Shinichi Hirose

Fujitsu Ltd., Kawasaki Si, Kanagawa, Japan

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Publications (29)43.35 Total impact

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    ABSTRACT: We investigated the electronic structures of so-called ‘double-capped’ InAs/InP quantum dot suitable for a 1.55-μm band single-photon emitter. From a transmission-electron-microscope image of the quantum dots, we extracted accurate information of the size and shape. We calculated the electron and hole eigenenergies by applying an 8-band k·p perturbation method into the quantum dot model based on the obtained size and shape. The calculated energies of the excitonic excited states agreed well with the resonant absorption peaks in a previously measured photoluminescence excitation spectrum with peaks that had been selected for quasi-resonant excitation to improve single-photon purity. This result suggests that the direct exciton creation into these states contributes to the suppression of excess carriers in the vicinity of the quantum dots, thus leading to a decrease in unwanted multiple-photon emission probabilities. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 02/2011; 8(2). DOI:10.1002/pssc.201000590
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    ABSTRACT: We have developed a high-performance single-photon source (SPS) operating at 1.5 mum wavelength. The source is an InAs/InP quantum dot with a horn-shaped nanostructure. A resonant excitation to the p-shell state helps achieve a single-photon efficiency of 5.8% after coupling into a single-mode fiber with a second-order correlation value of g(2)(0)˜ 0.055. The performance of the source has been assessed by integrating it into a conventional quantum key distribution system. We have successfully transmitted secure keys over a 50 km commercial fiber, exceeding the previously reported range for an SPS operating below 1.3 mum.
    Applied Physics Express 09/2010; 3(9). DOI:10.1143/APEX.3.092802 · 2.37 Impact Factor
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    ABSTRACT: Practical single-photon emitters (SPEs) using quantum dots (QDs) require high quantum efficiency in order to achieve high-rate and long-distance quantum communication. The strong Coulomb interactions between the carriers in QDs result in the dark-neutral and charged exciton states having distinct energies with bright-neutral excitons. High quantum efficiency can be achieved by suppressing the exclusive processes of the dark and charged excitons such as bright-neutral exciton recombination. However, the selective generation of bright-neutral or charged excitons has not been demonstrated for electrically pumped SPEs. We designed a p-i-n diode structure for constructing,an SPE with very high quantum efficiency. This structure enables resonant-tunneling injection of electrons from a quantum well into a CID with two holes. Two holes are induced by controlling a p-doped layer in the vicinity of the CID and the bias voltage. Electrons are injected into the OD one by one using the Coulomb blockade effect. This carrier injection efficiently forms positively charged excitons. In this positively charged exciton state, a single photon is generated each time an electron is injected. Calculation of the electronic structure and Coulomb energies showed that our structure enables high-rate, single-electron injection and subsequent highly efficient single-photon generation in the telecommunication band. (C) 2009 The Japan Society of Applied Physics
    Japanese Journal of Applied Physics 06/2009; 48(6). DOI:10.1143/JJAP.48.06FF01 · 1.13 Impact Factor
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    ABSTRACT: Changing the electric field applied to InAs quantum dots embedded in a p-i-n diode was found to modulate the radiative recombination rate of excitons in the dots. The quantum dots were capped with a strain-reducing layer to realize 1.3 μ m photoemission and a large dipole moment to the exciton states. The exciton states in a quantum dot were investigated by measuring the quantum-confined Stark shift for various applied electric fields and were compared with the theoretical electron and hole wave functions calculated using an eight-band k∙p model. When the absolute value of the applied electric field was reduced from -82.4 kV / cm to 0, the radiative recombination rate increased from 0.88 to 1.11 ns <sup>-1</sup> . Comparison of the experimental rate with the calculated one revealed that the increase in the radiative recombination rate was due to a decrease in the overlap integral between the electrons and holes. These optical characteristics of InAs quantum dots are especially important for developing optical devices that use single photons and single charges because the contribution of nonradiative processes is smaller than that of the radiative process.
    Journal of Applied Physics 08/2008; 104(1-104):013504 - 013504-8. DOI:10.1063/1.2947603 · 2.18 Impact Factor
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    ABSTRACT: Nearly background free 1.5-μm single-photon source has been demonstrated using quasiresonant pulsed excitation. We performed a micro-photoluminescence excitation together with photon-correlation experiments, and then we compared photon statistics in nonresonant and quasiresonant excitation case. When we tuned the pump laser to the quasiresonant state which is 18.6 meV higher than the ground state excitonic transmission, almost background-free luminescence was found at 1524.3 nm. The second-order correlation function at time delay of zero, g(2)(0), was reduced to 0.017 when we carefully estimated noise contributions of after-pulses and dark counts in InGaAs APD detectors. This result opens up the possibility of promoting high quality single-photon source at telecommunication wavelength. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 07/2008; 5(9):2699 - 2703. DOI:10.1002/pssc.200779270
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    H. Sasakura · S. Adachi · S. Muto · S. Hirose · H. Z. Song · M. Takatsu ·
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    ABSTRACT: The oscillations of excitonic and nuclear spin polarizations in an optically pumped single self-assembled In <sub>0.8</sub> Al <sub>0.2</sub> As / Al <sub>0.35</sub> Ga <sub>0.65</sub> As quantum dot (QD) were clearly observed under the excitation of a wetting layer edge at B=5 T . This indicates that an exciton pair with opposite spins is alternatively created via the half-localized state only by changing the delay time between cross-linearly polarized pulse pair. Furthermore, periodic modulation of Zeeman energy synchronizes the degree of circular polarization of photoluminescence from a single QD, indicating that the Overhauser field follows the optically created electron spin polarization in half-localized states of a QD, and the half-localized state in a QD consists of a confined electron in a discrete state and hole in the continuum state.
    Journal of Applied Physics 06/2008; 103(10-103):103530 - 103530-5. DOI:10.1063/1.2927448 · 2.18 Impact Factor
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    ABSTRACT: We succeeded in efficiently generating single-photon pulses from an InAs/InP quantum dot at a wavelength of 1.5 μm. Our optical structure, named a single photon horn, can propagate over 95% photon pulses in InP substrate. We extracted the photon pulses through an anti-reflection coating on a substrate, and then we injected them into an objective lens. Total extraction efficiency from the quantum dot to the lens reached ∼ 11%, which was estimated using a photon correlation measurement. Furthermore we directly observed the single-photon pulse width ∼ 1.6 ns as an exciton lifetime in the quantum dot, which opens up the possibility of operating the single photon horn over 100 MHz.
    Journal of Applied Physics 04/2007; 101(8):081720-081720-5. DOI:10.1063/1.2723177 · 2.18 Impact Factor
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    ABSTRACT: It is well-recognized that exciton-state energy splitting makes it difficult to generate entangled photon pairs (EPP) from a single quantum dot (QD). It will be shown that the splitting of the exciton states originates from not only QD anisotropy but also ionization of residual impurities. This result shows that the growth of highly pure barrier layers will be necessary for the generation of EPP. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 12/2006; 3(11):3908 - 3911. DOI:10.1002/pssc.200671611
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    ABSTRACT: An atomic-force microscope assisted technique is developed to control the position and size of self-assembled semiconductor quantum dots (QDs). Presently, the site precision is as good as ±1.5nm and the size fluctuation is within ±5% with the minimum controllable lateral diameter of 20nm. With the ability of producing tightly packed and differently sized QDs, sophisticated QD arrays can be controllably fabricated for the application in quantum computing. The optical quality of such site-controlled QDs is found comparable to some conventionally self-assembled semiconductor QDs. The single dot photoluminescence of site-controlled InAs/InP QDs is studied in detail, presenting the prospect to utilize them in quantum communication as precisely controlled single photon emitters working at telecommunication bands.
    Nanoscale Research Letters 11/2006; 1(2):160-166. DOI:10.1007/s11671-006-9012-x · 2.78 Impact Factor
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    H. Sasakura · S Adachi · S Muto · S Hirose · H. Z. Song · M. Takatsu ·
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    ABSTRACT: We have demonstrated experimentally the manipulation of exciton and nuclear spins in a single self-assembled In$_{0.75}$Al$_{0.25}$As/Al$_{0.3}$Ga$_{0.7}$As quantum dot. The oscillation of exciton and nuclear spin polarizations were clearly observed. The switching of the emissions in Zeeman split pair indicates that the exciton pair with opposite spins was created coherently via the continuum states and that we can control the electron and nuclear spin polarizations only by changing the delay time of the cross-linearly-polarized pulses. These suggest the high potentiality of electron and nuclear spin manipulation in a single QD via the continuum state.
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    ABSTRACT: We report on the generation of single-photon pulses from a single InAs/InP quantum dot in telecommunication bands (1.3-1.55 µm: higher transmittance through an optical fiber). First we prepared InAs quantum dots on InP (0 0 1) substrates in a low-pressure MOCVD by using a so-called InP 'double-cap' procedure. The quantum dots have well-controlled photo emission wavelength in the telecommunication bands. We also developed a single-photon emitter in which quantum dots were embedded. Numerical simulation designed the emitter to realize efficient injection of the emitted photons into a single-mode optical fiber. Using a Hanbury-Brown and Twiss technique has proved that the photons through the fiber were single photons.
    Journal of Physics Conference Series 05/2006; 38(1):140. DOI:10.1088/1742-6596/38/1/034
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    ABSTRACT: Triggered single-photon generation from InAlAs quantum dot (QD) was demonstrated for the first time. Emitted photon energy coincides with high detection efficiency range of Si single-photon detectors, which is highly suitable for free-space communication. Single-QD spectroscopy and crossed photon correlation measurements unambiguously revealed that several emitting lines observed in a single mesa structure originated from the identical QD, and two temporary competing decay processes associated with neutral states and charged states were identified. Presence of the competing process is also inferred from an analysis of steady-state photoluminescence intensities. Formation process of charged exciton in QD is also discussed.
    Physica E Low-dimensional Systems and Nanostructures 05/2006; 32(1-2). DOI:10.1016/j.physe.2005.12.027 · 2.00 Impact Factor
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    ABSTRACT: We succeeded in observing the electroluminescence and Stark shift of a single InAs/GaAs quantum dot in the O-band (O-band is a 1.3 mum band which has the lowest dispersion characteristics in optical fiber bands). In order to access a single quantum dot, we fabricated a p-i-n diode containing one quantum dot layer with a small ohmic contact area. The electroluminescence of a single exciton (lambda=1321.6 nm) and biexciton (lambda=1322.3 nm) were clearly observed at the center of the O-band at 7 K. This result is the longest wavelength attained up to now. The Stark shift of single quantum dots was also observed at around 1.32 mum at 7 K. These results are promising for the realization of electrically driven single-photon emitters at optical fiber bands.
    Japanese Journal of Applied Physics 04/2006; 45(4B):3621-3624. DOI:10.1143/JJAP.45.3621 · 1.13 Impact Factor
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    ABSTRACT: Carbon nanotubes (CNTs) have been successfully developed as thermal and source bumps for flip-chip high power amplifiers (HPAs). The newly developed 15 mum long CNT bumps exhibit thermal conductivity of 1400 W/m-K. A flip-chip AlGaN/GaN HEMT HPA with a gate width of 2.4 mm utilizing CNT bumps, operating voltage of 40 V, exhibits an output power of 39 dBm at, a frequency of 2.1 GHz without any degradation due to heat-up. To our knowledge, this is the first report about, a practical application of CNTs using their high thermal conductivity
    Electron Devices Meeting, 2005. IEDM Technical Digest. IEEE International; 01/2006
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    ABSTRACT: Regulated single-photon generation from an InAlAs quantum dot was demonstrated at a wavelength in which Si-avalanche photodiode single-photon detector has high quantum efficiency. Measured excitation power dependence of photon coincidence counts revealed that highly pure single-photon emission was realized at low excitation powers. Beyond a critical excitation, population of multi-exciton states in the quantum dot degraded the purity of single-photon emission. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    physica status solidi (c) 11/2005; 2(11). DOI:10.1002/pssc.200562027
  • T. Yokoi · S. Adachi · S. Muto · H. Sasakura · H.Z. Song · S. Hirose · T. Usuki ·
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    ABSTRACT: We report the polarization-dependent energy shift of excitonic emission in a self-assembled InAlAs/AlGaAs quantum dot (QD). The energy shift is well known as Overhauser shift and was observed in a naturally formed GaAs QD using monolayer fluctuation of a quantum well. However, there has been no observation so far in a self-assembled QD, which is suitable for formation of vertically coupled QDs. We demonstrate that the magnitude of the Overhauser shift is enhanced by the photo-injection of the highly polarized electron and is controllable by the polarization of the excitation light in a self-assembled InAlAs QD.
    Physica E Low-dimensional Systems and Nanostructures 11/2005; 29(3-4-29):510-514. DOI:10.1016/j.physe.2005.06.016 · 2.00 Impact Factor
  • Y. Sakuma · M. Takeguchi · K. Takemoto · S. Hirose · T. Usuki · N. Yokoyama ·
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    ABSTRACT: Structural and optical properties of InAs quantum dots (QDs) or InP (001) substrates were studied during low-pressure metalorganic chemical vapor deposition (MOCVD), using photoluminescence (PL) and transmission electron microscopy (TEM). To clarify how the properties of QDs can be controlled through surface reactions, two kinds of source gas supply sequence were compared. With a double-capping procedure, PL changed into a series of multiple peaks arising from different integer numbers of InAs monolayers. PL studies revealed that the spectra of double-capped samples are not affected by a long phosphine (PH3) exposure during growth interruption, indicating reactions relevant to arsenic (As)/phosphorus (P) exchange at the QD surface is self-limited. TEM observations clarified that the shape of double-capped QDs dramatically changes into a thin plate-shape with flat upper and lower interfaces while the density of QDs remains unchanged. In contrast, under the normal capping procedure, the structural and optical properties on QDs are sensitive to the duration of PH3 before InP capping. Results suggest that As/P exchange is not self-limiting, leading to decreased QD density and enlarged QD diameters. We discuss the role the thin InP initial cap layer plays in the anion exchange reaction during the double-capping procedure and the technical importance of controlling the emission wavelength of QDs.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 07/2005; 23(4). DOI:10.1116/1.1949216 · 1.36 Impact Factor
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    ABSTRACT: Micro‐photoluminescence spectroscopy and photon correlation measurement have been performed at telecommunication wavelength using single InAs/InP quantum dot. The single quantum dots were developed through a ‘double‐cap’ growth technique using metalorganic chemical vapor deposition. They show an ultranarrow luminescence line over a wide spectral range of the optical telecommunication band. A non‐classical light emission from an isolated quantum dot was clearly observed at 1277.1 nm in the O‐band (1.3 μm) through a single‐mode optical fiber. © 2005 American Institute of Physics
    06/2005; 772(1):743-744. DOI:10.1063/1.1994320
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    ABSTRACT: We have fabricated single quantum dot light emitting diodes and observed over 1.3μm electroluminescence for the first time. The linewidth was less than 60μeV and the intensity was increased linearly with current.
    Quantum Electronics and Laser Science Conference, 2005. QELS '05; 06/2005
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    ABSTRACT: Single-photon emitters and detectors are the key devices to realize secure communications with single-photon-based quantum cryptography and single-photon-based quantum computing. InAlAs quantum dots (QDs) cover the wavelength range with high quantum efficiencies of Si-based single-photon detectors. Clear photon antibunching was observed from an InAlAs single QD under weak excitations. To realize single-photon emitters on demand, complete population of the QD energy states before the photon emission events is necessary, but the measured antibunching properties were dependent substantially on the photo-excitation powers. The physical origin of this problem is discussed and the criterion to distinguish the real deviation from the photon antibunching condition and the artifact of the measurements is clarified. The capability of single-photon emissions on demand will be demonstrated with photon antibunching under pulsed operations.
    Japanese Journal of Applied Physics 06/2005; 44(24-27). DOI:10.1143/JJAP.44.L793 · 1.13 Impact Factor

Publication Stats

526 Citations
43.35 Total Impact Points


  • 2005-2011
    • Fujitsu Ltd.
      • Fujitsu Laboratories
      Kawasaki Si, Kanagawa, Japan
    • The University of Tokyo
      • Institute of Industrial Science
      Tokyo, Tokyo-to, Japan
  • 2004
    • Yokohama City University
      • Department of Otolaryngology (YCUH)
      Yokohama, Kanagawa, Japan