Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)




Stefan Ballmer

Second Advisor

Joshua Russell


AlGaAs, Coatings, Gravitational Waves, LIGO


With the first observation of a binary black hole merger in GW150914 \cite{150914Discovery}, LIGO heralded a new era in the field of observational astronomy. It was the first observation of any astronomical object by measuring its gravitational wave signature, opening a new window to the cosmos. A few years later, the first astronomical event to be observed by both electromagnetic waves and gravitational waves was made \cite{Cowperthwaite2017GW170817, Abbott2017GW170817}. Gravitational wave detectors are multi-kilometer long interferometers measure changes to their length imparted by passing gravitational waves that are smaller than an atomic nucleus. Such extreme precision requires extensive mitigation of many different noise sources, discussed in chapter 1. This work is primarily focused on thermal noise of the gravitational wave detector test mass coatings, detailed in chapter 2. Chapter 3 outlines the theory behind the cryogenic gentle nodal suspension \cite{Cesarini_GeNS}, the experiment the author constructed at Syracuse to measure coating thermal noise of possible test mass coatings. Chapters 4 and 5 will delve into the data that the author measured in the cryogenic gentle nodal suspension. There is a special focus in this work on AlGaAs/GaAs multi-layers, referred to as just AlGaAs for simplicity, as a possible test mass coating. This crystalline coating has shown remarkably low levels of loss at room temperature \cite{Cole2013OG, Penn2019MechAlGaAs}. This work serves as a follow-up to the room temperature loss measurements, with the goals of understanding the loss mechanisms inside an AlGaAs coating through the shape of its loss curve over temperature \cite{Martin2008measurements} and applying the findings to possible cold temperature gravitational wave detectors \cite{Akutsu2021, ET2020, CEHorizon2021}. The mechanical loss of the AlGaAs coating was found to be largely varied by mode, with the lowest mode displaying loss levels of $\phi_{\mathrm{coating}} \lesssim 1 \times 10^{-6}$ and the highest mode showing loss of $\phi_{\mathrm{coating}} \lesssim 1 \times 10^{-4}$. Investigations into the mechanisms contributing to excess loss in some of the modes are currently underway. The lowest loss modes are consistent with room temperature measurements.


Open Access