Temperature induced optical scatter changes in titania-germania coatings

TL;DR

This paper examines temperature-induced optical scatter in a 52-layer SiO2/TiO2:GeO2 high-reflective coating stack to assess its suitability for reducing coating thermal noise in ground-based GW detectors. Using an air annealing scatterometer with a 1041 nm probe at 8° incidence, it finds BRDF values near $1.1\times 10^{-7}\mathrm{str}^{-1}$ that can rise to $1.2\times 10^{-6}\mathrm{str}^{-1}$ during annealing in a measured region, while small regions exhibit substantially lower scatter. The results support TiO2:GeO2 coatings as a promising option for LIGO/Virgo upgrades and as a pathfinder for Cosmic Explorer, with ongoing deposition optimization and beam-profile-aware analyses. The work also notes spatial nonuniformity and the emergence of scatterers, underscoring the need for further microstructural characterization to fully establish performance.

Abstract

Titania doped with tantala is the high index material (high n) for the optical coatings used in LIGO and Virgo and its thermal noise limits LIGO/Virgo observations of astrophysical sources. In this paper, we study temperature induced changes to optical scatter of a multilayer highly reflective coating comprised of silica (low n) and titania doped with germania (high n) as a potential candidate to reduce coating thermal noise in ground-based observatories operating at room temperature. We observe that the scatter measured at 8 degree in a small region is low, with a median starting BRDF of $1.1 \times 10^{-7}\,\mathrm{str}^{-1}$ increasing to $1.2 \times 10^{-6}\,\mathrm{str}^{-1}$ through annealing. The results presented here show the potential of adopting titania doped with germania coatings for future upgrades to LIGO and Virgo and as a pathfinder coating for Cosmic Explorer, a next-generation detector.

Figures (2)

• Figure 1: Overview of the optical layout for the air annealing scatterometer. (a) shows the experimental schematic where a superluminiscent diode (1041 nm) is used to probe the optic installed inside the oven. The reflected beam from the optic makes a shallow 8$\degree$ angle with respect to the incident beam, and the scatter from the optic is continuously monitored over the course of the annealing run. (b) shows the design of the optic holder inside the oven.
• Figure 2: Optical scatter from annealing of the sample. (a) and (b) show the bright images of the sample at $t=0$ and $t=end$ respectively with the same exposure time. (c) shows the temperature induced scatter from the sample. The red curve shows BRDF from the large ROI (explained in text, illustrated in (a) and (b)) and the pink curve shows BRDF from a single subROI analysis where the ROI is highlighted in (d) and (e) corresponding to the $t=0$ and $t=end$ respectively. Both the red and pink curves bias the negative values from the image subtraction to zero. The blue curve in the above graph shows the BRDF from one subROI allowing for negative values from the image subtraction process.