Hi Mohammad,
Sorry for my late reply due to my trips… Again, as far as I understand your research problem, my first approach would be using dispersion equation for Harmonic Oscillator from Lorentz Model. I’m attaching a paper which describes this model briefly. When you open it, take a look at eqns (6)-(9) which directly related to Lorentz Model.
I can recommend famous book E. Hecht “Optics” for more details as well.