Visualization of nanometer scale defects responsible for optical loss and laser induced breakdown in binary coating materials for the UV spectral region
Final Report Abstract
The aim of the joint research project was to conceive novel insights into correlations of the power handling capability of UV-coatings to the internal field strength and especially to embedded defects on the nanometer scale. To extend and strengthen the fundamental understanding of the optical loss and laser-induced damage mechanisms of UV optics, samples with artificial damage ensembles were fabricated and studied. The research activities on the sample production increased the knowledge on the nanoparticle generation using a commercial sputtering nanoparticle source. Spectrophotometry, dark field microscopy, laser calorimetric absorption measurements as well as scanning electron microscopy were applied in order to gain insight into the fabricated defect ensembles. Produced model defect samples with a HR-mirror design were tested for their laser handling capability. The embedded nanoparticles reduced the LIDT of these mirrors linearly with the concentration, indicating differences in the particle size distribution. A comparison of the LIDT results with a theoretical model for thermal damage initiation by nanoabsorbers resulted in a good agreement to the estimation of the artificial defect ensemble obtained with the mentioned analyzing techniques. Nanocomposites of HfxSi1-xO2 were produced and studied regarding the interplay of crystallization, scattering and absorption. A dense amorphous structure was found for an Hf0.7Si0.3O2 nanocomposite enabling for example the special application in waterproof coatings. Furthermore, special investigations in the initiation and growth behavior of nanoscale to microscale pits generated novel knowledge on the defect induced damage of UV optics. Nanometer-sized pits are difficult to detect with the commonly used Nomarski microscopy and may therefore lead to an overestimation of the LIDT. The results of the LIDT tests indicated, that the initiation of nanopits occurs only rarely and is not relevant for the determination of the laser resistance. Round flat-bottom pits with a stable size over a large fluence were identified as typical damage morphology. No growth of the micropits was observed for the irradiation with up to 100k pulses. The damage morphology of entire test sites, which consisted of multiple micropits, was also independent of the total number of applied pulses, and catastrophic damage was also not observed. Therefore, coatings exhibiting such stable damage morphologies could be operated at fluences above the LIDT of these pits, if losses due to a few pits are acceptable. Anti-reflective surface structures (ARSS) were fabricated and different cleaning techniques were applied in order to improve the LIDT. It was shown that a significant increase in the LIDT of ARSS can be achieved with optimal ultrasonic cleaning. In comparison, oxygen plasma cleaning achieved only a small improvement in LIDT. A study dedicated to the improvement of the LIDT of HR-mirrors in the UV spectral region revealed a novel damage phenomenon. Theoretically, the electric field intensity in an HR-mirror decreases monotonically with an increasing angle of incidence. Due to this decrease of the EFI, an increase of the LIDT for a large AOI is expected. This was validated for small beam diameters, however, for large beam diameters a decrease of the LIDT was observed compared to the corresponding measurement under normal incidence. Additional tests with model defect samples revealed, that this phenomenon is independent of the defect material, the defect distribution as well as the used coating process. In order to investigate a resonant interaction between nanoparticles, numerical simulations were performed for a simplified structure of two layers and larger gold nanoparticles. However, this model, connecting predominant dipole mode scattering of spherical particles, was not able to reveal EFI intensifications high enough to explain the observed LIDT results. Therefore, a qualitative theoretical model, based on second order effects, was proposed. The cooperation on the topical research field of defects in optical coatings will be continued by the partners, and further joint research initiatives as well as publications are planned. Not least based on the leading results of this activity, a new project, "TheFastCoatings: Theory and control technique of optical loss in multilayer coatings used in femtosecond laser cavities ", has already been submitted.
Publications
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“Nanocomposite coatings for laser cavity applications”, The international Conference on Frontiers of Optical Coatings (FOC), Guangzhou, China, (2017)
Cheng, X., Paschel, S., Balasa, I., Ristau, D., Wang, Z.
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"Laser-induced pit formation in UV-antireflective coatings," Proc. SPIE 10805, Laser-Induced Damage in Optical Materials 2018: 50th Anniversary Conference, 108051N (16 November 2018)
Paschel, S., Balasa, I., Jensen, L. O., Cheng, X. , Wang, Z., Ristau, D.
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“Waterproof coatings for high-power laser cavities”, Light Sci Appl 8, 12 (2019)
Cheng, X., Dong, S., Zhi, S. et al.
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“Influence of the surface and subsurface contaminants on laser-induced damage threshold of anti-reflection sub-wavelength structures working at near infrared region”, Optics and Laser Technology 127, 106144, Elsevier, (2020)
Zhang, J., Liu, F., Jiao, H., et al.
