Optical properties of a thin-film stack illuminated by a focused field

Reflectance (R), transmittance (T), and absorptance (A) are calculated for a thin-film stack illuminated by a focused field. Based on Debye's integral representation, the electric and magnetic fields near focus are obtained, and the formulas for R, T, and A are represented as integrals of Poynting vectors. This formulation is applied to the case of a numerical aperture (N.A.) greater than 1.0 as well as to the case of a N.A. less than 1.0, and the corresponding numerical results are presented. They reveal that R, T, and A vary with N.A. and that the amount of variation increases with layer thickness.