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Takayoshi Kobayashi - One of the best experts on this subject based on the ideXlab platform.

  • Exciton-LO-phonon couplings in spherical semiconductor Microcrystallites
    SPIE milestone series, 2005
    Co-Authors: Shintaro Nomura, Takayoshi Kobayashi
    Abstract:

    Exciton-LO-phonon couplings in CdS x Sc 1-x semiconductor Microcrystallites (x=0.12±0.05) are investigated by measuring the temperature dependence of the width and energy of excitons by electroabsorption. The LO phonons are shown semiquantitatively to contribute to the experimentally obtained temperaure dependencies of the width and energy of excitons. The dependence of the coupling constant (the Huang-Rhys parameter) on the radius of Microcrystallites is calculated for CdSe and GaAs Microcrystallites. The phonon confinement effects are considered with "free-standing" and "rigid"-boundary conditions. As for the exciton state, nonparabolicity of the conduction band and the valence-band mixing are considered in order to obtain a precise exciton wave function, which is crucially important in calculating the Huang-Rhys parameter in a microcrystallite. The exciton-confined-optical-phonon interaction Harniltonian is constructed for a microcrystallite. It is found that the Huang-Rhys parameters have a minimum at a radius of 70 A for CdSe and 270 A for GaAs Microcrystallites. The size dependence of the Huang-Rhys parameter is also calculated for a microcrystallite with an extra charge at the spherical-particle center. The lowest (s,S 3/2 ) state in the trapped state is found to have small transition probability and g values of 1 in CdSe (R =30 A) and 0.01 in GaAs? = 100 A). The higher states are found to have larger transition probability and g values of 0.7 in CdSe (R =30 A) and 0.01 in GaAs (R = 100 A). These results suggest that large g values observed experimentally in CdS and CdSe Microcrystallites originate from extrinsic effects such as the presence of charged point defects inside the microcrystallite.

  • Large g value in CuCl semiconductor Microcrystallites
    Physical Review B, 1993
    Co-Authors: Shintaro Nomura, Kazuhiko Misawa, Yusaburo Segawa, Takayoshi Kobayashi
    Abstract:

    A magnetic circular dichroism measurement is applied to semiconductor Microcrystallites. The effective g values of CuCl semiconductor Microcrystallites embedded in polymer matrix are found to be 0.447 and -0.18 for the Z 3 and Z 12 exciton states, respectively, the former being 1.49 times enhanced from the bulk value. The result is a direct consequence of the quantum confinement effect, explained in terms of the change in amplitudes of the components with higher (index in the lowest exciton state

  • LO Phonons in CdS(x)Se(1-x) Semiconductor Microcrystallites,
    1992
    Co-Authors: Takayoshi Kobayashi, Shintaro Nomura
    Abstract:

    Abstract : It is of interest and importance to study semiconductor Microcrystallites as three dimensionally confined system in conjecture with one dimensionally confined quantum well systems. Because the oscillator strengths are concentrated on several discrete states, Microcrystallites are expected to have large optical nonlinearities. To accomplish this large optical nonlinearities, however, the spectral width play a major role. The intrinsic width is introduced by exciton-phonon coupling rather than an inhomogeneous width which originates from the distribution in the size of Microcrystallites. Recently, couplings to longitudinal optical (LO) phonons were investigated by various groups and it was suggested that a microcrystallite of smaller radius exhibits larger coupling to LO phonons because of the increase in couplings to shorter wavelength phonons by the confinement of the exciton wave functions in real space. In the former part of this paper, the temperature dependence of the exciton energy and the width is investigated by the modulation spectroscopy of the absorption. In the latter part, the exciton-LO phonon coupling constant (Huang-Rhys parameter) is investigated theoretically.

  • exciton lo phonon couplings in spherical semiconductor Microcrystallites
    Physical Review B, 1992
    Co-Authors: Shintaro Nomura, Takayoshi Kobayashi
    Abstract:

    Exciton — LO-phonon couplings in CdS Se& „semiconductor Microcrystallites (x=0.12+0.05) are investigated by measuring the temperature dependence of the width and energy of excitons by electroabsorption. The LO phonons are shown semiquantitatively to contribute to the experimentally obtained temperaure dependencies of the width and energy of excitons. The dependence of the coupling constant (the Huang-Rhys parameter) on the radius of Microcrystallites is calculated for CdSe and GaAs Microcrystallites. The phonon confinement effects are considered with "free-standing" and "rigid"-boundary conditions. As for the exciton state, nonparabolicity of the conduction band and the valence-band mixing are considered in order to obtain a precise exciton wave function, which is crucially important in calculating the Huang-Rhys parameter in a microcrystallite. The exciton-confined-optical-phonon interaction Hamiltonian is constructed for a microcrystallite. It is found that the Huang-Rhys parameters have 0 0 a minimum at a radius of 70 A for CdSe and 270 A for GaAs Microcrystallites. The size dependence of the Huang-Rhys parameter is also calculated for a microcrystallite with an extra charge at the spherical-particle center. The lowest (s, S3/2) state in the trapped state is found to have small transition probability and g values of 1 in CdSe (R =30 A) and 0.01 in GaAs (R =100 A). The higher states are found to have larger transition probability and g values of 0.7 in CdSe (R =30 A) and 0.01 in GaAs (R =100 A). These results suggest that large g values observed experimentally in CdS and CdSe Microcrystallites originate from extrinsic effects such as the presence of charged point defects inside the microcrystallite.

  • Excitonic Superradiance and Its Quenching by Confined-Acoustic Phonons in CdS Microcrystallites
    Nonlinear Optics, 1992
    Co-Authors: Takayoshi Kobayashi, Kazuhiko Misawa, Hiroshi Yao, Toyoharu Hayashi
    Abstract:

    ABSTRACT This paper reports the first observation of the excitonic superradiance from CdS Microcrystallites. We developed a new method to prepare CdS Microcrystallites embedded in polymer films, which shows enhanced band-edge emission due to the direct recombination of an exciton in the microcrystallite. The radiative decay rate of the microcrystallite was almost proportional to the particle volume, suggesting its superradiative decay property. The radiative decay rate was almost constant below a certain threshold temperature and it decreased nearly linearly with inverse temperature. This phenomenon is interpreted in terms of the quenching of the superradiance by acoustic-phonons confined in the microcrystallite. The threshold temperatures of Microcrystallites of 25 A and 29 A in radius were measured to be 45 K and 30 K, which correspond to the calculated frequencies of radial compression mode of 30 cm−1 (43 K) and 26 cm−1 (37 K), respectively.

Yasuo Oka - One of the best experts on this subject based on the ideXlab platform.

  • magneto optical properties of cd1 xmnxse Microcrystallites in sio2 glass prepared by rf sputtering
    Journal of Applied Physics, 1993
    Co-Authors: Kohei Yanata, Kyoichi Suzuki, Yasuo Oka
    Abstract:

    Cd1−xMnxSe Microcrystallites have been successfully prepared by rf sputtering in SiO2 matrix glass. Optical absorption edge shows high energy shifts of 20–550 meV to the high energy as compared to its position in bulk crystals; these shifts are interpreted as evidence for exciton confinement in the Microcrystallites. Annealing of the Cd1−xMnxSe Microcrystallites at 450–650 °C for 0.5–5 h increases the size of the Microcrystallites from 20 to 250 A. In an external magnetic field of 7 T the absorption edge shifts by 40 meV towards the low energy side due to the exchange coupling of the excitons with the d electrons of Mn2+ ions in the Cd1−xMnxSe Microcrystallites. The spectral red shifts of the exciton luminescence in the Cd1−xMnxSe Microcrystallites are also observed in the presence of magnetic fields. The corresponding effective g value varied from 138 at 1 T to 99 at 7 T.

  • Magneto‐optical properties of Cd1−xMnxSe Microcrystallites in SiO2 glass prepared by rf sputtering
    Journal of Applied Physics, 1993
    Co-Authors: Kohei Yanata, Kyoichi Suzuki, Yasuo Oka
    Abstract:

    Cd1−xMnxSe Microcrystallites have been successfully prepared by rf sputtering in SiO2 matrix glass. Optical absorption edge shows high energy shifts of 20–550 meV to the high energy as compared to its position in bulk crystals; these shifts are interpreted as evidence for exciton confinement in the Microcrystallites. Annealing of the Cd1−xMnxSe Microcrystallites at 450–650 °C for 0.5–5 h increases the size of the Microcrystallites from 20 to 250 A. In an external magnetic field of 7 T the absorption edge shifts by 40 meV towards the low energy side due to the exchange coupling of the excitons with the d electrons of Mn2+ ions in the Cd1−xMnxSe Microcrystallites. The spectral red shifts of the exciton luminescence in the Cd1−xMnxSe Microcrystallites are also observed in the presence of magnetic fields. The corresponding effective g value varied from 138 at 1 T to 99 at 7 T.

  • Preparation and Properties of Cd1-xMnxSe Microcrystallites Embedded in SiO2Glass Films
    Japanese Journal of Applied Physics, 1993
    Co-Authors: Kohei Yanata, Kyoichi Suzuki, Yasuo Oka
    Abstract:

    Cd1-xMnxSe Microcrystallites have been prepared by the rf-sputtering method. Subsequent annealing was carried out in an Ar atmosphere. In the presence of magnetic field, the photoluminescence spectra from the Cd1-xMnxSe Microcrystallites shift towards the low energy side due to the exchange interaction of the exciton with contained Mn2+ ions in the Microcrystallites. Observed red shifts in the high magnetic fields deviate from the Brillouin function-like dependence which is caused by the single Mn2+ ions. The results indicate that the role of the Mn2+ pairs and the antiferromagnetic interaction between the Mn2+ ions are more significant in the Microcrystallites than in bulk crystals.

Y Segawa - One of the best experts on this subject based on the ideXlab platform.

  • room temperature ultraviolet laser emission from self assembled zno microcrystallite thin films
    Applied Physics Letters, 1998
    Co-Authors: Zikang Tang, G K L Wong, Ping Yu, Masashi Kawasaki, Akira Ohtomo, Hideomi Koinuma, Y Segawa
    Abstract:

    Room-temperature ultraviolet (UV) laser emission of ZnO microcrystallite thin films is reported. The hexagonal ZnO Microcrystallites are grown by laser molecular beam epitaxy. They are self-assembled and parallelly arrayed on sapphire substrates. The facets of the hexagons form natural Fabry-Perot lasing cavities. The optical gain for the room-temperature UV stimulated emission is of an excitonic nature and has a peak value an order of magnitude larger than that of bulk ZnO crystal. The observation of room-temperature UV lasing from the ordered, nano-sized ZnO crystals represents an important step towards the development of nanometer photoelectronics. © 1998 American Institute of Physics.

Kohei Yanata - One of the best experts on this subject based on the ideXlab platform.

  • magneto optical properties of cd1 xmnxse Microcrystallites in sio2 glass prepared by rf sputtering
    Journal of Applied Physics, 1993
    Co-Authors: Kohei Yanata, Kyoichi Suzuki, Yasuo Oka
    Abstract:

    Cd1−xMnxSe Microcrystallites have been successfully prepared by rf sputtering in SiO2 matrix glass. Optical absorption edge shows high energy shifts of 20–550 meV to the high energy as compared to its position in bulk crystals; these shifts are interpreted as evidence for exciton confinement in the Microcrystallites. Annealing of the Cd1−xMnxSe Microcrystallites at 450–650 °C for 0.5–5 h increases the size of the Microcrystallites from 20 to 250 A. In an external magnetic field of 7 T the absorption edge shifts by 40 meV towards the low energy side due to the exchange coupling of the excitons with the d electrons of Mn2+ ions in the Cd1−xMnxSe Microcrystallites. The spectral red shifts of the exciton luminescence in the Cd1−xMnxSe Microcrystallites are also observed in the presence of magnetic fields. The corresponding effective g value varied from 138 at 1 T to 99 at 7 T.

  • Magneto‐optical properties of Cd1−xMnxSe Microcrystallites in SiO2 glass prepared by rf sputtering
    Journal of Applied Physics, 1993
    Co-Authors: Kohei Yanata, Kyoichi Suzuki, Yasuo Oka
    Abstract:

    Cd1−xMnxSe Microcrystallites have been successfully prepared by rf sputtering in SiO2 matrix glass. Optical absorption edge shows high energy shifts of 20–550 meV to the high energy as compared to its position in bulk crystals; these shifts are interpreted as evidence for exciton confinement in the Microcrystallites. Annealing of the Cd1−xMnxSe Microcrystallites at 450–650 °C for 0.5–5 h increases the size of the Microcrystallites from 20 to 250 A. In an external magnetic field of 7 T the absorption edge shifts by 40 meV towards the low energy side due to the exchange coupling of the excitons with the d electrons of Mn2+ ions in the Cd1−xMnxSe Microcrystallites. The spectral red shifts of the exciton luminescence in the Cd1−xMnxSe Microcrystallites are also observed in the presence of magnetic fields. The corresponding effective g value varied from 138 at 1 T to 99 at 7 T.

  • Preparation and Properties of Cd1-xMnxSe Microcrystallites Embedded in SiO2Glass Films
    Japanese Journal of Applied Physics, 1993
    Co-Authors: Kohei Yanata, Kyoichi Suzuki, Yasuo Oka
    Abstract:

    Cd1-xMnxSe Microcrystallites have been prepared by the rf-sputtering method. Subsequent annealing was carried out in an Ar atmosphere. In the presence of magnetic field, the photoluminescence spectra from the Cd1-xMnxSe Microcrystallites shift towards the low energy side due to the exchange interaction of the exciton with contained Mn2+ ions in the Microcrystallites. Observed red shifts in the high magnetic fields deviate from the Brillouin function-like dependence which is caused by the single Mn2+ ions. The results indicate that the role of the Mn2+ pairs and the antiferromagnetic interaction between the Mn2+ ions are more significant in the Microcrystallites than in bulk crystals.

Zikang Tang - One of the best experts on this subject based on the ideXlab platform.

  • room temperature ultraviolet laser emission from self assembled zno microcrystallite thin films
    Applied Physics Letters, 1998
    Co-Authors: Zikang Tang, G K L Wong, Ping Yu, Masashi Kawasaki, Akira Ohtomo, Hideomi Koinuma, Y Segawa
    Abstract:

    Room-temperature ultraviolet (UV) laser emission of ZnO microcrystallite thin films is reported. The hexagonal ZnO Microcrystallites are grown by laser molecular beam epitaxy. They are self-assembled and parallelly arrayed on sapphire substrates. The facets of the hexagons form natural Fabry-Perot lasing cavities. The optical gain for the room-temperature UV stimulated emission is of an excitonic nature and has a peak value an order of magnitude larger than that of bulk ZnO crystal. The observation of room-temperature UV lasing from the ordered, nano-sized ZnO crystals represents an important step towards the development of nanometer photoelectronics. © 1998 American Institute of Physics.