Koshihara Okimoto Lab.
Past News

Yoichi Okimoto, Ph.D
Yoichi Okimoto
03-5734-3895 okimoto.y.aam.titech.ac.jp
Associate Professor, Department of Chemistry

Mar. 1998 Ph. D, Department of Physics, University of Tokyo (Tokyo, Japan)

Apr.1998 - Mar.2002 Research scientist, Joint Research Center for Atom Technology (JRCAT)
Apr.2002 - Mar.2006 Researcher, Correlated Electron Research Center (CERC), Advanced Industrial Science and
Technology (AIST)
Apr. 2006- Present Position

Research field:
EOptical spectroscopy of strongly correlated materials (UV, visible, IR, THz, Raman, etc..)
EUltrafast control of electronic properties in solid state materials
Eh-index: 40 (web of science, @2015)

Recent activities:
EInvited seminar during 16-22 Nov.:
@Giving invited seminars in the following 4 places
@titled gFemtosecond spectroscopy and ultrafast dynamics in spincrossover cobalt perovskitesh.
@@-Jawaharial Nehru Univ. (JNU), 16 Nov.
@@-Indian institute of technology (IIT), 17 Nov.
@@-Indian Association for the Cultivation of Science (IACS), 21 Nov.
@@-Central Glass and Ceramic Research Institute (CGCRI), 22 Nov.
EInvited talk: gPhotoexcited states in cobalt perovskitesh, EMN meeting on ultrafast research, 16-19 Nov. 2015, Las Vegas, USA.
ELecture at Tsinghua univ. gLight and the interaction with materialshi8th-12th, Junej
ESeminar at Rennes 1st. Univ. (14th March)
EInvited talk: gUltrafast dynamics in spin crossover cobaltitesh, 5th International Conference on Photoinduced Phase Transitions and Cooperative Phenomena (PIPT5), 8 - 13 June 2014, Bled, Slovenia
ESeminar at Lawrence Berkeley National Laboratory (8th August)

My Research:

Photo-induced phase change of spin-crossover Co ceramics

It is well known that pedovskite-type cobalt oxides containing Co3{ ions often show a spin-crossover phenomenon with varying temperature. This is a spin configuration change in Co3{ ion between low spin (LS) and high spin (HS) or intermediate spin (IS) state in accordance with insulator-metal transition. We regard this cobaltite as a novel photoactive material, and performed femtosecond reflection spectroscopy on the cobalt perovskites. The followings are our recent activities:

(1) Pr0.5Ca0.5CoO3
EY. Okimoto et al., Phys, Rev. Lett. 103, 027402 (2009).
EY. Okimoto et al., Phys. Rev. B 83, 161101(R) (2011).

Pr0.5Ca0.5CoO3 undergoes the spin state transition between LS and IS state at ca. 90 K. After photoirradiation at 30 K, the reflectivity showed a sudden and gigantic increase with subsequent variation depending on the observed photon energy. A numerical calculation of Maxwellfs equations on the photoexcited region enabled us to see not only photoinduced I-M phase transition but also the real space dynamics of the photoinduced metallic domain at the velocity of ultrasonic wave. (This works were done by the collaboration with Itoh group in Materials and Structures Laboratory, Tokyo Institute of Technology.)

(2) RBaCo2O6- iR=Sm, Gd, and Tbj
EY. Okimoto et al., Phys. Rev. B, 84, 121102(R) (2011).

We performed femtosecond reflection spectroscopy in a cobalt oxide with double-perovskite structure, which undergoes the SC transition at around 350 K. In this system, the electronic transfer tA is controlled by changing the rare earth species, R. The transient reflectivity as well as the calculated optical conductivity showed an ultrafast change at room temperature, indicating (1) the appearance of a hidden state just after the photoirradiation different from the high temperature metallic state, which is assigned as the HS-hole bound state suggested by the recent theoretical calculation, and (2) that the transferred spectral weight by photoexcitation increases with increasing tA, which is also consistent with theoretical prediction by Ishihara group. These results indicate an important role of electronic correlation in the appearance of the photoinduced phase transition. (This work was done by the collaboration with Arima group at Univ. of Tokyo.)

(3) La1.5Sr0.5CoO4
EY. Okimoto et al., J. Phys. Soc. Jpn., 82, 187401 (2013).

A layered perovskite-type cobalt oxide, La1.5Sr0.5CoO4 is a K2NiF4-type and shows a checkerboard-type charge ordering of Co2+ and Co3+ at about 800 K. We performed femtosecond reflection spectroscopy in the cobalt oxide at room temperature and observed large change of reflection and transmittance. To investigate the spatial variation of the photoexcited area, we measured the transient changes of three independent optical quantities, i.e., transmittance, reflectivity, and backside reflectivity, as well as their fluence dependence at 0.25 eV. Using the numerical calculations based on Maxwell's equations, we calculated not only the dielectric constant but also the decay length of the photoexcited region (L) and analyzed the real-space dynamics of the photoexcited area in terms of the fluence and time dependence of L. With the information on the time dependence of L and Kramers-Kronig analysis, we numerically calculated the transient optical conductivity spectra, which showed instantaneous formation of a polaron-like absorption peak in the mid-infrared region. (This work was done by the collaboration with Itoh group and Sasagawa group in Materials and Structures Laboratory, Tokyo Institute of Technology.)

(4) BiCoO3
Another important aspect of perovskite-type oxides is ferroelectric behavior, observed in some lead titanates, for example. BiCoO3 is known to show a polar behavior similar to the lead titanate system. Besides, Oka et al. have recently revealed that BiCoO3 shows the spin crossover and the resultant structural transformation by applying external pressure. We performed ultrafast pump-probe spectroscopy in BiCoO3 using femtosecond laser pulses and investigated the ferroelectricity in the photoexcited state in terms of pump-probe nonlinear spectroscopy. After the photoirradiation of visible laser pulse at room temperature, relative change of the second harmonic intensity showed sudden reduction by 60%, implying ultrafast optical control of the polar structure.