Gravitational Wave Detectors

‘The Study of Dielectric Mirror Coatings for Ground Based Gravitational Wave Detectors’

BSc (Hons) Physics Project by Jordan Hill

Albert Einstein theorised in 1905 that what we perceive as the force of gravity is in fact a result of the curvature of space-time. This curvature is suggested to come from large cosmic objects moving through space-time. Humans first detected a Gravitational Wave (GW) on 14 Sept.2015 by Laser Interferometer Gravitational-Wave Observatory (LIGO). This GW came from a two binary black holes merging and, in 2017, the Nobel Prize was awarded to the lead scientists on the LIGO discovery team. In 2018, the LIGO and Virgo scientific collaboration released results from their search.

Since their detection in 2015, the study of Gravitational Waves (GWs) generated from known space phenomena - or new undiscovered ones - is considered a novel way to understand the Universe, providing unique information from remote places along the Universe being unreachable by state-of-the-art telescopes and space vehicles. In 2020-2021, the next generation of mirrors is planned to be implemented in the main GW detector facilities. Accordingly, it is timely to investigate the mechanisms governing the thermal noise in mirror coatings, and to explore new deposition techniques/materials to produce homogenous and large-area mirror coatings with high optical reflectance and low thermal noise.

UWS Physics student, Jordan Hill’s Honours project presents the design, fabrication and characterisation of novel mirror coatings for low mechanical loss, high-reflectance and wide-band GW detectors (GWDs). His project also shows the successful fabrication of a metrology system called Q-measurements system (QMS), to measure the mechanical loss of mirror coatings, consisting on single layer (i.e. Ta2O5) and multi-layers based quarter wavelength of Ta2O5 and SiO2. Samples have been annealed for different times and temperatures in order to study the resulting mechanical loss.

The investigation carried out in this Honours project aimed to characterise reduction of the thermal noise observed in mirror coatings at the range of frequencies where GWDs are more sensitive. To this end, this project has been developed at the Institute of Thin Films, Sensors & Imaging (University of the West of Scotland) led by Prof.Des Gibson, UWS in collaboration with Dr.Martin at the Institute for Gravitational Research (University of Glasgow), Prof.Reid (University of Strathclyde) and Prof.Jin Long (University of Tong Ji, Shanghai, China).

This project aimed to further extend the on-going research developed at the Institute of Thin Films, Sensors & Imaging (TFSI) at the UWS School of Computing, Engineering & Physical Sciences. The research group is working on different areas of investigation around semiconductor thin films deposited by plasma-assisted techniques. The high control over the material properties as a function of the deposition conditions, will allow the application of the resulting thin films in fields such as sensing, imaging and energy systems. Our researchers are focused on the accurate control of the deposition conditions, such as plasma properties, substrate preparation, substrate temperature, to achieve specific material properties - including high reflectivity, high sensitivity gas environments, and high selectivity. The characterisation of thin films is a crucial step towards the development of technology based on thin film materials.

Jordan was involved in the characterisation of mechanical loss in dielectric coatings using a Q-measurements set-up based on Mach-Zehnder interferometer. The thin films were deposited by plasma-assisted techniques at different growth conditions. Jordan’s studies comprised the use of laboratory technology including a Class-II He-Ne laser, a high-vacuum chamber and a high voltage amplifier, which he used with technician supervision and support. During his Honours project, he was involved in many aspects, including:

• Design and fabrication of a new Q-measurements system, which is the technique used to carry out the basic measurements of the coating mechanical loss at high-vacuum conditions. Jordan was in charge of the system maintaining and the preparation of system manual.
• Programming of the interface controlling the data acquisition of the ring-down measurements carried out during all the experiments.
• Analysis and simulation of resonance modes in mirror coatings.
• Comparison between the results obtained in systems available at different universities.

Dr.Carlos García Nuñez, Lecturer in Physics, UWS School of Computing, Engineering & Physical Sciences