Optical properties of amorphous coatings for low-temperature applications

Introduction:
The performance of optical coatings in cryogenic environments is critical for advanced applications such as high-precision optical systems; however, little data are available in the literature about their properties at low temperatures. At low temperatures, changes in refractive index and potential ice formation on coatings can significantly impact their optical properties, requiring a dedicated characterization. Silicon nitride coatings have been extensively studied in diverse fields, for applications both at room and low temperatures. In this work, the optical properties of non-stochiometric silicon nitride coatings at low temperatures are presented. SiN_x coatings are candidate materials for the mirrors of next-generation interferometric gravitational wave detectors such as the Einstein Telescope.

Methods:
A custom setup was employed for cryo-optic measurements to systematically study the optical properties of coatings at cryogenic temperatures. A cryostat with optical access, capable of reaching temperatures as low as 4 K, was designed to operate in ultra-high vacuum (10⁻⁸ mbar). In this study, we used real-time, in situ spectroscopic ellipsometry (iSE) coupled with the optical cryostat.

Results:

We acquired broadband SE spectra from room temperature down to liquid nitrogen temperatures. We modeled the ellipsometry spectra by means of a Cody–Lorentz dispersion function, obtaining the refractive index, extinction coefficient, and thickness of the samples as a function of the temperature.

Conclusion:
This study focuses on characterizing ion-beam-sputtered SiN_x optical coatings to investigate their behavior under cryogenic conditions. Characterization of the coatings at low temperatures assisted in the design of multilayer coatings operating at cryogenic temperatures. The accurate determination of the optical properties of the coatings was made possible by means of a proper modeling of few-nanometer cryodeposits (mainly water ice) that form on the cold surface of the coatings.

Acknowledgments:

We gratefully acknowledge the Project Einstein Telescope Infrastructure Consortium (ETIC)(IR0000004)-MUR call n. 3264 PNRR, Miss. 4-Comp. 2, Line 3.1.