Mitsuhiro Kusaba

Abstract The surface morphology of silicon solar cells irradiated with KrF excimer laser pulses (λ = 248 nm, τ = 20 ns) was investigated below the experimentally observed melting threshold fluence (F _th) of 0.47 J cm⁻² (±20%). At laser fluences of 0.23–0.48 J cm⁻² (equivalent to 0.49F _th to 1.0F _th), nanodot structures with a height and width of approximately 60–120 nm were periodically formed with an interdot spacing similar to the laser wavelength. The observed nanodot density (29 dots μm²) was higher than that previously obtained at longer wavelengths. Furthermore, crystallinity analysis by micro-Raman spectroscopy revealed a Raman shift of 519.56 cm⁻¹ after irradiation (N= 1500 pulses), compared with 518.27 cm⁻¹ prior to irradiation. A laser fluence of 0.41 J cm⁻² ( = 0.87F _th) was found to induce compressive stress on the silicon solar cell surface.

A tungsten carbide-cobalt (WC-Co) composite layer was formed on a stainless-steel type 304 (SS304) substrate using multibeam laser metal deposition (LMD) with blue diode lasers. This paper aims to provide WC-Co layer formation with low porosity and high layer formation efficiency by using the multibeam LMD process. The effects of process parameters such as laser output power and powder feed rate are tied together to explain the geometry of the melt layer as well as the fraction of the laser energy used for melting a material. The experimental results show that the porosity rate and layer formation efficiency were recorded at 0.3% and 0.0042 mm3/J, respectively, at the laser output power of 180 W and a powder feed rate of 75 mg/s. It was revealed that layer formation efficiency was dependent on the laser output power.

The Perpendicular Period and Phase Scanning (P³S) method can evaluate the uniformity of a laser-induced periodic surface structure (LIPSS). P³S assesses the uniformity of LIPSS using the standard deviation of the peak period and the average of the phase difference in the direction perpendicular to LIPSS. The P³S method demonstrates that LIPSS formed by two-color double-pulse irradiation is reduced to a quarter of the period dispersion, and the average phase difference of LIPSS is also reduced compared to the single-pulse irradiation. In addition, a 3D electromagnetic particle-in-cell simulation was performed to evaluate the possibility of an improved uniformity of LIPSS. The results confirm that the two-color double-pulse irradiation produces a uniform LIPSS and validates the effectiveness of the P³S method to assess the uniformity of LIPSS.

The carbon thin film as a target for laser-driven heavy ion acceleration has been developed using the carbonization of polyimide induced by the irradiation of a XeCl excimer laser. The relationship between the depth of the crater produced by the laser irradiation and the laser fluence were measured in order to clear the carbonization mechanisms of polyimide. The melting threshold of polyimide was estimated to 0.058 J/cm². It is found that the carbonization is induced by the irradiation with the laser fluence around or under the threshold.

Nano/microfabrication experiments on the surface of silicon solar cell are performed by ultraviolet femtosecond laser in the fluence range from 0.10-0.70 J/cm². The surface morphology change of sample was observed by a laser scanning microscopy and a scanning electron microscope. The crystallinity of surface irradiated by laser pulses was evaluated by Raman spectroscopy. The ablation threshold for silicon was estimated to be 0.10 J/cm². We successfully produced nano-structures on the surface of silicon solar cell maintaining single-crystalline with the irradiation at the laser fluence of 0.10 J/cm².

The authors have investigated the uniformity of laser-induced periodic surface structures (LIPSSs) generated on titanium surfaces irradiated with a two-color double-pulse cross-polarized beam with a time delay of Delta t=0-200ps. The double-pulse beam consisted of 800nm pulses with a duration of 150fs and 400nm pulses with a duration of >150fs. The fundamental-pulse fluence F-800 and the second-harmonic pulse fluence F-400 were set to be near the corresponding ablation thresholds of F-800th=0.108J/cm(2) and F-400th=0.090J/cm(2), respectively. The authors found that uniform LIPSSs could be produced on titanium surfaces using laser fluences of 1.5F(400th)+0.9F(800th) and a delay of Delta t=0-2ps. The periodicity and direction of the LIPSSs were characterized by the wavelength and electric field of the fundamental (800nm) pulse. The results suggest that the longer-wavelength pulse influences surface plasma wave generation and improves uniformity by the second harmonic pulse even though laser plasma is produced on the surface.

Laser-induced periodic surface structures (LIPSS) were formed on silicon solar cell surfaces by using a XeCl excimer laser to irradiate them with nanosecond UV laser pulses in the fluence range 0.2-0.6 J/cm(2) near the melting threshold fluence (F-th = 0.5 J/cm(2)) in air. We measured the reflectance of the silicon solar cells after laser irradiation and evaluated their crystallinity by Raman spectroscopy. We found that reflectance and crystallinity depended on laser fluence and number of pulses. For fluence of 0.5 J/cm(2), the produced LIPSS contributed to a reduction of Delta R = 3.3% in reflectance at a wavelength of 500 nm. The crystallinity of the fine structures with reduced reflectance was maintained at laser fluences near the melting threshold.

We have demonstrated the suppression of ablation rate on a silicon surface irradiated by a double-pulse beam with two color laser in time delays of Δt = -900 - 900 ps. The double pulse beam consists of 810nm with 40fs pulse and 405nm with &gt 40fs pulse. The fundamental-pulse fluence F810 is kept below ablation threshold (Fth, 810nm = 0.190 J/cm2) while the second harmonic pulse fluence F405 are kept above the ablation threshold (Fth, 405nm = 0.050 J/cm2). We find that ablation rate of silicon is drastically decreased at delay times of 600ps.

<p>In order to investigate the effect of surface roughening due to laser irradiation in two-step thermochemical water-splitting with CeO₂ system, we irradiated reactive ceramics of CeO₂-NiO system and formed groove-like structures on the surface of these samples using the two types of lasers, such as a CW laser (λ=1070nm) and a nanosecond (Ns) pulsed-laser (λ=532nm). We observed structural and characteristic changes after laser irradiation and thermochemical cycling, and estimated the amount of the syngas (H₂ and O₂ gases) during thermochemical cycling. The following findings can be derived from the experimental results. (1) The surface of the samples irradiated by a CW laser became much smoother and its color changed from milky white to blue, while the sample surface after Ns pulsed-laser irradiation was almost the same as that before laser irradiation. (2) Glass-like material was observed on the surface of the sample after thermochemical cycling, and groove-like structures partly changed into melted-like structures. (3) The amount of the syngas from the all of the irradiated samples was markedly higher than that from the unirradiated samples only at the early stage of thermochemical cycling. This reduction after a few cycles would result from the sintering of CeO₂ samples under high temperature and from the separation of inorganic adhesive used as a binder. (4) Even though with lower surface area, the samples by Ns pulsed-laser irradiation emitted more H₂ gas than those of other samples, because they have no a melted-like and smooth surface.</p>

The ablation rate is investigated on a silicon surface irradiated by a double-pulse beam crosspolarized in time delays of -1000-1000 ps. The reduced ablation rate is found clearly at delay times of 600ps.

In a conventional laser peening (C-LP) without a protective coating, a treated metallic surface is always melted after the C-LP processing, resulting in the generation of tensile residual stress. In order to establish a new LP technique without surface melting, we have investigated the effect of an LP (Ink-LP) using a black liquid ink (an indian ink) as an absorber. The laser wavelength and pulse duration of the laser used were 532 nm and ∼5 ns, respectively. We changed the pulse energy and spot diameter of the laser beam, resulting in the corresponding power density of ∼1 - 15 GW/cm². The pulse number was 1. The metal sample used was an aluminum alloy (A2017). As a reference, we have conducted similar experiments for the C-LP processing, which in the metallic surface are directly irradiated, and have compared surface deformation and surface melting between the Ink-LP and C-LP processing. As a result, it was found that the Ink-LP processing can provide us with a promising LP effect with a wider treatment area and no surface melting in comparison with those of the C-LP processing. Thus, we have showed the usefulness of the Ink-LP processing using a black ink.

The photoreduction of Sm3+ to Sm2+ was studied by a means of the nanosecond laser flash photolysis using a KrFexcimer laser with the oscillation wavelength of 248 nm. Sample was air-saturated methanol solution of 15-crown-5 with SmCl3. The photoreduction occurred via a one-photon process. The reduction yield at 1 μs after the laser irradiation was estimated to be 0.37±0.05.

Developments of two types of the electron gun are presented. One is a compact DC electron gun using Field Emitter Array (FEA) as the cold cathode, the other is a photoinjector which outputs sub-pico second electron beam pulse. The FEA electron gun achieved to producing a 2 mA/50 kV electron beam. The photoinjector experiments revealed an exotic space-charge effect on the ultra-shot electron beam pulse. © 2014 The Institute of Electrical Engineers of Japan.

メタノール中のSm3+が高強度フェムト秒レーザー照射により、Sm2+へ還元されることが分かった。この還元反応は、レーザー中心波長がSm3+の4f&#039;-4f遷移による吸収線のピーク波長に相当する403nmおよび800nmのときに観測された。403nm励起では、4f&#039;-4f吸収を経た多光子過程による反応で、800nm励起では、溶媒のイオン化によるSm3+の電子捕捉によるものと思われる。生成したSm2+の蛍光特性について調べた。

A compact electron gun using field emitter array has been developed. With a simple triode configuration consisting of FEA, mid-electrode and anode electrode, the electron gun produces a parallel beam with a diameter of 0.5 mm. This electron gun is applicable for compact radiation sources such as Cherenkov free-electron lasers. (c) 2008 Elsevier B.V. All rights reserved.

Formation of the metal particles of Cd has been studied by nanosecond laser flash photolysis using an ArF excimer laser with the oscillation wavelength of 193 nm. The sample was a 10% methanol solution of CdCl(2). The solvated electron, e(aq)(-), was generated by the excitation of charge transfer bands of Cl(-) to solvent. The ea(aq)(-) efficiently reduced Cd(2+) to Cd(+) during a laser pulse of 30 ns (FWHM). The Cd particles were produced by the reaction between Cd(+) and Cd(+). In the air saturated solution, the Cd(+) was easily oxidized by dissolved oxygen. The concentration of the Cd particles formed in the degassed solution was approximately twice of that in the air saturated solution. Crown Copyright (c) 2008 Published by Elsevier B.V. All rights reserved.

Chemical actinometer for UV laser nanoprocessing has been developed using infrared spectroscopy for the photoproduct of UV irradiated acetylacetone. Sample was hexafluoroacetylacetone, HFA, in which the H atoms of two methyl groups of acetylacetone are replaced by F atoms. It was irradiated by a KrF excimer laser with the oscillation wavelength of 248 nm during 100 seconds and the photoproducts were analyzed by transmittance measurement of FT-IR in the spectral region between 3500 cm^-1 and 1300 cm^-1. One of the photoproducts was CO and its formation efficiency was estimated to be approximately 0.6. The photodissociation reaction of HFA is expected to be useful for the actinometer by IR spectroscopy of the photoproduct.

Design of a Cherenkov free-electron laser operating in millimeter wave range was carried out as a test bench for a compact terahertz radiation source. A CW output of 1.8 W is expected at the frequency of 46.4 GHz with 50 keV electron beam. The experimental apparatus consists of Spindt-cathode, double-silicon-slab resonator and super-conducting coil.

Synthesis of single-wall carbon nanotubes (SWNTs) was carried out by an ablation method using a XeCl excimer laser with the oscillation wavelength of 308 nm. It was irradiated onto a graphite target containing Co and Ni at the temperatures of 1273 K, 1373 K, 1473 K and 1623 K under the atmosphere (0.1 M Pa) of Ar gas with the flow rate of 12 ml/min. It was found that SWNTs were formed in ablated carbonaceous soot. The diameter distribution and the length were estimated to be 1.2 similar to 1.7 nm and &gt; 2 mu m, respectively, by the measurements of Raman spectrum and images of scanning and transmission electron microscopes. The yield of SWNTs was the highest for the laser ablation at the highest temperature of 1623 K. (c) 2005 Elsevier B.V. All rights reserved.

A compact strong test hybrid helical microwiggler has been designed and developed with ten periods, each with length 10 mm and four poles, and gap diameter of 5 mm. One period is constructed with four segments. Each segment has a thickness of 2.5 mm and consists of pentagonal permanent magnets and permendur poles. Each segment is stacked along the wiggler axis and is rotated by an angle of 90degrees to the adjacent segment. The outer width is only 72 mm. The wiggler is divided into two sections where two kinds of permanent magnet - one with apex angle of 90degrees and one with 120degrees - were installed in order to find out the effect of geometrical size for the field. Peak fields of 0.360 and 0.381 T were achieved with small rms variations of about 1.1 % and 0.6 %, respectively, depending on the geometrical size of the permanent magnet.

The nu (1) + nu (3) and the nu (1) + nu (2) + nu (1)(4) + nu (-1)(5) combination bands of (C2H2)-C-13 at 6521.9 and 6535.1 cm(-1), respectively, are studied with diode laser spectroscopy. Line center wavelengths have been determined with 0.00012-nm standard uncertainty up to J = 40 for nu (1) + nu (3) and up to J = 25 for nu (1) + nu (2) + nu (1)(4) + nu (-1)(5). Linestrengths are modeled with a Herman-Wallis expansion F(m) = 1 + c(1)m + c(2)m(2) + ... with c(1) = -1.33 x 10(-3), c(2) = 0.85 x 10(-4), c(3) = 0, c(4) = 1.13 x 10(-7) for nu (1) + nu (3) and c(1) = 1.11 x 10(-3), c(2) = -3.85 x 10(-4) for nu (1) + nu (2) + nu (1)(4) + nu (-1)(5). Collision broadening parameters are extracted from Voigt fits to the line profiles and can be modeled for both bands by the analytical expression gamma = 5.221 - 0.0839 \m\ + 1.46 exp(-0.45 \m\) MHz/mbar, indicating independence of vibrational state. Collisional narrowing is studied on selected lines and the model dependence of linestrengths and broadening parameters is determined. Pressure shifts are studied in nu (1) + nu (3), and a characteristic difference in the shifts for the P-branch and the R-branch is explained in terms of a simple ad hoc assumption about the level shifts. These bands have a potential for providing wavelength standards for optical fiber communication. (C) 2001 Academic Press.

A test hybrid helical microwiggler with 4 poles per period has been designed and developed with parameters such as period of 10 mm, gap diameter of 5 mm and periodical number of 10. One period of its wiggler is constructed with four segments. Each of them has a thickness of 2.5 mm and consists of pentagonal permanent magnets and permendurs as ferromagnets. Each segment is stacked along wiggler axis rotating byangle of 90°C It is so small that the outer width is 72 mm. The measured peak field is 0.35 to 0.4 T depending on the gap space of 5.2 to 4.8 mm.

Multiphoton reduction of Eu3+ to Eu2+ in methanol was induced by irradiation with the second harmonic (394 nm, 2 ps) of a Ti:sapphire laser. The conversion efficiencies were increased to 0.3 for short pulse excitation, as compared to on the order of 10-5 for a nanosecond dye laser pulse. It appears that a three-photon process pumps a deep charge-transfer state from which Eu3+ efficiently dissociates to Eu2+. On the basis of the excitation spectra, the first intermediate state is the 5L6 state, which is one of the f-electronic excited states. The Eu2+ concentration increased shot by shot and approached a plateau. The saturation behavior will be explained in terms of the back photooxidation of Eu2+ to Eu3+.

Photoreduction of Sm3+ to Sm2+ has been studied by means of the laser flash photolysis using a KrF excimer laser. The linear dependence of the concentration of Sm2+ on the laser fluence indicated that the photoreduction of Sm3+ to Sm2+ in the solution occurs via the one-photon process. The photoreduction yield of Sm3+ was determined to be 0.34+/-0.09. The lifetime of Sm2+ in air saturated sample was estimated to be 4.6 mu s.

We have been developing a tungsten field emission cathode for a Cherenkov FEL or Smith-Purcell FEL. The properties of the electron beam from a tungsten tip have been investigated using numerical simulation and some experiments. Although the 2nd gale reduced the emission current due to a decrease of the effective field at the tungsten tip, it focused and enlarged the electron beam current in a certain potential range of the 2nd gate depending on the cathode voltage and the distance between Ist and 2nd gates. The electron beam diameter was reduced to similar to 2 mm on the fluorescent screen at the anode. (C) 1999 Elsevier Science B.V. All rights reserved.

We have started to develop a tungsten (W) field emission cathode for Cherenkov free electron laser (FEL) or Smith-Percell FEL. In this work, elementary properties of electron beam current from W tip have been investigated by numerical simulation and experiment using a system consisting of W tip, 1 st- and 2nd- gate, and anode. When there was no 2nd gate, the current increased up to-20μA with the absolute voltage of cathode according to the Fowler-Nordheim equation. The addition of 2nd gate decreased the current half to 5th lower than expected value from the simulation. However, it focused and enlarged the current of the electron beam in a certain voltage range of 2nd gate. These phenomena agreed qualitatively with those of the simulation. In further study, improved experiment will be made to get higher current and parallel smaller size of an electron beam.

A hybrid planar wiggler with a period of 20 mm has been studied as the simplest one which gives the strong field including some higher harmonic components by selecting proper sizes of the ferromagnetic and the permanent magnet. Small gap length of the wiggler and small width of permendur satisfy these conditions to a certain degree. Gain analysis of FEL suggests that for high wiggler field of K&gt 1 ∼ 1.6, higher harmonic gains are improved primarily due to its strong field, and for low wiggler field of K&lt C 1 ∼ 1.6, they are mainly due to the modification of the wiggler field distribution.

1,2,3,4,5,6,7,8-Octafluorophenazine (F-Phen) has an absorption spectrum in the longer wavelength extending to the visible-light region and has a more positive oxidation potential than for unfluorinated phenazine. F-Phen has been photolysed with water in acetonitrile under visible-light irradiation to produce stoichiometrically 1,3,4,5,6,7,8-heptafluoro-2-hydroxyphenazine (F-Phen-2-OH). Photolysis of F-Phen with water in the presence of benzene leads to the formation of phenol followed by the disappearance of F-Phen. EPR analysis and laser flash photolysis reveal that the photoexcited triplet state of F-Phen participates in water oxidation to hydroxyl radicals with concurrent conversion to F-Phen-2-OH. The mechanism is discussed with the results of semi-empirical molecular orbital calculations.

Laser flash photolysis has been carried out to elucidate relaxation dynamics of highly excited naphthalene in the gas phase. Highly vibrationally excited 'hot' naphthalene in the ground state was formed through rapid internal conversion after 193 nm laser excitation. Hot naphthalene was effectively deactivated by nitrogen molecules, and the energy transferred per collision was similar to that of azulene or hexafluorobenzene. The molecule was found to absorb a second photon under high photon density conditions to undergo an intramolecular chemical reaction.