Dissertation Defense: Elizabeth Gretarsson, "Parameter Characterization for Coating Thermo-optic Noise and Coating Brownian Noise in Optical Cavities with Emphasis on Crystalline GaAs/AlGaAs Coatings"

    Thursday, November 18, 2021 - 9:00am

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    Passcode: mirrors


    Thermal noise sources in optical coatings set the fundamental noise limits in optical cavities, including the LIGO gravitational wave detectors.  Characterization of these noise sources is needed to design coatings for future detector upgrades.  Ringdown measurements are used to characterize the loss in large-area coatings but rely on the assumption that the losses are amplitude independent.  Here I present enhanced ringdown measurements of crystalline candidate coatings and show that the loss is amplitude independent in the amplitude-gap between traditional ringdown measurements and rms thermal noise.  To search for excess loss, I compare my measurements to a loss model where the largest contributor is thermo-elastic loss.

    In addition to reducing coating Brownian noise, future coatings must be optimized for thermo-optic noise reduction.  These designs are sensitive to both the thermal expansion and thermo-optic coefficients of the materials.  Measurement of these properties proves difficult because thin film properties often do not match bulk properties of the same material.  I demonstrate that by measuring the wavelength shift with temperature of the transmission spectra for single- and multi- layered coatings, we can obtain a combination of these parameters and predict the level of thermo-optic noise.  I also demonstrate that by using a laser to monitor the movement of the band edge for an HR coating we can achieve a higher precision measurement, but at the cost of reduced systematic accuracy.