Modeling of laser-material interaction

Laser beam is absorbed by free electrons within skin layer of metals, and the temperature of material rises abruptly with 10-12 [K/s] rate. If a laser fluence is sufficient, material will be melt, even vaporized. To calculate this phenomena, classical heat conduction equation is used. Also evaporated plume is considered with ambient gas by Euler equation and scalar transport equation. High pressure, velocity and temperature plume results in the formation of shock-front and contact-front as you see pictures. If laser fluence is very high, plasma which ablated plume is ionized (ionization ratio is obtained by Saha equation) by ablated plume is formed. And the plasma absorbed the irradiated laser beam above the substrate. This absorption makes heat transfer changed. About 104 [K] for plume temperature, the degree of ionization of plume can not be neglected, and the absorption of plume of laser beam is dominant. Therefore we must consider the shielding effect of plasma for high fluences.

If the laser pulse width is reduced to subpicosecond (e.g. 200fs), the dynamics of electrons and phonons become quite different. The relaxation times of electron-electron and electron-phonon are about 100fs and a few ps to tens ps, respectively. So the temperatures of electron and phonon must be considered in calculation of heat transfer of metal by femtosecond laser pulse irradiation. To model two carriers (electrons and phonons) simultaneously, we use the two temperature model [Anisimov, 1974]. This model assumes that the coupling constant between electrons and phonons is constants and the diffusion term for phonons can be neglected with comparing the diffusion velocity for electrons.

[Characteristics]

• Nonthermal ablation
• Ultrashort time scale (High intensity)
• Ablation for transparent material (Multiphoton absorption)

[Appilcation]

• Nano size manufacturing/Patterning
• Recovering of photomask (removal of absorbent on photomask)
• Analysis of chemical reaction and dynamics (chemical time scale is comparable to femtosecond)
• Medical application