Optical coatings for gravitational waves detectors

4th August 2017, 12.30.43 Italian time: A gravitational wave crossed the Earth. To generate it, the fusion of two black holes (with 31 and 25 solar masses, respectively, and 1.8 billion light years away from us) was necessary. The total mass of the new black hole produced by the fusion was 53 solar masses: this means that in the process, 3 solar masses have been converted into gravitational wave energy. To detect the gravitational waves there are two Michelson interferometers: Two LIGO antennas in USA and Virgo antenna in Italy. Since the expected signals would be extremely weak, reducing the instrument “background noise” is mandatory. In the most sensitive frequency band (50 ÷ 300 Hz), where the first gravitational signals have been detected the sensitivity of the presently operating detectors (Virgo and LIGO) is dominated by the thermal noise in the highly reflective optical coatings of the cavity mirrors. Several strategies have been proposed in order to reduce coating thermal noise, such as optimizing the coating design (layers’ thicknesses) and/or materials, using nanolayered mixtures of different amorphous materials or crystalline compounds. We study innovative reflective optical coatings, based on dielectric oxides (TiO2, SiO2, ZrO2, NbO2,...), obtained by starting from a Bragg-like mirror configuration (alternance of two materials with high and low refractive index and thickness equal to a quarter of wavelength /4). In particular we optimize the design with respect to the thermal noise by reducing the thickness of the material with higher optical losses and replacing homogeneous layers with stratified nano-composites. The main advantage of this strategy is the inhibit of crystallization upon annealing. Indeed, while on one side the annealing process is necessary to relax intrinsic material stress/strain, to guarantee smooth surfaces and interfaces, and to set-up the correct material stoichiometry, on the other it induces the formation of crystallites, which will work as scattering centres for the light, with a consequent blow up of optical losses and thermal noise. Surface roughness, layer thickness and crystallinity of the as-grown multilayers and post annealing samples have been investigated by means of atomic force microscopy (AFM) and by high resolution X ray diffraction (XRD).