In our daily life, we are surrounded by photonic materials, which work as photocatalysts, 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 scales -- 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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Detection mechanisms of coherent phonons: allowed and forbidden Raman scattering
Once coherent phonons are generated in a crystal, they modify the bulk optical property periodically and thus enable detection through reflectivity and transmission. We have revealed that various Raman scattering processes are at work to associate the nuclear motions with the dielectric function. For a probe wavelength resonant with the absorption of a crystal, “forbidden” Raman scattering processes can contribute in addition to “allowed” Raman processes. When the polarization angle of the probe light is rotated, the forbidden and allowed processes are observed as interfering isotropic and anisotropic coherent phonon amplitudes.
(See more detail about coherent phonon detection mechanisms.)
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Charge injection and recombination dynamics at interfaces of dye-sensitized solar cells
Dye-sensitized solar cells are long expected to be the next generation, low cost photovoltatic cells. Their device performance is determined by competition between microscopic processes -- charge injection from dyes to TiO2, reduction of dye cations by electrolytes, recombination of conduction band electrons and dye cations -- which typically occur on time scales from femtosecond to millisecond. We are aiming to provide feedback to the development of novel dyes and additives from view point of optical physics by revealing the ultrafast charge dynamics at the interface of dye-sensitized solar cells.
NEWS!
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Our paper, titled “Allowed and forbidden Raman scattering mechanisms for detection of coherent LO phonon and plasmon-coupled modes in GaAs” has been published online in Phys. Rev. B.