Wideband antireflection coatings of porous MgF2 films by using glancing angle deposition

In this paper, a new method to design and fabricate a wideband antireflection (AR) coating with a porous MgF2 film by using glancing angle deposition (GLAD) is proposed. Various glancing angles and substrate rotations in GLAD were employed to control the refractive indices and the microstructures of MgF2 films. The relationship between the refractive index and the glancing angle of MgF2 films deposited by using GLAD is investigated. The results show that as the glancing angle is increased from 0 degrees to 80 degrees, the porosity of MgF2 films increases due to the shadow effects, and the effective refractive index of MgF2 films drastically decreases from 1.378 to 1.185 at 550 nm. Two-layer wideband AR coatings with porous and dense MgF2 films were also deposited by using GLAD. The results show that the two-layer AR coating has low and wideband reflection for wavelengths between 400 and 1200 nm and that the average reflectance is 0.38 %. We found that porous MgF2 films deposited by using GLAD at high glancing angles provide very low refractive indices, and so are very useful for wideband AR coatings.