Detecting ultrathin ice on materials for optical coatings at cryogenic temperatures

Abstract

The performance of optical cavities in Gravitational Wave Detectors (GWD) is negatively affected by the growth of ice layers when operating at cryo temperatures. Loss of performance begins when the ice overlayer is only a few-nm thick. Careful planning is then required to minimize, monitor and take into account the presence of ultrathin ice on cryo-cooled optical surfaces. Here we employed spectroscopic ellipsometry (SE) to study icing on the surfaces of SiO2 and Ti:Ta2O5 thin films, two materials used in the high-reflective mirrors of current GWD. SE measurements were performed at 75 K. The data presented suggest that SE is a most convenient
tool to monitor in operando the ice formation on the surfaces of GWD mirrors.
Furthermore, ultrathin ice layers can affect the evaluation of the optical properties of materials at low temperatures, a valuable task for those next-generation GWD that will operate at cryogenic temperatures. The characterization of an ultrathin ice overlayer (<10 nm) allowed to determine for the first time the low-temperature optical properties of Ti:Ta2O5. The same approach could be applied to determine the low-temperature optical properties of other dielectric films, thus helping to screen new materials for cryo-operated GWD mirrors.

Monitoring the evolution of optical coatings during thermal annealing with real-time, in situ spectroscopic ellipsometry

“…The acquisition time for a single SE spectrum was below 3 s. SE spectra consist of two values, conventionally labeled Ψ and ∆, which are related to the complex Fresnel coefficients r p , r s of the sample under investigation: Ψ = arctan(|r p |/|r s |) and ∆ = δ p − δ s , where |r p,s | and δ p,s are the modulus and the phase of r p,s , respectively [37]. The nature of Ψ and ∆ implies that SE measurements do not need any reference beam, thus making SE a particularly well suited tool for real-time, in situ investigations where the temperature of the sample is varied [27,28,31,38,39]. The quantities of interest, such as the refractive index and thickness of a coating, are extracted from Ψ, ∆ by means of a model-based data analysis [37].…”