On measurements of the refractive index dispersion in Porous Silicon

Attention is drawn to the limited applicability of the well-known simplified expressions for determining the dispersion of the refractive index in thin films from the Fabry-Perot interference. It is emphasized that, instead of the true curve n(λ), which characterizes the change in the phase velocity of light, one may arrive at the λ dependence of the group velocity of a wave packet. A method is proposed to extend the technique for measuring n(λ) with the application of a Jamin interferometer combined with a spectrograph in the case when the zero-order fringe falls outside the field of the spectrograph.