Satoshi Kako

Room-temperature triggered single photon emission from a III-nitride site-controlled nanowire quantum dot.

We demonstrate triggered single photon emission at room temperature from a site-controlled III-nitride quantum dot embedded in a nanowire. Moreover, we reveal a remarkable temperature insensitivity of the single photon statistics, and a g((2))[0] value at 300 K of just 0.13. The combination of using high-quality, small, site-controlled quantum dots with a wide-bandgap material system is crucial for providing both sufficient exciton confinement and an emission spectrum with minimal contamination in order to enable room temperature operation. Arrays of such single photon emitters will be useful for room-temperature quantum information processing applications such as on-chip quantum communication.

Exciton and biexciton luminescence from single hexagonal GaN/AlN self-assembled quantum dots

We report single dot spectroscopy of hexagonal GaN∕AlN self-assembled quantum dots grown by metalorganic chemical vapor deposition. Through the reduction of the number of quantum dots using submicron mesa structures, we have obtained several isolated photoluminescence peaks emitted by individual quantum dots. With increasing excitation power, an additional peak emerges in the higher-energy side of an excitonic ground state. This additional peak shows quadratic power dependence, and is attributed to the photoluminescence from a biexciton state. The biexciton binding energy is negative and the magnitude is about 30meV, which is explained by the effects of a strong built-in electric field.

Photon correlation studies of single GaN quantum dots

We present measurements of the second-order coherence function on emission from single GaN quantum dots. In some cases a large degree of photon antibunching is observed, demonstrating isolation of a single quantum system. For a selected quantum dot, we study the dependence of photon antibunching on excitation power and temperature. Using pulsed excitation, we demonstrate an ultraviolet triggered single-photon source operating at a wavelength of 358nm.

Size-dependent radiative decay time of excitons in GaN/AlN self-assembled quantum dots

Size-dependent radiative decay time of excitons in GaN/AlN self-assembled quantum dots is reported. Two samples having different average size of quantum dots (QDs) have been investigated at the temperature of 3.5 K. The measurement has revealed that larger-QD sample shows longer photoluminescence (PL) decay time and smaller emission energy than smaller one. The dependence of radiative decay time of the samples on emission energy smoothly connects with each other reflecting the size distribution. The radiative decay time strongly increases by almost three orders magnitude, reaching microseconds, upon increasing the size of QDs. The increase of PL decay time with increasing the size of QDs is attributed to the reduction of oscillator-strength due to the strong built-in electric field in the GaN/AlN heterostructures.

AlN air-bridge photonic crystal nanocavities demonstrating high quality factor

The authors report an achievement of high quality AlN ultraviolet photonic crystal nanocavities. Convex AlN air-bridge structures with embedded GaN quantum dots have been formed by utilizing photoelectrochemical etching of 6H-SiC substrates. Room-temperature microscopic photoluminescence measurements reveal the high quality of the nanocavities. For the lowest-order cavity mode of a 150-nm-period nanocavity with seven missing holes, the highest Q factor (>2400) ever reported in nitride-based photonic crystals has been obtained.

Time-of-Flight Measurement of Lateral Carrier Mobility in Organic Thin Films

Lateral carrier transport in a copper phthalocyanine (CuPc) thin film has been investigated by the time-of-flight technique using a micro-excitation system. Drift mobility in the CuPc film has been estimated from the transient photocurrent measured for various electric field strengths. The drift mobility has been compared to the field-effect mobility of a thin-film transistor with CuPc as the channel material. The field-effect mobility was comparable to the drift mobility measured by the TOF technique.

Single-photon emission from cubic GaN quantum dots

We report the demonstration of single-photon emission from cubic GaN/AlN quantum dots grown by molecular beam epitaxy. We have observed spectrally clean and isolated emission peaks from these quantum dots. Clear single-photon emission was detected by analyzing one such peak at 4 K. The estimated g(2)[0] value is 0.25, which becomes 0.05 when corrected for background and detector dark counts. We have also observed the single-photon nature of the emission up to 100 K (g(2)[0] = 0.47). These results indicate that cubic GaN quantum dots are possible candidates for high-temperature operating UV single-photon sources with the possibility of integration into photonic nanostructures.

Formation and optical properties of stacked GaN self-assembled quantum dots grown by metalorganic chemical vapor deposition

Multiple-layer stacked GaN quantum dots (QDs) with intense photoluminescence (PL) have been successfully grown by the Stranski-Krastanow growth mode in metalorganic chemical vapor deposition. Transmission electron microscopy (TEM) and scanning TEM analyses showed vertically aligned QDs resulting from a strain field induced by the buried islands. A redshift and linewidth narrowing of the PL spectra for the multilayered structures indicate that the size of the QDs becomes larger and more uniform throughout the layering process. The PL intensity increased with increasing the number of stacked layers, indicating that the QDs can be stacked without introducing new nonradiative recombination centers.

Observation of enhanced spontaneous emission coupling factor in nitride-based vertical-cavity surface-emitting laser

We demonstrate an enhancement of spontaneous emission coupling factor β in a nitride-based vertical-microcavity surface-emitting laser. The 2.5λ vertical microcavity, the quality factor of which reaches 740, is sandwiched between a nitride and an oxide distributed Bragg reflector. From input–output measurements and analyses of the rate equations, the β of the lasing mode at a wavelength of 396.1 nm is estimated to be 1.6×10−2. The estimated β can be well accounted for by a simple theoretical model.

High-Q (>5000) AlN nanobeam photonic crystal cavity embedding GaN quantum dots

We report on the fabrication and optical characterization of high-quality-factor AlN one-dimensional nanobeam photonic crystal cavities embedding GaN quantum dots and operating in the ultra-violet range. By means of electron-beam lithography, dry etching and photoelectrochemical etching, we implement a high-frequency nanobeam cavity design in an AlN epilayer containing GaN quantum dots. Room-temperature microphotoluminescence characterization of the fabricated nanobeams exhibits resonances with wavelengths as short as 320 nm and quality factors as high as Qexp = 5.0 × 103 at 380 nm. This constitutes a significant improvement over previously reported group-III nitride photonic crystal cavities in terms of operating wavelength.