Coating Thermal Noise in Gravitational-Wave Detectors

Abstract

The direct detection of gravitational waves, originating from cataclysmic events such as black hole and neutron star mergers, has ushered in a new era of observational astronomy. These signals offer unique insights into astrophysical phenomena and fundamental physics, but fully realizing their potential requires continued improvements in detector sensitivity. A primary factor limiting the performance of current ground-based interferometers like Advanced LIGO and Advanced Virgo is thermal noise arising from the highly reflective multilayer coatings on the test mass mirrors. Reducing this coating thermal noise, particularly its Brownian component, while simultaneously maintaining exceptionally low optical absorption and scatter is necessary to advance detector capabilities.

This thesis addresses this challenge through the characterization and development of alternative coating materials and designs. Central to this work is a dedicated experimental apparatus employing a high-finesse folded optical cavity and a multimode co-resonance technique. This system enables direct, high-precision measurements of coating thermal noise in the frequency band relevant to gravitational-wave detectors and allows for relatively rapid evaluation of candidate coatings, providing timely feedback for materials development.

Coating materials such as niobia-based oxides, hafnia-tantala mixtures, and substoichiometric silica, were explored employing strategies like compositional optimization, post-deposition annealing, and multimaterial designs with buried layers. Progress toward lower-noise coatings is demonstrated. Highly reflective coatings based on optimized titania-silica, titania-germania, and ternary silicon nitride structures achieved thermal noise levels approximately 75% that of current detector coatings. These coatings also exhibited exceptionally low optical absorption, reaching levels near 1 part-per-million following appropriate heat treatment. While challenges related to defect formation during annealing and discrepancies between different noise measurement methodologies were identified, ongoing research, particularly on defect mitigation in materials like titania-germania, continues to advance the field. The findings presented here contribute to the materials science foundation for improving current gravitational-wave detectors and guiding the design of future observatories.