Microwave plasma assisted sputtering of a combined Ta₂O₅/SiO₂ and a-Si:H/SiO₂ two stack optical coating design concept for gravitational wave detectors

In this work, a new optical coating design concept is proposed for gravitational wave detector (GWD) utilising, microwave plasma assisted sputtering of a high reflector (HR) multi-material stack coating consisting of hydrogenated amorphous silicon (a-Si:H), Ta₂O₅ and SiO₂ for GWD’s mirror coatings. This work includes the study of the optical, mechanical, structural and morphological properties of the coating to evaluate the potential of the materials for GWDs. Benefits of the microwave plasma assisted sputter deposition technique are described. a-Si is a promising alternative material for GWD mirror coatings, as it has been demonstrated to have reduced thermal noise, however, has optical absorption above the required level for GWDs. The HR multi-material coating design concept is a way potentially mediate the high absorption a-Si whilst taking advantage of its reduced thermal noise, incorporating the a-Si material into a Ta₂O₅ and SiO₂ based HR multilayer coating. Hydrogenating a-Si (a-Si:H) is another method to reduce the absorption with the hydrogen concentration being an important parameter, this work combines these techniques to further lower the absorption. The multi-material coating consists of an ‘upper stack’ of Ta₂O₅/SiO₂ low absorbing material on the incident side of the coating used to reflect the majority of the incident laser power and a ‘lower stack’ of a a-Si:H/SiO₂ higher absorbing materials at the bottom of the coating where there is less laser power to be absorbed. In addition to investigating the effectiveness of the two-stack approach for optimum compromise between optical and mechanical loss reduction, this work also studies the effect of annealing on the properties of the multi-material coating. Moreover, effects of the deposition parameters such as deposition rate on each material are investigated and utilised to optimise properties the two-stack coating approach.