Optical-constants model for semiconductors and insulators

The refractive index and the extinction coefficient, i.e., the real and imaginary parts of the complex index of refraction, for semiconductors and insulators are derived as a function of photon energy. These derivations apply f-sum rules and symmetry relations to critical-point transitions from the valence band to the conduction band. Comparison with measured optical data reveals that present formulations are valid over a wide range of photon energies immediately above the band gap to the first ionization threshold for inner shells. The present work shows an improvement and extension over the theory of Forouhi and Bloomer, which applies for a narrow range of photon energies above the absorption edge.