A.W. Lussier¹, A. Markosyan³, S.C. Tait⁴, M. Chicoine¹, B. Baloukas², S. Roorda¹, M.M. Fejer³, G. Vajente⁴, L. Martinu ², F. Schiettekatte^1
1Université de Montréal, Montréal, Canada
²Polytechnique Montréal, Montréal, Canada
³Stanford University, Stanford, CA
⁴California Institute of Technology, Pasadena, CA
In their most sensitive frequency range, current gravitational wave detectors (GWD) like LIGO are limited by coating thermal noise (CTN) a phenomenon due to the structural fluctuation in amorphous materials and linked to internal mechanical dissipation (IMD). GWDs feature Bragg mirrors made of amorphous oxides, located at the ends of the 4 km long Michelson interferometer, possess very low absorptance (<1 ppm) and scattering (<100 ppm), while minimizing the CTN. Indeed, random fluctuations of the mirror’s surfaces induce a noise comparable to the shot noise in these instruments.
In order to reduce CTN for the next generations of GWDs, we investigate the possibility of fabricating Bragg reflectors from materials possessing the lowest possible IMD. In particular, this research focuses on hydrogenated amorphous silicon (a-Si:H) and amorphous silica (a-SiO2). Indeed, a-Si presents a low IMD, similar to a-SiO₂ and the pair features a very high refractive index contrast, requiring less materials, both aspects contributing to reduce CTN.
However, the absorption of a-Si is typically too high, especially at the current operating wavelength of 1064 nm. a-Si:H has a similar structure to a-Si but has a much smaller absorption potentially compatible with the high-power laser of these instruments. However, its IMD has never been studied in detail within the most common hydrogen concentration range of 5 to 30 at. % at room temperature.
Our project aims at investigating how to simultaneously lower the total absorption of a Bragg mirror containing a-Si:H, while increasing the refractive index and lowering the IMD of a-Si:H through the optimization of deposition conditions and post-treatments. Currently, our best a-Si:H features an absorption coefficient of 0.15-0.30 cm^-1 at 1550 nm and an IMD 8-16 times lower than the high-index material in current LIGO mirrors. Stacks with a-SiO₂ feature an absorptance of 7.0 ppm at 1550 nm and 2.75 ppm at 2000 nm. Sufficiently low absorption could be obtained if coupled to a few layers of another, low-absorption material at the top of the reflector.