Keiji Kuroda

Excitonic and biexcitonic decoherence in self-assembled GaAs quantum dots as observed by phase-locked interferography

We observe single-photon interferograms for emission of a single self-assembled GaAs quantum dot. A phase stabilizer is applied to the interferometer, enabling us to trace autocorrelations of weak emissions with a photon-counting level. At low excitation, where the average number of excitons in a dot is less than one, interferography reveals a single exponential decay, reflecting excitonic decoherence. At moderately high excitation, polarization interference between the exciton-biexciton transitions is found to appear on the interferogram. The decoherence time and the binding energy of biexcitons are determined simultaneously.

Emission studies on photonic crystals fabricated using dyed polystyrene colloids

Three dimensionally ordered photonic crystals are fabricated with rhodamine B dyed polystyrene colloidal spheres using the inward growing self-assembling method in less than 3 h. This avoids the difficulties due to infiltration of active materials into passive photonic crystals. The superior optical quality of the photonic crystals fabricated using this method results in high reflectance values even at large angles of incidence. The study of emission characteristics on these functionalized photonic crystals shows a clear trend dependent on the angle of emission, wavelength, and the angle-resolved transmittance. The dip observed in the emission spectrum clearly matches the photonic stop band position at different angles of observation. The emission spectrum measured at different angles was found to follow a simple functional form related to the intrinsic emission of the dye and the stop band effect due to the photonic crystal environment.

Doubly enhanced spontaneous emission due to increased photon density of states at photonic band edge frequencies

Double enhancement of spontaneous emission due to increased photon density of states at the emission frequency and the small group velocity of light at the excitation frequency was clearly demonstrated by angle-resolved photoluminescence experiments for dielectric multilayers composed of Ta2O5 and SiO2 with oxygen vacancies as light emitters. Theoretical emission profiles given by the weak modulation approximation agreed well with the experimental observations.

Self-assembly of quantum dots and rings by droplet epitaxy and their optical properties

Developments in the self-assembly by droplet epitaxy in our research group have enabled us to fabricate various GaAs quantum nanostructures of high optical quality such as quantum dots, single quantum rings, and concentric double quantum rings. We clarified their electronic states and relaxation processes by micro photoluminescence experiments. We achieved lasing of lattice-matched GaAs quantum dots and their excitonic Rabi oscillation by resonant excitation. We succeeded in the control of their photon emission rate by photonic crystal micro cavities.

Final-state readout of exciton qubits by observing resonantly excited photoluminescence in quantum dots

The authors report on an approach to detect excitonic qubits in semiconductor quantum dots by observing spontaneous emissions from the relevant qubit level. The ground state of excitons is resonantly excited by picosecond optical pulses. Emissions from the same state are temporally resolved with picosecond time resolution. To capture weak emissions, the authors greatly suppress the elastic scattering of excitation beams, by applying obliquely incident geometry to the microphotoluminescence setup. Rabi oscillations of the ground-state excitons appear to be involved in the dependence of emission intensity on excitation amplitude.

Enhanced spontaneous emission observed at one-dimensional photonic band edges

We recently reported experimental evidence for double enhancement of spontaneous emission due to increased photon density of states and small group velocity at photonic band-edge frequencies by observing angle-resolved emission and excitation spectra of photoluminescence [K. Kuroda et al., Opt. Express 17, 13168 (2009)]. The specimen we used was a one-dimensional photonic crystal composed of periodic multilayers of Ta2O5 and SiO2 with oxygen vacancies as light emitters. In the present study, we report on the lack of any excitation intensity dependence of the emission peak height and width, which excludes possibilities of nonlinear effects, the polarized emission spectra, and their comparison with theoretical calculations to further confirm our finding.

Acceleration and suppression of photoemission of GaAs quantum dots embedded in photonic crystal microcavities

We fabricated optical microcavities in a photonic crystal slab embedded with GaAs quantum dots by electron beam lithography and droplet epitaxy. The Purcell effect of exciton emission of GaAs quantum dots was confirmed for the first time by microphotoluminescence and lifetime measurements.

Photon Correlation in GaAs Self-Assembled Quantum Dots

We report on photon coincidence measurement in a single GaAs self-assembled quantum dot (QD) using a pulsed excitation light source. At low excitation, when a neutral exciton line was present in the photoluminescence (PL) spectrum, we observed nearly perfect single photon emission from an isolated QD at 670 nm wavelength. For higher excitation, multiple PL lines appeared on the spectra, reflecting the formation of exciton complexes. Cross-correlation functions between these lines showed either bunching or antibunching behavior, depending on whether the relevant emission was from a biexciton cascade or a charged exciton recombination.

Recovery of population inversion in an erbium-doped fiber amplifier observed by temporally resolving amplified spontaneous emissions

We report on the recovery of population inversion in a CW-pumped erbium-doped fiber amplifier. Spontaneous emissions after the input of signal pulses were temporally resolved. The recovery time for population inversion was estimated to be a few hundred microseconds. The pump power and signal width dependences of the recovery were measured. We discuss the dependences by applying an analytical formula derived for the temporal evolution of population inversion.

Temperature Dependence of the Raman Shift and Raman Width of Solid Parahydrogen: Effect of Vibron–Phonon Scattering

We present a model for vibron–phonon scattering in molecular crystals. A general kinetic equation for the vibrons is derived. We use this equation to calculate the frequency shift and dephasing rate of the vibrons as functions of temperature. We find good agreement between the calculated results and the experimental observations for the Raman shift and Raman width of the fundamental vibrational Raman transition of solid parahydrogen.