The Search for an Amorphous Coating with Low Mechanical Loss and High Refractive Index

The next major upgrade to gravitational wave detector LIGO, called Advanced LIGO+ or A+, is planned to operate at room temperature and requires 4 times improvement in coating mechanical loss to meet its design sensitivity. The coatings are made up of alternating layers of high and low refractive index materials to make the mirrors highly reflective. The current coatings have amorphous silica as the low index layer and titania-doped tantala as the high index layer. The mechanical loss of high index layer dominates the coating loss and the thermal noise requirement for A+ could be met by finding a high index layer with low enough mechanical loss. In this talk, I will describe the research efforts aimed at finding such a coating. In particular, I will talk about how the atomic structure measurements and modeling of amorphous oxides are being used to guide the coating development process. I will highlight some interesting correlations in the structure and loss measurements and draw some empirical conclusions. Finally, I will discuss the thermally activated relaxation processes (TARP) in amorphous oxides based on atomic simulations of two-level systems (TLSs). TARP is believed to be the source of mechanical loss in amorphous materials.