Shivanand and Kevin J. Webb
An exact theory describing the electromagnetic plane-wave force density on a scattering slab having various constitutive parameters and embedded in a background material with complex impedance is presented. It is shown that the constitutive parameters of the background medium contribute to the force density only through the impedance and not the refractive index. Asymptotic expressions show that the total force per unit area for sufficiently thick slabs having overall loss or gain remains positive, irrespective of the refractive index sign in the slab. However, example material responses indicate that for thin slabs the total force per unit area can be negative for both positive and negative refractive index slabs with gain.