Cecil Moore wrote:
To the best of my knowledge,
there's no mention of complex indices of refraction in _Optics_.

Born and Wolf has a chapter on the Optics of Metals. Chapter 13.1 is
called Wave Propagation in a Conductor. In it, they use a complex wave
number, complex dielectric constant, complex phase veloctity, and a
complex refractive index.

73, Jim AC6XG