Functional laser-induced damage threshold of aluminum coating deposited by magnetron sputtering with different power supply modes

Abstract

The need for high-power, high-intensity laser facilities has driven the scientific community to develop optics capable of withstanding higher fluences. The main challenge in increasing power levels is the limited damage threshold of optics. In this paper, we report the laser-induced damage threshold (LIDT) of aluminum coatings deposited via magnetron sputtering. Aluminum coatings (150 nm) were investigated as 45° s-polarization mirrors, a geometry widely employed for beam steering in high-power laser systems. The coatings were produced using three plasma techniques: direct-current (DCMS), radiofrequency (RFMS), and high-power impulse (HiPIMS). Our results show that the RFMS coating is rougher than the DCMS and HiPIMS coatings, which affects its reflectance efficiency (at 45° in s-polarization), even at 1060 nm, decreasing from 96% for DCMS and HiPIMS to 91% for RFMS. 1k-on-1 LIDT tests using a femtosecond laser beam (110 fs) at 1060 nm demonstrate that coatings deposited by HiPIMS and DCMS withstand higher fluences (0.260 J/cm2) compared to RFMS (0.112 J/cm2). Finally, the LIDT performance was compared with the Functional LIDT (F-LIDT), defined as the incident fluence at which the reflected beam decreases by less than 5%. In this study, we observed a discrepancy between the 1k-on-1 LIDT and the F-LIDT values for DCMS and HiPIMS. The F-LIDT for DCMS was higher (0.288 J/cm2) than that of HiPIMS (0.232 J/cm2), which may be partly attributed to the larger crystallite size observed in DCMS. Additionally, the potential use of the HiPIMS power supply mode for developing high-performance optical coatings is discussed.