Investigation of the refractive index profile of inhomogeneous iron garnet films containing bismuth

The refractive index n of quasihomogeneous iron garnet films with the composition
(YGdYb)_3 – xBi_xFe_5 – yAl_yO₁₂ increased considerably as a result of reduction in the growth temperature T_g. This was attributed mainly by an increase in x and a reduction in y. The dependences n(T_g) were used to select the conditions for growth of inhomogeneous (across the thickness) films and to predict the refractive index profile n(z), where z was measured from the free surface along the normal to the film. The spectra of optical modes were analyzed in the WKB approximation and this yielded the n(z) profiles for films with a refractive index n decreasing monotonically from the free surface and for films with the symmetric profile n(z) (buried waveguides). The n(z) profiles were compared with the distributions of the compensation temperature T_c(z), and of x(z) and y(z) across the thickness.