This research program explores two of the primary noise sources in Advanced LIGO and other gravitational wave interferometers: upconversion noise and coating thermal noise. Coating thermal noise can be reduced by understanding and minimizing the mechanical loss in the mirror materials. Traditional high reflectivity coatings are multilayers of dielectric material, usually amorphous metal-oxides, with alternating high and low index. The materials that comprise the high-index layers are the primary source of mechanical loss; the low-index material is fused silica, which has an anomalously low loss among amorphous dielectric materials. In this research program, the PI seeks to develop a stabilized, high index material with a coefficient of thermal expansion matched to the low index material, so that the loss of the composite coating can be reduced through high temperature annealing. In addition, crystalline coatings, such as Aluminum Gallium Arsenide (AlGaAs), have been shown to have a mechanical loss about a factor 10 lower than most amorphous dielectric coatings. The AlGaAs coating could be developed as mirror materials for the 1.5 micron lasers planned for third generation detectors. However, current samples have been limited to the centimeter scale. This research program will explore the source of mechanical loss in these crystalline coatings including any issues related to the scaling and application of the coatings for large optics. Finally, upconversion noise is the coupling of low frequency noise, primarily seismic noise, into the detection band of the interferometer. Upconversion noise can be difficult to characterize because the noise peak is located at the sum or difference of the frequencies of the coupled mechanisms. This research program will develop a data analysis tool that will use bicoherence, the higher order form of coherence, to determine the sources of the upconverted noise.
Coating thermal noise is the leading noise source in the central frequency band of Advanced LIGO and is a primary limit to overall detector sensitivity. Reducing coating thermal noise leads to a direct increase in the detector sensitivity and a cubed increase in its expected event rate. Thus even a small reduction in the noise is important. A significant improvement in coating noise will hasten the day when LIGO will make a direct detection of gravitational waves and launch the era of gravitational wave astronomy. Beyond the study of gravitational waves, coating thermal noise has now become an important noise source in other areas of physics, including precision optics and in precision experiments that utilize microresonators. Finally, with the lower noise floor of Advanced LIGO, upconversion is expected to be a much more prominent noise source than in Initial LIGO. The commissioning teams will welcome a tool that can identify these noise peaks with their source. And since upconversion is a common noise problem within the precision physics community, the development of an effective tool could be useful well beyond the gravitational wave community.
