# Temperature induced optical scatter changes in titania-germania coatings

## TL;DR

This study investigates how temperature affects optical scatter in titania-germania coatings, which are promising for reducing thermal noise in gravitational wave detectors like LIGO and Virgo.

## Contribution

It provides the first detailed analysis of temperature-induced optical scatter changes in titania-germania coatings for gravitational wave observatories.

## Key findings

- Optical scatter increases with annealing temperature.
- Low initial scatter measured at 8 degrees, with a median BRDF of 1.1e-7 sr^{-1}.
- Potential for improved coatings in future gravitational wave detectors.

## 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.