A diffractive lens works by having small holes in a plate, at least as I understand it. The diffractive pattern then interferes constructively at the focal poin

A diffractive lens works by having small holes in a plate, at least as I understand it. The diffractive pattern then interferes constructively at the focal point. So the transparent material is just 'air' or more likely, vacuum. It doesn't need to be solid. The problem I forsee is that it doesn't let much light through. Because most of the lens isn't holes, but instead solid non transparent material.