Mitsuhiro Higashihata

Large improvement in quantum fluorescence yield of Er3+-doped fluorozirconate and fluoroindate glasses by Ce3+ codoping

The fluorescence characteristics of different Ce3+:Er3+-codoped fluoride host glasses, such as fluorozirconate and fluoroindate glass, are reported. It is shown that Ce3+ codoping into Er3+ doped fluoride glasses resulted in a quenching of Er3+ ions from the 4I11/2 to the 4I13/2 state, and the branching ratio for the Er3+ 4I11/2→4I13/2 transition increased from 0.20 to over 0.80 by codoping 2.0 mol % Ce3+. Further, the fluorescence quantum yield at 1.55 μm was also significantly improved. A Ce3+:Er3+-codoped fluorozirconate fiber laser operating at 1.55 μm band with 980 nm excitation has been realized for the first time, which clearly indicates the effectiveness of Ce3+ codoping.

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.

Density-Controlled Growth of ZnO Nanowires Via Nanoparticle-Assisted Pulsed-Laser Deposition and Their Optical Properties

Vertically aligned ZnO nanowires with controllable density were successfully synthesized by nanoparticle-assisted pulsed-laser deposition (NAPLD), by which nanoparticles formed in the gas phase by the condensation of the ablated species are transported onto the substrate, and nanoparticles play an important role in density control. The effects of synthesis conditions on the density and optical properties of ZnO nanowires were investigated in detail. With the increase in repetition rate and laser energy, ZnO nanowires with a higher density can be obtained as a result of a larger quantity of nanoparticles formed in the gas phase. The density-controlled growth also depends on substrate–target distance. UV emission was observed from all the obtained nanowires at about 390 nm without visible fluorescence near 500 nm due to their excellent crystallinity and the oxygen-deficient defect.

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.

Catalyst/dopant-free growth of ZnO nanobelts with different optical properties from nanowires grown via a catalyst-assisted method

The paper reported a simple but effective gold catalyst-tailored chemical vapour deposition (CVD) method to grow ZnO nanowires and nanobelts with different photoluminescence (PL) behaviours. ZnO nanowires along the c-axis were synthesized via a gold catalyst-assisted CVD method through the conventional vapour–liquid–solid (VLS) mechanism. But, ZnO nanobelts grew along the a-axis enclosed with {010} and {0001} base planes without using any catalyst or dopant through the vapour–solid (VS) mechanism. The nanowires and nanobelts were proved to be of different optical quality, which was related with their growth process. Besides the different origin of the near-band-gap emission at room temperature, laser emission was only observed from ZnO nanobelts while the nanowires show anomalous photoluminescence behaviour under higher excitation density.

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.