Sum-rule constraints in reflectance extrapolation for Kramers-Kronig analysis

Abstract
A criterion is given by which the quality of an extrapolation may be judged when using Kramers-Kronig analysis of reflectance data to obtain the optical constants. The method is particularly useful is the case of small-band-gap materials where the more usual constraints present difficulties. Sum rules on the calculated optical constants are used as guides in choosing an adjustable parameter in the extrapolation function. An analytical model is used to develop a constraint condition with which the adjustable parameter may be chosen. The method is applied to the KK calculation of the optical constants of Cd3As2.