Dielectric V-ridge gratings: transition from antireflection to retroreflection.

We present a systematic study of the optical properties of dielectric gratings with symmetric V-shaped ridges having a 90 degree apex angle and refractive index n. Such structures exhibit completely different optical properties if the dimensions are scaled with respect to the vacuum wavelength λ ₀ of light. In the subwavelength domain, where the grating period d is less than λ ₀ /n, the grating behaves as an antireflection layer. In the large-period domain d≫λ ₀ (with normal incidence from the dielectric side), the grating turns into a micro-retroreflector array. The transition between these well-known domains is studied using rigorous diffraction theory. The results are verified experimentally by fabricating and characterizing V-profile gratings with the aid of wet etching of silicon using a process that defines a 90 degree apex angle and replication into a polymer.