Annealing Effect on the Nano-meter Scale Tit.aniay Silica Multi-layers for Mirror Coatings of the Laser Interferometer Gravitational Waves Detector

Thermal noise from the mirror coatings is the dominant noise source for the Laser Interferometer Gravitational-wave Observatory (LIGO) detectors at its most sensitive frequency range of ~100 Hz. The thermal noise of the coatings is directly related to the mechanical loss of the coating materials. Reduction of the mechanical loss of the coatings both at room temperature for the current detector and at cryogenic temperature for the next generation detector is the focus of this research. Thermal annealing of the coatings is a common method to reduce the mechanical loss in many coating materials. We propose to replace the conventional quarter-wave layers in a dielectric high reflector by a stack of layers composed of alternating titania and silica layers each in the nanometer scale thickness range. We report that the crystallization temperature of the 1.8 nm titania nano-layer has increased to in between 700°C and 800°C, which allows the titania/silica nano-layer to be annealed at a higher temperature and remain amorphous. We also report that the room temperature and the cryogenic mechanical loss of the titania/silica nano-layer were reduced significantly after 600°C thermal annealing.