependence. Optical reflection spectroscopy is one ofgood probes to investigate the character of carriers. In theSmB6 case, the signal from the thermally excited carriers appears in the THz region. The change of the fitting
parameters of the Drude function as a function of temperature must reflect the property at the energy gap edge.
Then we measured the temperature dependence of thereflectivity spectrum in the THz region.
Figure 2(a) indicates the temperature dependence ofreflectivity spectrum [R(ω )] of SmB6 by thick lines. AtT =5 K, the reflectivity does not approach to unity withdecreasing photon energy. This indicates the insulating
character. With increasing temperature, the reflectivity
grows up from the lower energy side. The high reflectivity originates from the Drude component due to thermally excited carriers. The Drude component indicatesthe electronic structure thermally excited area near theenergy gap edge.
The fitted curve using one Drude and two Lorentz
functions are indicated by thin solid lines in Fig. 2(a).
The fitting function is as follows;ε̂ (ω ) = ε∞ −n4π Ne f f e24π Ni e2+∑ m{(ω 2 − ω 2 ) − iω /τi }m(ω 2 − iω /τ ) i=1i0¯¯¯ 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 themeasured energy region, Ne f f and τ the effective number and the relaxation time of carriers, respectively, mthe rest mass of an electron, Ni , ωi0 and τi are the intensity, resonance frequency and the relaxation time of i-thbound state including energy gaps of electronic structure
and optical phonons, reflectivity. The two Lorentz functions were set to the main gap at 15 meV and to the impurity state at 5 meV. The fitting functions can reproduce

Cryostat for DAC
Si bolometerTHzmicroscope
Schwarzschildmirrors

Manipulator

DAC

FIGURE 3. Photo of terahertz microscope with a high pressure diamond anvil cell (DAC) at BL6B of UVSOR-II.

the experimental curves well. The obtained Ne f f and τ
derived from the fitting parameters are shown as the function of temperature in Fig. 2(b). If an activation type behavior [Ne f f ∝ exp(−2∆/kB T ), where ∆ is the energy
gap and kB the Boltzmann constant] is expected, log Ne f fshould be proportional to 1/T . However log Ne f f is proportional to T as shown in Fig. 2(b). This indicates thatthe energy gap can not be explained by the rigid bandmodel but it may indicate the energy gap shrinks withincreasing temperature. Two-state model might be assumed to explain the behavior [15]. On the other hand,τ rapidly increases with decreasing temperature below
20 K. The temperature is coincident with that of thegrowth of the magnetic excitation at 14 meV [14]. Thismeans that the thermally excited carriers strongly relates
to the magnetic excitation at 14 meV.

TERAHERTZ SPECTROSCOPY OF SMS
UNDER PRESSURES
The infrared synchrotron radiation (IRSR) is a powerfultool for microspectroscopy and imaging not only in theIR but also in the THz regions because of its high brilliance. Almost all IRSR beamlines in the world provide
commercial IR microscopes end stations that are easy
to use. Since Schwarzschild mirrors in commercial IR
microscopes are small, the microscope is available onlyin the mid-IR region, i.e., it is not suitable for the THz
region. Then, we install a new microscope with large
Schwarzschild mirrors in order to cover the IR region
down to the THz region as shown in Fig. 3.
Some of the main features are; 1) Large working distance because some specific experiments (at very low
temperatures, under high pressures, a near-field spectroscopy, etc.) are planned. 2) We need to cover the THz
region because quasiparticle states of correlated materials and finger print vibration mode of proteins appear

only in this spectral range. In order to achieve thesegoals, we use a large-size Schwarzschild mirrors (Diameter = 140 mm, NA = 0.5, working distance = 106 mm,magnification = ×8) to reduce the diffraction effect in theTHz region.
After installation of the THz microscope, we checked
the spatial resolution in the different wavenumber ranges
in which the microscope covers [7]. The THz microscope in conjunction with UVSOR-II covers down to 30
cm−1 in contrast that with a globar lamp covers above
500 cm−1 . The spatial resolution and the intensity using UVSOR-II is much higher than that using a globar
lamp in the whole wavenumber range. This indicates thatthe THz microscope using UVSOR-II is a very efficientmeans of microspectroscopy in the IR and THz regions.
We started the THz spectroscopy under high pressures. To detect the charge dynamics, the optical reflection spectroscopy [R(ω )] in the THz region must be performed as mentioned in the previous section. The highpressure is produced by a diamond anvil cell. Since thesample area in the pressure cell is smaller than 1 mm indiameter, we have to use a microscopic technique. Thenthe THz spectroscopy under high pressures is difficult toperform using conventional FTIRs.
A membrane-type diamond anvil cell was employed
to produce high pressures to samples. The schematic figure combined with THz microscope is shown in Fig 3.
The pressure applied to samples is controlled by thehelium gas pressure filled in the membrane. Since theareas of the culet plane of diamond and of the membrane are 1.13 mm2 and 1030 mm2 , respectively, thepressure of samples is about 103 times larger than thehelium gas pressure. The sample with a typical size of
0.4×0.4×0.05 mm3 was set in a diamond anvil cell with
Apiezon-N grease as the pressure medium and with agold film as a reference and ruby tips as for a pressurereference. The pressure was calibrated by a ruby fluorescence measurement.
SmS is an insulator (it is called “black phase”) withthe gap size of 1000 K at ambient pressure [16]. Above
about 0.7 GPa, the sample color changes to gold (goldenphase) and the Sm-ion changes from the divalence tomixed valence [17]. To investigate the mechanism of thetransition, we performed the THz reflection spectroscopy
under pressures.
The obtained R(ω ) of SmS at 300 K as a function ofpressure were shown by thick lines in Fig. 4(a). At theambient pressure, the spectrum indicates the insulating
character because the low energy limit does not approachto unity and a clear large peak due to the TO-phonon
between Sm2+ and S2− ions appears. When a pressure
was applied, the background intensity increased with increasing pressure. The background indicates the appearance of carriers. Therefore the carrier density increases
with increasing pressure. The fitting curve of the combi-(a)SmS T=300K(b)0.65GPa-2

Neff (/SmS)

100.45GPa

EG=1000K

EG=850K-3

101.0

Golden phase (N ~ 1)

P=0.85GPa

We would like to thank Prof. S. Kunii, Prof. N.K. Satoand Dr. K. Matsubayashi for their providing samples reported in this paper. This work was a joint study programof Institute for Molecular Science and was partially supported by Grants-in-Aid for Scientific Research (GrantNo. 18340110) from MEXT of Japan and by the Research Foundation for Opto-Science and Technology.

R(Z)0.20GPa

REFERENCES

0.50.00.1MPa

0.0204060

0.5

1.0

P (GPa)80´Z (meV)

FIGURE 4. (a) Pressure dependence of reflectivity spectrum[R(ω )] of SmS (thick lines) in the black phase at 300 K. The
fitting curve of the combination of a Drude and a Lorentz
functions are also plotted by thin solid lines. Successive curves
are offset by 0.5 for clarity. (b) Pressure dependence of energy
gap evaluated by the Drude and Lorentz fitting of R(ω ) spectra.
See the text for details.

nation of a Drude and a Lorentz functions is shown by
thin solid lines in the same figure. The Lorentz function
was set to reproduce the TO-phonon. To fit the obtainedspectra, the Ne f f in the Drude function is only changed
and the other parameters (τ in the Drude function andall parameters in the Lorentz function) are fixed. The obtained Ne f f is plotted in Fig. 4(b). The log Ne f f is proportional to the pressure up to 0.65 GPa. The pressure
dependence indicates that the energy gap closes with increasing pressure. The energy gap size (2∆) at the ambient pressure was evaluated to be about 1000 K [16]. Byevaluating the gap size from Ne f f , the energy gap at theblack-golden phase boundary is 850 K and then the gap
suddenly closes to 100 K in the golden phase by the firstorder transition [18].

CONCLUSION
In conclusion, two examples of research activities of
the terahertz spectroscopy and microspectroscopy at theinfrared-terahertz beamline BL6B of UVSOR-II were reported. The performance of BL6B is excellent comparedwith a conventional globar light source. Some other experiments that cannot be performed by using conventional spectrometers, for instance, infrared magnetic circular dichroism [19], infrared reflection-absorption spectroscopy of adsorbed molecules on surfaces [7], subterahertz spectroscopy of superionic conductors and soon, are performed at the beamline. In the future, new terahertz spectroscopies will be produced at the beamline.

1.

T. Nanba, Y. Urashima, M. Ikezawa, M. Watanabe, E.
Nakamura, K. Fukui, and H. Inokuchi, Int. J. Infrared andMillimeter Waves 7,1769 (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, and T. Tanaka,
AIP Conf. Proc. 705, 49 (2004).
3. S. Kimura, E. Nakamura, T. Nishi, Y. Sakurai, K. Hayashi,
J. Yamazaki, and M. Katoh, Infrared Sci. Tech. 49, 147(2006).
4. 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).
5. 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).
6. S. Kimura, Jpn. J. Appl. Phys. 38 Suppl. 38-1, 392 (1999).
7. S. Kimura, Y. Sakurai, E. Nakamura, and T. Mizuno, AIP
Conf. Proc. 879, 595 (2006).
8. 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).
9. G. Travaglini and P. Wachter, Phys. Rev. B 29, 893 (1984).
10. T. Nanba, H. Ohta, M. Motokawa, S. Kimura, S. Kunii,and T. Kasuya, Physica B 186-188, 440 (1993).
11. S. Kimura, T. Nanba, S. Kunii, and T. Kasuya, Phys. Rev.
B 50, 1406 (1994).
12. G. Aeppli and Z. Fisk, Comments Condens. Matter Phys.
16, 155 (1992).
13. 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).
14. 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).
15. B. Gorchunov, N. Sluchanko, A. Volkov, M. Dressel, G.
Knebel, A. Loidl, and S. Kunii, Phys. Rev. B 59, 1808
(1999).
16. K. Matsubayashi, K. Imura, H.S. Suzuki, T. Mizuno, S.
Kimura, T. Nishioka, K. Kodama, and N.K. Sato, J. Phys.
Soc. Jpn. (2007) in press.
17. J.L. Kirk, K. Vedam, V. Narayanamurti, A. Jayaraman,and E. Bucher, Phys. Rev. B 6, 3023 (1972).
18. T. Mizuno, S. Kimura, K. Matsubayashi, K. Imura, H.S.
Suzuki, and N.K. Sato, submitted.
19. S. Kimura, M. Okuno, H. Iwata, H. Kitazawa, G. Kido,
F. Ishiyama, and O. Sakai, J. Phys. Soc. Jpn. 71, 2200
(2002).