General proof of optical reciprocity for nonlocal electrodynamics

Recent studies have established certain specific results concerning the optical reciprocity of a source and an observer in the presence of a dielectric medium with a nonlocal response (i.e. spatial dispersion). These include the case of a linear dielectric response with dependence on the electric field gradient, as well as the case of a general anisotropic nonlocal medium in the long wavelength quasistatic limit. Here we present a more rigorous study of this problem by extending the previous results to the most general case of an anisotropic nonlocal medium in the context of exact electrodynamics with reference to the symmetry of the Green dyadic. It is established that for nonmagnetic materials within linear optics, reciprocity will remain valid as long as the dielectric tensor satisfies the condition . Possible applications of the results are briefly discussed. © 2009 IOP Publishing Ltd.