Dispersive 2nd-Order Nonlinear Material

In this model, the electrical displacement D is

where ε_L is the linear relative permittivity and X⁽²⁾ is the second order isotropic
susceptibility. They are the real values. In order to simulate second order nonlinear
effect, you should input two parameters: the linear relative permittivity  ε_L and the
second order isotropic susceptibility X⁽²⁾.