Pulse duration dependence of dry laser peening effects in the femtosecond-to-picosecond regime

Dry laser peening was conducted using an ultrashort-pulse laser with a laser intensity that exceeded the air breakdown threshold, and the pulse duration dependence of peening effects was clarified. Under the experimental condition for which the pulse duration was shorter than 1 ps, the peening effects decreased due to a decrease in fluence. This is because the refractive index decreased, and the laser irradiation area was expanded by nonlinear phenomena. Moreover, when the pulse duration was longer than 1 ps, the peening effects decreased due to the significant thermal effect. In conclusion, an optimum pulse duration was found for dry laser peening in the femtosecond-to-picosecond regime, in which the laser intensity exceeded the air breakdown intensity. These findings serve as a basis for the optimization of ultrashort-pulse laser processing at intensities that exceed the air breakdown threshold, and can significantly expand the scope of industrial applications.