LIGO Document G1700345-v1

Quality Factor Measurements of Materials for Gravitational Wave Detector Mirror Coatings

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One of the main limiting factors in detection of gravity waves at LIGO is the mechanical losses in the reflective coatings deposited on the test masses. These losses are due to thermal exchanges with the environment and their magnitudes are determined by the Q factor of the coating system.
In the past, the Q factor of various coatings and coating methods has been measured when they have been deposited on thick test masses. This causes much of the measurement of the Q to be dominated by the substrate and not by the coating itself. Our plan is to coat the front and back of silicon nitride membranes that are 100 nm thick. By coating both sides of such a thin material we should be able to measure the Q factor of the coatings alone.
We have constructed and will be implementing the use of a small Michelson interferometer that will have the coated membrane at the end of one arm and a static mirror at the other. We will kick the membrane with an electrode and observe the time it takes for our laser output to ring down. This is how we will measure the Q factor for the coating.
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