The first direct observations of gravitational waves have been made by the Advanced LIGO detectors.
However, the quest to improve the sensitivities of these detectors remains, and epitaxially grown single-crystal
coatings show considerable promise as alternatives to the ion-beam sputtered amorphous mirror
coatings typically used in these detectors and other such precision optical measurements. The mechanical
loss of a 1 μm thick single-crystalline gallium phosphide (GaP) coating, incorporating a buffer layer region
necessary for the growth of high quality epitaxial coatings, has been investigated over a broad range of
frequencies and with fine temperature resolution. It is shown that at 20 K the mechanical loss of GaP is a
factor of 40 less than an undoped tantala film heat-treated to 600 °C and is comparable to the loss of a
multilayer GaP/AlGaP coating. This is shown to translate into possible reductions in coating thermal noise
of a factor of 2 at 120 K and 5 at 20 K over the current best IBS coatings (alternating stacks of silica and
titania-doped tantala). There is also evidence of a thermally activated dissipation process between 50 and
70 K
