A method for modeling the complex refractive index of doped, strained or ultra-thin semiconductors starts with a model for a standard bulk material which may be in any form such as a pre-existing lookup table, a dispersion model derived from an effective medium approximation (EMA) or a critical point (CP) model. The modeling method perturbs the .di-elect cons..sub.2 curve of the bulk material by enhancing, suppressing or shifting the strong features of the curve. A Kramers-Kronig transformation is then applied to the .di-elect cons..sub.2 perturbation to obtain the corresponding perturbation to the .di-elect cons..sub.1 curve. The combination of the perturbed .di-elect cons..sub.2 curve and the correspondingly perturbed .di-elect cons..sub.1 curve are then used to obtain the complex dielectric function or complex refractive index of the modified material.