The linear susceptibility is the frequency response of a linear system to an applied electric field, which is causal, and therefore real and imaginary parts obey Kramers-Kroenig Relations \[\chi_r (\O...The linear susceptibility is the frequency response of a linear system to an applied electric field, which is causal, and therefore real and imaginary parts obey Kramers-Kroenig Relations χr(Ω)=2π∫∞0ωχi(ω)ω2−Ω2dω=n2(Ω)−1,χi(Ω)=−2π∫∞0Ωχr(ω)ω2−Ω2dω.
