  At the planar interface between a material and vacuum, the complex surface
response S(omega)=[eps(omega) 1]/[eps(omega)+1], with eps(omega) the relative
complex dielectric permittivity of the material, exhibit resonances, typical of
the surface polariton modes, when eps(omega) ~ 1. We show that for a moderately
sharp resonance, S(omega) is satisfactorily described with a mere (complex)
Lorentzian, independently of the details affecting the various bulk resonances
describing ) eps(omega). Remarkably, this implies a quantitative correlation
between the resonant behaviors of Re[S(omega)] and Im[S(omega)], respectively
associated to dispersive and dissipative effects in the surface near-field. We
show that this "strong resonance" approximation easily applies, and discuss its
limits, based upon published data for sapphire, CaF2 and BaF2. Extension to
interfaces between two media or to a non planar interface is briefly
considered.
