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Summary of my research work

Tohru Suemoto

1. Overview

Since my graduate course at the University of Tokyo, I have been working in the field of

spectroscopy on the solid state materials. The wavelength (frequency) of the spectroscopy I comitted

has not been restricted within optical region, but ranged from terahertz to soft X-ray. I concerned with

variety of materials including semiconductors, oxides, metal complexes, magnets, metals, semimetals

and biomolecules, through collaborations with material scientists and chemist.

In the following the main subjects I have been engaged in so far are summarized.

1) Self-trapped excitons in solid rare gases (doctor work at the University of Tokyo)

2) Quasi-elastic light scattering in superionic conductors (Tohoku University)

3) Resonant Raman scattering in semiconductor quantum well structures

(Max Planck Institute for Solid State Research)

4) Persistent hole-burning in rare earth ion doped solids

5) Carrier dynamics in semiconductors studied with time-resolved electronic Raman scattering.

6) Resonant excitation spectroscopy of semiconductor nanostructures i.e., porous silicon.

7) Nuclear wave-packet dynamics and carrier dynamics studied with femtosecond luminescence

spectroscopy.

8) Dynamics of photoinduced phase transitions.

9) Dynamics of spin systems studied with terahertz spectroscopy.

10) Observation of laser ablation processes by time resolved soft X-ray interferometer.

I will describe the outline of the topics, which have continued until 2016 (items 7-10) in the following

sections.

2. Femtosecond luminescence spectroscopy

Luminescence phenomenon has long been known to provide versatile tools for investigating the

excited electronic states with high sensitivity and straight forward interpretation. For time-resolved

observation of luminescence, high speed photodiodes, streak cameras, optical Kerr shutters, etc. are

widely used. Among them, the up-conversion method has the highest time resolution in measurement

of luminescence. This situation has not changed since the 1990s up to the present time[1].

2.1 Dynamics of wave-packet

In the up-conversion method, as shown in Fig. 1 (a), light emitted from a sample and the gating

pulse from a laser are mixed in a nonlinear optical crystal to generate sum frequency (up-conversion)

light, and this is directed to a spectrometer and detected by a photomultiplier or semiconductor devices.

As the sum frequency is generated only when the luminescence light and the gating light are present in

the crystal at the same time, the luminescence can be cut out in time by adjusting the relative time delay

between the pumping and gating pulses. We used this method to observe the nuclear wave-packet

dynamics in the localized excited states. During the wave-packet motion on the adiabatic potential

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surface as shown in Fig. 1 (b), the luminescence photon energy changes depending on the position of

the wave-packet on the potential surface [2]. By using this fact, not only the position of the wave packet,

but also its shape can be captured.

Figure 1 (c) shows the temporal evolution of the luminescence spectrum observed for the self-

trapped excitons generated by a pumping pulse in bromine-bridged platinum complex [3]. It can be seen

that the peak position of the spectrum oscillates with time. Furthermore, the spectrum cut out at each

time point reflects the shape of the wave-packet. Therefore, we refer to the frames arranged in

chronological order as "wave packet movies". In the subsequent research, it was possible to visualize

the splitting of the wave-packet into oscillating part and the traveling part, the latter corresponding to

the transition of the lattice arrangement to a new state overcoming a potential barrier. In the same

experiment, the nodes and antinodes of the wave function constituting the wave packet are resolved.

2.2 Topological insulator

The topological insulator is a substance that basically is an insulator inside, but has a metallic band

structure with continuous density of states on the surface. The topologically protected surface state is

the two dimensional Dirac electron system with low carrier scattering rate and attracting attention from

engineering view point as a platform for high speed electronic devices. The carrier dynamics in this

system is now extensively studied by means of time-resolved photoemission spectroscopy. However,

the luminescence approach has not been reported. Recently, we succeeded in observing the short lived

hot luminescence in the infrared region below band gap of the bulk by using up-conversion time-

resolved luminescence spectroscopy [5]. Figure 2(b) shows the band structure of the topological

insulator TlBiSe2, which we studied in this work. The insulating bulk band structure having a band gap

of 0.35 eV is shown by blue hatching and the surface Dirac cone is shown by crossing straight lines.

The important character of this system is the spin orientation locked to the momentum, that is, the spin

orientation on the red line and blue line are opposite. Figure 2(a) shows the time-development of

luminescence, where the rise time and the decay time are very different below and above the band gap

Fig. 1 (a) Principle of time-resolved luminescence measurement by up-conversion method. (b) Generation of wave-packet on the adiabatic potential surface by photoexcitation and the luminescence process. (c) Time evolution of emission spectrum from self-trapped exciton in Br-bridged Pt complex.

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0.35 eV. Fig. 2(c) summarizes this behavior. The luminescence below band gap cannot be assigned to

bulk and should be ascribed to the surface state. Unlike photoemission spectroscopy, luminescence

experiment can be performed even in the atmosphere, so it is expected to provide a useful tool for

studying the dynamic characteristics of the device in operation. We also studied the ultrafast relaxation

of Dirac electrons in graphite [6] and graphene [7] by the same method.

3. Dynamics of photoinduced phase transition

The arrangement of atoms, polarization, magnetization, conductivity, etc. in materials can be

changed by applying pressure, magnetic field or changing temperature or other environment parameters.

These are known as phase transitions. In some cases, the phase transitions can be induced by light

irradiation and are called as "photoinduced phase transitions" (PIPT). Here, we discuss the PIPT in a

cyano-bridged metal complex (Prussian blue analog) RbxMn [Fe (CN) 6] y nH 2O [9]. As shown in Fig.

3 (a), the valence state of the metal ion at room temperature is Fe3+ - Mn2+ (HT - phase), but as the

temperature is lowered, a charge transfer transition occurs resulting in Fe2+- Mn3+ configuration (LT -

phase), and at the same time, the magnetic susceptibility. Since the frequency of the stretching mode of

the CN molecule bridging Fe and Mn reflects the valence of the adjacent metal ions, this vibrational

spectroscopy can be used to determine the valence of Fe and Mn [10]. Since the pairs Fe2+ - Mn2+ or

Fe3+ - Mn3+ appear at the boundary between the two phases as shown in the figure, it is possible to

quantify the amount of boundary from vibration spectra. Figure 3 (b) shows the time evolution of the

amounts of the high-temperature (HT) phase, the low-temperature (LT) phase and the boundary during

light irradiation obtained from the Raman scattering intensity. Here, we can see a conversion of the HT

Fig. 2 (a) Time evolution of luminescence in the topological insulator TlBiSe2 under excitation at a photon energy of 1.55 eV. The photon energy of luminescence is shown on the left end of each curve. (b) Band structure of TlBiSe2. (c) Photon energy dependence of rise time and decay time for luminescence.

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phase into LT phase along with a marked increase of the boundary component. The amount of the

observed boundary component is far larger than that observed during HT to LT conversion by cooling

the sample. This indicates that the LT phase domain size in PIPT is very small [11]. Transient generation

of a large amount of boundary component is also verified by time-resolved infrared absorption

measurements [12]. This was a rare example showing the dynamics of the nanometer scale domains

associated with PIPT.

4. Terahertz magnetic resonance

4.1 Coherent control of spin precession

Most of the research activities utilizing terahertz (THz) spectroscopy have been devoted to the

phenomena induced via electric dipole moment transitions.

Fig. 3 (a) Structure of RbxMn [Fe(CN)6]y nH2O. (b) Time evolution of high temperature phase, low temperature phase and boundary components during light irradiation.

HT phase LT phase

Sum

Boundary(Fe3+-Mn3+)x2

15 %

1.0

0.8

0.6

0.4

0.2

0.010080604020

Time (min.)

Fra

ctio

nCNFe2+ Mn3+

3+

3+

LT phase

2+

2+

2+

HT phase

2+

2+ 3+

3+

3+

3+

Fe3+ - Mn2+Fe2+ - Mn3+

2+

Fe3+ Mn2+

(a) (b)

Fig. 4 (a) Excitation of a spin by pulsed magnetic field. (b) Circularly polarized FID radiation observed in -iron oxide. (c) Selective excitation of F and AF modes in YFeO3.

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This is because the magnetic dipole transition is very weak in general (106 times lower than that of

electric dipole transition in atomic systems). However, the magnetic dipole transition has an essential

importance in the magnetic resonance phenomena. As the frequencies of ferromagnetic and anti-

ferromagnetic resonances are quite high and falls in sub-THz and THz region, the possibility of the

magnetic resonance experiment utilizing THz radiation has become recognized since around 2010.

During experiments evaluating the THz transmission properties of a ferromagnetic material (-iron

oxide), we found a free induction decay (FID) signal with circular polarization from a magnetized

sample under excitation by linearly polarized THz radiation [13]. Figure 4(a) illustrates the

magnetization vector in a ferromagnetic material. The magnetic moment M initially parallel to z-axis is

tilted to y-axis direction by applying an impulsive magnetic field HTHz in x direction. Then M starts to

make a precession motion around z-axis. The screw-like trajectory in Fig. 4(b) shows the FID radiation

with circular polarization emitted from a magnetized -iron oxide in time domain. As the turning

direction was inverted when we inverted the magnetization, this was ascribed to the precession motion

of the magnetization. Figure 4(d) shows a coherent control of spin precession in orthoferrite, which has

two magnetic modes at 0.3 THz(F-mode) and 0.53 THz (AF-mode) [14]. When the sample is excite by

a single pulse, both modes are excited as shown by a black curve in Fig. 4(d). However, when we use a

pair of pulses with a time separation in-phase or out-of-phase of each mode as shown in Fig. 4(c), we

can selectively excite AF-mode or F-mode as shown by red or blue curve.

4.2 Coupling of spin excitation and metamaterial structure

A metal C-shaped split ring resonator (SRR) of several tens of micrometers (Fig. 5 (a)) has a

resonance in sub-THz region.

When we induce an electric current in the ring, using the electric field component of the THz wave, the

magnetic flux passing through the ring is enhanced by nearly one order of magnitude than that contained

in the same THz wave. This strong magnetic field can be used to drive the spins in the substrate. The

Fig. 5 (a) Illustration of the sample with a split ring resonator. (b)(c) Waveforms and Fourier spectra of Faraday rotation signal. Upper panels and lower panels are experiment and calculation results, respectively.

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precession motion monitored by Faraday rotation is shown in the upper part of Fig. 5 (b). The resonance

frequency of the spin can be tuned by varying the temperature and the amplitude significantly increases

when it matches the resonance frequency of the ring. Figure 5 (c) shows the Fourier spectra of the

oscillation. The lower parts of (b) and (c) are the calculation results based on the model assuming a

coupling between the spin system and the LC resonance circuit, showing very good agreement with the

experimental results [15].

4.3 Production of macroscopic magnetization by THz wave

ErFeO3 undergoes a spin reorientation phase transition between 87 and 96 K, that is, the

magnetization direction is parallel to the a-axis at low temperature, it starts to rotate above 87 K toward

c direction, and finally it aligns parallel to the c-axis at 96 K. Usually, in the case of temperature induced

reorientation, the domains with +c and -c orientation occur with equal probability and the macroscopic

magnetization becomes nearly zero. However, if we excite the precession motion of the spins by THz

magnetic field and input a heating pulse at an appropriate timing, we can make totally +c or -c polarized

macroscopic volume selectively [16]. The result is shown in Fig. 6. The dotted curve is the fluctuating

weak magnetization induced by the THz pulse irradiation at t=0. If the heating pulse (800nm) hits the

sample at 57 ps when the fluctuation is upward, the magnetization is amplified to about 100 times (red

curve) in positive direction. Similarly, the final magnetization is in negative direction (blue curve), if

the heating pulse comes at 65 ps. At the final stage (180ps), 80% of the spins are aligned in the same

direction in both cases. This demonstrates the possibility of ultrafast manipulation of macroscopic

magnetization, by using THz and optical pulses combined.

5. Single shot soft X-ray imaging

In most of the research on ultrafast phenomena using pump and probe technique, repeatable

phenomena are targeted, because it is required to repeat the same phenomena for collecting information

in time sequence. Although this method is applicable to the phenomena such as electron dynamics, in

which the system is believed to recover precisely the initial state after some waiting time. However,

photoinduced phase transition, chemical reaction and destructive phenomena such as laser ablation are

Fig. 6 Magnetic fluctuation induced by THz wave (dotted curve) and generation of macroscopic magnetization (red and blue curves) by THz pulse and heating pulse combined.

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inherently irreversible and for investigating these phenomena requires a single-shot measurement

technique. In order to perform such measurement, a single shot soft X-ray interferometer was

constructed using a soft X-ray laser (wavelength; 13.9 nm, pulse width; 7 ps) equipped at QTS Kansai

Laboratory as a light source. Figure 7(a) illustrates the outline of the system. The soft X-ray beams

reflected at the ablated area and the reference area on the same sample are merged at a very small angle

by using a double Loyd's mirror and incident on the X-ray CCD camera placed at a large distance. From

the shift of the interference fringes on the CCD camera, we can measure the dilation of the sample

surface with a picosecond time resolution [17].

Figure 7 (b) shows an interference image after irradiating the Au film with a Gaussian beam of

near-infrared (800 nm, 70 fs) laser pulse to cause ablation. One fringe corresponds to 20 nm, and

displacement can be read with an accuracy of about 1 nm. Although the expansion of the order of

micrometer using visible light has been reported, observation at the initial stage of expansion with a

nanometer scale was made possible by the use of soft X-ray for the first time. Figure 7 (c) shows the

reflectance image during ablation. Since the ablation is caused by the Gaussian beam, the excitation

density varies depending on the distance from the center. Existence of clear ring-shaped boundary lines

indicates that the ablation scheme changes discontinuously at certain excitation density.

References

[1] Dynamics of nuclear wave packets at the F center in alkali halides (review), T. Koyama and T.

Suemoto, Rep. Prog. Phys. 74, 076502 (2011).

[2] quasi-one-dimensional halogen-bridged Pt complex, S. Tomimoto, S. Saito, T. Suemoto, K. Sakata, J.

Takeda, and S. Kurita, Phys. Rev. B 60, 7961-7965 (1999).

[3] Real-time capturing of the nuclear wave-packet shape in self-trapped excitons, T. Matsuoka, J. Takeda,

S. Kurita, and T. Suemoto, Phys. Rev. Lett. 91, 247402 (2003).

[4] Splitting dynamics of nuclear wave packets in a Peierls insulator using femtosecond laser spectroscopy,

Y. Takahashi, K. Yasukawa, S. Kurita, and T. Suemoto, Phys. Rev. B 81, 081102(R) (2010).

Fig. 7 (a) is a conceptual diagram of a time-resolved soft X-ray interferometer. (b) Time resolved interference image associated with laser ablation. (c) Soft X-ray reflection image during laser ablation.

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[5] Optically detecting the edge-state of a three-dimensional topological insulator under ambient conditions

by ultrafast infrared photoluminescence spectroscopy, S. Maezawa, H. Watanabe, M. Takeda, K.

Kuroda, T. Someya, I. Matsuda, and T. Suemoto, Sci. Rep. 5, 16443 (2015).

[6] Access to hole dynamics in graphite by femtosecond luminescence and photoemission spectroscopy, T.

Suemoto, S. Sakaki, M. Nakajima, Y. Ishida, and S. Shin, Phys. Rev. B 87, 224302 (2013).

[7] Layer number dependence of carrier lifetime in graphenes observed using time-resolved mid-infrared

luminescence, H. Watanabe, T. Kawasaki, T. Iimori, F. Komori, and T. Suemoto, Chem. Phys. Lett.

637, 58-62 (2015).

[8] Photoinduced metallic state in VO2 proved by the terahertz pump-probe spectroscopy, M. Nakajima, N.

Takubo, Z. Hiroi, Y. Ueda, and T. Suemoto, Appl. Phys. Lett. 92, 011907 (2008).

[9] Dynamics of photoinduced phase transitions in a Prussian blue analog studied by CN vibrational

spectroscopy (review), T. Suemoto, R. Fukaya, A. Asahara, H. Watanabe, H. Tokoro, and S. Ohkoshi,

Current Inorganic Chemistry, 6, 10-25 (2016).

[10] Photoinduced charge-transfer process in rubidium manganese hexacyanoferrate probed by Raman

spectroscopy, R. Fukaya, M. Nakajima, H. Tokoro, S. Ohkoshi, and T. Suemoto, J. Chem. Phys. 131,

154505 (2009).

[11] Growth Dynamics of Photoinduced Phase Domain in Cyano-Complex Studied by Boundary Sensitive

Raman Spectroscopy, A. Asahara, M. Nakajima, R. Fukaya, H. Tokoro, S. Ohkoshi, and T. Suemoto,

Acta Physica Polonica A 121, 375-378 (2012).

[12] Ultrafast dynamics of reversible photoinduced phase transitions in rubidium manganese

hexacyanoferrate investigated by midinfrared CN vibration spectroscopy, A Asahara, M Nakajima, R

Fukaya, H Tokoro, S. Ohkoshi, and T. Suemoto, Phys. Rev. B 86, 195138 (2012).

[13] Ultrafast time domain demonstration of bulk magnetization precession at zero magnetic field

ferromagnetic resonance induced by terahertz magnetic field, M. Nakajima, A. Namai, S. Ohkoshi,

and T. Suemoto, Opt. Exp. 18, 18260-18268 (2010).

[14] Coherent Control of Spin Precession Motion with Impulsive Magnetic Fields of Half-Cycle Terahertz

Radiation, K. Yamaguchi, M. Nakajima, and T. Suemoto, Phys. Rev. Lett. 105, 237201(2010).

[15] Enhanced spin-precession dynamics in a spin-metamaterial coupled resonator observed in terahertz

time-domain measurements, T. Kurihara, K. Nakamura, K. Yamaguchi, Y. Sekine, Y. Saito, M.

Nakajima, K. Oto, H. Watanabe, and T. Suemoto, Phys. Rev. B 90, 144408 (2014).

[16] Macroscopic Magnetization Control by Symmetry Breaking of Photoinduced Spin Reorientation with

Intense Terahertz Magnetic Near Field, T. Kurihara, H. Watanabe, M. Nakajima, S. Karube, K. Oto,

Y. Otani and T. Suemoto, Phys. Rev. Lett. 120, 107202-1-6 (2018)

[17] Single-shot picosecond interferometry with one-nanometer resolution for dynamical surface

morphology using a soft X-ray laser, T. Suemoto, K. Terakawa, Y. Ochi, T. Tomita, M. Yamamoto, N.

Hasegawa, M. Deki, Y. Minami, and T. Kawachi, Opt. Exp. 18, 14114-14122 (2010).