ERI: Birefringence and Optical Defect Mapping of Crystalline Mirror Coatings Using a Scanning and Rastering Optical Cavity

Nontechnical description: Novel optical coatings made by techniques, such as molecular-beam epitaxy for growing crystalline coatings, need careful characterization of both optical and mechanical properties. Careful characterization allows us to develop improved mirrors for the most sensitive experiments such as world-leading precision timing and interferometric gravitational wave detection. Crystalline Al0.92Ga0.08As/GaAs (AlGaAs) mirrors are currently used in the most sensitive timing experiments and are a candidate for future LIGO upgrades. However, these mirrors suffer from birefringence effects (differences in how orthogonal polarizations of light are reflected). The sources of non-uniformity in the birefringence across the surface of the mirror are not well understood and could be a limiting factor in the sensitivity of future instruments. This award supports research that maps the birefringence across the surface of AlGaAs mirrors. The results can be used to improve the manufacturing process and estimate the birefringence noise in next-generation instruments including gravitational wave detectors. Technical description: Crystalline Al0.92Ga0.08As/GaAs (AlGaAs) mirrors serve as critical components in ultra-low noise optical cavities due to very low optical loss and 10x lower thermal noise than amorphous oxides. Therefore, AlGaAs mirrors are being considered for future upgrades of interferometric gravitational-wave detectors. This project will develop a high-finesse optical cavity where one cavity mirror, the AlGaAs mirror under test, is mounted on a translation stage to map the birefringence across the surface. By scanning through the cavity resonances at each location and measuring the resonance peak splitting, we will produce high-resolution maps of birefringence and birefringence inhomogeneities in the test mirror. Building on our prior expertise in thin-film optical characterization, resonant cavities, and optical defect mapping, the proposed research will advance understanding of the variation of birefringence across the surface of AlGaAs mirrors. The data this project will generate can be used to model birefringence-induced noise in next-generation gravitational-wave detectors and related optical systems. This work establishes a cornerstone experiment at a primarily undergraduate institution, while providing a highly significant hands-on research opportunity for students. 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.