LIGO Document T2100186-v1

Effective Medium Description of Multilayer Coatings

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T - Technical notes
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The elastic properties of multilayer coatings determine the thermal noise imposed on beams reflected from interferometer mirrors. Those properties are not simple averages of the properties of the individual layers making up the coating, and the averaged properties that enter into modeling thermal noise are different from those entering into measurement of the loss via the Q of mechanical resonators. It is the purpose of this document to establish a unified description of these elastic multilayers through the use of an effective medium theory for both isotropic (amorphous) and cubic (AlGaAs, AlGaP) layers that is applicable both for noise calculations and for analyzing loss measurements.
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