To design a photonic device, one needs to know the wavelength dependence of the complex refractive index of the constituent materials. We show that refractive indices of a dispersive material can be reliably computed from the reflection and transmission spectra of a thin film of the material using an optimization method. We also apply Kramers-Kronig relation to compute the real part of the refractive index of Rhodamine B using the corresponding values of its imaginary part, computed from the transmission spectrum of a thin film of Rhodamine B. Such methods are useful to determine the refractive indices of lesser-known materials.
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