Al2O3/HfO2 Nanolaminate Gradient-Index Structure Boosts Damage Resistance of Picosecond Laser Reflection Coatings

Ultra-Intense Laser Science Excellence Innovation Brief

(Issue 821)

September 15, 2026

SIOM Advances Damage Resistance of Picosecond Laser Reflection Coatings Based on Al₂O₃/HfO₂ Nanolaminate Graded-Refractive-Index Structures

The research team led by Shao Jianda and Zhu Meiping in the High-Power Laser Component Technology and Engineering Department of the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has made new progress in studying the damage resistance of picosecond laser reflection coatings based on Al₂O₃/HfO₂ nanolaminate graded-refractive-index structures. The related findings, titled "Enhancing laser-induced damage threshold of picosecond laser mirrors using graded-refractive-index multilayer structures", were published in Journal of Applied Physics.

Conventional optical coatings typically use alternating stacks of high- and low-refractive-index materials, with SiO₂ serving as the low-index material and HfO₂ and similar materials as the high-index material. Increasing the refractive index of the high-index material helps broaden the reflection bandwidth, but such materials generally have narrower optical bandgaps and are more susceptible to damage under intense laser irradiation. At the same time, the electric field in the film decays gradually from the air side toward the substrate, making the high-field region near the air side the most likely site for damage initiation. Balancing reflection bandwidth and laser-induced damage threshold is therefore a key challenge in the design of picosecond laser mirrors.

The research team proposed a design strategy for picosecond laser reflective thin films based on gradient-index composite materials. UsingAl₂O₃/HfO₂nanolaminate materials andAl₂O₃as the high-index layers, andSiO₂as the low-index layer, the researchers designed and fabricated a gradient-index mirror,M-Graded, and compared it with anM-Constantmirror that used a fixed-compositionAl₂O₃-HfO₂mixed material as the high-index layer. InM-Graded, the high-index layer nearest the air side usesAl₂O₃, which has a wider optical bandgap and higher laser damage resistance, while the high-index layer near the substrate usesAl₂O₃/ HfO₂nanolaminate material. Each nanolaminate consists of four groups ofAl₂O₃/ HfO₂sublayers; by varying the sublayer thickness ratio in different nanolaminates, the equivalent refractive index is gradually increased from1.63on the air side to1.86on the substrate side. The referenceM-Constantuses co-evaporatedAl₂O₃- HfO₂mixed material with anAl₂O₃toHfO₂ratio of0.69:0.31, essentially matching the overall ratio of the two materials inM-Graded.

The study systematically compared the spectral performance, surface roughness, film stress and picosecond laser damage characteristics of the two mirrors. The results show that both mirrors achieves-polarized reflectance above99.5%near1053 nm. The high-reflectance bandwidth ofM-Gradedis81 nm, only slightly below the87 nmofM-Constant. Under laser irradiation at1053 nmwith a pulse width of about8 ps, the laser-induced damage threshold ofM-Gradedreached9.4 J/cm²,1.49times higher than the6.3 J/cm²ofM-Constant. Damage morphology and depth analysis indicate that damage initiation in both mirrors is closely related to nanoscale absorbing defects on the surface, withM-Gradedshowing a deeper and more dispersed damage distribution. The study shows that using the wide-bandgap materialAl₂O₃in the near-surface high-electric-field region and employingAl₂O₃/HfO₂nanolaminates to gradually raise the refractive index of the deeper high-index layers can significantly improve the laser damage resistance of picosecond laser reflective thin films at the cost of only a slight reduction in reflectance bandwidth.

This work was supported by the Shanghai Academic Leaders Program, the Shanghai Leading Talent Program and the Chinese Academy of Sciences Basic Research Young Scientists Project.

Original article link

Fig.1 (a) Schematic of the coating structure of theM-Gradedmirror and (b) theM-Constantmirror.

Fig.2 (a) Cross-sectional images of nanolaminates NG₁, NG₅and NG₉.(b) Elemental composition of Al and Hf in the M-Constant mirror and the M-Graded mirror.

Fig.3 Laser thin-film performance:(a) reflectance spectra,(b) surface roughness,(c) laser damage probability curves,(d)relationship between damage depth distribution and the electric field distribution within the coating.