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

εL is the linear relative permittivity
XNL is the nonlinear susceptibility
τ is the response time
εR is Raman model permittivity
ωR is the Raman model resonant frequency

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