A sapphire crystal doped with titanium ions (Ti3+) at a density of 3 x 1019 cm-3 has a strong absorption line centered on λ = 500 nm due to the titanium ions. We will assume that the refractive index of the undoped sapphire host is constant and 1.76, and that one electron per ion contributes to the resonance.
a) Given these parameters and ignoring local field corrections, find the difference in high frequency (ω → ∞) and low frequency (ω → 0) refractive indices due to the titanium ions.
b) Calculate the same, but now taking into account local field corrections. Explain in your own words which formulation of the local field correction you use and why.