Antireflection

Antireflection is a set of techniques used to minimize reflections at optical interfaces, thereby increasing transmitted light, contrast, and overall efficiency. It can be achieved with thin-film coatings or by structuring the surface itself to alter its effective refractive index.

The standard approach uses interference in dielectric thin films. For a simple single-layer coating designed for a specific wavelength, the film thickness is approximately one-quarter of that wavelength in the coating (t ≈ λ/4n_c). A coating with a refractive index near the geometric mean of the adjacent media (n_c ≈ sqrt(n_substrate · n_air)) can substantially cancel reflections at that wavelength. More practical designs employ multiple alternating layers of high and low refractive index materials to broaden the antireflection band and tolerate angle and polarization changes.

Beyond conventional dielectric stacks, other approaches include gradient-index coatings, where the refractive index changes gradually through the layer, and subwavelength surface textures (moth-eye structures) that suppress reflection by providing a gradual impedance transition without distinct interfaces.

Materials and fabrication commonly involve combinations of low-index materials such as magnesium fluoride or silicon dioxide and high-index materials like titanium dioxide or tantalum pentoxide. Deposition methods include evaporation, sputtering, and chemical vapor deposition, chosen to match substrate properties and environmental durability.

Applications span camera and eyeglass lenses, solar panels, displays, and optical sensors. Performance is typically characterized by residual reflectance across the operating band and angle of incidence; coatings that perform well near normal incidence may be less effective at oblique angles or off-design wavelengths. The field has evolved since the early 20th century and remains essential in modern optics.