Toshihiro Nakaoka

Controlling the spontaneous emission rate of single quantum dots in a two-dimensional photonic crystal

We observe large spontaneous emission rate modification of individual InAs quantum dots (QDs) in a 2D photonic crystal with a modified, high-Q single-defect cavity. Compared to QDs in a bulk semiconductor, QDs that are resonant with the cavity show an emission rate increase of up to a factor of 8. In contrast, off-resonant QDs indicate up to fivefold rate quenching as the local density of optical states is diminished in the photonic crystal. In both cases, we demonstrate photon antibunching, showing that the structure represents an on-demand single photon source with a pulse duration from 210 ps to 8 ns. We explain the suppression of QD emission rate using finite difference time domain simulations and find good agreement with experiment.

Optically Probing Spin and Charge Interactions in a Tunable Artificial Molecule

We optically probe and electrically control a single artificial molecule containing a well defined number of electrons. Charge and spin dependent interdot quantum couplings are probed optically by adding a single electron-hole pair and detecting the emission from negatively charged exciton states. Coulomb- and Pauli-blockade effects are directly observed, and tunnel coupling and electrostatic charging energies are independently measured. The interdot quantum coupling is shown to be mediated by electron tunneling. Our results are in excellent accord with calculations that provide a complete picture of negative excitons and few-electron states in quantum dot molecules.

Room temperature continuous wave lasing in InAs quantum-dot microdisks with air cladding.

We demonstrated the first room temperature continuous wave lasing in InAs quantum-dot microdisk lasers with a standard air-cladding optical confinement structure. The spectrum shows the single strong lasing peak at a wavelength of 1280 nm. The threshold pump power is 410 muW, and the corresponding effective threshold obtained by considering the absorption efficiency is 81 muW. This achievement is mainly attributed to the increase in Q factor by the improved disk shape.

Lasing characteristics of InAs quantum-dot microdisk from 3 K to room temperature

We fabricated a microdisk laser with five-stacked InAs quantum-dot (QD) active region, and demonstrated the lasing operation from 3K to room temperature by femtosecond pulsed photopumping. At room temperature, the threshold power was estimated to be 0.75mW, when the influence of the surface recombination at the disk edge was neglected. The lasing wavelength was 1.2–1.3μm, which corresponded to excited states of the QDs. The temperature dependence of the threshold, slope efficiency, lasing wavelength, and linewidth are explained by the rapid increase in nonradiative recombination and internal absorption at critical temperatures of 200–230K.

Exciton dynamics in current-injected single quantum dot at 1.55μm

We investigate the exciton dynamics in a current-injected single InAs quantum dot (QD) which emits 1.55μm photons. Photon antibunching behavior is observed using a single electroluminescence line of a single QD. The radiative lifetime of this line determined by time-resolved measurement is 1.59ns. The single exciton recombination time agrees with the lifetime calculated with an eight-band kp model. We examine a high drive rate operation of the device by changing the delay time between two electrical pulses. These results demonstrate that our device has the potential to achieve telecommunication band subgigahertz single-photon emission with electrical pulses.

Direct observation of acoustic phonon mediated relaxation between coupled exciton states in a single quantum dot molecule

We probe acoustic phonon mediated relaxation between tunnel coupled exciton states in an individual quantum dot molecule in which the inter-dot quantum coupling and energy separation between exciton states is continuously tuned using static electric field. Time resolved and temperature dependent optical spectroscopy are used to probe inter-level relaxation around the point of maximum coupling. The radiative lifetimes of the coupled excitonic states can be tuned from ~2 ns to ~10 ns as the spatially direct and indirect character of the wavefunction is varied by detuning from resonance. Acoustic phonon mediated inter-level relaxation is shown to proceed over timescales comparable to the direct exciton radiative lifetime, indicative of a relaxation bottleneck for level spacings in the range

InAsSb Quantum Dots Grown on GaAs Substrates by Molecular Beam Epitaxy

\Delta E\

Fine and Large Coulomb Diamonds in a Silicon Quantum Dot

~3-6 meV.

Carrier relaxation in closely stacked InAs quantum dots

InAsSb quantum dots were grown on GaAs substrates by two methods. One was grown by conventional molecular-beam epitaxy in which arsenic and antimony were irradiated simultaneously. The other was grown by irradiating previously grown InAs dots with antimony to prevent antimony flux from functioning as a surfactant. Although the photoluminescence spectrum of the dots grown by the conventional method had two peaks, the photoluminescence spectrum of the dots grown by the second method had a single peak. Peaks at wavelengths longer than 1.4 µm were observed in the photoluminescence spectrum of the InAsSb quantum dots grown by the second method.

Carrier-density dependence of photoluminescence from localized states in InGaN/GaN quantum wells in nanocolumns and a thin film

We experimentally study the transport properties of silicon quantum dots (QDs) fabricated from a highly doped n-type silicon-on-insulator wafer. Low noise electrical measurements using a low temperature complementary metal-oxide-semiconductor (LTCMOS) amplifier are performed at 4.2 K in liquid helium. Two series of Coulomb peaks are observed: long-period oscillations and fine structures, and both of them show clear source drain voltage dependence. We also observe two series of Coulomb diamonds having different periodicity. The obtained experimental results are well reproduced by a master equation analysis using a model of double QDs coupled in parallel.