Search Results - (Author, Cooperation:M. Nishioka)

2014-12-20

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

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Antigen-Presenting Cells/immunology ; Autoimmune Diseases/*immunology ; CD8-Positive T-Lymphocytes/*immunology ; CTLA-4 Antigen ; *Clonal Anergy ; Humans ; Lymphocyte Activation ; Receptors, Antigen, T-Cell/immunology ; *Self Tolerance ; T-Lymphocytes, Regulatory/*immunology ; Vitiligo/immunology

1089-7550

AIP Digital Archive

Physics

Successful fabrication of thin GaAs quantum wires (120–200 A(ring))×(200–300 A(ring)) by a novel metal-organic chemical-vapor-deposition growth technique is reported. The GaAs quantum wires were grown on a V groove formed by two GaAs triangular prisms which were selectively grown on SiO2 masked substrates. The V groove has a very sharp corner at the bottom, which results in reduction of the effective width of the quantum wire structures. The measurement of photoluminescence and photoluminescence excitation spectra with polarization dependence indicate the existence of the quantized state in the quantum wires.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We fabricate semi-insulating InGaAs/GaAs multiple quantum wells and observe the excitonic enhancement of the photorefractivity in the Franz–Keldysh geometry at wavelengths of 0.92–0.94 μm. A maximum two-wave mixing gain of 138 cm−1 and a maximum diffraction efficiency of 1.5×10−4 are obtained. The saturation intensity and the spatial resolution are also measured by four-wave mixing. The diffraction efficiency is saturated at a high external electric field. The dominant cause of this saturation is the deviation of the excitonic electroabsorption from its quadratic law. © 2001 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We investigate enhanced and inhibited spontaneous emission effects in a vertical λ-microcavity structure having two kinds of quantum wells (QWs) with the thicknesses of 76 and 114 A(ring), measuring both photoluminescence intensity and carrier lifetime. The 76 and 114 A(ring) QWs are placed at the maximum and at the nodes of the emitted standing wave in the microcavity, respectively. When the λ-microcavity mode is tuned to the quantized band-gap energy of the 76 A(ring) QWs (enhanced condition), the PL intensity is enhanced compared with the case that the cavity mode is tuned to the quantized band-gap energy of the 114 A(ring) QWs (inhibited condition). In addition, the increase of the carrier lifetime is also observed under the inhibited condition. These results demonstrate existence of enhanced and inhibited spontaneous emission effects in the microcavity structures.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We demonstrated a novel selective growth technology for the fabrication of quantum microstructures utilizing in situ patterning of contamination resist in the metalorganic chemical vapor deposition system. The results indicate that a GaAs quasi-quantum wire structure, as narrow as 700 nm, can be successfully fabricated, showing that this technique may be applied to fabrication technologies for quantum microstructures.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The differential gain which is an important parameter for modulation dynamics in semiconductor lasers is evaluated experimentally by measuring the gain coefficient and the carrier lifetime in GaAs/AlGaAs double-heterostructure (DH) lasers, quantum well (QW) lasers, and p-modulation-doped quantum well (p-MDQW) lasers. The results indicate that the differential gain of the QW laser is 2.4 times as high as that of the DH laser, which is consistent with the theory. In addition, it is found that improvement of the differential gain using the p-MDQW structure is not so large as that expected by the theory. This result suggests that enhanced energy broadening due to the reduction of the equivalent dephasing time τeqin, which includes both the dephasing time τin due to the intraband relaxation and the band tailing effects, significantly affects the gain spectra in the p-MDQW lasers, which is confirmed by the measurement of the spontaneous emission spectra.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We have fabricated and studied the dynamics of In0.16Ga0.84As λ-cavity lasers focusing on the influence of carrier transport/capture and gain flattening by using optical resonant pumping and off-resonant pumping. With the off-resonant pumping, shoulders in the lasing pulse shape were observed, while no shoulder appeared with the on-resonant pumping. From rate equation analysis and time-resolved photoluminescence measurement, it was concluded that the shoulders result from carrier transport/capture effects. Saturation of the inverse rise time was observed at high pump levels. This results from gain flattening due to the steplike density of states of 2D carriers. The pulse width obtained from the laser was as narrow as 7.7 ps. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A picosecond pulse (〈1.8 ps) at 8570 A(ring) is successfully generated by a gain switching method in an optically pumped GaAs/AlGaAs multiquantum well laser with a cavity length of 155 μm. This is the narrowest pulse width so far achieved in semiconductor lasers without the external cavity. We believe that this short pulse generation results from the enhanced differential gain due to the two-dimensional properties of the carriers in the quantum wells.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Piezoelectric activity in reactively sputtered lead zirconate titanate [Pb(Zr,Ti)O3 or PZT] thin films has been confirmed through the fabrication of surface acoustic wave delay lines on poled polycrystalline films of PZT deposited on glass substrates. Films of varying thickness ranging from 3.4 to 6 μm show a dispersion in resonant frequency for interdigital transducers designed to resonate at 44 MHz on bulk PZT material. The electromechanical coupling coefficient k2 was in the range 0.57–0.79% for hk=0.21–0.37.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We discuss measurement results of the exciton radiative lifetime in GaAs quantum wires grown by metalorganic chemical-vapor selective growth technique at 9 K. By changing the lateral width of the quantum wires in the range of 7–35 nm systematically, the lateral width dependence of the radiative exciton lifetime was measured. The results show that the measured lifetime increases from 260 to 422 ps with the decrease of the lateral width from 25 to 7 nm. This increase of the radiative lifetime can be explained by taking account of both reduced exciton coherence length and spreading of the wave function of electrons in the barrier region.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

GaAs triangular-shaped quantum wires with the lateral width of ∼10 nm are fabricated by metalorganic chemical vapor selective deposition growth technique. The lateral dimension is determined by both photoluminescence (PL) measurement and a high-resolution scanning electron micrograph observation. A systematic change in the size of the quantum wire exhibits consistent blue shifts of the PL peak keeping high intensities, which demonstrates enhanced two-dimensional quantum confinement with the material of high quality.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We succeeded in the fabrication of GaAs wires by an electron beam induced selective growth technique, for the first time. In situ irradiation of the electron beam, with simultaneous supply of tri-methyl-gallium (TMG) and cracked AsH3, formed a GaAs quasiquantum wire structure as narrow as 300 nm selectively. Auger analysis and dependence of the growth on source materials and types of substrate suggest that the selective growth results from the decomposition of TMG by the electron beam irradiation.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We investigated magnetophotoluminescence spectra up to 40 T of GaAs quantum wires with various lateral widths (7–30 nm) grown by the selective metalorganic chemical vapor deposition. The photoluminescence peak shift due to the magnetic fields is more suppressed with decreasing the wire width. The observed energy shift was in good agreement with the calculation based on a variation method. These results clearly demonstrate existence of the two-dimensional confinement effect and enhanced binding energy of excitons. In the narrowest wire (7 nm), however, comparison of the experimental results with the calculation indicates penetration of exciton wavefunction from the two-dimensional potential to the barrier region. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report new results on a photon-exciton interaction in a λ cavity having two kinds of quantum wells (QWs) with the thickness of 62 and 90 A(ring) which are sited at the antinode and two nodes of the standing wave, respectively. By measuring the reflectivity spectrum of the quantum microcavity, mode splitting (two dips) is observed when the photons in the cavity are resonant with the excitons of 62 A(ring) QWs at 14 K. However, no mode splitting (one dip) is observed when the microcavity is tuned to the quantized energy of the excitons arising from the 90 A(ring) QWs. The reason is the difference of the coupled interaction between photons and excitons due to the standing wave distribution of the optical field in the microcavity. The above results clearly demonstrate the controllability of quantum electrodynamics effects due to the confined optical mode (i.e., the standing wave) in a high-Q microcavity.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report on in situ fabrication and the photoluminescence spectra of pyramid-shaped GaAs dot structures grown on (100) GaAs substrates using selective epitaxial growth by metalorganic chemical vapor deposition. The dot structures have lateral size of 25 nm and the period of 140 nm, showing a clear photoluminescence peak with strong intensity. In addition, energy change of magnetophotoluminescence spectra demonstrates the enhancement of exciton binding energy due to lateral confinement.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A strained InGaAs quantum wire laser with a vertical microcavity structure was fabricated for the first time. In this laser structure, quantum wires with a lateral width of about 10 nm were grown by a selective metalorganic chemical vapor deposition technique. The length of the microcavity was 4λ(λ=883 nm), with AlAs/AlGaAs distributed Bragg reflectors. The cavity effect was demonstrated by the measurement of photoluminescence with and without the cavity. Lasing oscillation was observed at 77 K by optical pumping.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We observed a very sharp photoluminescence peak from a single GaAs/AlGaAs quantum dot structure by using a microphotoluminescence measurement technique. The spectral linewidth was more suppressed by decreasing the excitation laser power, which is mainly due to reduction of the filling effect of quantized energy levels. The minimal spectral linewidth with low excitation laser power was 0.9 meV. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Self-alignment of InGaAs quantum dots was achieved by growing the quantum dots on the multiatomic steps in metalorganic chemical vapor deposition. In this technique, first GaAs epilayer with multiatomic step structures along straight lines was grown on a vicinal GaAs substrate under appropriate growth conditions. Then, the InGaAs quantum dots were grown selectively on the multiatomic step edges using strain effects. This growth technique results in spontaneously aligned InGaAs quantum dots without any preprocessing technique prior to the growth. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report the direct deposition of strained InGaAs-dot structures with a diameter of about 15 nm on GaAs surfaces by metalorganic chemical vapor deposition growth. High resolution scanning electron micrographs show highly uniform quantum-sized dots formed by the Stranski–Krastanow growth mode. The sharp photoluminescence emission band of buried dot structures indicates efficient carrier capture and a homogeneous heterointerface. The average dot size and area dot density can be controlled accurately by growth temperature, and InGaAs deposition thickness, respectively.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We fabricated GaAs arrowhead-shaped quantum wires utilizing both the selective growth technique and the difference in the stabilized crystal facet between GaAs and Al0.4Ga0.6As; the stabilized facet of the GaAs layer is (111)A and that of the Al0.4Ga0.6As layer is (311)A. A systematic change in the size of the quantum wire exhibits blue shifts of the photoluminescence peak, which is due to enhancement of the two-dimensional quantum confinement effect.

Electronic Resource