Substrate-Dependent Damage and Thermal Stress Mitigation of 2 μm High-Transmittance Films

2-5 μm mid-infrared (MIR) lasers, particularly optical parametric systems, have significant application in gas detection, extreme ultraviolet lithography and electro-optical countermeasures, but their output performance are constrained by the laser induced damage thresholds (LIDTs) of 2 μm optical components. This study investigated the substrate-dependent damage mechanism of 2 μm high-transmittance (HT) films on fused quartz, calcium fluoride (CaF2), and sapphire. Furthermore, the influence of adding an Al2O3 buffer layer to alleviate thermal stress on the laser-induced damage performance was also studied. The results show the LIDTs of the films without the Al2O3 layer follow the order of sapphire (40.5 J/cm2)> quartz (22.1 J/cm2)> CaF2 (11.0 J/cm2), attributed to the differences in the thermal conductivity and coefficient of thermal expansion among the substrates. After adding the Al2O3 layer, the LIDTs of the films on quartz and sapphire decrease due to additional interfacial and nodule defects, whereas the LIDT of the film on CaF2 increases to 17.8 J/cm2 owing to mitigated interfacial stress mismatch and enhanced film adhesion. These findings demonstrate the thermophysical properties of substrates determine the LIDTs of MIR HT films, and the buffer layer exhibits a substrate‑dependent competition between defect introduction and stress mismatch alleviation.