Abstract
We report on the development and extensive characterization of co-sputtered tantala–zirconia (Ta2O5-ZrO2) thin films, with the goal to decrease coating Brownian noise in present and future gravitational-wave detectors. We tested a variety of sputtering processes of different energies and deposition rates, and we considered the effect of different values of cation ratio η = Zr/(Zr + Ta) and of post-deposition heat treatment temperature T a on the optical and mechanical properties of the films. Co-sputtered zirconia proved to be an efficient way to frustrate crystallization in tantala thin films, allowing for a substantial increase of the maximum annealing temperature and hence for a decrease of coating mechanical loss φ c. The lowest average coating loss was observed for an ion-beam sputtered sample with η = 0.485 ± 0.004 annealed at 800 °C, yielding rad. All coating samples showed cracks after annealing. Although in principle our measurements are sensitive to such defects, we found no evidence that our results were affected. The issue could be solved, at least for ion-beam sputtered coatings, by decreasing heating and cooling rates down to 7 °C h−1. While we observed as little optical absorption as in the coatings of current gravitational-wave interferometers (0.5 parts per million), further development will be needed to decrease light scattering and avoid the formation of defects upon annealing.
Highlights
The detection of gravitational-wave (GW) signals from astrophysical systems [9] has ushered in a new era in astronomy [10, 11], allowing the study of previously invisible systems
In the most sensitive region of the detection band of Advanced LIGO [7] and Advanced Virgo [15], between 50 and 300 Hz, the sensitivity is limited by an equal contribution of quantum noise [26] and thermal noise, the latter being dominated by coating Brownian noise [72]
This paper focuses on collaborative work, carried out by several groups within the LIGO and Virgo Collaborations to produce and characterize co-sputtered tantala–zirconia thin films, for possible use as high refractive index layers for the new sets of mirrors of the Advanced LIGO + and Advanced Virgo + detector upgrades
Summary
The detection of gravitational-wave (GW) signals from astrophysical systems [9] has ushered in a new era in astronomy [10, 11], allowing the study of previously invisible systems. The sensitivity of current detectors to signals from very large and massive astrophysical systems is limited by random noise in microscopic systems, that is, fluctuations resulting from the internal mechanical dissipation in the material constituting the mirrors used as test masses [42]. Marginal or no improvement is necessary for the low refractive index material For this reason, most of the coating research in the Virgo and LIGO collaborations focused on finding an alternative material to replace the tantala–titania layers [33, 84], while keeping the same outstanding optical properties (absorption, scattering, etc) of the HR coatings. Higher annealing temperatures (up to 900 ◦C) and duration would be beneficial for the silica layers, decreasing their mechanical loss [17, 31] as a result of structural relaxation, leading to HR coatings with overall lower loss angle and lower thermal noise.
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