  It is well known that the quasinormal modes (or resonant states) of photonic
structures can be associated with the poles of the scattering matrix of the
system in the complex-frequency plane. In this work, the inverse problem, i.e.,
the reconstruction of the scattering matrix from the knowledge of the
quasinormal modes, is addressed. We develop a general and scalable
quasinormal-mode expansion of the scattering matrix, requiring only the complex
eigenfrequencies and the far-field properties of the eigenmodes. The theory is
validated by applying it to illustrative nanophotonic systems with multiple
overlapping electromagnetic modes. The examples demonstrate that our theory
provides an accurate first-principle prediction of the scattering properties,
without the need for postulating ad-hoc nonresonant channels.
