Thin-film garnet materials with zero linear birefringence for magneto-optic waveguide devices (invited)

Abstract

The many advantages of thin-film waveguide magneto-optic devices have been difficult to realize because of the linear birefringence of these films, which makes it very difficult to achieve the required Faraday rotation. In epitaxial garnet films, this linear birefringence (the difference between the refractive indices of the TE and TM modes) can be reduced to small values by (1) growing single-mode multilayer films to minimize the shape effect, (2) growing the films in compression to control the photoelastic effect, and (3) annealing at high temperatures to eliminate the growth-induced effect. The remaining birefringence can be reduced to zero by growing the top active layer too thick so that the shape effect is smaller in magnitude than the stress effect, and then thinning it by chemical etching until the effects exactly cancel each other at a particular wavelength. An alternative method is to grow the top layer too thin and then to deposit a dielectric layer such as silicon nitride of the proper thickness to reduce the shape effect so that it exactly cancels the stress effect. Theoretical results on multilayer magneto-optic waveguides and successful etch-tuning experiments at 1.32 and 1.51 μm are presented. These tuning procedures can produce polarization-independent waveguide devices such as isolators or polarization rotators which may replace bulk or thick-film devices.