Biomedical subjects

M Kuwata-Gonokami

Publications and source records attributed to M Kuwata-Gonokami.

10 recordsLinked to original sources

PubMed2005

Study of orthoexciton-to-paraexciton conversion in Cu(2)O by excitonic Lyman spectroscopy.

Using time-resolved 1s-2p excitonic Lyman spectroscopy, we study the orthoexciton-to-paraexciton transfer, following the creation of a high density population of ultracold 1s orthoexcitons by resonant two-photon excitation with femtosecond pulses. An observed fast exciton-density dependent conversion rate is attributed to spin exchange between pairs of orthoexcitons. Implication of these results on the feasibility of Bose-Einstein condensation of paraexcitons in Cu(2)O is discussed.

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PubMed2002

Efficient two-photon light amplification by a coherent biexciton wave.

A reversible coupling between photon pair states and a long-lived, highly coherent biexciton wave in CuCl allows efficient phase-sensitive two-photon amplification or attenuation of ultrashort light pulses. We demonstrate a gain of 350 cm(-1) for a pump intensity of 1 MW/cm(2), nearly 2 orders of magnitude higher than that achievable with conventional parametric crystal amplifiers. We develop a theoretical model that describes this new type of parametric converter where the light pump is replaced by a coherent biexciton wave and show that it is well suited for the generation of entangled photons and the squeezing of an optical beam with ultrafast time gating.

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PubMed2001

Electron-hole droplet formation in direct-gap semiconductors observed by mid-infrared pump-probe spectroscopy.

Mid-infrared pump-probe measurements with subpicosecond time resolution reveal the existence of a metastable condensed phase of the electron-hole ensemble in a direct-gap semiconductor CuCl. High-density electrons and holes are directly created in a low-temperature state by the resonant femtosecond excitation of excitons above the Mott transition density. Strong metallic reflection with a plasma frequency Planck's over 2pi(omega)p approximately 0.5 eV builds up within 0.3 ps. Within a few picoseconds, the mid-infrared reflection spectrum is transformed from metalliclike into colloidlike. The observed resonance feature at Planck's over 2pi(omega)p/sqrt[3] allows us to obtain the carrier density in the metastable electron-hole droplets of 2x10(20) cm(-3).

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PubMed2000

Ultrafast optical nonlinearity in the quasi-one-dimensional mott insulator Sr2CuO3

We report strong instantaneous photoinduced absorption in the quasi-one-dimensional Mott insulator Sr2CuO3 in the IR spectral region. The observed photoinduced absorption is to an even-parity two-photon state that occurs immediately above the absorption edge. Theoretical calculation based on a two-band extended Hubbard model explains the experimental features and indicates that the strong two-photon absorption is due to a very large dipole coupling between nearly degenerate one- and two-photon states. Room temperature picosecond recovery of the optical transparency suggests the strong potential of Sr2CuO3 for all-optical switching.

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PubMed2000

Ultrafast spin dynamics and critical behavior in half-metallic ferromagnet: Sr2FeMoO6

Ultrafast spin dynamics in ferromagnetic half-metallic compound Sr2FeMoO6 is investigated by pump-probe measurements of the magneto-optical Kerr effect. The half-metallic nature of this material gives rise to anomalous thermal insulation between spins and electrons and allows us to pursue the spin dynamics from a few to several hundred picoseconds after the optical excitation. The optically detected magnetization dynamics clearly shows the crossover from microscopic photoinduced demagnetization to macroscopic critical behavior with universal power law divergence of relaxation time for a wide dynamical critical region.

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PubMed1996