Ichirou Fukumoto

Pulse duration effect on the distribution of energetic particles produced by intense femtosecond laser pulses irradiating solids

The energy distribution of hot electrons produced by a very short, intense laser pulse (I=2/4×1018 W/cm2, 60 fs, λ=800 nm, obliquely incident p polarized) is investigated theoretically via particle simulation and experimentally via measurements of the electron distribution in the MeV region and the Doppler-shifted emission spectrum of fast ions. This energy distribution is shown to be greatly different from the known two-temperature distribution. The hot electrons with energies near the maximal (∼2 MeV) constitute the distribution with an effective temperature Th considerably higher than that of lower-energy electrons, which dominate the emission of energetic ions. The temperature scaling with the laser intensity differs from the known Th∼I1/2.

Observation of MeV multicharged ions and hot electrons accelerated by a 65-fs laser pulse

The generation of fast particles (both ions and electrons) in a femtosecond laser-produced plasma has been studied experimentally and the possibility of applying particle-in-cell (PIC) codes to the description of the experimental results has been investigated. The energy distribution function of fast ions has been measured directly by means of X-ray spectroscopy. It is shown that this diagnostic can also be used as an indirect method to measure fast electrons inside the plasma. The hot electron distribution in the 0:1–2 MeV energy range was measured directly using a standard electron magnetic spectrometer. The comparison of the experimental results with the numerical simulations show: (1) the electromagnetic PIC-FC code can be successfully applied for simulating plasmas created by the interaction of femtosecond-laser pulses with solid targets and (2) the existence of a transient population of hot electrons with a non-Maxwellian distribution typical of interaction experiments with short-pulse lasers. ? 2001 Elsevier Science Ltd. All rights reserved.

Modified molecular dynamics simulation on ultrafast laser ablation of metal

Recently, short-pulse lasers have been applied to microfabrication in the field of various industries. It becomes more difficult to experimentally observe the microfabrication phenomena as pulse width becomes short. Numerical analysis with a continuum model has a limit in elucidation of such phenomena, therefore, computer simulations at the atomic or molecular level must be important. The authors have conducted the molecular dynamics simulation of laser materials processing over the years. In this paper, a modified molecular dynamics method for metal developed by the authors, where calculation of the molecular dynamics is carried out while revising the heat conduction by free electrons at each time step, was applied to elucidate the laser microfabrication phenomena. Pulse width dependence of damage threshold, evaporation process, velocity distribution of evaporation particles and temperature profile near the surface were investigated.Recently, short-pulse lasers have been applied to microfabrication in the field of various industries. It becomes more difficult to experimentally observe the microfabrication phenomena as pulse width becomes short. Numerical analysis with a continuum model has a limit in elucidation of such phenomena, therefore, computer simulations at the atomic or molecular level must be important. The authors have conducted the molecular dynamics simulation of laser materials processing over the years. In this paper, a modified molecular dynamics method for metal developed by the authors, where calculation of the molecular dynamics is carried out while revising the heat conduction by free electrons at each time step, was applied to elucidate the laser microfabrication phenomena. Pulse width dependence of damage threshold, evaporation process, velocity distribution of evaporation particles and temperature profile near the surface were investigated.

Molecular dynamics simulation of ablation process with ultrashort-pulsed laser

Ultra-Short pulsed lasers are highly useful tools in the field of microfabrication. In microfabrication, the laser pulse usually is very short in the pico-second or subpico- second range: therefore, it is very difficult to observe the transient material processing phenomena experimentally. Over the years, the authors have conducted molecular dynamics (MD) simulation to study the ablation process with ultra-short pulsed laser irradiation. The MD method has been modified to simulate the laser ablation of metals by updating heat conduction effect by free electrons at each calculation time step. In this paper, a review of the modified MD simulations on ablation and shock phenomena for metal with pico-second laser irradiation is presented.

Scattering of particles ablated by ultrashort-pulse laser

Ablation phenomena when Gaussian beam of the fourth harmonics of Nd:YAG laser is irradiated to an aluminum substrate during picoseconds were simulated using the modified molecule dynamics method that Ohmura and Fukumoto have developed. Scattering velocity of ablation particles were displayed by vectors, and both molten pool and slip planes in the material were visualized. At the same time, the angle, size and velocity distributions of the scattering particles, both potential and kinetic energies per ablation atom, and ablation energy were examined quantitatively. Authors have already clarified that there are two types in ablation form under constant laser fluence. One is explosive ablation and the other is relatively calm ablation. The former occurs when pulse width is extremely short. The simulation results showed that the angle and size of scattering particles depend on the ablation forms. These differences appear because ablation energy in the explosive process is much larger than that in the relatively calm ablation, that is, kinetic energy per ablation atom becomes very large when the pulse width is extremely short. Velocity of particles also depends on the ablation forms, but the tendency is comparatively weak. Scattering angle in both ablation forms has almost a normal distribution, but the standard deviation in the explosive ablation is much smaller.Ablation phenomena when Gaussian beam of the fourth harmonics of Nd:YAG laser is irradiated to an aluminum substrate during picoseconds were simulated using the modified molecule dynamics method that Ohmura and Fukumoto have developed. Scattering velocity of ablation particles were displayed by vectors, and both molten pool and slip planes in the material were visualized. At the same time, the angle, size and velocity distributions of the scattering particles, both potential and kinetic energies per ablation atom, and ablation energy were examined quantitatively. Authors have already clarified that there are two types in ablation form under constant laser fluence. One is explosive ablation and the other is relatively calm ablation. The former occurs when pulse width is extremely short. The simulation results showed that the angle and size of scattering particles depend on the ablation forms. These differences appear because ablation energy in the explosive process is much larger than that in the relativel...

Molecular dynamics analysis of picosecond pulse laser ablation

Laser ablation process of aluminum with the fourth harmonics of Nd:YAG laser is simulated, using the modified molecular dynamics method, which has been developed by Ohmura and Fukumoto. It has been clarified in the previous studies that there are two types in laser ablation process. One is explosive ablation and the other is calm ablation. In this paper, the ablation processes of these two types were visualized first by classifying the ablated atoms by the start time of scatter. It was cleared that the transition of kinetic and potential energies of ablated atoms and the energy ratio used for ablation to the absorbed energy depend on these ablation types. It was also verified that the transition of the number of ablated atoms and lumps of atoms also depend on the ablation types.

Molecular Dynamics Analysis of Ultrafast Laser Ablation Phenomena.

Laser ablation phenomena of aluminum with the fourth harmonics of Nd:YAG laser is analyzed using the modified molecular dynamics (MD), which has been developed previously by Ohmura and Fukumoto. In the modified MD method, the MD calculation for metal is carried out compensating the heat conduction by free electrons at each time step. Main conclusions are summarized as follows: (1) When the fluence is constant, there is an optimal irradiance of the laser pulse to make the number of evaporated atoms be the maximum. The ratio of evaporation energy to the laser energy absorbed in the material becomes the maximum at this optimal irradiance. (2) There are two types in laser ablation process. One is explosive evaporation which occurs when pulse width is extremely short, and the other is relatively calm evaporation which occurs when pulse width is comparatively long. In the former process, a comparatively large particles scatter, and in the latter process, the size of scattering particles is relatively small. (3) For the ultrafast laser irradiation, the evaporation threshold fluence of aluminum is about 20 mJ/cm2.

Molecular dynamics analysis on physical phenomena of metal with evaporation induced by laser irradiation

Recently, ultra-short pulsed lasers with high peak power have been developed, and their application to the materials processing is expected for a tool of precision microfabrication. During surface generation process with laser ablation, lattice defects such as dislocations, vacancies, grain boundaries, are also generated beneath the surface. Lattice defects influence the quality or accuracy of materials processing, therefore it is important for laser precision microfabrication to elucidate the generation mechanism of them. In this paper, laser ablation phenomena of metal were analyzed using the modified molecular dynamics method, which has been developed by Ohmura and Fukumoto. Main results obtained are summarized as follows: (1) The shock wave induced by the Gaussian beam irradiation propagates radially from the surface to the interior. (2) A lot of dislocations are generated near the surface by the propagation of shock wave. (3) Many grains are generated in the resolidification process after the end of laser pulse. They are metastable and some crystal-orientations of them change to one of the base metal and the grain boundaries disappear in the cooling process.