Papers - Watanabe, Shinichi

Observation of ultrafast photoinduced closing and recovery of the spin-density-wave gap in (TMTSF)(2)PF6

Shinichi Watanabe, Ryusuke Kondo, Seiichi Kagoshima, Ryo Shimano

PHYSICAL REVIEW B (AMER PHYSICAL SOC)  80 ( 22 ) 220408(R) 2009.12

Accepted,  ISSN  1098-0121

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Photoexcited dynamics of a spin-density-wave (SDW) state in an organic linear-chain compound bis-(tetramethyl-tetraselenafulvalene)hexafluorophosphate, (TMTSF)(2)PF6, is investigated by optical pump and terahertz probe experiments. After the ultrashort laser-pulse excitation, a sudden closing followed by a recovery of the SDW gap is clearly observed in the far-infrared spectrum. The recovery time is found to diverge toward the phase-transition temperature, T-SDW, while the gap remains open at T-SDW. The observed critical slowing down behavior is interpreted in terms of three-dimensional fluctuation which enlarges near the phase-transition temperature.

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Ultrafast terahertz probe of spin-density wave dynamics in organic conductor (TMTSF)(2)PF6

S. Watanabe, R. Kondo, S. Kagoshima, R. Shimano

2009 CONFERENCE ON LASERS AND ELECTRO-OPTICS AND QUANTUM ELECTRONICS AND LASER SCIENCE CONFERENCE (CLEO/QELS 2009), VOLS 1-5 (IEEE)     2993 - + 2009

Accepted

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Photo-induced terahertz reflectivity change of the spin-density wave (SDW) gap in (TMTSF)(2)PF6 is observed. We find an ultrafast (similar to ps) increase of terahertz conductivity indicating a destabilization of the SDW states by a weak photo-excitation. (C) 2009 Optical Society of America

Spin-density-wave gap in (TMTSF)2PF6 probed by reflection-type terahertz time-domain spectroscopy

S. Watanabe, R. Kondo, S. Kagoshima, and R. Shimano

Physica Status Solidi (b) 245 ( 12 ) 2688-2691 2008.10

Research paper (scientific journal), Joint Work, Accepted

高空間分解能を持つ超小型テラヘルツ分光装置の開発

渡邉 紳一, 島野亮

分光研究 57 ( 2 ) 81 2008

Research paper (scientific journal), Joint Work

Compact terahertz time domain spectroscopy system with diffraction-limited spatial resolution

S. Watanabe, R. Shimano

Review of Scientific Instruments (AMER INST PHYSICS)  78 ( 10 ) 103906 2007.10

Research paper (scientific journal), Joint Work, Accepted,  ISSN  0034-6748

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A compact and rigid terahertz time domain spectroscopy system is presented. The size of the device is 20 mm diameter times 61 mm length with four parabolic-shaped concave mirrors dug in it to effectively focus a terahertz beam on a sample. The device has no chromatic aberration over the whole bandwidth of the beam (0.3-2 THz), and an effective numerical aperture of about 0.45 is achieved, which has a capability to image the structure whose size is almost the same as the wavelength. Frequency resolved images clearly show this performance. We also show that quantitative retrieval of the complex refractive index of the structure as small as twice of the wavelength is possible. (C) 2007 American Institute of Physics.

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Narrow (~4 meV) inhomogeneous broadening and its correlation with confinement potential of pyramidal quantum dot arrays

K. Leifer, E. Pelucchi, S. Watanabe, F. Michelini, B. Dwir, E. Kapon.

Applied Physics Letters 91 ( 8 ) 081106 2007.08

Research paper (scientific journal), Joint Work, Accepted

Narrow (approximate to 4 meV) inhomogeneous broadening and its correlation with confinement potential of pyramidal quantum dot arrays

K. Leifer, E. Pelucchi, S. Watanabe, F. Michelini, B. Dwir, E. Kapon

APPLIED PHYSICS LETTERS (AMER INST PHYSICS)  91 ( 8 ) 81106 2007.08

Accepted,  ISSN  0003-6951

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The inhomogeneous broadening in the luminescence spectra of ordered arrays of pyramidal GaAs/AlGaAs semiconductor quantum dots (QDs) was studied as a function of the dot size. Dot arrays with inhomogeneous broadening as small as 4.1 meV and a corresponding ground state to first excited state transition separation of 28 meV were obtained. By evaluating the QD energy levels using a multiband k center dot p model, the authors estimated that the observed inhomogeneous broadening corresponds to dot height fluctuations of about 1-2 ML across the array.(c) 2007 American Institute of Physics.

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Mechanisms of quantum dot energy engineering by metalorganic vapor phase epitaxy on patterned nonplanar substrates

E. Pelucchi, S. Watanabe, K. Leifer, Q. Zhu, B. Dwir,P. De Los Rios, E. Kapon.

Nano Letters (AMER CHEMICAL SOC)  7 ( 5 ) 1282-1285 - 1285 2007.03

Research paper (scientific journal), Joint Work, Accepted,  ISSN  1530-6984

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A novel technique for tuning the strength of quantum confinement in site-controlled semiconductor quantum dots (QDs) is introduced and investigated theoretically and experimentally. The method makes use of controlled local growth rates during metalorganic vapor phase epitaxy on patterned arrays of inverted pyramids. A model accounting for precursor migration and adatom incorporation predicts the tuning in QD thickness as a function of the pattern parameters. The results are in good agreement with experimental findings. This technique offers means for designing QD photonic structures with potential applications in QD-based cavity quantum electrodynamics and quantum information processing.

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Very compact THz-TDS imaging system with diffraction limited spatial resolution

Shinichi Watanabe, Ryo Shimano

2007 JOINT 32ND INTERNATIONAL CONFERENCE ON INFRARED AND MILLIMETER WAVES AND 15TH INTERNATIONAL CONFERENCE ON TERAHERTZ ELECTRONICS, VOLS 1 AND 2 (IEEE)     56 - 57 2007

Accepted

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Development of a very compact terahertz time domain spectroscopy (THZ-TDS) imaging system with diffraction-limited spatial resolution is presented. The size of our device made of a cupper rod is 20 mm diameter times 61 mm length. Four parabolic-shaped concave mirrors dug in the rod effectively focus a THz beam on a sample, achieving similar to 0.45 effective numerical aperture.

Patterning of confined-state energies in site-controlled semiconductor quantum dots

S. Watanabe, E. Pelucchi, K. Leifer, A. Malko, B. Dwir, E. Kapon.

Applied Physics Letters (AMER INST PHYSICS)  86 ( 24 ) 243105 2005.06

Research paper (scientific journal), Joint Work, Accepted,  ISSN  0003-6951

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We demonstrate control of the confined-state energies of semiconductor quantum dots (QDs) grown on prepatterned substrates. The InGaAs/AlGaAs QDs self-order at the apex of self-limiting, inverted pyramids whose locations are fixed by lithography. The confinement energy in the dots is systematically varied across the substrate by changing the pattern of the pyramid array in their vicinity. The resulting energy- and site-controlled QDs show systematic and reproducible shifts of their emission wavelengths as well as antibunched photon emissions from confined single excitons. Such QDs should be useful for applications in quantum information processing and quantum communication devices, e.g., multiple-wavelength single-photon emitters. (c) 2005 American Institute of Physics.

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Site- and energy-controlled pyramidal quantum dot heterostructures

E. Kapon , E. Pelucchi, S. Watanabe, A. Malko, M. H. Baier, K. Leifer, B. Dwir, F. Michelini, and M. A. Dupertuis

Physica E (ELSEVIER SCIENCE BV)  25 ( 2-3 ) 288-297 - 297 2004.08

Research paper (scientific journal), Joint Work, Accepted,  ISSN  1386-9477

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The formation mechanisms, structure and optoelectronic properties of Ga(In)As/(Al)GaAs quantum dot (QD) heterostructures grown by organometallic chemical vapor deposition on patterned (111)B GaAs substrates are reviewed. With this approach, it is possible to prepare high-quality semiconductor QDs whose position on a substrate is precisely controlled via a pre-growth photolithography step. The dots are formed at the center of an inverted, tetrahedral pyramid and are surrounded by distinct, low-dimensional barriers (quantum wires and quantum wells). Reproducible neutral and charged exciton states are observed in the QDs and are probed using microphotoluminescence and photon correlation spectroscopy. Single- and correlated-photon emissions are systematically detected from specific single and biexciton states. The emission spectra of light emitting diodes incorporating single pyramidal dots indicate the possibility of preferential injection of charge carriers via self-ordered wires connected to the dots. Finally, photonic crystal structures incorporating pyramidal QDs with controlled energy states are demonstrated. (C) 2004 Elsevier B.V. All rights reserved.

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Site-controlled quantum dots grown in inverted pyramids for photonic crystal applications

E. Pelucchi, S. Watanabe, K. Leifer, B. Dwir, E. Kapon.

Physica E (ELSEVIER SCIENCE BV)  23 ( 3-4 ) 476-481 - 481 2004.07

Research paper (scientific journal), Joint Work, Accepted,  ISSN  1386-9477

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We report on the growth and optical properties of various configurations of sub-micron pitch dense arrays of pyramidal quantum dots (QDs) grown by organometallic chemical vapour deposition on patterned substrates. We show that the effective growth rate of these QDs is influenced by the ratio between the free {1 1 1}B area and {1 1 1}A exposed facets surrounding them. This provides a powerful technique for engineering the energy level structure of ordered QD arrays by means of geometrical patterning of the growth template. Such technique should be particularly useful for applications in photonic crystals incorporating QDs with tailored absorption and/or emission properties. (C) 2004 Elsevier B.V. All rights reserved.

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Growth and optical characterization of dense arrays of site-controlled quantum dots grown in inverted pyramids

S. Watanabe, E. Pelucchi, B. Dwir, M. Baier, K. Leifer, E. Kapon.

Physica E (ELSEVIER SCIENCE BV)  21 ( 2-4 ) 193-198 - 198 2004.03

Research paper (scientific journal), Joint Work, Accepted,  ISSN  1386-9477

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We report on the growth and optical properties of dense arrays of single GaAs/AlGaAs quantum dot (QD) heterostructures with pitches as small as 300 nm. The samples were grown by organometallic chemical vapor deposition in dense inverted pyramids on {1 1 1}B GaAs substrate pre-patterned using electron beam lithography and wet chemical etching. The growth conditions such as deoxidation and growth temperatures, growth rates, and V/III ratio, had to be chosen quite differently from those employed with micron-size pyramids. Low-temperature micro-photoluminescence and cathodoluminescence spectra of the samples show distinct luminescence from the QDs with a linewidth of less than 1 meV and uniform emission energy for an ensemble of 900 QDs. The possibility of incorporating such QD arrays inside optical microcavity structures is also discussed. (C) 2003 Elsevier B.V. All rights reserved.

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Dense uniform arrays of site-controlled quantum dots grown in inverted pyramids

S. Watanabe, E. Pelucchi, B. Dwir, M. Baier, K. Leifer, E. Kapon.

Applied Physics Letters (AMER INST PHYSICS)  84 ( 15 ) 2907-2909 - 2909 2004.02

Research paper (scientific journal), Joint Work, Accepted,  ISSN  0003-6951

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We report on the growth and optical properties of homogeneous, dense arrays of site-controlled, single GaAs/AlGaAs quantum dot (QD) heterostructures with periodicities as small as 300 nm. The samples were grown by organometallic chemical vapor deposition on (111)B GaAs substrates containing dense inverted pyramid recess patterns prepared by electron beam lithography and wet chemical etching. Low-temperature microphotoluminescence spectra of the samples show distinct luminescence from the QDs with 1-3 meV linewidth. Low-temperature cathodoluminescence spectrally resolved images reveal uniform emission energy within an ensemble of 900 QDs. (C) 2004 American Institute of Physics.

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High uniformity of site-controlled pyramidal quantum dots grown on prepatterned substrates

M. Baier, S. Watanabe, E. Pelucchi, E. Kapon

Applied Physics Letters (AMER INST PHYSICS)  84 ( 11 ) 1943-1945 - 1945 2004.01

Research paper (scientific journal), Joint Work, Accepted,  ISSN  0003-6951

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We studied the uniformity of site-controlled, pyramidal InGaAs/AlGaAs semiconductor quantum dots (QDs) grown by organometallic chemical vapor deposition on prepatterned substrates. The inhomogeneous broadening of the QD ground state emission has been determined to be 7.6 meV by statistical single QD photoluminescence spectroscopy on a set of 120 individual QD structures. Taking into account the ground-to-excited state separation of 55 meV, such a small value has not yet been observed in QD systems where other growth mechanisms are employed. Moreover, a high reproducibility of the sharp QD emission features in the single exciton regime has been observed. (C) 2004 American Institute of Physics.

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Vertically polarized lasing and photoluminescence in a ridge quantum-wire laser

S. Watanabe, S. Koshiba, M. Yoshita, M. Baba, H. Sakaki, and H. Akiyama

Physical Review B 68 ( 19 ) 193304 2003.12

Research paper (scientific journal), Joint Work, Accepted

Imaging of emission patterns in a T-shaped quantum wire laser

Y. Takahashi, S. Watanabe, M. Yoshita, H. Itoh, Y. Hayamizu, H. Akiyama, L. N. Pfeiffer, and K. W. West,

Applied Physics Letters (AMER INST PHYSICS)  83 ( 20 ) 4089-4091 - 4091 2003.09

Research paper (scientific journal), Joint Work, Accepted,  ISSN  0003-6951

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Spatially and spectrally resolved microscopic images of spontaneous and stimulated emissions are imaged at the mirror facets of a GaAs T-shaped quantum wire laser with high uniformity. Laser emission from the one-dimensional ground state reveals a circular image located at the core of a T-shaped optical waveguide but significantly smaller in area than the low power spontaneous emission from the same waveguide. These images unambiguously allow assignment of all spontaneous and laser emissions to the wire ground state and respective intersecting wells in the structure. (C) 2003 American Institute of Physics.

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High-quality InxGa1-xAs/Al0.30Ga0.70As quantum dots grown in inverted pyramids

E. Pelucchi, M. Baier, Y. Ducommun, S. Watanabe, E. Kapon

Physica Status Solidi (b) (WILEY-V C H VERLAG GMBH)  238 ( 2 ) 233-236 - 236 2003.07

Research paper (scientific journal), Joint Work, Accepted,  ISSN  0370-1972

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We report here on the fabrication by organometallic chemical vapour deposition of InxGa1-xAs quantum dots (QDs) with x up to 35% grown in pyramidal recess patterns with Al0.30Ga0.70As barriers. We show QD luminescence with narrow PL lines, tuning of the emission energies varying by more than 180 meV and ground state to first excited state separations as large as 80 meV.

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Polarization dependence of the optical interband transition defined by the spatial variation of the valence p-orbital Bloch functions in quantum wires

S. Watanabe, M. Yoshita, S. Koshiba, and H. Akiyama

Japanese Journal of Applied Physics (INST PURE APPLIED PHYSICS)  41 ( 10 ) 5924-5936 - 5936 2002.10

Research paper (scientific journal), Joint Work, Accepted,  ISSN  0021-4922

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We develope a method to express wave functions of hole states in semiconductor quantum wire (QWR) structures based on spatial variation of the valence p-orbital Bloch functions, to show how envelope wave functions relate to polarization-dependent interband transition. A wave function of a hole state is obtained solving the Schrodinger equation based on the 4 x 4 Luttinger Hamiltonian, and then recomposed by means of six bases of three p-orbital Bloch functions with two spin components. As a result, the hole wave function is expressed by six envelope wave functions for the six bases, Then. interband optical transition matrix elements with x-, y-, and z-polarizations are separately given by overlap integrals between envelope wave functions of holes for p(x), p(y), and p(z) orbitals and those of electrons. We also calculate the wave functions for a modeled ridge QWR structure with mirror symmetry as well as for an asymmetric structure, and discuss the polarization dependence of the optical transition.

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Lasing from a single-quantum wire

Y. Hayamizu, M. Yoshita, S. Watanabe, H. Akiyama, L. N. Pfeiffer, and K. W. West

Applied Physics Letters 81 ( 26 ) 4937-4939 2002.10

Research paper (scientific journal), Joint Work, Accepted