Scattering from anisotropic particles: A challenge for the optical theorem?

The optical theorem is a very general law of scattering theory that has been discussed almost exclusively for spherically symmetric scatterers. In this work we present the extension to the case of anisotropic scatterers, by treating explicitly the problem within the Rayleigh-Gans approximation. Working formulas for the fluctuating components of the forward-scattering amplitude S VV(0) and S VH(0) are given, and a paradox concerning the applicability of the optical theorem is solved. While the S VH(0) cannot interfere with the incoming vertical polarized beam, we show that S VV(0) fluctuates around a non-zero average so to compensate at any instant for the integrated scattered intensity at both polarizations. The results are relevant for the design and interpretation of experiments of dynamic depolarized light scattering in the forward direction.