Raman model allows another way to simulate the nonlinear phenomenon where the
nonlinear susceptibility was modeled by a second-order derivative equation which is
related to the resonant wavelength and the response time

FDTD - equations 37 - 39

where

εLis the linear relative permittivity
XNLis the nonlinear susceptibility
τis the response time
εRis Raman model permittivity
ωRis the Raman model resonant frequency

In order to simulate the Raman effect, set four parameters for Raman model: εL, εR,
τ, and ωR.