Kota Okazaki

Lasing characteristics of an optically pumped single ZnO nanosheet.

We report the lasing characteristics of a single ZnO nanosheet optically pumped by ultraviolet laser beam. The ZnO nanosheets were synthesized by a carbothermal chemical vapor deposition method. The ZnO nanosheets dispersed on a silica glass substrate were excited by the third-harmonic of a Q-switched Nd:YAG laser (λ = 355 nm, τ = 5 ns) and photoluminescence from a single ZnO nanosheet was observed. The observed emission spectra showed the obvious lasing characteristics having modal structure and threshold characteristics. The threshold power for lasing was measured to be 50 kW/cm2, which was much lower than 150 kW/cm2, the threshold power of the reference ZnO nanowire. It indicates that the ZnO nanosheet is a superior gain medium for an ultraviolet laser. The oscillation mechanism inside a ZnO nanosheet is attributed to the micro-cavity effect, based on the three-dimensional laser-field simulation.

Ultraviolet whispering-gallery-mode lasing in ZnO micro/nano sphere crystal

We report ultraviolet (UV) whispering-gallery-mode (WGM) lasing in a zinc oxide (ZnO) micro/nanosphere crystal fabricated by simply ablating a ZnO sintered target, which was much more productive method without any time-consuming crystal-growth process. The lasing spectral mode spacing was controlled by changing the diameters, and single-mode lasing was realized from a ZnO nanosphere. Experimental results were in good agreement with predictions from WGM theories. Since the ZnO sphere can operate as an active WGM refractometric sensor for small molecules in UV region, high sensitivity enhanced by high quality factor, refractive index, and wavelength dispersion can be expected.

Effect of laser annealing on photoluminescence properties of phosphorus implanted ZnO nanorods.

The effect of the nanosecond laser annealing on the photoluminescence (PL) property of phosphorus ions (P(+)) implanted ZnOnanorods (NRs) has been investigated. The nanosecond laser annealing was performed with the third harmonic of a Q-switched Nd:YAG laser (355nm, 10ns/pulse) at a fluence of 100mJ/cm(2). It turned out that nanosecond laser annealing is more effective in the recovery of the PL property compared with the thermal annealing using an electric furnace. As the results, the I-V characteristics of the p-n homojunctions along ZnO NRs showed rectifying property with a threshold voltage of approximately 6V.

Influence of ZnO buffer layer on ZnO nanowire growth by nanoparticle-assisted pulsed laser deposition

We have succeeded in growing various ZnO nanocrystals, such as nanowires, nanorods, and nanowalls, by a nanoparticle-assisted pulsed-laser deposition (NAPLD). In this study, low-density ZnO nanowires were synthesized by introduction of a ZnO buffer layer. Low-density hexagonal cone-shape ZnO cores are formed on the buffer layer, and vertically-aligned ZnO nanowires are grown on the cores. The density of the nanowires was clearly decreased with increasing the thickness of the Buffer layer. The buffer layer can be used as one of the effective additives to control the growth density of the ZnO nano-crystals synthesized by NAPLD.

Responses of polymers to laser plasma EUV light beyond ablation threshold and micromachining

We have investigated responses of PDMS, PMMA and acrylic block copolymers (BCP) to EUV light from laserproduced plasma beyond ablation thresholds and micromachining. We generated wide band EUV light around 100 eV by irradiation of Ta targets with Nd:YAG laser light. In addition, narrow band EUV light at 11 and 13 nm were generated by irradiation of solid Xe and Sn targets, respectively, with pulsed CO2 laser light. The generated EUV light was condensed onto samples, using an ellipsoidal mirror. The EUV light was incident through windows of contact masks on the samples. We found that through-holes with a diameter of 1 μm can be fabricated in PDMS sheets with thicknesses of 10 μm. PDMS sheets are ablated if they are irradiated with EUV light beyond a threshold power density, while PDMS surfaces were modified by irradiation with the narrow band EUV light at lower power densities. Effective ablation of PMMA sheets can be applied to a LIGA-like process for fabricating micro-structures of metals using the practical apparatus. Furthermore, BCP sheets were ablated to have micro-structures. Thus, we have developed a practical technique for microma chining of PMMA, PDMS and BCP sheets in a micrometer scale.

Comprehensive photonics-electronics convergent simulation and its application to high-speed electronic circuit integration on a Si/Ge photonic chip

We developed a design technique for a photonics-electronics convergence system by using an equivalent circuit of optical devices in an electrical circuit simulator. We used the transfer matrix method to calculate the response of an optical device. This method used physical parameters and dimensions of optical devices as calculation parameters to design a device in the electrical circuit simulator. It also used an intermediate frequency to express the wavelength dependence of optical devices. By using both techniques, we simulated bit error rates and eye diagrams of optical and electrical integrated circuits and calculated influences of device structure change and wavelength shift penalty.

Growth of periodic ZnO nano-crystals on buffer layer patterned by interference laser irradiation

Zinc oxide (ZnO) nano-crystal is great interest for optoelectronic applications in particular ultraviolet (UV) region such as UV-LEDs, UV-lasers, etc. For the practical optoelectronic applications based on the ZnO nanocrystals, control of nanowire growth direction, shape, density, and position are essentially required. In our study, we introduced a ZnO buffer layer and interference laser irradiation to control the growth position of ZnO nanocrystals. In this presentation, structural and morphological characteristics of periodic ZnO nano-crystals synthesized by the nanoparticle-assisted pulsed laser deposition will be discussed.

Design technique with equivalent circuit for photonics-electoronics convergence system

We developed a design technique for a photonics-electronics convergence system using an equivalent circuit of optical devices. This technique enables overall design and optimization of the photonics-electronics convergence system in an electrical circuit simulator.

Laser Nano-Soldering of ZnO Nanowires and GaN Thin Film for Fabrication of Hetero p-n Junction

We have investigated the laser nano soldering between ZnO nanowires and the p-type GaN thin film for the realization of the hetero p-n junction with low thermal loading. When the ZnO nanowires were irradiated with 355 nm laser beam at a fluence of 0.25 J/cm2, only a tip of the nanowires was melted due to the field enhancement effect near the tip of the nanowires, and a small bead with a diameter of 40 nm was formed at the tip of each nanowires. This phenomenon was applied to solder the junction between the ZnO nanowires and a p-type GaN thin film, by irradiating the junction through the GaN film with 375 nm laser beam which is transparent for the p-type GaN thin film but opaque for the ZnO nanowires. As a result, the improvements of the I–V characteristics of the junction and the increment of the UV electro-luminescence were clearly observed, demonstrating the effectiveness of the nano soldering in fabrication of the hetero p-n junction between the ZnO nanowires and the GaN thin film.

Control of ZnO nano-crystals synthesized by nanoparticle-assisted pulsed laser deposition using buffer layer and laser irradiation

Various ZnO nanocrystals, such as nanowires, nanorods, and nanowalls, have been successfully synthesized by a nanoparticle-assisted pulsed laser deposition (NAPLD). In this study, we have succeeded in controlling the growth density and position of the ZnO nano crystals with a ZnO buffer layer and a buffer layer patterned by interference laser irradiation, respectively. Vertically aligned ZnO nanowires with low lateral density were grown on the ZnO buffer layer, and each nanowire was grown at the tip of the hexagonal cone-shape ZnO core formed on the layer. The lateral density of the ZnO nanowires can be controlled by the buffer layer thickness. In addition, laser irradiation to the buffer layer can also control the density, because the density of the nanowire grown on the laser-irradiated layer was clearly decreased as compared with no-irradiated layer. Furthermore, patterned growth of ZnO nano crystals was demonstrated using four beam interference patterning. The buffer layer and interference laser irradiation can be used as one of the effective additives to control the growth of the ZnO nano crystals synthesized by NAPLD.