where g k and f k are the k th mode's magnitude. The parameters τ k , ω k and Ψ k are the k th mode's characteristic time, characteristic angular frequency and phase factor, respectively. Only the values of τ 1 and τ 2 are fixed based on the results obtained from the continuous relaxation time distribution model during the regression process. Then, the complex permittivity, ϵ(ω), is obtained by the Eq. 3. The spectral amplitude of each mode ∆ϵ i is defined by