Residual Energy Measurement at Picosecond Laser Impact on Metals

Residual energy or thermal coupling coefficient shows what fraction of laser pulse energy is spent for target heating. High residual energy rates lead to low impact efficiency and sometimes to unwanted effects like phase transitions and chemical reactions. Experimental works on its measurement under nano- and femtosecond laser irradiation are not numerous but still can be found, while picosecond range is poorly studied. The latter is interesting since thermal coupling effects are close to ultrashort impact, while lasers efficiency, complexity, and price are closer to nanosecond devices. We used a calorimeter to evaluate heat flux through a target for focused and non-focused irradiation. Our data obtained for aluminum irradiated at 1064 nm, 71 ps, 15 Hz show higher residual energy rates than could be found in published sources: 2.5-3 times higher than calculated for 100 ps, ca. 1.25-fold higher than experimental evaluation for nano- and femtosecond impact. We assume that the difference may be due to a change in the actual absorptivity of the target upon repeated impact, the spatial shape of the beam (homogeneous or Gaussian), and the experimental methods used (target temperature or through the heat flux). The results obtained are highly novel and important for correct modeling of thermal coupling and adequate transition from laboratory to real systems using repetitively pulsed picosecond impacts.