Abstract
The Laser Interferometer Gravitational Wave Observatory (LIGO) detects gravitational waves with a large Michaelson interferometer and is used to observe astronomical phenomena. Signal of gravitational wave is weak, it is important that the noises should be reduced. The sensitivity of the Laser Interferometer Gravitational wave Observatory (LIGO) is mainly limited by coating Brownian noise at 100 Hz. This noise comes from the high reflective optical coating on the mirror. According to the fluctuation-dissipation theorem, noise of the film is proportional to mechanical loss, thus the level of thermal noise can be known from measuring the mechanical loss of the film. To measure mechanical loss, one can let an excited silicon cantilever ring down decay, while observing the decay time to obtain mechanical loss. Furthermore, from Fluctuation-dissipation theorem, it can also be known that system temperature is proportional to thermal noise, thus it is desirable to know the property of the film material under low temperature. To increase the system's sensitivity, thermal noise on the reflecting film must be minimized; therefore, we attempt to search for a material for the film that has low mechanical loss, and can be measured at low temperature. The first part of the paper describes a new cryogenic apparatus which allows the measurement of the mechanical Q-factor – as a measure of internal losses – in a temperature range from 5 K up to 300 K. This closed-loop cryogenic system is able to measure mechanical loss of the coatings on the cantilevers. And the whole measurement process is fully automatic without human attendance. The second part is crystallization following thermal annealing of thin film stacks consisting of alternating nm-layer(Titania/Silica) was investigated. Coatings of the mirror is composed of pairs of alternating high and low refractive index thin films with thickness of quarter-wavelength. Currently, the materials used in the coatings are silica and Titania in amorphous. It was found that thermal annealing also reduces coating mechanical losses, and thermal noise . However, excessive thermal annealing eventually leads to crystallization. Mechanical losses due to friction among crystallites, as well as scattering from the grain boundaries, make the coatings unsuitable. It was found that the Titania layers eventually crystallized forming the Anatase phase. However, progressively thinner layers exhibited progressively higher threshold temperatures for crystallization onset. Accordingly it can be expected that composites with thinner layers will be able to sustain higher annealing temperatures without crystallizing. In order to increase the annealing temperature, we use nano-multilayer film structure to replace the high reflector of the high refractive index layer. The structure of nano-layer has the same total optical thickness. Notice that nano-layer with more numbers of layers has thinner TiO2 layer thickness, which translates into tolerance for higher annealing temperature. Then, apply different thermal annealing processes to the structure of the 11,15,19 layer of the nano-layer and discuss how the annealing of each structure affects mechanical loss. Key word: mechanical loss, cantilever, closed-loop cryogenic system, low vibration noise, temperature gradient, clamping torque