Abstract
In 1915, gravitational waves in the theory of general relativity has been propounded by Albert Einstein, after that, in 1974, Hulse and Taylor had indirectly observed the existence of gravitational waves in a binary pulsar system. However, no one had directly confirmed its existence until 14th January 2015: the two detectors located at Hanford, WA and Livingston, LA of the Laser Interferometer Gravitational-Wave Observatory (LIGO) had simultaneously observed a gravitational wave. The finding has not only confirmed the existence of gravitational wave in the theory of general relativity, but also contributed greatly to astronomical observations. However, the signal of gravitational wave is extremely weak, so the noise should be reduced as low as possible. Much research in our laboratory has been devoted to investigating the coating Brownian noise, which is the thermal noise caused by the optical coating on the high reflective mirror. According to Fluctuation-Dissipation theorem, the thermal fluctuation (thermal noise) is proportional to mechanical dissipation (mechanical loss) in the material. Therefore, as long as we measure the mechanical loss of films, we can deduce the thermal noise of films as well. We investigated plasma-enhanced chemical vapor deposition (PECVD) method aimed for LIGO to deposit HR mirror coating. It has many advantages such as low processing temperature, fast deposition, and good uniformity for film. So far, amorphous silicon (a-Si), silicon dioxide (SiO2), and silicon nitride (SiNx) deposited by PECVD method had been studied. The different compositions of silicon nitride (SiNx) films can be deposited by PECVD with different gas flow ratio between SiH4 and NH3 while the stress and refractive index varied with nitrogen concentration. In this research, we selected SiO2 to be low index material and SiN0.40 to be high index material for optical coating because of the high index difference and stress compensation between these two materials. Therefore, we investigated the mechanical loss of SiN0.40/SiO2 quarter-wave (QW) stacks from 1 to 4 pairs at room temperature which were deposited by PECVD system furthermore the QW were the thickness of a quarter wavelength at 1550nm. The results of this study, the mechanical loss of SiNx/SiO2 stacks with 1 to 4 pairs were investigated at room temperature, and the loss were in 10-5 order at 100 Hz, lower than the material used in current gravitational wave detector. The most important thing is that the coating loss would more reduce by thermal annealing and thus SiNx/SiO2 is worthy of further study as HR mirror coating for LIGO.