Electromagnetic scattering from active objects: invisible scatterers.

The scattering properties of active particles are studied and compared with those of particles with a complex conjugate (passive) index of refraction. It is shown that the extinction cross section of active particles is zero at certain frequencies and that only at certain frequency bands it is amplified. For most frequencies, the interference between diffracted and refracted waves causes the extinction cross section to behave like that of passive particles. A comparison of backscattered and forwardscattered intensities between active and passive particles with n(a) = n(p) * shows that as +/- Im(n) ? + infinity the intensities converge to the same value. For nonsymmetric scatterers, such as cylindrical fibers of elliptic cross section, it is shown that in the resonance region the major portion of the scattered field is not in the forward direction. In addition, the ratio of backscattered to forward scattered intensity is found to be greater than unity for active media for certain frequencies beyond the low frequency region.