Optical Path and Birefringence Characterization of Crystalline Coatings for Gravitational-wave Detectors

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This award supports studies of thermal and optical properties of aluminum gallium arsenide (AlGaAs) crystalline mirrors. Crystalline mirrors are extremely thin (micrometer-scale) monocrystalline coatings laid on top of a suitable substrate such as fused silica glass. These crystalline coatings have the potential to significantly reduce the noise in interferometric gravitational wave detectors like those operated by the Laser Interferometer Gravitational-Wave Observatory, LIGO. The Advanced LIGO detectors inaugurated the field of gravitational wave astronomy with dozens of detections in the last five years. To increase the astrophysical reach of the detectors, both near-term and future upgrades are being planned. The work performed under this award supports these upgrades. It also supports indirect development of science through the training of future scientists, and contains a significant outreach component to high-school students and senior citizens.Coating thermal noise (CTN) in currently available mirror coatings now constrains our ability to increase the astrophysical reach of upgraded detectors. Crystalline AlGaAs coatings have the potential to solve this problem since they can reduce CTN by a factor of at least five. Yet, significant work needs to be done to realize an AlGaAs coating suitable for installation in interferometric gravitational wave detectors. Most of the studies are aimed at characterizing AlGaAs coating birefringence, birefringence uniformity across the coating, thermo-optic effect, and thermo-optic uniformity. A specialized Michelson interferometer will be constructed to map birefringence over the full face of 3 inch diameter AlGaAs coatings. Also, thermo-optic noise cancellation relies on uniformity of the thermo-optic effect across the coating. Existing thermo-optic apparatus will therefore be modified to map the thermo-optic effect over the full coating. The broader impacts of the proposal include the involvement of cross-generational teams in scientific inquiry. Project “Tap Into Spacetime” will be launched where teams constituted of at least one senior citizen and one or more high school students will work together to build a capable, 5-30 MHz, antenna. They will use free software tools developed for radio reception to perform spectrographic and aural analysis of the data received by their antenna. This activity "graduates" them to perform a similar analysis of public gravitational wave data using the same tools. Each signal has a distinct aural character and spectrographic signature that together give clues about its origin. The teams will find, save, and categorize such events and features. They discuss their findings on a dedicated Slack workspace. The teams will report their results with talks at Embry-Riddle Aeronautical University and some will attend a small scientific meeting such as the Society of Amateur Radio Astronomer’s annual meeting or the regional APS four-corners meeting.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.