  We study the interaction of an electromagnetic field with a non-absorbing or
absorbing dispersive sphere in the framework of complex angular momentum
techniques. We assume that the dielectric function of the sphere presents a
Drude-like behavior or an ionic crystal behavior modelling metallic and
semiconducting materials. We more particularly emphasize and interpret the
modifications induced in the resonance spectrum by absorption. We prove that
"resonant surface polariton modes" are generated by a unique surface wave,
i.e., a surface (plasmon or phonon) polariton, propagating close to the sphere
surface. This surface polariton corresponds to a particular Regge pole of the
electric part (TM) of the S matrix of the sphere. From the associated Regge
trajectory we can construct semiclassically the spectrum of the complex
frequencies of the resonant surface polariton modes which can be considered as
Breit-Wigner-type resonances. Furthermore, by taking into account the Stokes
phenomenon, we derive an asymptotic expression for the position in the complex
angular momentum plane of the surface polariton Regge pole. We then describe
semiclassically the surface polariton and provide analytical expressions for
its dispersion relation and its damping in the non-absorbing and absorbing
cases. In these analytic expressions, we more particularly exhibit
well-isolated terms directly linked to absorption. Finally, we explain why the
photon-sphere system can be considered as an artificial atom (a ``plasmonic
atom" or "phononic atom") and we briefly discuss the implication of our results
in the context of the Casimir effect.
