Effects of mixing and annealing on the optical and mechanical properties of TiO$_{2}$:Ta$_{2}$O$_{5}$ amorphous coatings

TL;DR

This study systematically investigates how mixing ratio and annealing affect the optical and mechanical properties of amorphous TiO$_{2}$:Ta$_{2}$O$_{5}$ coatings, crucial for optimizing Bragg reflectors in gravitational wave detectors.

Contribution

It provides a comprehensive analysis of the effects of composition and annealing on optical absorption and mechanical losses, identifying optimal conditions for low-loss coatings.

Findings

Urbach energy has a minimum around 20% TiO$_{2}$, correlating with lowest optical absorption.

The coating loss angle shows weak dependence on frequency and broad Ti content range.

Optimal annealing parameters depend on specific cation ratios for minimal mechanical loss.

Abstract

Amorphous mixed titania-tantala coatings are key components of Bragg reflectors in the gravitational wave detectors (GWDs). Attaining the lowest possible values of optical absorption and mechanical losses in the coatings is of paramount importance for GWDs, and this requires a complex optimization of the coating deposition and post-deposition annealing. We present here a systematic investigation of the optical properties and internal friction of amorphous mixed titania-tantala coatings grown by ion beam sputtering. We consider coatings with six different cation mixing ratios and we study them both in the as-deposited and annealed state. By exploiting spectroscopic ellipsometry data and modelling, along with ancillary techniques, we retrieved the dielectric function of the coatings in a wide spectral range. When varying the mixing ratio and performing the annealing we find monotonic trends for most of the aforementioned properties. Remakably, the post-annealing Urbach energy displays a definite minimum for a mixing ratio around 20%, very close to the composition of the coatings showing the lowest optical absorption for GWDs applications. We suggest that the observed minimum in the Urbach energy depends not only on the mixing ratio, but also on the annealing parameters. On the other hand, the minimum coating loss angle was found to be weakly dependent on the considered measurement frequency and to lie within a rather broad range of Ti content, suggesting that the search for an absolute minimum following post-deposition annealing should be rather sought in the study of the best annealing parameters for each specific cation ratio considered. This work constitutes a reference for the optical properties of the amorphous mixed titania-tantala coatings, and highlights the relevance of the Urbach energy in the optimization process of materials for high-performing Bragg reflectors.