We present spectra of transmittance, reflectance, and Faraday rotation of transmitted and reflected light for a periodic garnet multilayer structure with a central defect layer. The multilayer consists of alternating layers of bismuth and yttrium iron garnet, is 1.5 μm thick, and was prepared by pulsed laser deposition. For the reflection measurements, a silver mirror was evaporated on top of the multilayer. Faraday rotation is strongly enhanced at resonances in transmission and reflection. The peak value obtained at 748 nm in transmission is 5.3 deg and at 733 nm in reflection is 18 deg. A single layer BIG film of equivalent thickness shows 2.2 deg Faraday rotation at 748 nm. We find rather good agreement between measured and calculated spectra. Using calculations of the distributions of light intensities at different wavelengths inside the multilayer, we are able to give consistent qualitative explanations for the enhancement of Faraday rotation. We also find numerically that - at moderate strengths of the optical resonances - a linear relation exists between Faraday rotation and the intensity integrated over all magneto-optically active layers, if absorption is neglected.
We suggest to modify the usual sensor film for magneto-optical imaging by introducing a Bragg mirror consisting of heteroepitaxial garnet layers between the substrate and sensor film. For one example situation, we show by calculation that the quality factors of image contrast and optical efficiency can be higher for heteroepitaxial garnet multilayers than for single-layer iron garnet films currently in use as sensor films.