Dear Mohammad,
I’m a little confused what kind of relations you are looking looking for… Since you are studying the application of Electro-optic effect for modulation, I’d refer you to look at Pockel’s effect phenomenon where the refractive index is changing linearly with the electric field applied. In my opinion, books Kasap “Optoelectronics and Photonics: Principles and Practices” and Agrawal “Fiber-Optic Communication Systems” are good sources for review.