Defect induced thermal-plasma coupling damage in optical films under nanosecond pulse laser irradiation

Thermal-plasma coupling damage induced by defect under 1.064 mu m nanosecond pulse laser irradiation in HfO2/SiO2 multilayer films was investigated. Based on the typical damage morphology obtained from the experimental results, thermal-plasma coupling damage caused by the localized absorbing defects was discussed. Theoretical analysis and dynamic model was also introduced to illustrate the thermal-plasma coupling damage process and corresponding damage morphologies. It was found that on fixed incident laser fluence conditions, the defect-lodging depth determined the final damage morphologies in thermal-plasma coupling damage. The defect lodging in shallow sites cannot be enough to create shock compression wave due to plasma-ball expanding rapidly to the surface of film layer owing to the shorter duration of ionization. At this moment, the high heat plasma energy releases owing to defect ejection. And finally high thermal plasmas scald the film material resulting in plasma scalding damage morphology. Only relatively deeper lodging defect can induce a sufficient shock compression wave in plasma expanding process and as a result the circle ripples damage morphology forms.