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Abstract: Laser measurement and laser processing techniques have been gaining strong attention from various applications. This research aims at the development of a novel optical device, and in order to fulfill the objective, characteristics of the thermal lens effect is studied. This phenomenon has the optical property of a divergent lens since the refractive index distribution on the optical axis is formed when the liquid medium is irradiated. One reason for the refractive index distribution is the temperature distribution in the liquid medium when it is irradiated. In this research, as a fundamental step to develop the fluidic optical devices, the interaction between a refractive angle of the probe beam and the temperature distribution on the thermal lens effect is investigated. [Copyright &y& Elsevier]

Highlights • A numerical model has been developed using an open source software FreeFem++. • The numerical model is used to estimate the water content, temperature and gas crossover. • An In-situ experiment using microprobe technique to determine gas crossover. • The results show that gas diffusion coefficient as a function of temperature and relative humidity. However, the effect of temperature seems to more complicated due to significantly effect of crystalline region at higher temperature range of 80–100 °C. Abstract Gas crossover through the membrane is one of the key factors for membrane degradation and performance loss of proton exchange membrane fuel cell (PEMFC). The permeation of hydrogen and oxygen through the membrane are consumed with the generation of heat and water but without the generating of useful work, leading to a fuel inefficiency. On the other hand, hydrogen peroxide (H 2 O 2) is most probably formed by the reaction of crossover hydrogen and oxygen at both the anode and cathode side. In addition, the chemical reaction of gas crossover can produce peroxide (HO) and hydroperoxide (HOO) radicals, which could accelerate the membrane degradation. In this study, a numerical simulation model has been built using the partial differential equation solver FreeFem++. A steady-state, two-dimensional, single-phase and non-isothermal model of a single PEMFC has been considered to determine the water content distribution, temperature profile, and permeation of gaseous species within the membrane. An in-situ microprobe technique has been also applied to determine the properties of the oxygen crossover with a range of relevant fuel cell operating temperatures and reactant humidity. The numerical results are compared with experimental data of the diffusion coefficient of reactants gases in the membrane, in order to investigate the gaseous species transport characteristics in the membrane at very low current density. [ABSTRACT FROM AUTHOR]

The fluidic optical device by means of the thermal lens effect has attracted increasing attention as a flexible beam shaper. However, onset of natural convection inhibits operation of the fluidic optical device as it causes asymmetric distortion on shaped beam profile. For implementation of the fluidic beam shaper, this adverse effect should be controlled in terms of the beam profile change. From this perspective, we investigated the influence of natural convection on the propagating beam profile through the fluidic optical device by means of CW laser induced the thermal lens effect. Numerical method taking advantage of beam propagation method was used to calculate modulation of the probe beam profile. The calculated results demonstrated good agreements with several experimental results. Using this numerical model, the parametric study to control asymmetry and quality of the annular beam was performed quantitatively in the dual beam thermal lens system. The core of this study is that the solution of the simple pentadiagonal equation can provide the crucial investigation on the modulation of the beam profile in the fluidic optical device with natural convection. [ABSTRACT FROM AUTHOR]

Abstract: Laser drilling of silicon carbide (SiC) wafer in air (dry ablation) and underwater by using ns pulsed infrared (1064nm) Nd:YAG laser is investigated. In order to suggest optimal parameters of via processing in SiC wafer, the effects of pulse number, laser fluence, water film thickness, and focus position are evaluated. As compared with dry ablation vias, decreasing etching rate, increasing via diameter, and generation of cracks in high-energy regime are observed in liquid-assisted processing. However, it is found that it can create vias without debris, HAZ, cracks. Also, optimal parameter set for infrared pulse laser processing under water is found to be the laser fluence of less than 10J/cm2 and water thickness of 1mm. [Copyright &y& Elsevier]

Abstract: This research presents a novel fluidic lens based on thermal lens effect. Effects of the pump power and the pump beam intensity distribution to the probe beam profile in the dual thermal lens system are investigated experimentally and theoretically. A model, which accounts for heat conduction, natural heat convection and ray tracing in inhomogeneous medium is developed to predict the characteristics of the thermal lens system. Numerical results show the advantage of the uniform pump beam in reducing the spherical aberration compared with the Gaussian pump beam. An experiment with the uniform pump beam is carried out to confirm the numerical prediction. Experimental results show a good agreement with the calculated results. Finally, the pump power is varied to adjust the focal length of the system. [Copyright &y& Elsevier]

Abstract: Laser measurement and laser processing techniques have been gaining strong attention from various applications. This research aims at the development of a fluidic laser beam shaper, and in order to fulfill the objective, thermal lens effect characteristics are studied. This phenomenon has the optical property of a divergent lens since the refractive index distribution on the optical axis is formed when a liquid is irradiated. In this research, effects of the pump power and the propagation distance to the probe beam profile are investigated experimentally and theoretically, with the purpose of developing a fluidic laser beam shaper. It is indicated that, by controlling some parameters in thermal lens system such as the pump power (in the regime of linear optics) and absorption coefficient, an input Gaussian beam can be converted into a flat-top beam profile. The relationship between the distance to obtain the flat-top beam, the pump power and absorption coefficient is investigated to show the flexibility of the fluidic laser beam shaper in many fields of laser application. [Copyright &y& Elsevier]

Nanosecond laser Bessel beam scribing on the TCO thin film was investigated to improve processing precision and robustness of optical system. Fundamental wave (1064 nm) of Nd:YAG laser was shaped into high-quality Bessel beam by using novel optical system consisting of axicons and convex lens. Spatial FWHM of the beam was only 1.5 μm in the present context, and significantly precise scribing with minimum width of 2.3 μm was achieved on 600–700 nm-thick FTO film with electrical isolation. Furthermore, due to the critically deep focal length of millimeters-order, robustness on sample positioning was greatly improved. Additionally, experimental results showed that single shot removal of entire film can be achieved using film side irradiation unlike conventional Gaussian beam. Temperature distribution during the process was calculated by a numerical model in which we have taken into account beam propagation inside the film to give comparison with a Gaussian beam irradiation. The calculation results showed that only Bessel beam is self-reconstructed behind plasma shielding so that entire film can be removed by single shot. Our findings suggest that Bessel beam can be used for efficient IR scribing with significantly high quality without selecting substrate material. [ABSTRACT FROM AUTHOR]

Substrate side irradiation is widely used for a thin film removal process because high absorption at the film/substrate or film/film interface leads to complete isolation of thin film by single shot irradiation of laser pulse with low energy. However, in the transparent thin film removal process, large thermal expansion or local phase change at the interface cannot be created by substrate side irradiation because of its large optical penetration depth compared to its small thickness. Nevertheless, substrate side irradiation works obviously for single shot film isolation process compared to film side irradiation, and the mechanism of the process was not clear in terms of difference in the irradiation direction. In order to investigate the effect of the irradiation direction, this study focused on the transient interaction between the material and nanosecond laser pulse. Experimental results showed that film was thermally ablated. Variation of temporal profile of nanosecond laser pulse during the process was experimentally investigated to detect plasma shielding. Pulse width and energy transmittance of transmitted pulse decreased by plasma shielding as pulse energy increases regardless of irradiation direction. In addition, temperature distribution in the film during the process was investigated using a 2-dimensional thermal model, which accounts for melting, vaporization, and laser induced plasma shielding. Calculated temperature distribution was used to support the scenario of the process mechanism which was investigated in the experiments. Our findings demonstrate that laser induced backward ablation is a single shot TCO film removal mechanism, and plasma shielding is dominant factor to interrupt absorption of beam thorough the film in the film side irradiation process. [ABSTRACT FROM AUTHOR]

Abstract: Potential profile in PEFC electrolyte membrane has been investigated by using microprobe technique in order to help understand membrane degradation mechanism. In particular, potential distribution in the membrane under several operation conditions has been measured by using microprobes inserted in the membrane. A theoretical model has been developed to predict the potential distribution under different cell current density with cells of different rib/channel width. The model showed good agreement with experimental result. [Copyright &y& Elsevier]