Practical Definition Method of Laser Ablation Threshold in Nano-second Order Pulse Duration Considering Intensity Distribution at Focal Point

The threshold of laser ablation means the minimum laser intensity that enables to remove material. This value should be inherent to the material but independent of the optical system configuration essentially. In this study, the novel definition of the threshold is proposed, and then, the values for various materials are experimentally determined to evaluate its validity. In the experiments, the pulsed YAG laser with the pulse width of 8ns and the wavelength of 355nm is scanned on silicone, aluminum and silver for groove machining. The ablation threshold can be determined as the power density corresponding to the groove edge. In case of silicone, the values of threshold are always 80MW/cm 2 independent of the laser intensity and its distribution. Furthermore, since the change in the focal length of the objective lens does not affect to the threshold determination, it is confirmed that the threshold is also independent of the optical system configuration. In case of aluminum and silver, the values of threshold are 135 and 40MW/cm 2 respectively, and are also independent of the laser fluence and the optical system configuration. The proposed definition enables estimation the ablation dimensions corresponding to the laser intensity and material independent of the optical system configuration.