Thermal Noise Reduction in Ternary Optical Coatings: From Ti::GeO$_2$-Based Ternary Systems to High Index Materials

TL;DR

This paper explores advanced ternary optical coating designs to reduce thermal noise in gravitational wave detectors, analyzing material properties, optimization strategies, and potential for achieving target noise reduction.

Contribution

It introduces a novel optimization approach for low-noise dielectric stacks and evaluates the performance limits of various ternary material systems.

Findings

Standard ternary coatings fall short of target noise reduction.

Alternative high-index materials show promise for meeting noise reduction goals.

Optimization strategies reveal trade-offs between absorption, noise, and material choices.

Abstract

Minimizing coating thermal noise is crucial for enhancing gravitational wave detector sensitivity, with a target Amplitude Spectral Density Reduction Factor (ASD RF) of $0.5$ relative to standard coatings. This study investigates the design of low-noise dielectric stacks using the 'Double Stack of Doublet' strategy, explored via ad-hoc optimization heuristics specifically developed for efficient parametric analysis of coating performance. We analyze the performance limits of ternary coatings based on SiO$_2$, Ti::SiO$_2$, and Ti::GeO$_2$, considering material property uncertainties and absorption constraints. Optimization results show that this system, even with relaxed absorbance constraint (1 ppm), falls short of the target, achieving a best ASD RF of $\sim 0.69$. Consequently, we explore alternative ternary 'Double Stack of Doublet' designs incorporating higher-refractive-index materials. Simulations demonstrate that incorporating alternative high-index materials offers a promising pathway, potentially enabling the achievement of the project target. We discuss the optimization strategies, performance trade-offs, design robustness, and implications of using high-index, potentially higher-loss materials for next-generation optical coatings.