Tetsuya Shimogaki

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.

Patterned growth of ZnO nanowalls by nanoparticle-assisted pulsed laser deposition

We have succeeded in controlling the growth density of ZnO nanowires synthesized by nanoparticle-assisted pulsed laser deposition (NAPLD) using a ZnO buffer layer and laser irradiation. The density of the nanowires decreased with increasing thickness of the buffer layer. In addition, the nanowire density also decreased by laser irradiation to the buffer layer. Patterned growth of ZnO nanowalls using NAPLD was successfully achieved with the help of the buffer layer patterned by interfering laser beams.

Synthesis and characterization of SB doped ZNO microspheres by pulsed laser ablation

We succeeded in synthesizing Sb-doped ZnO microspheres by ablating a ZnO sintered target containing of Sb with Nd:YAG laser in the air. The structural properties of the microspheres were investigated by Raman scatting studies. The Zn-Sb related local vibrational mode was detected 238 cm-1. Room-temperature photoluminescence properties of the microsphere were investigated under 325 nm He-Cd laser or 355 nm Nd:YAG laser excitation. An ultraviolet (UV) emission and lasing in whispering gallery mode (WGM) were observed from the photoexcited microsphere.

Ultraviolet electroluminescence from hetero p-n junction between a single ZnO microsphere and p-GaN thin film.

We report ultraviolet electroluminescence from a hetero p-n junction between a single ZnO microsphere and p-GaN thin film. ZnO microspheres, which have high crystalline quality, have been synthesized by ablating a ZnO sintered target. It was found that synthesized ZnO microspheres had a high-optical property and exhibit the laser action in the whispering gallery mode under pulsed optical pumping. A hetero p-n junction was formed between the single ZnO microsphere/ p-GaN thin film, and a good rectifying property with a turn-on voltage of approximately 6 V was observed in I-V characteristic across the junction. Ultraviolet and visible electroluminescence were observed under forward bias.

Catalyst-free growth of ZnO nanowires on various-oriented sapphire substrates by pulsed-laser deposition

Catalyst-free growth of one-dimensional zinc oxide (ZnO) nanowires is reported. ZnO nanowires were synthesized on ZnO buffer layers deposited on various-oriented sapphire substrates. Syntheses of ZnO buffer layers and nanowires were performed by ultraviolet pulsed-laser deposition. ZnO nanowires number density was the lowest in the case of using m-cut sapphire substrates. ZnO nanowires grown on a-cut sapphire substrates had vertical alignment with distances of tens to hundreds of nanometers. On the other hand, ZnO nanowires grown on c-cut sapphire substrates had the biggest nucleation rate. The dependence of crystalline orientation of ZnO buffer layers on the orientation of sapphire substrates were investigated by electron back scatter diffraction measurement. From their results, the growth models of ZnO buffer layers were suggested and the variations in morphological properties of ZnO nanowires were discussed.

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.

Fabrication of ZnO crystals by UV-laser annealing on ZnO nanoparticles prepared by laser ablation method

Various zinc oxide (ZnO) nanocrystals are expected as new building blocks for optoelectronic devices. Among them, we have studied about fabricating ZnO nanowires using nanoparticle-assisted pulsed laser deposition (NAPLD). Recently, we achieved to fabricate the periodically-aligned ZnO nanowires with a period of from 4 to 5 μm using interfering four-beams of nanosecond ultraviolet (UV) laser processing. ZnO nanowires with diameters of several dozen nanometers were grown on the ZnO buffer layer prepared by pulsed laser deposition at the low-chamber pressure of 3 Pa. Additionally, crystallization of ZnO nanoparticles collected on a sapphire substrate was achieved by UV-laser annealing. In this method, ZnO nanoparticles were collected at room temperature, then they were laser-annealed with a KrF excimer laser. The particle size increased by instantaneous melting and aggregation of ZnO nanoparticles because of the high absorption efficiency of ZnO in the UV spectral region. It was found that the optical property was improved by UV-laser annealing process. Additionally, their x-ray diffraction peaks of wurtzite ZnO crystals had narrower full width half maximum than those before laser annealing.

Fabrication and bandgap engineering of doped ZnO microspheres by simple laser ablation in air

We synthesized magnesium (Mg)-doped ZnO microspheres by laser ablation of a ZnO sintered target containing magnesium oxide (MgO) with the fundamental of a Nd:YAG laser at 1064 nm. The well-spherical ZnO microcrystals with diameters of 1-20 μm were collected on a substrate which was put near the ablation spot. X-ray diffraction and micro-Raman spectrum indicate that the ZnO microspheres have a crystalline structure. Room-temperature photoluminescence properties of the microsphere were investigated under third harmonic generation of a Nd:YAG laser excitation at 355 nm. An ultraviolet (UV) lasing in whispering gallery mode (WGM) and blue-shift of the UV WGM peaks were observed from the Mg-doped ZnO microsphere.

Synthesis of Sb-doped ZnO microspheres by pulsed laser ablation and their photoluminescence properties

We succeeded in synthesizing antimony (Sb)-doped ZnO microspheres by ablating a ZnO sintered target containing 5 wt% of Sb with a Nd:YAG laser at a fluence of 25 J/cm2 in air. The well-spherical ZnO microcrystals with diameters of 1-20 μm were collected on a substrate which was put near the ablation spot. Most of the ZnO microspheres have a crystalline structure. In addition, Raman peak of the Sb-doped ZnO microspheres was shifted toward lower frequency side, indicating substitutional Sb3+ at Zn antisite. Room-temperature photoluminescence properties of the microsphere were investigated under 325 nm He-Cd laser or 355 nm Nd:YAG laser excitation. An ultraviolet (UV) emission and lasing in whispering gallery mode were observed from the photoexcited microsphere.