Development of ion-beam sputtered silicon nitride thin films for low-noise mirror coatings of gravitational-wave detectors
A. Amato, M. Bazzan, G. Cagnoli, M. Canepa, M. Coulon, J. Degallaix, N. Demos, A. Di Michele, M. Evans, F. Fabrizi, G. Favaro, D. Forest, S. Gras, D. Hofman, A.… 展开作者
A. Amato, M. Bazzan, G. Cagnoli, M. Canepa, M. Coulon, J. Degallaix, N. Demos, A. Di Michele, M. Evans, F. Fabrizi, G. Favaro, D. Forest, S. Gras, D. Hofman, A. Lemaitre, G. Maggioni, M. Magnozzi, V. Martinez, L. Mereni, C. Michel, V. Milotti, M. Montani, A. Paolone, A. Pereira, F. Piergiovanni, V. Pierro, L. Pinard, I. M. Pinto, E. Placidi, S. Samandari, B. Sassolas, N. Shcheblanov, J. Teillon, I. Vickridge, M. Granata
发表信息：
A. Amato et al., Development of ion-beam sputtered silicon nitride thin films for low-noise mirror coatings of gravitational-wave detectors, Phys. Rev. D 111, 042003 (2025)
薄膜涂层的布朗热噪声是基于光学谐振腔的高精度实验（如引力波干涉仪）的一个基本限制。本文介绍了一项研究活动的成果，该活动旨在开发噪声更低的离子束溅射氮化硅薄膜，并满足引力波干涉仪对光学损耗极为严格的要求。为了检验薄膜合成条件与其元素组成及光学和力学性能之间存在相关性的假设，我们改变了溅射离子束的电压、电流强度和组成，并开展了广泛的表征工作。研究发现，折射率单调依赖于束电压，并随N/Si比线性变化，而光学吸收似乎对其他尚未识别的因素高度敏感。然而，通过系统地改变沉积参数，找到了一个最佳工作点。由此我们表明，在900 $^{\circ}$C热处理后，我们的薄膜在约2.8 kHz处的损耗角可低至$(1.0 \pm 0.1) \times 10^{-4}$ rad，在1064 nm处的消光系数可低至$(6.4 \pm 0.2) \times 10^{-6}$。据我们所知，该损耗角值是此类薄膜上迄今测得的最低值。随后，我们利用所开发的氮化硅薄膜设计并制备了一种多材料镜面涂层，其在100 Hz处的热噪声振幅为$(10.3 \pm 0.2) \times 10^{-18}$ m Hz$^{-1/2}$，比Advanced LIGO和Advanced Virgo干涉仪现有镜面涂层低25%，且在1064 nm处的光学吸收低至$(1.6 \pm 0.5)$ parts per million。
英文摘要：
Brownian thermal noise of thin-film coatings is a fundamental limit for high-precision experiments based on optical resonators such as gravitational-wave interferometers. Here we present the results of a research activity aiming to develop lower-noise ion-beam sputtered silicon nitride thin films compliant with the very stringent requirements on optical loss of gravitational-wave interferometers.
In order to test the hypothesis of a correlation between the synthesis conditions of the films and their elemental composition and optical and mechanical properties, we varied the voltage, current intensity and composition of the sputtering ion beam, and we performed a broad campaign of characterizations. While the refractive index was found to monotonically depend on the beam voltage and linearly vary with the N/Si ratio, the optical absorption appeared to be strongly sensitive to other factors, as yet unidentified. However, by systematically varying the deposition parameters, an optimal working point was found. Thus we show that the loss angle and extinction coefficient of our thin films can be as low as $(1.0 \pm 0.1) \times 10^{-4}$ rad at $\sim$2.8 kHz and $(6.4 \pm 0.2) \times 10^{-6}$ at 1064 nm, respectively, after thermal treatment at 900 $^{\circ}$C. To the best of our knowledge, such loss angle value is the lowest ever measured on this class of thin films.
We then used our silicon nitride thin films to design and produce a multi-material mirror coating showing a thermal noise amplitude of $(10.3 \pm 0.2) \times 10^{-18}$ m Hz$^{-1/2}$ at 100 Hz, which is 25\% lower than in current mirror coatings of the Advanced LIGO and Advanced Virgo interferometers, and an optical absorption as low as $(1.6 \pm 0.5)$ parts per million at 1064 nm.
