Optical behaviour of gradient-index multilayer films

Abstract

Gradient-index films (i.e. films in which the refractive index varies continuously along the normal to the film surface) have been prepared by depositing alternate layers of two materials of different refractive indices. The optical behaviour of gradient-index films of the ZnS-MgF₂ combination has been investigated. The results show that, for small step thicknesses (much less than the wavelength of light used for the measurements) of each layer, a gradient-index multilayer film behaves like an inhomogeneous film. A periodically stratified medium obtained by depositing a number of identical gradient-index ZnS-MgF₂ multilayer films behaves like a band reflecting filter. The band reflectance increases with the number of periods and tends to nearly 100% for 20 periods. Further, the reflectance band is observed to shift towards longer wavelengths on increasing the step thicknesses of the layers in the periods. The reflectivity of the stratified medium has been calculated theoretically by assuming that the gradient-index film is made of a stack of homogeneous films. The experimental and theoretical results are compared and discussed.