Enhancing Titania-Tantala Amorphous Materials as High-Index Layers in Bragg Reflectors of Gravitational-Wave Detectors

Abstract

Amorphous mixed titania-tantala coatings are key components of Bragg reflectors in gravitational-wave detectors (GWDs). Attaining the lowest possible values of optical absorption and mechanical losses in coatings is of paramount importance for GWDs, and this requires a complex optimization of the coating deposition and postdeposition 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defined as Ti/(Ti + Ta)and we study them both in the as-deposited and annealed states. All coatings have been subject to the same annealing of 500 °C for 10 h in air, which is the postdeposition treatment adopted so far for Bragg reflectors in GWD applications. By exploiting spectroscopic ellipsometry data and modeling, 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in some cases, almost lineartrends for most of the aforementioned properties. Remakably, the postannealing 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 GWD 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 (cation ratios of 21 and 44%), suggesting that the search for an absolute minimum following postdeposition 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 as an additional parameter to guide the optimization process of materials for high-performing Bragg reflectors.

Role of substrate and TiO2 content in TiO2:Ta2O5 coatings for gravitational wave detectors

“…Here we complement the study presented in [13], by investigating the role of the substrate on the morphological and structural properties of TiO 2 :Ta 2 O 5 coatings. From the morphological point of view, we performed a comprehensive study based on AFM: figure 1 shows the AFM topographies, 10 µm × 10 µm in lateral size, of the as-deposited TiO 2 :Ta 2 O 5 coatings with different compositions, nominally 0%, 25%, 80%, and 100%.…”

“…Each study requires a slightly different support. As described in [13], Si wafers are used during optical investigations, to avoid backside reflections, while SiO 2 disk are preferred for mechanical measurements.…”

“…Finally, we used SE to determine the broadband refractive index of the TiO 2 :Ta 2 O 5 coatings grown on Si, in their as-deposited form as well as after annealing at 500 • C. All measurements were performed in a wavelength range going from 620 to 2100 nm, except for pure Ta 2 O 5 coating (0% TiO 2 ), where data are only available up to 1100 nm. SE data were modeled with a Cauchy dispersion relation, following the approach described in [13]. The resulting refractive index is reported in figure 6 as a function of %TiO 2 .…”

“…The SiO 2 disk were made of Corning 7980 A and polished on both surfaces to a residual root mean square (RMS) roughness of 1 nm, which is about 5 times higher than that of GWDs' substrates [15]. Thin SiO 2 disk were also used, in the past, for mechanical measurements on twin samples, as reported elsewhere [6,13]. Table 1 provides the details of the investigated coatings, including the nominal Ti/(Ta + Ti) concentration, the correspondent value measured by energy dispersive spectroscopy (EDS) (reported in supplementary material), and their substrate.…”

“…On the contrary, a net worsening of the coating loss angle is measured in pure TiO 2 , after annealing at the same temperature. Amato et al [13] have extensively studied the optical and mechanical properties of TiO 2 :Ta 2 O 5 coatings, by systematically varying the cation mixing ratio, before and after the standard annealing procedure. They found out that, while the mechanical losses are quite low over a broad range of Ti/(Ta + Ti) ratio, the Urbach energy function, a parameter measuring the structural disorder, has a clear minimum in correspondence of the annealed 20% TiO 2 :Ta 2 O 5 , which justifies the enhanced performance of the used mixture over other cation ratios.…”