Ultrafast Spectroscopy Group
In our daily life, we are surrounded by photonic materials, which work as photocatalyst, solar cells, light emitting diodes (LEDs), lasers, etc. The optical functions are associated with microscopic processes such as photo-excitation of electron-hole pairs, their energy-relaxation, transport, and radiative recombinations. The efficiency of the photonic materials often relies on the time scale -- typically between femtosecond and microsecond -- of the competing microscopic processes.
We study the ultrafast optical response of the photonic materials by using a pump-probe technique. Femtosecond optical pulses induce coherent phonons in the materials, with which we can monitor the collective atomic motions in the real time and extract information on the time-dependent electron-phonon coupling.
RECENT RESEARCH TOPICS
Sub 10fs ultrashort optical pulses can excite such high-frequency phonons as the in-plane C-C stretching (so called G mode) of graphite. The frequency of the coherent in-plane phonon upshifts with increasing excitation density. Time-resolved analysis reveals that the frequency upshifts instantaneously at the photoexcitation, and then relaxes to the stationary value in picosecond time scale.
Through collaboration with theoretical group, we have revealed the frequency upshift is due to the "non-adiabatic" effect that is characteristic to the quasi-2D electronic structure of graphite. This implies that electrons near the Fermi level cannot follow the nuclear motion, and the screening effect (or Kohn anomaly) is transiently weakened as a result.
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Electron-phonon interaction in low-dimensional systems
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Coherent phonons in wide-gap semiconductors
Wide-gap semiconductors such as GaN, ZnO and diamond are the current and next generation materials for ultraviolet LEDs and lasers. We examine the electron-phonon coupling in these systems by monitoring coherent phonons. For single crystal diamond, the lifetime of the coherent phonons was found to be > 7 picoseconds, which is extraordinarily long compared with the vibrational period of 24 femtoseconds. (figure top right of this page)
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Bismuth and antimony --- model systems of coherent phonon study
Coherent phonons of group V semimetals Bi and Sb have been experimentally studied for 20 years, yet there remain controversies in their generation mechanisms and their “collapse-revival” under extremely intense excitation (right figure). The systems attract revived attention because they offer ideal target for time-resolved x-ray diffraction study.
