Tetsuya Kouno

Two-dimensional light confinement in periodic InGaN/GaN nanocolumn arrays and optically pumped blue stimulated emission

Two-dimensional (2D) light diffraction in a uniform array of GaN nanocolumns arranged in a rectangular lattice dramatically enhanced the light intensity at a specific wavelength, indicating the function of 2D distributed feedback (DFB). Here a GaN rectangular-lattice nanocolumn array, which integrated InGaN/GaN multiple quantum wells (MQWs) in the top region of the nanocolumns, was grown by rf-plasma-assisted molecular beam epitaxy (rf-MBE). At a specific wavelength of 471.1 nm, the first observation of stimulated emission from 2D-DFB in an InGaN-based nanocolumn array was obtained. The specific wavelength is calculated by the 2D finite-difference time domain (2D-FDTD) method on the assumption of a refractive index dispersion of GaN; a simple expression for specific wavelength, which is a function of the array period L and the hexagon side length S of each nanocolumn, is proposed, which is convenient for producing a simple design of a GaN nanocolumn array structure in a square lattice.

Lasing Action on Whispering Gallery Mode of Self-Organized GaN Hexagonal Microdisk Crystal Fabricated by RF-Plasma-Assisted Molecular Beam Epitaxy

GaN hexagonal microdisks were fabricated on Ti-mask (4 nm in thickness) nanohole-patterned m-plane (10-10) GaN substrates by radio frequency-plasma-assisted molecular beam epitaxy. The GaN hexagonal microdisks, which were supported by ~300 nm-diameter GaN nanocolumns, consisted of thin hexagonal c-plane plates with a hexagon side length of 1-2 m and a typical thickness of 200 nm. The GaN hexagonal microdisks were optically pumped under a high optical excitation density with a 355-nm-wavelength Nd:YAG laser, and ultraviolet lasing actions on the quasi-whispering gallery mode (WGM) were observed at room temperature. The lasing wavelength was 372 nm and the threshold excitation density was approximately 250 kW/cm2. The quasi-WGM resonance was numerically analyzed using a simple plane wave model and a 2-D-flnite difference time domain method, the behaviors of the WGM and quasi WGM were analyzed, revealing that the quasi-WGM has a higher Q-factor, thus, it was clarified that the laser actions of the GaN hexagonal microdisks occurred on the quasi-WGM resonance.

Lasing Actions in GaN Tiny Hexagonal Nanoring Resonators

We achieved optically pumped lasing actions of extremely small optical resonators, which have 1.4-2-μm-diameter GaN hexagonal nanorings with wall widths of 60-140 nm. The hexagonal nanorings were grown by rf-plasma molecular beam epitaxy (rf-MBE) with Ti-mask selective area growth (SAG). Due to the nanometer scale of the wall width, the propagation of threading dislocations through the wall was suppressed, and high crystalline quality of the nanoring resonators was achieved. The hexagonal nanorings possessed a fine cavity configuration of crystallographically flat outer walls and consisted of hexagonally connected sixfold side sections. The periodic boundary condition for each section with a short length limits the number of longitudinal resonance wavelengths, and narrowing the wall suppresses the occurrence of lasing in higher transverse modes. Therefore, both contribute to the lasing action in a single longitudinal mode of the hexagonal nanoring.

Well-arranged novel InGaN hexagonal nanoplates at the tops of nitrogen-polarity GaN nanocolumn arrays

Periodically arranged novel InGaN hexagonal nanoplates were fabricated at the tops of square-lattice N-polarity GaN nanocolumn arrays. The key finding in this work is that the growth of InGaN on N-polarity GaN nanocolumns led to a peculiar nanoplate structure. The InGaN nanoplates with thicknesses of 50-100 nm extended outward from the narrow nanocolumns with diameters of 100-150 nm, to form larger hexagonal nanoplates with a typical side length of 250 nm.

Optical microresonant modes acting in thin hexagonal GaN microdisk

Optical microresonant modes acting in a thin hexagonal GaN microdisk with a side length of approximately 1.5 µm were investigated by experiment and numerical analysis. Step-by-step room-temperature photoluminescence measurement under a high-excitation source was carried out, in which geometry of a thin hexagonal GaN microdisk was changed by laser ablation. As a result, the existence of the whispering gallery mode (WGM) and Fabry–Perot mode acting in the microdisk were confirmed. In addition, the results obtained by a numerical analysis of three-dimensional finite difference time domain method also indicate that these modes potentially contribute to light confinement in the thin hexagonal GaN microdisk, but the quasi-WGM preferentially contributes to light confinement in the microdisk.

Light confinement in hexagonal GaN nanodisk with whispering gallery mode

We demonstrated light confinement in hexagonal GaN nanodisks. The nanodisks were fabricated by crystal growth via radio-frequency plasma-assisted molecular beam epitaxy, and the side length of the nanodisks was approximately 300 nm. Sharp peaks appeared at a wavelength of approximately 370 nm in the room-temperature photoluminescence spectrum of the nanodisks, indicating that the nanodisks acted as an optical nanoresonator. In addition, the results obtained by a finite-difference time domain method suggest that the resonant mode acting preferentially in such nanodisks is the whispering gallery mode. These results indicate that the GaN-based simple nanostructures can be used for nanolasers.

Quasi-Whispering Gallery Mode Lasing Action in an Asymmetric Hexagonal GaN Microdisk

The optical properties of an asymmetric hexagonal GaN microdisk, fabricated via crystal growth by radio-frequency plasma-assisted molecular beam epitaxy (rf-MBE), were investigated by experimental and theoretical methods to clarify the contribution of the light confinement modes of quasi-whispering gallery modes (QWGMs). Under the optically pumped condition, a lasing action was observed from the asymmetric GaN microdisk, indicating that there is a resonant scheme of QWGMs in such a microcavity of a GaN microdisk. In addition, the analysis of the light response of hexagonal GaN microdisk models by a three-dimensional finite difference time-domain method (3D-FDTD) indicates that the QWGMs preferentially contribute to a lasing action even in a microcavity with a regular hexagonal configuration with side lengths of 1 to 2 µm. The results of the experiment and the numerical analysis strongly indicate that it is important to take not only the WGM but also the QWGMs into account to clarify the optical responses of resonant modes in a hexagonal microdisk configuration.

Periodic Radiation Patterns and Circulating Direction of Lasing Light by Quasi Whispering Gallery Mode in Hexagonal GaN Microdisk

We have experimentally elucidated the periodic radiation patterns and circulating direction of the lasing light generated by the quasi-whispering-gallery mode (QWGM) in a hexagonal GaN microdisk. The radiated lasing light from the microdisk is highly directional, with the high intensities of the obtained radiated lasing light having a periodic spacing of 120° in the planar direction. The results show that the QWGM-generated lasing light circulates in a single direction in the microdisk, namely, either clockwise or counter-clockwise.

Excitation area dependence of lasing modes in thin hexagonal GaN microdisks

Thin, hexagonal GaN microdisks act as optical microcavities based on the whispering gallery mode (WGM) and the quasi-WGM that can cause lasing action. We investigated the relationship between the excitation area in regular and asymmetric thin, hexagonal GaN microdisks as well as the characteristics of these lasing modes using a room temperature photoluminescence mapping technique under high-excitation conditions. The lasing modes that are preferentially exhibited in a thin asymmetric hexagonal GaN microdisk were found to depend on the most strongly excited area in the microdisk. This behavior is potentially useful for optical micro switches and similar applications.

Influences of growth parameters on the film formation of hexagonal boron nitride thin films grown on sapphire substrates by low-pressure chemical vapor deposition

Hexagonal boron nitride (h-BN) films were grown on c-plane sapphire substrates by low-pressure chemical vapor deposition with BCl3 and NH3 as the boron and nitrogen sources, respectively, and the influences of growth parameters on the film quality were investigated for samples with a thickness of about 1 µm. The dependence of X-ray diffraction on the growth temperature (T g) indicated that the crystalline quality is most improved in the sample grown at 1200 °C, in which the epitaxial relationship of {100}h-BN ∥ {110}sapphire and {001}h-BN ∥ {001}sapphire was confirmed. This condition enhanced lateral growth, resulting in the formation of grains with flat top surfaces. The T g dependence was discussed in relation to the amorphous AlN formed on the substrate surface and the reaction between BCl3 and NH3 in the vapor phase. The correlation between the structural and luminescent properties, which was found from the T g dependence of CL, was also discussed.