Thin films for non-reciprocal magneto-optic devices

Abstract

Magneto-optic devices such as isolators, circulators, switches and field sensors can be made with thin films of transparent magnetic materials. The materials of choice for optical communications applications are bismuth-doped rare earth iron garnets, similar to those which were developed for magnetic bubble memories, but with planar rather than uniaxial magnetic anisotropy. These garnets can have a very low optical absorption and large Faraday rotation at the important wavelengths of 1.3 and 1.55 μm. Films a few optical wavelengths thick, grown by liquid phase epitaxy on single-crystal substrates such as gadolinium gallium garnet, can be used in the waveguide mode. Advantages over bulk single crystals or thick films include ease and versatility of crystal growth, lower bias field requirements, guided optical modes throughout to eliminate the need for lenses and difficult alignment procedures, and compatibility with integrated optics. The major disadvantage is the optical birefringence inherent in thin films which interferes with the Faraday effect and must be minimized to produce useful devices. Waveguide isolators with ratios of between -32 and -37 dB over a broad band of wavelengths near 1.5 μm have been reported. This was achieved with triple garnet films, each a few microns thick, with carefully controlled magnetic and optical properties. Practical applications will depend on further improvements in these properties and their variation with temperature. For shorter wavelengths, where the iron garnets have high optical absorption, other materials such as diluted magnetic semiconductors ( e.g. Cd x Mn 1− x Te) in thin film form will be required.