Abstract(s)
The noise caused by internal mechanical dissipation in the high refractive index amorphous
thin films in dielectric mirrors is an important limitation for gravitational wave detection. The
objective of this study is to decrease this noise spectral density, which is linearly dependent on
such dissipation and characterized by the loss angle of the Young’s modulus, by adding zirconia to
titania-doped tantala, from which the current mirrors for gravitational wave detection are made.
The purpose of adding zirconia is to raise the crystallization temperature, which allows the material
to be more relaxed by raising the practical annealing temperature. The Ta, Ti and Zr oxides
are deposited by reactive magnetron sputtering in an Ar:O2 atmosphere using radio-frequency
and high power impulse plasma excitation. We show that thanks to zirconia, the crystallization
temperature rises by more than 150◦C, which allows one to obtain a loss angle of 2.5 × 10−4
, that
is, a decrease by a factor of 1.5 compared to the current mirror high-index layers. However, due to
a difference in the coefficient of thermal expansion between the thin film and the silica substrate,
cracks appear at high annealing temperature. In response, a silica capping layer is applied to
increase the temperature of crack formation by 100◦C.