  Thermal noise is a limiting factor in many high-precision optical
experiments. A search is underway for novel optical materials with reduced
thermal noise. One such pair of materials, gallium arsenide and
aluminum-alloyed gallium arsenide (collectively referred to as AlGaAs), shows
promise for its low Brownian noise when compared to conventional materials such
as silica and tantala. However, AlGaAs has the potential to produce a high
level of thermo-optic noise. We have fabricated a set of AlGaAs crystalline
coatings, transferred to fused silica substrates, whose layer structure has
been optimized to reduce thermo-optic noise by inducing coherent cancellation
of the thermoelastic and thermorefractive effects. By measuring the
photothermal transfer function of these mirrors, we find evidence that this
optimization has been successful.
