Amorphous mirror coatings for ultra-high precision interferometry

The dominant noise source in aLIGO is Brownian thermal noise, due to mechanical losses in the atomic structure of the amorphous titania doped tantala end test-mass mirror coatings. This thesis investigates the structural source of these losses. The effect of titania doping and thermal annealing upon the atomic structure of amorphous tantalum pentoxide coating preparations are studied using advanced electron diffraction techniques. Significant differences between the coating atomic structures have been identified for the first time in detail. The tantala based coatings studied have been demonstrated as better described by a heterogeneous phase separated model, rather than the continuous random network model for covalently bonded amorphous metal-oxides. The short-range ordering (SRO) of the coating atomic structures was investigated using pair-distribution function analyses, with an upper limit found to be ~4 Å. Correlations spanned ~9 Å, and have been related to model structures; between 4 - 5 Å, correlations were identified as signatures for 3D structural ordering. Fluctuation Electron Microscopy (FEM) was employed to investigate the MRO of the coating atomic structures. A novel approach to FEM was developed by the author during this PhD, in which the structural variance was computed using normalised cross-correlation coefficients. This made absolute intensity irrelevant, with the shape and the spatial distribution of the diffracted intensity taking precedence. The method is insensitive to poor SNR, illumination conditions, slight differences in experimental facility, and slight thickness variations in the samples. Virtual Dark-Field (VDF) imaging was adapted to amorphous structures in novel ways for the first time in this thesis. Simultaneous representation of the FEM data in real and reciprocal space, spatially resolved the structures responsible for the FEM signal. Correlation analyses were performed between VDF images of the structural ordering that relate to specific atom-pair correlations, including the use of novel annular variance images. The images and correlations clearly highlight the heterogeneous ordering and phase separation within the structures. Mechanisms responsible for the coating mechanical losses have been proposed, relating to the MRO, tensile-stress, as well as its reduction by titanium doping