Description
The sensitivity of 3rd generation gravitational wave detectors is currently projected to be
limited by the level of Brownian coating thermal noise produced from highly reflecting
mirror coatings. In the current detectors layers of Ta$_2$O$_5$, a material with higher
refractive index (n = 2.14), has higher levels of optical absorption compared the lower index
layers of SiO$_ {2}$ (n=1.44). To improve detector sensitivity and duty cycle, the optical and
mechanical properties of new doped coating materials are being investigated. This work
presents measurements of the optical absorption of SiO$_2$ and SiO$_2$ doped with
TiO$_2$ layers as part of a highly reflecting coating stack. Utilising the photothermal
common-path interferometry technique, we discuss the effects of heat treatment and
crystallisation on the optical performance of the material compared to current aLIGO
coatings.
