Kramers–Kronig constrained variational analysis of optical spectra

A universal method of extraction of the complex dielectric function ϵ(ω)=ϵ1(ω)+iϵ2(ω) from experimentally accessible optical quantities is developed. The central idea is that ϵ2(ω) is parameterized independently at each node of a properly chosen anchor frequency mesh, while ϵ1(ω) is dynamically coupled to ϵ2(ω) by the Kramers–Kronig (KK) transformation. This approach can be regarded as a limiting case of the multioscillator fitting of spectra, when the number of oscillators is on the order of the number of experimental points. In the case of the normal-incidence reflectivity from a semi-infinite isotropic sample the new method gives essentially the same result as the conventional KK transformation of…