Gravitational waves (GW) are detected by interferometers with km-long detector arms. The sensitivity of these detectors is limited by the noise originating from the reflective coating of the detector mirrors. The state-of-the-art coating materials do not longer comply with the increasingly strict requirements regarding uniformity, high reflectivity, low optical absorption and low mechanical loss for future GW detectors. Due to the lack of a fundamental understanding of how the structure impacts the performance of the existing materials, together with the increasing design complexity of future detectors, it is still challenging to find materials and coating techniques that can meet these requirements. In this project, I aim to address these challenges. Firstly, atomic layer deposition (ALD) will be explored as a novel deposition method to tailor mirror coatings with superior uniformity for next-generation GW detectors. Plasma ALD will be explored as novel method to tailor the coating structural properties. Delta-doping will be used to decrease the mechanical loss of tantala coatings, and silica-titania nanolayered laminates deposited with ALD will be explored. Secondly, by using advanced X-ray total scattering experiments and subsequent pair distribution function analysis I aim to unravel relations between atomic structure and optical-mechanical properties. This is the fundamental insight needed to push the research in mirror coatings for GW detectors to the next level.