Negative index of refraction

Patent Granted We can choose to avoid directly considering the Poynting vector and wavevector or a propagating light field, and consider instead the response of the materials directly: that is, we consider what values of permittivity ε and permeability µ result in negative phase velocity (NPV). Since both ε and µ are in general complex, their imaginary parts do not have to be negative for a passive (i.e. lossy) material to display negative refraction.