Abstract
In electromagnetism, the electric displacement field D represents how an electric field E affects in a given medium. The actual permittivity ε is calculated by =ε_r ε_0=(1+χ)ε_0, where χ is the electric susceptibility of given material. The electric susceptibility χ satisfies the Kramers-Krönig relation. From the wave equation, electric susceptibility χ, the refractive index n and attenuation coefficient q have the relation χ=〖(n+iq)〗^2. However we can only measure n and q within a finite bandwidth. In this paper, we will find out a subset of the kernel of the Kramer-Krönig relation, and then use a minimization with a regularized extension to recover χ outside the measured bandwidth. We will recover the data of silicon and silver. For silver, we observe the singularity of its data χ_s and apply the KK-relation on χ-χ_(s ). For the singularity of conductors, we put the detail in the section 1.3.