An optical medium has a graded effective refractive index with a high
maximum refractive index change. The medium is formed using alternating
layers of two or more materials having significantly different refractive
indices. The thickness of the layers of at least one of the materials is
substantially less than the effective light wavelength of interest. The
effective index of refraction in a local region within the medium depends
on the ratio of the average volumes of the two materials in the local
region. A graded index of refraction is provided by varying the relative
thicknesses of the two materials.