Broadband 920-nm mirror thin film damage competition

• Published in: Optical engineering 2024-10, Vol.64 (3)
• Main Authors: Negres, Raluca A., Kafka, Kyle R. P., Smith, Chris, Stehlik, Marek, Olandt, Sarah, Rigatti, Amy, Demos, Stavros G.
• Format: Article
• Language: English
• Subjects: aluminum mirrors; broadband high reflectors; deposition processes; design; femtosecond pulses; group delay dispersion; laser damage growth; laser damage initiation; laser damage threshold; laser induced damage; multilayer dielectric coatings; near-IR; optical coatings; optical engineering; pulsed laser operation; thin film coatings; thin films
• ISSN: 0091-3286
• DOI: 10.1117/1.OE.64.3.031002

The 2023 Laser Damage conference thin-film damage competition was devoted to a survey on the state-of-the-art broadband near-IR multilayer dielectric (MLD) mirrors designed for ultra-short pulsed laser applications. The requirements for the coatings were a minimum reflection of 99.5% at 45-deg incidence angle for S-polarization from 830 nm to 1010 nm and group delay dispersion (GDD) < ± 50fs2. The participants were allowed to select the coating materials, coating design, and coating deposition method. Samples were damage tested at a single testing facility to enable direct comparison among the participants using a 25 ± 5 fs optical parametric chirped-pulse amplification (OPCPA) laser system operating at 5 Hz. The testing results from this set of 37 samples showed that dense coatings by ion-beam sputtering (IBS), magnetron sputtering (MS), and electron-beam ion assisted deposition (e-beam IAD) exhibited highest damage initiation onset (laser-induced damage threshold or LIDT) while e-beam coatings were low performers. In addition, multilayer coatings using tantala and/or hafnia as high index materials were top performers. Furthermore, this competition included for the first time the measurement of the damage growth onset (laser-induced damage growth threshold or LDGT). This latter performance metric plays an important role in establishing the safe operational conditions for larger aperture ultrashort pulsed lasers. Information pertaining to the morphology of the damage sites and their evolution under subsequent exposure to different laser fluences leading to damage growth is presented. Finally, not all coating samples in the survey met the GDD requirements stated above and associated measurements are discussed in the context of the present and past thin-film damage competitions focused on similar broadband near-IR MLD coatings.