  We present a formulation for the pole expansion of the scattering matrix of
open optical resonators, in which the pole contributions are expressed solely
in terms of the resonant states, their wavenumbers, and their electromagnetic
fields. Particularly, our approach provides an accurate description of the
optical scattering matrix without the requirement of a fit for the pole
contributions or the restriction to geometries or systems with low Ohmic
losses. Hence, it is possible to derive the analytic dependence of the
scattering matrix on the wavenumber with low computational effort, which allows
for avoiding the artificial frequency discretization of conventional
frequency-domain solvers of Maxwell's equations and for finding the optical
far- and near-field response based on the physically meaningfull resonant
states. This is demonstrated for three test systems, including a chiral
arrangement of nanoantennas, for which we calculate the absorption and the
circular dichroism.
