Naoaki Fukuda

Shape control of elemental distributions inside a glass by simultaneous femtosecond laser irradiation at multiple spots

The spatial distributions of elements in a glass can be modulated by irradiation with high repetition rate femtosecond laser pulses. However, the shape of the distribution is restricted to being axially symmetric about the laser beam axis due to the isotropic diffusion of photo-thermal energy. In this study, we describe a method to control the shape of the elemental distribution more flexibly by simultaneous irradiation at multiple spots using a spatial light modulator. The accumulation of thermal energy was induced by focusing 250 kHz fs laser pulses at a single spot inside an alumino-borosilicate glass, and the transient temperature distribution was modulated by focusing 1 kHz laser pulses at four spots in the same glass. The resulting modification was square-shaped. A simulation of the mean diffusion length of molten glass demonstrated that the transient diffusion of elements under heat accumulation and repeated temperature elevation at multiple spots caused the square shape of the distribution.

Modulation of laser induced-cracks inside a LiF single crystal by fs laser irradiation at multiple points

Crack formations inside a LiF single crystal after femtosecond laser irradiation at multiple points were investigated. In the case of sequential laser irradiation at three points, the propagations of some cracks were prevented by the dislocation bands generated by the previous laser irradiation. On the other hand, in the case of simultaneous laser irradiation at three points with a spatial light modulator, cracks in all the <100> directions from the photoexcited regions were generated clearly, but the length of one crack depended on the distribution of laser irradiation positions. The simulation of elastic dynamics after fs laser irradiation at three points elucidated that the interference of laser induced stress waves depended on the distributions of the irradiation positions. We found that the constructive interference of stress waves at a crack tip should have prevented the crack from propagating further and the tensile stress by destructive interference of stress waves along a crack should have facilitated the propagation of the crack.

Transient strain distributions during femtosecond laser-induced deformation inside LiF and MgO single crystals

Strain and stress dynamics inside MgO and LiF single crystals after photoexcitation by a focused femtosecond laser pulse were investigated by the observation of transient distributions of birefringence around the photoexcited region using a time-resolved polarization microscope. Both in MgO and LiF, propagation of two stress waves, which were attributed to quasi-longitudinal and quasi-transverse elastic waves, were observed, but crack propagation was observed only in LiF. Inside MgO, the observed strain distributions could be reproduced by elastic simulation, whereas inside LiF the strain distributions during crack propagation were largely different from the simulated ones; strain was widely distributed between cracks and the 〈110〉 regions in a stress wave, the strained region around the photoexcited region was smaller, and the strains in the 〈110〉 region and near the crack tips were larger than those by the simulation. The amplitudes of strain and stress in stress waves and temperature change in the photoexcited region were evaluated, and the origins of strain distribution change due to crack generation were discussed based on the differences between MgO and LiF.

Condensation of Si-rich region inside soda-lime glass by parallel femtosecond laser irradiation

Local melting and modulation of elemental distributions can be induced inside a glass by focusing femtosecond (fs) laser pulses at high repetition rate (>100 kHz). Using only a single beam of fs laser pulses, the shape of the molten region is ellipsoidal, so the induced elemental distributions are often circular and elongate in the laser propagation direction. In this study, we show that the elongation of the fs laser-induced elemental distributions inside a soda-lime glass could be suppressed by parallel fsing of 250 kHz and 1 kHz fs laser pulses. The thickness of a Si-rich region became about twice thinner than that of a single 250 kHz laser irradiation. Interestingly, the position of the Si-rich region depended on the relative positions between 1 kHz and 250 kHz photoexcited regions. The observation of glass melt during laser exposure showed that the vortex flow of glass melt occurred and it induced the formation of a Si-rich region. Based on the simulation of the transient temperature and viscosity distributions during laser exposure, we temporally interpreted the origin of the vortex flow of glass melt and the mechanism of the formation of the Si-rich region.

Polarization imaging camera with a waveplate array fabricated with a femtosecond laser inside silica glass

In this study, we demonstrate a polarization imaging camera with a waveplate array of a silica glass fabricated by femtosecond (fs) laser direct writing. To use a waveplate array of silica glass for polarization imaging, non-uniformity of the transmittance and retardance in the waveplates must be considered. Therefore, we used a general method of polarization analysis with system matrices determined experimentally for all the units in the waveplate array. We found that a figure of merit based on the determinant of the system matrix could be applied to improve the accuracy of analysis and the robustness to the retardance dispersion for both the simulated and the fabricated waveplate array.

Modification of flow of glass melt and elemental distributions by parallel irradiation with femtosecond laser pulses

Local melting can be induced inside a glass by focusing femtosecond (fs) laser pulses at high repetition rate (>100kH). As the results, the spatial distributions of glass elements are modified in the molten region. Because various propertied of glasses depends on the composition of elements, the modification of spatial distribution of glass elements using fs laser will make it possible to control glass properties in three dimensional manner. The important point of the control of elemental distribution is how to control the flow of glass melt during laser irradiation. In this study, to elucidate how parallel laser irradiation affects the flow of glass melt during laser irradiation, we investigated the relationship between the flow of glass melt and various irradiation parameters by in-sites observation of flow of glass melt inside a sodalime glass during repetitive photoexcitation by 1 kHz and 250 kHz fs laser pulses at multiple spots.

Sum-frequency generation of continuous-wave tunable ultraviolet coherent light in BBO-installed external cavity

Recently, we have tried to develop a continuous wave (CW), tunable, and ultraviolet (UV) coherent light source through sum-frequency generation (SFG) using a BBO nonlinear crystal with a two-stage frequency-conversion system using two different external cavities for the enhancement of CW lights. In the first stage, we obtained the 532-nm light with the second harmonic generation (SHG) of the 1064-nm light. A bow-tie external cavity incorporating four mirrors, whose cavity length was controlled by the frequency stabilization method proposed by Hänsch and Couillaud, was employed there. In the second stage, to generate the 312-nm light, we demonstrated doubly resonant sum frequency generation of the 532-nm light from the first-stage and the 754-nm light from a single-frequency CW Ti:Sapphire laser. Considering a nonlinear coefficient, it should be preferable to use a BiBO crystal for high-efficient SFG, but the 312-nm light might be absorbed by the BiBO crystal. Therefore, we chose a BBO as a nonlinear crystal to avoid the absorption of the 312-nm light.

Study on prevention of welding defects in high power CO2 laser materials processing

For CO2 laser welding of large output, when a deep penetration welding in single pass is done in the ambient atmosphere, it is known that blowholes may occur because of the in-keyhole gas being entrapped in molten metal. Keeping this problem in mind and changing the parameters, we conducted welding tests with full-penetration bead-on-plate welding, and checked by radiographic test for welding defects. The experimental results demonstrated that the larger the specimen thickness, the more frequently welding defects occur, and that generation of welding defects depends upon the amount of welding heat input. Welding defects such as blowholes remain in metal, because the gas once entrapped into keyhole floats up in molten metal, and it is enclosed in the course of solidification. From this, we can verify the theory that a larger welding heat input, in the case of the good appearance full-penetration bead-on-plate welding, may be favorable for preventing welding blowholes. It can be assumed, therefore, that a larger heat input may hinder cooling of molten metal, and need a longer time for metal solidification; in this longer span of time, in-molten metal gas may escape while the metal is sufficiently heated.For CO2 laser welding of large output, when a deep penetration welding in single pass is done in the ambient atmosphere, it is known that blowholes may occur because of the in-keyhole gas being entrapped in molten metal. Keeping this problem in mind and changing the parameters, we conducted welding tests with full-penetration bead-on-plate welding, and checked by radiographic test for welding defects. The experimental results demonstrated that the larger the specimen thickness, the more frequently welding defects occur, and that generation of welding defects depends upon the amount of welding heat input. Welding defects such as blowholes remain in metal, because the gas once entrapped into keyhole floats up in molten metal, and it is enclosed in the course of solidification. From this, we can verify the theory that a larger welding heat input, in the case of the good appearance full-penetration bead-on-plate welding, may be favorable for preventing welding blowholes. It can be assumed, therefore, that a l...

Study of Interaction between Unsteady Supersonic Jet and Vortex Rings Discharged from Elliptical Cell

Pulsed laser ablation with an elliptical cell gives well-defined thermodynamic conditions to the growth of high-quality thin films. The unsteady supersonic jet formed by the shock tube with small high-pressure chamber was used as a simple alternative model of pulsed laser ablation. The vortex ring formed by the shock wave is important to reveal behavior of unsteady supersonic jet discharged from elliptical cell. However, there has been little effort to investigate the interaction between the vortex ring and the jet. The purpose of the present study is to investigate the behavior of the vortex rings and the jet. The experiment and numerical calculation were carried out by schlieren method and by solving the axisymmetric two-dimensional compressible Navier-Stokes equations, respectively. The system of the calculation and the experiment is a model of laser ablation of a certain duration followed by a discharging process through the exit. Moreover, a parametric study was performed to demonstrate the effect of pressure ratio on the interaction among vortex rings and the supersonic jet. The interaction between the supersonic jet and the vortex rings increased the velocity of the supersonic jet up to the magnitude of the velocity at the center of the vortex rings. Closing a distance between the vortex ring and the jet is higher interaction between the vortex rings.

Formation of composite nanoparticles behind shock waves generated by dual pulsed laser ablation

In this study, dual pulsed laser ablation was used to prepare composite nanoparticles. Formation of composite nanoparticles was attempted by controlling the amount of time delay between the laser ablation processes, which was accomplished by imposing time delay on the pulse arrival at two different target materials. By monitoring the emission spectra of the laser plume, the relation between the time evolution of the emission spectra and the possibility of composite nanoparticle formation was investigated. In addition, numerical simulations were conducted to compare the rate of nanoparticle formation in homogeneous and heterogeneous nucleation. The results indicated that, under particular conditions in the heterogeneous nucleation, the plume emission spectra represents the time evolution and the conditions of composite nanoparticle formation by the dual laser ablation.