Studies of mechanical and optical properties of thin film coatings for future gravitational wave detectors

Tait, Simon C. (2021) Studies of mechanical and optical properties of thin film coatings for future gravitational wave detectors. PhD thesis, University of Glasgow.

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Gravitational radiation in the form of gravitational waves was the last prediction to be verified from Einstein's general theory of relativity. Einstein suggested that when a body or bodies with an asymmetric distribution of mass accelerated, energy from the motion would create distortions in space-time which would propagate in all directions at the speed of light.

Until the first confirmed observation of gravitational waves from the coalescence of two black holes, this theory had not been experimentally proven. The first gravitational wave (GW) event, `GW150914' confirmed Einstein's predictions, with the event releasing 3 solar masses worth of energy as gravitational radiation during the collision. Since this event, more than 50 confident GW events have been detected, including the first observation of an extremely rare kilonova event after the collision of two neutron stars.

Gravitational waves exert fluctuating strains on space as they propagate, resulting in changes in the length of objects that they pass through. Current gravitational wave detectors use laser interferometry to measure this effect using a single laser source and beamsplitter. Two perpendicular laser beams are created and used to monitor the positions of suspended mirrors at the ends of km-scale perpendicular arms. The laser beams reflected from the mirrors are recombined at the beam splitter, creating an interference pattern. Any changes in these mirrors' position then result in a differential change in the arm length inside the detector, altering the generated interference pattern. As the expected change in differential arm length produced by a gravitational wave event is 1x10^-18m all other sources of motion, or noise, must be reduced to exceedingly low levels to maximise the sensitivity to such events.

Throughout the range of frequencies to which a gravitational wave detector is sensitive too, its highest sensitivity occurs between approx 50 Hz and 150 Hz. In this frequency band, thermal noise stemming from thermal vibrations in the materials used to create highly reflecting mirror coatings for each test mass limits the sensitivity of the detector. Each material's contributions to the level of thermal noise are proportional to its level of mechanical loss, its temperature and dimensions of the laser beam on its surface.

This thesis will focus on the development of coating materials with low mechanical loss and low optical absorption, which can be used to decrease levels of thermal noise inside and increase the stability of a gravitational wave detector. As the amount of laser light absorbed into the coating layer also dictates the test mass's thermal state, a large part of this research will also focus on this aspect of coating measurement. A large part of the work in this thesis involves the first experimental verification of the so-called `multimaterial coating' principle, through testing the optical absorption and room-temperature and cryogenic mechanical loss, of an example of this type of novel coating design.

Chapter 1 describes the nature of gravitational radiation and its possible sources. An introduction to the experimental interferometry techniques used in a gravitational wave detector is considered, and notable sources of noise are summarised.

Chapter 2 provides a detailed summary of coating thermal noise in gravitational wave detectors. This chapter also introduces some of the recent advancements and current avenues of research in HR gravitational wave detector coatings.

Chapter 3 is an account of work carried out by the author at the LIGO Livingston Observatory to develop a technique for monitoring the absorption of the detector mirrors in situ. By studying the resonant frequencies of coated test masses in a gravitational wave detector, a relationship between frequency and the change in test mass temperature by laser heating can be produced. If the total laser power and the level of optical absorption of the coated optic are known, predictions of how its resonant frequencies will change can be modelled using finite element analysis (FEA). If the optical absorption of the coating material at the time of deposition was known, the shift in resonant frequencies of the test mass in response to laser heating could be used to predict any changes in the absorption of the optic.

Chapter 4 discusses the experimental Photothermal Common-path Interferometry (PCI) technique used by the author to measure the optical absorption of thin-film coatings. This technique is used to study the peculiar changes in optical absorption of tantalum pentoxide Ta2O5 coatings after heat treatment in a laboratory atmosphere and under a low vacuum. Implementation of a polarisation stabilisation and power correction system into the PCI techniques has allowed for 2-dimensional absorption maps to produce these samples. In this chapter, measurements of a novel multimaterial coating designed to decrease coating thermal noise and optical absorption are studied. The material is subjected to two different heat treatment studies, and its optical absorption is characterised using 1064 nm, 1550 nm and 2000 nm laser light.

In Chapter 5, the methods and techniques used to measure the mechanical loss (internal friction) of coating materials are introduced. Throughout this chapter, the development of an automated measurement technique used to characterise the mechanical loss diameter = 3" (76.2 mm), t = 2. 6 mm is discussed and compared to existing measurement techniques.

Chapter 6 describes the use of the technique developed in Chapter 5 to study the mechanical loss of the same novel multimaterial coating as a function of heat-treatment temperature. These measurements allow the level of coating thermal noise produced by each multimaterial coating to be calculated using the methods described in Chapter 2.

The development of the gentle nodal support described in Chapter 5 is continued in Chapter 7, upgrading the apparatus to function at cryogenic temperatures (80 K<T<293 K). By creating an automated gentle nodal support that operates at cryogenic temperatures, the mechanical loss of coating materials can be characterised for third-generation gravitational wave detector applications. This chapter describes the development of the CryoGeNS system and the characterisation of uncoated diameter = 2" (50.8 mm), t=360um crystalline silicon disks.

Chapter 8 details cryogenic mechanical loss measurements of the prototype multimaterial coating, carried out using the CryoGeNS nodal support. Coating loss calculated from each disk is compared to measurements of the same samples carried out in a second commercial cryostat and cryogenic measurements of cSi cantilevers coated in the same materials. This allows the level of thermal noise improvement of these coatings to be calculated as a function of temperature.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Coatings, optics, multimaterial, tantalum, silica, silicon, gravitational waves, LIGO, highly reflecting mirror coatings, thermal noise.
Subjects: Q Science > QC Physics
Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
Funder's Name: Science and Technology Facilities Council (STFC)
Supervisor's Name: Martin, Dr. Iain W. and Rowan, Professor Sheila
Date of Award: 2021
Depositing User: Mr Simon C. Tait
Unique ID: glathesis:2021-82514
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 22 Oct 2021 07:15
Last Modified: 26 Oct 2021 14:32
Thesis DOI: 10.5525/gla.thesis.82514

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