@abc.abstractproperty<EOL><INDENT>def _eps(self, wavelength=None):<DEDENT>
return NotImplementedError<EOL>
The permittivty of the desired material.

Each new material needs this method implemented.

Args:
    wavelength (float, list, None): The wavelength the permittivty
        will be evaluated at.

Returns:
    float, list: The permittivty at the target wavelength.