Rugate

Unifacial, superior surface transversely rugate, with minute oririmes at the bottom of the furrows

Source: wiktionary

Rugate coatings typically consist of a single layer deposited onto a substrate where the refractive index of the layer continuously changes as a function of depth.

Source: wiktionary

Researchers have focused on various manufactured periodically nonhomogeneous dielectric materials, including rugate filters and periodically multilayered materials.

Source: wiktionary