Variation of Transmittance Spectra of Porous Antireflection Coatings with Index Profile

摘要

Transmittance spectra were calculated for single-layer coatings with (1) homogeneous refractive index, (2) index profiles that increase monotonically with distance into the coating, and (3) index profiles that rise in discrete steps. The calculations predict that large transmittance at tuned wavelengths can be obtained using any of these coating designs and known, porous silica coating materials. The spectral bandwidth of transmittance maxima increases when the index profile is altered from homogeneous to any profile whose index increases monotonically with thickness in the coating. Bandwidth is greatest for the linear-ramp coating whose index rises linearly from that of air to that of the substrate, but this design cannot currently be used because we are unable to fabricate silica coating material with the extremely large porosity needed to provide a coating index near unity at the air interface. For currently attainable fractional silica contents, estimated to be in the range 0.4 to 0.5, the optimum bandwidth is provided by a coating with a single discrete step in its index profile. (ERA citation 10:044721)