著者

Tanaka Masanori Sawai Shinya Sengoku Masaya Kato Manabu Masumoto Yasuaki 舛本 泰章

出版者

American Institute of Physics

雑誌

Journal of applied physics (ISSN:00218979)

巻号頁・発行日

vol.87, no.12, pp.8535-8540, 2000-06

被引用文献数

30

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Nanocrystalline particles of ZnS:(Ag, Al) semiconductor phosphor, whose sizes are mostly 3–5 nm in diameter, are prepared by the gas-evaporation method with cw CO2 laser heating. The Raman scattering spectrum as well as the transmission electron microscope observation demonstrates that the crystallization of the nanoparticles was caused successfully through the gas-phase condensation. Under irradiation of ultraviolet light, the nanoparticles exhibit blue luminescence, as in the case of the starting material of ZnS:(Ag, Al) bulk powder. The peak of the luminescence spectrum of the nanoparticles shifts to lower energy with increasing delay time and also with decreasing excitation intensity, showing that the luminescence originates from the donor–acceptor pair recombination. However, it is concluded that the luminescence of the nanoparticles is not ascribed to the blue Ag luminescence mechanism responsible for the luminescence of the bulk powder, by taking into account the spatial confinement of an electron trapped at the donor and a hole at the acceptor. It is argued that the luminescence mechanism of the nanoparticles is the so called self-activated luminescence, which involves zinc vacancies.

著者

Sugisaki Mitsuru Ren Hong-Wen Nair Selvakumar V. Nishi Kenichi Masumoto Yasuaki

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.66, no.23, pp.235309, 2002-12

被引用文献数

56 47

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The effects of external electric and magnetic fields on InP self-assembled quantum dots (QDs) were investigated by means of single dot spectroscopy. By systematically changing a bias applied to the sample, successive energy shifts of the photoluminescence (PL) peaks from excitons and biexcitons due to the quantum confined Stark effect were clearly observed. The quadratic Stark coefficient was evaluated to be of the order of 10-31 Fcm2. The energy separation of the PL peaks arising from the excitons and biexcitons changed with the applied electric field, reflecting a slight difference of the Stark coefficient between the exciton and biexciton states. The existence of permanent dipole moments was also revealed in both the exciton and biexciton states. The spatial separations between the electrons and holes along the growth direction in a QD were estimated to be 7 Å for the exciton state, and 8 Å for the biexciton state. Further, the diamagnetic shift and the Zeeman splitting of the exciton states were clearly observed in a magnetic field. It was found that the diamagnetic coefficient gradually decreases on decreasing the QD size. A simple qualitative model can explain that this result is due to competition between quantum confinement and magnetic confinement.

著者

Tomimoto Shinichi Kurokawa Atsushi Sakuma Yoshiki Usuki Tatsuya Masumoto Yasuaki

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.76, no.20, pp.205317, 2007-11

被引用文献数

14 10

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Recombination dynamics of excitons confined in disk-shaped InAs/InP quantum dots is studied by time-resolved photoluminescence measurements. By comparing the result with that in a homologous ultrathin quantum well, it is revealed that the lateral confinement of excitons suppresses the thermal variation of the radiative recombination lifetime. The oscillator strength of the radiative transition is reduced with the decrease of the disk height, which is attributed to the increasing tunneling into the InP barrier of the exciton wave function.

著者

Tomimoto Shinichi Nozawa Shinsuke Terai Yoshikazu Kuroda Shinji Takita Kôki Masumoto Yasuaki

出版者

The American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.81, no.12, pp.125313, 2010-03

被引用文献数

11

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We have studied the spin dynamics of confined electrons in an ultrathin CdTe/ZnTe quantum well andself-assembled quantum dots by time-resolved Kerr-rotation technique. The dependence of the spin-precessionfrequency on the magnetic field direction shows the anisotropy of the g-factor tensor, which is the opposite ofthe usual quantum wells with moderate widths. The geometrical anisotropy of the confinement also affects theinitial orientation of the electron spins created optically, as revealed clearly with the use of oblique-incidencepump pulses.

著者

Sun Jianhui Zhao Jialong Masumoto Yasuaki

出版者

American Institute of Physics

雑誌

Applied physics letters (ISSN:00036951)

巻号頁・発行日

vol.102, no.5, pp.053119, 2013-02

被引用文献数

49

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We demonstrate the electron transfer (ET) processes from CuInS2/ZnS core/shell quantum dots (QDs) into porous anatase TiO2 films by time-resolved photoluminescence spectroscopy. The rate and efficiency of ET can be controlled by changing the core diameter and the shell thickness. It is found that the ET rates decrease exponentially at the decay constants of 1.1 and 1.4 nm–1 with increasing ZnS shell thickness for core diameters of 2.5 and 4.0 nm, respectively, in agreement with the electron tunneling model. This shows that optimized ET efficiency and QD stability can be realized by controlling the shell thickness.

著者

Tomimoto Shinichi Kawana Keisuke Murakami Akira Masumoto Yasuaki

出版者

Elsevier B.V.

雑誌

Journal of Luminescence (ISSN:00222313)

巻号頁・発行日

vol.133, pp.162-168, 2013-01

被引用文献数

1

---

We have experimentally observed the spin polarization process of single electrons in InP/InGaP quantum dots by time-resolved Kerr rotation measurements. It is found that the inversion of the spin polarization direction occurs with the variation of the intensity of the optical pulse. The spin coherence lifetime abruptly changes on the occurrence of the inversion. We have reproduced the inversion in numerical simulations using the density operator of the electron–trion four-level system, assuming a small deviation of the optical pulse from circular polarization. The change of the spin lifetime is attributed to the qualitative change of the four-level system in the electric polarization state.

著者

Zhao Jialong Nair Selvakumar V. Masumoto Yasuaki

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.63, no.3, pp.033307, 2001-01

被引用文献数

7 11

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The size dependence of excitonic states and vibrational modes in CuCl cubic quantum dots embedded in NaCl crystals was studied by means of site-selective persistent spectral hole burning spectroscopy. It is shown that the interaction of the longitudinal optical (LO) phonon with the exciton results in the formation of coupled exciton-phonon modes when the energy of the LO phonon approaches the energy spacings between the ground and excited states of the exciton. The energy anticrossings of the LO phonon modes with two higher optically allowed excited states of the exciton in CuCl QD’s were clearly observed at confinement energies of about 10 and 6 meV, respectively. In addition, the linewidth of the LO phonon sideband in CuCl QD’s in NaCl crystals was found to increase abruptly from 0.3 meV to about 1.2 meV at a confinement energy of about 23 meV, which appears to result from the exciton-phonon mixing of the LO phonon mode with the first excited state of the exciton. The experimental results are qualitatively understood by the LO phonon renormalization theory.

著者

Ignatiev Ivan V. Kozin Igor E. Davydov Valentin G. Nair Selvakumar V. Lee Jeong-Sik Ren Hong-Wen Sugou Shigeo Masumoto Yasuaki

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.63, no.7, pp.075316, 2001-02

被引用文献数

38 48

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Phonon resonances observed in the photoluminescence (PL) spectra of InP and In0.35Ga0.65As self-assembled quantum dots (QD’s) in an external electric field are studied in detail. The resonances are shown to arise from fast phonon-assisted relaxation of hot carriers, and to become observable when the PL is quenched by nonradiative losses from excited states. A simple model is developed that considers tunneling of the carriers from the QD’s into the barrier layer as the main process responsible for PL quenching in the presence of an electric field. From this model, the depth of the potential well for holes is estimated to be 10–20 meV for the InP QD’s. The PL kinetics measurement is performed with a time resolution of 6 ps. Clear evidence of surprisingly fast carrier relaxation with emission of high-energy acoustic phonons is found. Further acceleration of the carrier relaxation is observed under strong optical pumping. We consider this effect to be caused by Auger-like carrier-carrier scattering processes. Acceleration of the relaxation observed at elevated temperatures is ascribed to stimulated phonon emission.

著者

Matsumoto Takahiro Suzuki Jun-ichi Ohnuma Masato Kanemitsu Yoshihiko Masumoto Yasuaki

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.63, no.19, pp.195322, 2001-05

被引用文献数

62

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The optical absorption spectrum in nanocrystalline silicon (n-Si) was determined from both light transmittance and reflectance measurements. We observed that n-Si has a phonon structure in the optical absorption spectrum. This structure originates from momentum-conserving TO phonon absorption and emission, and provides direct evidence that n-Si is an indirect-band-gap semiconductor with quantum size effects. By using small-angle x-ray scattering to measure the nanocrystal size distribution, we found that the band-gap widening varies as (1/L)1.6 with decreasing nanocrystal diameter L.

著者

Tokunaga Eiji Ivanov A. L. Nair Selvakumar V. Masumoto Yasuaki

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.63, no.23, pp.233203, 2001-06

被引用文献数

11 6

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We report observation of the inverse polariton series and a detailed experimental study of the exciton components in polaritons (Hopfield coefficients). Spontaneous emission of excitonic molecules into outgoing polaritons associated with the i(=1,2,3,4,5)th exciton states in bulk CuCl is detected and analyzed by the bipolariton model. Because the intensities of the emission lines, which form the inverse polariton series, are determined by the exciton components in the final-state polaritons, we are able to measure the Hopfield coefficients for the highly composite outgoing polaritons. Quantum interference in the optical decay of excitonic molecules, due to the multiple exciton components in the polariton and biexciton states, is also demonstrated.

著者

Tokunaga Eiji Kurihara Kazuyoshi Baba Motoyoshi Masumoto Yasuaki Matsuoka Masahiro

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.64, no.4, pp.045209, 2001-07

被引用文献数

10 9

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Radiative decay of biexcitons wave-vector (K) selectively generated in a low-K region (0<~

著者

Masumoto Yasuaki Hirata Yuuki Mohan Premila Motohisa Junichi Fukui Takashi

出版者

American Institute of Physics

雑誌

Applied physics letters (ISSN:00036951)

巻号頁・発行日

vol.98, no.21, pp.211902, 2011-05

被引用文献数

9

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The photoluminescence from single InP/InAs/InP core-multishell nanowires (CMNs) was highly linearly polarized in perpendicular to the nanowire long axis. Taking account of dielectric confinement of light field, we found the internal optical dipole squared perpendicular to the nanotube long axis d0x2 is larger than that parallel to the axis d0z2 by two orders of magnitude. With the increase in the thickness of InAs nanotubes, the ratio β = d0x2/d0z2 increases. This strong anisotropy in β comes from nearly perfect wurtzite structure of InP/InAs/InP CMNs and small inclusion of pz-character into the Γ7(B)-hole state.

著者

Masumoto Yasuaki Yoshida Seitaro Ikezawa Michio Tomimoto Shinichi Sakuma Yoshiki

出版者

American Institute of Physics

雑誌

Applied physics letters (ISSN:00036951)

巻号頁・発行日

vol.98, no.6, pp.061905, 2011-02

被引用文献数

5 4

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We observed that the biexciton binding energy in InAs quantum rhombic disks (QRDs) is enhanced by twice compared with that for InAs quantum dots (QDs) so far reported around 1.24 μm nearby the telecommunication wavelength. The heterodyne-detected four-wave-mixing detected the exciton-biexciton quantum beat superposed on photon echo decay, giving the biexciton binding energy of 3.4 meV to 3 monolayer (ML) InAs QRDs and 4.1 meV to 4 ML InAs QRDs, respectively. The largest biexciton binding energy of 4.1 meV in InAs QDs is ascribed to increased electron-hole overlap in confined geometry with a minimized strain distribution.

著者

Fujiwara Seiji Bando Kazuki Masumoto Yasuaki Sasaki Fumio Kobayashi Shunsuke Haraichi Satoshi Hotta Shu

出版者

American Institute of Physics

雑誌

Applied physics letters (ISSN:00036951)

巻号頁・発行日

vol.91, no.2, pp.021104, 2007-07

被引用文献数

30 27

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Laser oscillation of whispering gallery modes was observed in microring structures of semiconducting thiophene/phenylene co-oligomer (TPCO) crystals at room temperature. Microring structures were formed by dry etching from thin film crystals of TPCO. The thresholds for the laser oscillation of a microring and a thin film crystal are 200 and 1400 µJ/cm2 for picosecond excitation, respectively. Therefore, the threshold for the microring was reduced to 1/7 of that for the thin film crystal. The dramatic reduction of threshold clearly demonstrates the importance of microcavity in making efficient organic semiconductor lasers.

著者

Tomimoto Shinichi Kawana Keisuke Murakami Akira Masumoto Yasuaki

出版者

American Physical Society

雑誌

Physical review B (ISSN:10980121)

巻号頁・発行日

vol.85, no.23, pp.235320, 2012-06

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We have experimentally studied the spin dynamics of excitons, electrons, and trions in charge-tunable InP/InGaP quantum dots (QDs) excited by picosecond resonant laser pulses by observing the time-resolved Kerr rotation. In singly charged QDs, inversion of the spin polarization direction of doped electrons is found to be caused simply by variation in the pulse intensity, which is accompanied by an abrupt change of the spin coherence time. This phenomenon is reproduced by density-matrix calculations allowing for the reaction on the QD electron-trion four-level system during its coherent radiation emission. This result means that the optical coherence is another critical factor affecting electron spin coherence.