Laser Induced Damage of Pure and Mixture Material High Reflectors for 355nm and 1064nm Wavelength

High reflecting multilayer coatings play a key role for many applications of pulsed Nd:YAG high power lasers in industry and science. In the present contribution, improvements in the optical properties and the radiation resistance of high reflectors for 355nm and 1064nm wavelength on the basis of mixture materials are discussed. Within a co-operation between the LASEROPTIK GmbH and the Laser Zentrum Hannover e. V., several deposition processes including Ion Beam Sputtering, Magnetron Sputtering, and Electron Beam Evaporation could be addressed for this study. The selected material combinations HfO2+ZrO2/SiO2, HfO2+Al2O3/SiO2, HfO2+SiO2/SiO2 and HfO2/SiO2 were deposited using a zone target assembly for the IBS technique or defined material mixtures for the evaporation process. Single layers of the applied mixtures were analyzed by UV/Vis/NIR spectroscopy to correlate the optical constants with the atomic compositions quantified by Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS). In addition to pure material reference mirrors and reflecting multilayer coatings with high index material mixtures, also interference coatings consisting of nanolaminates as well as multilayer systems with refractive index profiles were produced. The laser induced damage thresholds at 1064nm wavelength for nanosecond pulse durations were measured in a 1000on1 experiment complying with the standard ISO11254. For the 355nm high reflectors, the radiation resistance was determined in a 10000on1 procedure, furthermore, the radiation-induced absorption was measured by laser calorimetry according to ISO11551. Finally, the layer interfaces and the amorphous microstructure of selected multilayers were analyzed by Transmission Electron Microscopy (TEM) to obtain detailed information about possible partial crystallinity. The results are interpreted in the context of former investigations on the power handling capability of coating systems involving material mixtures.