Tatsuo Okada

Comparative study on emission characteristics of extreme ultraviolet radiation from CO2 and Nd:YAG laser-produced tin plasmas

The direct comparison of the emission characteristics of an extreme ultraviolet (EUV) light between the CO2 and the Nd:YAG laser-produced plasmas (LPP) with a solid tin target is reported. In the case of the Nd:YAG LPP, the conversion efficiency (C.E.) peaked at a laser intensity of about 5×1010W∕cm2 and decreased at higher laser intensity. In the case of the CO2 LPP, the C.E. monotonically increased up to 2×1010W∕cm2, where the C.E. is comparable to the maximum C.E. of the Nd:YAG LPP. The spectral efficiency of the Nd:YAG LPP within the 2% bandwidth around 13.5 nm decreased with laser intensity. The corresponding spectral efficiency of the CO2 LPP was almost constant. This observation indicates the potential of the CO2 laser-produced LPP as the EUV light source for the EUV lithographic systems.

Synthesis of ZnO Nanorods by Nanoparticle Assisted Pulsed-Laser Deposition

Nano-structured ZnO thin films were synthesized by the nanoparticle assisted pulsed-laser deposition in an oxygen background gas. Crystallized and c-axis oriented ZnO nanorods with a size of approximately 300 nm in average diameter and 6 µm in length were grown on sapphire substrates heated at approximately 700°C by the pulsed-laser deposition technique without any catalyst. Strong photoluminescence near the band-gap was observed from nanorods under excitation at 308 nm. The Rayleigh scattering diagnostics of the plume was also conducted, revealing that the nanorods grew from ZnO nanoparticles which formed in the plume and were transported onto the substrate.

Fabrication of dot matrix, comb, and nanowire structures using laser ablation by interfered femtosecond laser beams

A line of periodic dot structure with an interval of 6.25 μm on a gold thin film was fabricated with a single shot of interfered femtosecond laser beams split by a diffraction beam splitter. The total length of the structure was 6 mm. In addition, dot matrix and comb structures were fabricated with transportation of samples at an arbitrary speed during the process. The samples worked as transmission and reflection gratings. In addition, nanowires were fabricated by peeling the comb structure, of which the thickness was 200 nm.

Transport of YO molecules produced by ArF laser ablation of YBa2Cu3O7-δ in ambient oxygen gas

One‐dimensional‐imaging laser‐induced fluorescence spectroscopy (1D‐LIF) has been applied to investigate the dynamics of the nonemissive neutral particles (YO molecules) during the ArF excimer laser ablation of YBa2Cu3O7−δ in an ambient oxygen gas. Investigating the 1D‐LIF observation, the propagation of particles through the ambient gas at appropriately high pressures is categorized into two phases, the propagation phase and the diffusion phase. In the propagation phase, the point source blast wave model (shock model) describes well the dynamics at high background gas pressures. Particles propagate according to the shock model over a finite distance from the pellet surface after the ablation, and almost stop there. The propagation distance depends on the ambient gas pressure and the ablation fluence. After the propagation ceases, the particles start to diffuse through the background gas; that is the diffusion phase. Rotational temperature variations of YO molecules in the different phases are also measur...

Particle dynamics during nanoparticle synthesis by laser ablation in a background gas

Particle dynamics during Si nanoparticle synthesis in a laser-ablation plume in different background gases were investigated by laser-spectroscopic imaging techniques. Two-dimensional laser induced fluorescence and ultraviolet Rayleigh scattering techniques were used to visualize the spatial distribution of the Si atoms and nanoparticles grown, respectively. We have developed a visualization technique called re-decomposition laser-induced fluorescence to observe small nanoparticles (hereafter called clusters) which are difficult to observe by the conventional imaging techniques. In this article, the whole process of nanoparticle synthesis in different background gases of He, Ne, Ar, N2 and O2 was investigated by these techniques. In He, Ne, Ar and N2 background gases at 10 Torr, the clustering of the Si atoms started 200, 250, 300 and 800 μs after ablation, respectively. The growth rate of the clusters in He background gas was much larger than that in the other gases. The spatial distributions of the Si n...

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.

Two‐dimensional laser‐induced fluorescence imaging of a pulsed‐laser deposition process of YBa2Cu3O7−x

Two‐dimensional time‐resolved density distributions of ground state barium (Ba) and copper (Cu) atoms as well as ground state yttrium oxide (YO) molecules have been measured by two‐dimensional laser‐induced fluorescence during a pulsed‐laser deposition (PLD) process of YBa2Cu3O7−x. The gas phase of PLD has been investigated at ambient oxygen gas pressures between vacuum and 1 Torr. Characteristic behaviors have been observed for each of the measured ground state species. This is due to different oxidation schemes with the ambient oxygen gas. Whereas YO molecules mainly formed near the expanding front of the ablation plume, Ba atoms immediately oxidized at the plume front. Cu atoms, however, did not react during the gas phase of PLD. In addition, a well defined boundary of the expanding front of the ablation plume has been observed at oxygen gas pressures above 100 mTorr. Measurements of density distributions of emissive Ba, Cu, and YO are also discussed.

Deposition of ZnO film by pulsed laser deposition at room temperature

Crystalline zinc oxide (ZnO) films were successfully deposited on sapphire and quartz glass substrates by pulsed laser deposition (PLD) at room temperature. Transmittance measurements showed that crystalline films have a clear cut off at about 350 nm. After annealing at 1000 °C for 10 min, ultraviolet photoluminescence at about 390 nm was observed under 308 nm excitation.

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.

Observation of Nano-Particle Formation Process in a Laser-Ablated Plume Using Imaging Spectroscopy

The behavior of atomic species and of growing nano-particles in the laser-ablated plume was observed by laser-induced fluorescence (LIF) and Rayleigh scattering (RS) imaging techniques, and the results on the ablation of silicon (Si) in a He ambient are presented. The particles were formed in a limited region in the plume with a characteristic spatial distribution. The growth of the particles was completed about 20 ms after ablation at a helium (He) ambient pressure of 1.33 kPa and 50 ms at 0.67 kPa. The spatial distributions of the particles was retained for more than one second.