Effect of protective layer thickness on sub- 10fs laser damage to metal mirrors

A systematic study of sub-10 femtosecond pulse laser induced damage threshold (LIDT) determination was performed for the metal mirrors, (i.e. silver mirrors, aluminum mirrors, and aurum mirrors, etc.) with different thickness of protective layers. The damage morphology of metal mirrors with different thickness protective films at fluences below the single-pulse LIDT was studied to investigate the mechanisms leading to the onset of damage. The study found that the increase in the thickness of a single protective layer has little effect on the initial location of damage, and the use of a protective layer does not necessarily increase the damage threshold of a metal mirror. The damage threshold of the metal mirror is affected by the competition between the electric field in the protective layer and the material band gap and the degree of integration of the electric field with the metal-dielectric interface. However, the metal film without additional treatment has a lower degree of integration at the metal-dielectric interface and is more susceptible to damage. Therefore, even if the thickness change of the protective layer affects the peak intensity of the electric field in the protective layer, the degree of bonding at the metal-dielectric interface is still the decisive factor in the damage threshold of the metal-dielectric film. This work is helpful to find new technologies to improve the damage threshold of metal mirrors used in ultrafast high-power laser systems.