Details of a Researcher - Ota, Yasutomo

Papers - Ota, Yasutomo

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Nanocavity-enhanced optical stark shift in a single quantum dot under extremely low excitation power

Takagi H., Ota Y., Kumagai N., Ishida S., Iwamoto S., Arakawa Y.

Optics Infobase Conference Papers 2012.12

Research paper (international conference proceedings), Joint Work, Accepted

Optical properties of site-controlled InGaAs quantum dots embedded in GaAs nanowires by selective metalorganic chemical vapor deposition

Tatebayash J., Ota Y., Ishida S., Nishioka M., Iwamoto S., Arakawa Y.

Japanese Journal of Applied Physics 51 ( 11 PART2 ) 2012.11

Research paper (scientific journal), Joint Work, Accepted, ISSN 00214922

We report the optical properties of site-controlled InGaAs quantum dots (QDs) embedded in GaAs nanowires (NWs) grown by selective metalorganic chemical vapor deposition (MOCVD). InGaAs/GaAs QD-in-NWs with various QD heights and In compositions are realized on patterned GaAs(111)B substrates in the form of InGaAs/GaAs heterostructures, and identified by structural analyses using scanning transmission electron microscopy, photoluminescence (PL) characterization, and numerical analyses of the band structure using a single-band effective mass approximation. Room temperature (RT) light emission is observed at 1.03 μm from InGaAs/GaAs QD-in-NWs which is indicative of the formation of high-quality InGaAs QD-in-NWs. Sharp emission peaks from exciton and biexciton of single In(Ga)As QD-in-NWs are observed by using μ-PL characterization at 10 K. © 2012 The Japan Society of Applied Physics.

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Cavity quantum electrodynamics and lasing oscillation in single quantum dot-photonic crystal nanocavity coupled systems

Arakawa Y., Iwamoto S., Nomura M., Tandaechanurat A., Ota Y.

IEEE Journal on Selected Topics in Quantum Electronics 18 ( 6 ) 1818 - 1829 2012.09

Research paper (scientific journal), Joint Work, Accepted, ISSN 1077260X

Our recent advances in solid-state cavity quantum electrodynamics and lasing oscillation in single quantum dot (QD)-photonic crystal (PhC) nanocavity coupled systems are discussed. These include the fabrication of high-quality 2-D PhC nanocavities $(Q 50 000)$, which enabled the generation of spontaneous two-photon emission from a single QD, and the realization of lasing oscillation with single QD gain in the strong coupling regime. Moreover, we have fabricated high-quality 3-D PhC nanocavities $(Q \sim 38 500)$, which have facilitated the realization of both lasing oscillation and the Purcell effect. Lasing oscillation in a 1-D PhC nanobeam cavity with gain produced by a few QDs has also been demonstrated. © 1995-2012 IEEE.

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High guided mode-cavity mode coupling for an efficient extraction of spontaneous emission of a single quantum dot embedded in a photonic crystal nanobeam cavity

Enderlin A., Ota Y., Ohta R., Kumagai N., Ishida S., Iwamoto S., Arakawa Y.

Physical Review B Condensed Matter and Materials Physics 86 ( 7 ) 2012.08

Research paper (scientific journal), Joint Work, Accepted, ISSN 10980121

We demonstrate efficient extraction of a single InAs quantum dot spontaneous emission through an air-suspended waveguide to which a photonic crystal nanobeam cavity is coupled. First, the quantum dot emission into the cavity mode is enhanced due to a high Purcell effect arising from a large quality factor and a small mode volume of the cavity mode. Second, the cavity photons can be efficiently transferred to the guided mode by optimizing the design of a structure that exhibits a high coupling rate between the two modes. This configuration has lead to the observation of a high extraction efficiency (79%) of the quantum dot emission through the waveguide. Moreover, as a result of the high extraction efficiency to the guided mode, the unwanted microphotoluminescence coupling to the free space modes and detected from the top surface of the sample is strongly suppressed. Indeed, we show that it exhibits a negative peak as a function of the detuning between the cavity mode energy and the quantum dot transition energy. © 2012 American Physical Society.

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Site-controlled formation of InAs/GaAs quantum-dot-in-nanowires for single photon emitters

Tatebayashi J., Ota Y., Ishida S., Nishioka M., Iwamoto S., Arakawa Y.

Applied Physics Letters 100 ( 26 ) 2012.06

Research paper (scientific journal), Joint Work, Accepted, ISSN 00036951

We report the demonstration of site-controlled InAs/GaAs quantum dots (QDs) embedded in GaAs nanowires (NWs) by selective metalorganic chemical vapor deposition. InAs/GaAs quantum-dot-in-nanowires (QD-in-NWs) with various InAs thicknesses are realized on patterned GaAs(111)B substrates in the form of InAs/GaAs heterostructures and identified by structural analyses using scanning transmission electron microscopy and photoluminescence characterization. Sharp excitonic emission peaks at 10 K from single QD-in-NWs with the narrowest exciton linewidth of 87 μeV are observed. Light emission from the single QD-in-NW shows photon antibunching, along with biexciton-exciton cascaded emission process, which evidences single photon emission from high-quality QD-in-NWs. © 2012 American Institute of Physics.

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Wavelength tunable quantum dot single-photon source with a side gate

Nakaoka T., Tamura Y., Miyazawa T., Watanabe K., Ota Y., Iwamoto S., Arakawa Y.

Japanese Journal of Applied Physics 51 ( 2 PART 2 ) 2012.02

Research paper (scientific journal), Joint Work, Accepted, ISSN 00214922

We demonstrate photon antibunching from a wavelength controlled quantum dot single-photon source with a side gate. The photoluminescence peaks from the quantum dots embedded in the side-gate structure are clearly identified as a neutral exciton and a neutral biexciton by the studies of excitation-power dependence, polarization dependence, and photon correlation. The neutral exciton energy is controlled by the side gate via the quantum confined Stark effect. Measurement of the second-order autocorrelation function indicates g (2)(0) = 0.07 at 0 V, 0.14 at 0.8 V, and 0.24 at 0.9 V. The results show g (2)(0) are below the 0.5 limit necessary for classification as a single photon source even under applied gate voltage. We also show a biexciton-exciton cascade which can be used to create entangled photon pairs. © 2012 The Japan Society of Applied Physics.

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Nanocavity-enhanced optical Stark shift in a single quantum dot under extremely low excitation power

Takagi H., Ota Y., Kumagai N., Ishida S., Iwamoto S., Arakawa Y.

2012 Conference on Lasers and Electro Optics CLEO 2012 2012

Research paper (international conference proceedings), Joint Work, Accepted

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Spontaneous two-photon emission from a single quantum dot

Ota Y., Iwamoto S., Kumagai N., Arakawa Y.

Physical Review Letters 107 ( 23 ) 2011.11

Research paper (scientific journal), Joint Work, Accepted, ISSN 00319007

Spontaneous two-photon emission from a solid-state single quantum emitter is observed. We investigated photoluminescence from the neutral biexciton in a single semiconductor quantum dot coupled with a high Q photonic crystal nanocavity. When the cavity is resonant to the half energy of the biexciton, the strong vacuum field in the cavity inspires the biexciton to simultaneously emit two photons into the mode, resulting in clear emission enhancement of the mode. Meanwhile, the suppression of other single photon emission from the biexciton was observed, as the two-photon emission process becomes faster than the others at the resonance. © 2011 American Physical Society.

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Competing influence of an in-plane electric field on the Stark shifts in a semiconductor quantum dot

Nakaoka T., Tamura Y., Saito T., Miyazawa T., Watanabe K., Ota Y., Iwamoto S., Arakawa Y.

Applied Physics Letters 99 ( 18 ) 2011.10

Research paper (scientific journal), Joint Work, Accepted, ISSN 00036951

We report on the fabrication of a side-gate structure which enables a purely lateral electric field to be applied onto a self-assembled quantum dot. The lateral field produces an unconventional M'-shaped exciton energy shift-a blueshift followed by a redshift. The unconventional energy shift is reproduced by calculation. The calculation shows that only the positively charged exciton shows the unconventional shift. The origin is attributed to the field-induced hole-concentration in the bottom-corner of the dot, which strongly enhances the repulsive direct Coulomb interaction and reduces the exciton binding energy. © 2011 American Institute of Physics.

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New method to isolate and distribute photoluminescence emissions from InAs quantum dots over a wide-wavelength range

Ohkouchi S., Kumagai N., Shirane M., Igarashi Y., Nomura M., Ota Y., Yorozu S., Iwamoto S., Arakawa Y.

Journal of Crystal Growth 323 ( 1 ) 250 - 253 2011.05

Research paper (scientific journal), Joint Work, Accepted, ISSN 00220248

We propose a new method to isolate and distribute the photoluminescence emission wavelengths of InAs quantum dots (QDs) over a wide-wavelength range by the growth of the partial GaAs capping layer at low temperature and annealing processes. When the partial capping layer was grown at 485°C, the emission wavelengths of the QDs were distributed within a small range around the mean wavelength. On the other hand, when the partial capping layer was grown at a lower temperature of 423°C, the emission wavelengths of the individual QDs were isolated and distributed over a wide range. Thus fabricated QDs are effective for the QD based devices such as single photon sources. © 2010 Elsevier B.V. All rights reserved.

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Strong coupling between a photonic crystal nanobeam cavity and a single quantum dot

Ohta R., Ota Y., Nomura M., Kumagai N., Ishida S., Iwamoto S., Arakawa Y.

Applied Physics Letters 98 ( 17 ) 2011.04

Research paper (scientific journal), Joint Work, Accepted, ISSN 00036951

We demonstrated the strong coupling between a one-dimensional photonic crystal nanobeam cavity and a single quantum dot (QD). Thanks to a high quality factor (∼25 000) with small mode volume [0.38× (n/λ) 3] of the nanobeam cavity, an anticrossing behavior with a vacuum Rabi splitting of 226 μeV was observed. The ratio of the QD-cavity coupling strength to the cavity decay rate, which is a figure of merit of quantum optical applications, is estimated to 2.1. This is the highest value among any QD-based cavity quantum electrodynamics systems reported so far. © 2011 American Institute of Physics.

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Effects of growth temperature of partial GaAs cap on InAs quantum dots in In-flush process for single dot spectroscopy

Kumagai N., Ohkouchi S., Shirane M., Igarashi Y., Nomura M., Ota Y., Yorozu S., Iwamoto S., Arakawa Y.

Physica Status Solidi C Current Topics in Solid State Physics 8 ( 2 ) 248 - 250 2011.02

Research paper (scientific journal), Joint Work, Accepted, ISSN 18626351

We have investigated effects of growth temperature of partial GaAs cap (T<inf>cap</inf>) on InAs quantum dots (QDs) in indium-flush (In-flush) process with atomic force microscopy (AFM) and micro photoluminescence (μ-PL) measurements. T<inf>cap</inf> in In-flush process are varied from 385 °C to 485 °C as an experimental parameter. In order to investigate the effects of T<inf>cap</inf> on structural property of single InAs QD, shape of QD in initial capping process are characterized by AFM measurements in air at room temperature. While isotropic shape of QDs is kept in low T<inf>cap</inf> conditions below about 440 °C, in-plane anisotropy of initial shape of QDs are increased with increasing of T<inf>cap</inf> due to elongation to the [1̄0] direction. μ-PL measurements are performed at 7K for the investigation of the effects of T<inf>cap</inf> on optical property in single InAs QDs. In low T<inf>cap</inf> conditions from 410 °C to 435 °C, distinct emissions from excitonic states in single QDs are observed. In the same T<inf>cap</inf> range, the AFM result shows that isotropic shape of QDs is kept. On the other hand, in high T<inf>cap</inf> conditions above about 440 °C, indefinite peaks and broad emissions appear remarkably. Excitonic emissions which are observed in low T<inf>cap</inf> range are hardly observed in this high T<inf>cap</inf> range. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Zero-cell photonic crystal nanocavity laser with quantum dot gain

Nomura M., Ota Y., Kumagai N., Iwamoto S., Arakawa Y.

Applied Physics Letters 97 ( 19 ) 2010.11

Research paper (scientific journal), Joint Work, Accepted, ISSN 00036951

We demonstrate laser oscillation in a hexagonal-lattice photonic crystal nanocavity using an InGaAs quantum dot gain material by optical pumping at 5 K. The cavity comprises a defect created by shifting several air holes in a two-dimensional photonic crystal slab structure without removing any air holes to achieve both small mode volume and a high cavity quality factor. The measured cavity quality factors and estimated mode volume for the nanocavity are ∼33 000 and 0.004 μm3 [0.23 (λ<inf>0</inf> /n)3]. The laser threshold is compared between the zero-cell and L3-type nanocavity lasers, and the zero-cell nanolasers are found to have lower thresholds of about one-third of the L3-type nanolasers. This result suggests that a higher Purcell factor of the zero-cell nanolaser is reflected as a lower laser threshold. © 2010 American Institute of Physics.

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Charged and neutral biexcitonexciton cascade in a single quantum dot within a photonic bandgap

Shirane M., Igarashi Y., Ota Y., Nomura M., Kumagai N., Ohkouchi S., Kirihara A., Ishida S., Iwamoto S., Yorozu S., Arakawa Y.

Physica E Low Dimensional Systems and Nanostructures 42 ( 10 ) 2563 - 2566 2010.09

Research paper (scientific journal), Joint Work, Accepted, ISSN 13869477

We investigate the recombination dynamics of positively charged and neutral biexcitons and excitons in a single InAs/GaAs quantum dot (QD) within a two-dimensional (2D) photonic bandgap (PBG). The 2D PBG makes the exciton lifetime four times longer and enhances photon-extraction efficiency compared to those without the PBG. Photon cross-correlation measurements demonstrate the cascade emissions of both charged and neutral biexcitonsexcitons from the same QD. In the charged case, a hole in the p-shell relaxes into the s-shell between the cascade, and the corresponding transition is confirmed based on the spin configuration. The long exciton lifetime with the PBG helps us to reveal the spin dynamics that did not clearly appear in intrinsic QDs. © 2009 Elsevier B.V. All rights reserved.

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Observation of unique photon statistics of single artificial atom laser

Nomura M., Kumagai N., Iwamoto S., Ota Y., Arakawa Y.

Physica E Low Dimensional Systems and Nanostructures 42 ( 10 ) 2489 - 2492 2010.09

Research paper (scientific journal), Joint Work, Accepted, ISSN 13869477

We demonstrate a photonic crystal nanocavity laser essentially driven by a self-assembled InAs/GaAs single quantum dot gain and its unique photon statistics. Gain tuning measurements and photon correlation measurements indicated that a single quantum dot plays a substantial role in the laser oscillation. Photon correlation measurements showed a distinct transition from anti-bunching to Poissonian via photon bunching around the threshold with the increase of the excitation power. Numerical simulations, including contributions of other light sources besides a single quantum dot, indicated that the photon bunching feature around the threshold can be enhanced by the interfusion of incoherent photons into the cavity mode. © 2009 Elsevier B.V. All rights reserved.

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Suppression of indefinite peaks in InAs/GaAs quantum dot spectrum by low temperature capping in the indium-flush method

Kumagai N., Ohkouchi S., Nakagawa S., Nomura M., Ota Y., Shirane M., Igarashi Y., Yorozu S., Iwamoto S., Arakawa Y.

Physica E Low Dimensional Systems and Nanostructures 42 ( 10 ) 2753 - 2756 2010.09

Research paper (scientific journal), Joint Work, Accepted, ISSN 13869477

We have investigated effects of growth temperature of thin GaAs capping layer in the initial stage of indium-flush process using atomic force microscopy and microscopic photoluminescence (μ-PL) methods. The shape of capped InAs quantum dot (QD) and its μ-PL properties are sensitive to the growth temperature of thin GaAs capping layer. In the case of the high temperature cap, the QD shape in initial capping stage is elongated along the [1 1 -0] direction, and μ-PL spectrum shows several peaks accompanied with indefinite peaks. On the other hand, the low temperature case, the QD shape is kept in isotropic and μ-PL spectrum shows distinctive emissions from excitonic states of the QD with suppressed indefinite peaks. These results indicate that the low temperature capping is effective to keep an isotropic shape of QD and suppress indefinite peaks. © 2009 Elsevier B.V. All rights reserved.

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Spin dynamics of excited trion states in a single InAs quantum dot

Igarashi Y., Shirane M., Ota Y., Nomura M., Kumagai N., Ohkouchi S., Kirihara A., Ishida S., Iwamoto S., Yorozu S., Arakawa Y.

Physical Review B Condensed Matter and Materials Physics 81 ( 24 ) 2010.06

Research paper (scientific journal), Joint Work, Accepted, ISSN 10980121

We measure photon cross-correlation between two positively charged biexcitonic emission lines and a positively charged excitonic line for a single InAs/GaAs quantum dot (QD). Marked difference in the correlation function is observed, which originates from multiple spin configurations of intermediate states, i.e., excited positively charged exciton (trion) states. With the aid of a rate equation simulation, we evaluate the transition rate with p -shell hole-spin flip to be 0.8-1.0 GHz, which is almost comparable to the radiative decay rate of the ground charged exciton, presumably due to the spin scattering between carriers in a QD and a wetting layer and/or mixing of the excited trion states. In contrast, the relaxation rate with conserving p -shell hole-spin projection is estimated to be about one order of magnitude higher than that with hole-spin flip. © 2010 The American Physical Society.

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Laser oscillation in a strongly coupled single-quantum-dot-nanocavity system

Nomura M., Kumagai N., Iwamoto S., Ota Y., Arakawa Y.

Nature Physics 6 ( 4 ) 279 - 283 2010.04

Research paper (scientific journal), Joint Work, Accepted, ISSN 17452473

The strong coupling of photons and matter 1 in semiconductor nanocavities has been a test bed for cavity quantum electrodynamics 2,3 (QED). Vacuum Rabi oscillation 4-8 -the coherent exchange of a single quantum between a single quantum dot (SQD) and an optical cavityand highly efficient cavity-QED lasers 9-19 have both been reported. The coexistence of vacuum Rabi oscillation and laser oscillation seems to be contradictory, but it has recently been predicted theoretically that the strong-coupling effect could be sustained in laser oscillation 20 . Here, we demonstrate the onset of lasing in the strong-coupling regime in an SQD-cavity system. A high-quality semiconductor optical nanocavity and strong SQD-field coupling enabled the onset of lasing while maintaining the fragile coherent exchange of quanta. © 2010 Macmillan Publishers Limited. All rights reserved.

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Neutralization of positively charged excitonic state in single InAs quantum dot by Si delta doping

Kumagai N., Ohkouchi S., Shirane M., Igarashi Y., Nomura M., Ota Y., Yorozu S., Iwamoto S., Arakawa Y.

Journal of Physics Conference Series 244 2010

Research paper (scientific journal), Joint Work, Accepted, ISSN 17426588

Positively charged excitonic state in single InAs quantum dot (QD) has been neutralized by Si delta doping and the optical properties of the fabricated samples are characterized by micro-photoluminescence measurements (μ-PL) at low temperature. An InAs QD sample having dominant emissions from positively charged excitonic states is prepared for a reference. By optimizing Si delta doping conditions, positively charged QDs are neutralized successfully and distinctive emissions from neutral exciton and biexciton from single QDs are achieved. For excessive doping condition, μ-PL spectra show optical degradation of lowering of peak intensity and broadening of peak width as doping concentration increases. © 2010 IOP Publishing Ltd.

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Investigation of the spectral triplet in strongly coupled quantum Dot-nanocavity system

Ota Y., Kumagai N., Ohkouchi S., Shirane M., Nomura M., Ishida S., Iwamoto S., Yorozu S., Arakawa Y.

Applied Physics Express 2 ( 12 ) 2009.12

Research paper (scientific journal), Joint Work, Accepted, ISSN 18820778

We experimentally investigated the excitation power dependence of a strongly coupled quantum dot (QD)-photonic crystal nanocavity system by photoluminescence measurements. At a low excitation power regime, we observed a vacuum Rabi doublet emission at the QD- cavity resonance condition. With increasing excitation power, in addition to the doublet, a third emission peak appeared. This observed spectral change is unexpected from conventional atomic cavity quantum electrodynamics. The observations can be attributed to featured pumping processes in the semiconductor QD-cavity system. © 2009 The Japan Society of Applied Physics.

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