Abstract
Laser-Induced optical breakdown often occurs unexpectedly at optical intensities far lower than those predicted by ultra-short pulse laser experiments, and is usually attributed to contamination. To determine the physical mechanism, optical coatings were contaminated with carbon and steel microparticles and stressed using a 17 kW continuous-wave laser. Breakdown occurred at intensity levels many orders of magnitude lower than expected in clean, pristine materials. Damage thresholds were found to strongly follow the bandgap energy of the film. A thermal model incorporating the particle absorption, interface heat transfer, and free carrier absorption was developed, and it explains the observed data, indicating that surface contamination heated by the laser thermally generates free carriers in the films. The observed bandgap dependence is in direct contrast to the behavior observed for clean samples under continuous wave and long-pulse illumination, and, unexpectedly, has similarities to ultra-short pulse breakdown for clean samples, albeit with a substantially different physical mechanism.
Highlights
Five decades of research have created optics and coatings capable of operating at average powers of hundreds to thousands of kWcm−2 in controlled environments while maintaining consistent optical properties[1,2,3]
Both the λ/2 and high reflectivity (HR) samples were designed for a center wavelength of 1064–1070 nm, corresponding to the emission wavelengths of Nd3+:YAG solid-state and ytterbium-doped fiber lasers
Carbon contaminated distributed Bragg reflectors (DBR) coatings failed at irradiances as low as 17 kWcm−2 for titania-silica while hafnia-silica started to fail at 2.25 MWcm−2
Summary
Five decades of research have created optics and coatings capable of operating at average powers of hundreds to thousands of kWcm−2 in controlled environments while maintaining consistent optical properties[1,2,3]. Understanding how surface contamination initiates damage and determining what can be done to mitigate its effects are crucial for designing optics that can survive in typical (dirty) conditions. This work finds that the physical mechanism behind contamination-induced breakdown is thermal free carrier generation and subsequent absorption. This mechanism predicts a strong bandgap dependence for breakdown, which is clearly observed in experimental testing. One gains little or no improvement in breakdown characteristics when using protective layers of high bandgap material with typical optical coating thicknesses, a result that can be mathematically predicted by calculating the volume of the optical substrate that is heated by contaminant evaporation. The experimental findings of the bandgap dependence demonstrates that large bandgap materials are more reliable when used in high power applications
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