Ion beam deposited optical mirror coatings for next generation gravitational wave detectors

  • Gavin Wallace

Student thesis: Doctoral Thesis


Gravitational waves are a form of radiation, generated from the largest astrophysical bodies in the Universe, which were first postulated by Albert Einstein in 1915 as part of his general theory of relativity. A century after this the gravitational waves from a binary black hole merger were measured on the 14th of September 2015, by the LIGO scientific community. This was the first measurement of its kind and has heralded a new form of astronomy.Gravitational waves are measured by laser interferometry, where light is reflected from a mirror over two 4 km arm cavities, in the case of aLIGO, and combined to form an interference condition. As the displacement caused by a gravitational wave passing through the Earth are so small (~ 10ˉ18 m) the detectors can be subject to strict sensor noise budgets, with sources of such noise ranging from earthquakes on the other side of the planet to thermal fluctuations of the very molecules in the detector.The thermal fluctuations in the detector associated with the highly reflective coatings used on the interferometry mirrors, can be characterised as coating Brownian thermal noise. This source of noise is at its highest between 50 Hz - 150 Hz, the most sensitive band of the detector. As such there is a large effort in the community to reduce this noise source significantly for the next generation of gravitational wave detectors. This thesis focuses on the use of ion beam sputtering deposition, the current method used to produce gravitational wave detector mirrors, to create low optical and mechanical loss coatings for the next generation of gravitational wave detector. The materials chosen to be studied optically, mechanically, compositionally and structurally were amorphous silicon and silicon nitride due to their desirable optical and mechanical properties for gravitational wave detectors.Chapter 1 describes the nature of gravitational waves, confirmed and unconfirmed Cosmic sources of such waves and methods used, both historically and currently, to detect them. Fundamental limits to the detectors are discussed with an explanation to the methods used to mitigate such limits. Notable gravitational wave detections are explained in addition to the current generation of detectors. Finally an overview is given of the next generation of detectors.Chapter 2 describes Brownian thermal noise, both its origins and methods currently used by gravitational wave detectors to mitigate its effects on observational reach. Chapter 3 details the coating techniques which have been used, either as part of research or production, for optical mirror coatings of gravitational wave observatories. Specifically ion beam deposition is described in more detail, both from a theoretical and experimental standpoint.Chapter 4 describes the metrology used in this research to characterise the thin film coatings optically, mechanically, compositionally and structurally. Chapter 5 and 6 details the research conducted on amorphous silicon and silicon nitride coatings and the evaluation of such coating use for gravitational wave detectors both as deposited and after heat treating at increasing temperatures.
Date of Award9 Jun 2023
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorStuart Reid (Supervisor) & Terry Gourlay (Supervisor)

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