Published papers

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.

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.

We demonstrate photonic crystal nanocavity laser with nearly-single quantum dot gain. Continuous-wave lasing was achieved at photocarrier generation rate of 5 x 109/s, corresponding to several carrier injection into nearly-single quantum dot. © 2008 OSA / CLEO/QELS 2008.

We demonstrate photonic crystal nanocavity laser with nearly-single quantum dot gain. Continuous-wave lasing was achieved at photocarrier generation rate of 5 × 109/s, corresponding to several carrier injection into nearly-single quantum dot. © 2008 IEEE.

We utilized simultaneous coupling of higher and ground states of a single QD with two different resonant modes of a PhC nanocavity for efficient excitation and emission. Enhanced emission with strongly suppressed background was demonstrated. © 2008 OSA/CLEO/QELS 2008.

We utilized simultaneous coupling of higher and ground states of a single QD with two different resonant modes of a PhC nanocavity for efficient excitation and emission. Enhanced emission with strongly suppressed background was demonstrated. © 2008 IEEE.

We successfully fabricated photonic crystal nanocavities for generating entangled photon pairs using single quantum dots. The fabricated cavities have a minimum splitting between two orthogonal dipóle modes of ∼250 μeV and measured cavity Qs of ∼2,000. A maximum fidelity of 0.95 for entangled photon pairs is expected with this cavity.

We fabricated photonic crystal nanocavities which have two orthogonal dipole modes with a minimum splitting of ∼250 μeV and measured cavity Qs of -2,000. Those cavities are applicable for generating entangled photon pairs using quantum dots. © 2007 IEEE.

An enhanced optical Stark shift in a single quantum dot is demonstrated using a newlydesigned H1-type photonic crystal nanocavity. A 70μeV shift is observed at 450nW; 110-times lower than excitation powers reported in the literature. © OSA 2012.

An enhanced optical Stark shift in a single quantum dot is demonstrated using a newly-designed H1-type photonic crystal nanocavity. A 70μeV shift is observed at 450nW; 110-times lower than excitation powers reported in the literature. © 2012 OSA.

We demonstrate a photonic crystal nanocavity laser essentially driven by single quantum dot gain. A diluted quantum dot density (∼0.4/cavity) resulted in clear single quantum dot feature and distinct phase transition in photon correlation measurements. ©2009 IEEE.

We demonstrate a photonic crystal nanocavity laser essentially driven by single quantum dot gain. A diluted quantum dot density (~0.4/cavity) resulted in clear single quantum dot feature and distinct phase transition in photon correlation measurements. © 2009 OSA.

We report here the first observation of vacuum Rabi splitting in a single quantum dot (QD) embedded in a H1 photonic crystal nanocavity by photoluminescence measurement. The QD emission was tuned into a cavity mode by controlling the temperature. At the resonance condition, clear anticrossing with a Rabi splitting of ∼124 μeV was observed, where the cavity mode possesses the smallest mode volume V∼0.43 (λ/n)3 among strongly coupled QD-cavity systems reported to date. © 2009 American Institute of Physics.

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.

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 (λ0 /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.

Conventional air-bridge two-dimensional photonic crystal (2D-PhC) nanocavities emit light with an equal power distribution for upward and downward out-of-plane directions. Some applications, however, require concentration of the radiation in a preferred direction. In this Letter, we design a 2D-PhC nanocavity that radiates dominantly into one of the out-of-plane directions with a narrow far-field distribution. Our design is based on a conventional airbridge L3 photonic crystal nanocavity. The asymmetric out-of-plane power distribution is achieved solely by adding periodic shallow holes on the slab surface, which also function as a second-order grating for the directional beaming.

Circular dichroism covering the telecommunication band is experimentally demonstrated in a semiconductor-based three-dimensional chiral photonic crystal (PhC). We design a rotationally stacked woodpile PhC structure where neighboring layers are rotated by 60° and three layers construct a single helical unit. The mirror-asymmetric PhC made from GaAs with sub-micron periodicity is fabricated by a micro-manipulation technique. Due to the large contrast of refractive indices between GaAs and air, the experimentally obtained circular dichroism extends over a wide wavelength range, with the transmittance of right-handed circularly polarized incident light being 85% and that of left-handed light being 15% at a wavelength of 1.3 μ m. The obtained results show good agreement with numerical simulations. © 2014 AIP Publishing LLC.

We experimentally demonstrate a three-dimensional photonic crystal (3D PC) nanocavity in a 〈110a〉-layered diamond structure with a quality factor (Q-factor) of 12800 at a wavelength of 1.1 μm. The observed Q is 1.2 times higher than that of a 3D PC nanocavity in a woodpile structure with the same in-plane size and the same number of stacked layers. This result indicates the potential importance of the 〈110a〉-layered diamond structure for getting high Q 3D PC nanocavities within a limited in-plane space.

We design and demonstrate a photonic crystal nanobeam cavity with a Q factor controllable by a micro-electro-mechanical system (MEMS). The cavity is evanescently coupled to an adjacent nanobeam waveguide, and its Q factor is controlled by electro-mechanically adjusting the gap distance between them. We experimentally demonstrate control of the Q factor from 2,250 to 2,750, by applying voltage from 0 to 11 V. © 2013 The Japan Society of Applied Physics.

We measure polarization resolved photoluminescence from excitonic complexes in tens of single InAs/GaAs quantum dots (QDs) over a wide range of energies in two samples: one as-grown, the other subjected to rapid thermal annealing (RTA). Using the linear polarization of the X+ emission as a probe of the valence band mixing (VBM), we find that RTA enhances QD VBM, attributed to the increase in QD height. We complement our measurements with 8-band k · p calculations, which suggest the increase in height on annealing is responsible for this enhancement. © 2013 The Japan Society of Applied Physics.