  Quasinormal mode (QNM) expansion is a popular tool to analyze light-matter
interaction in nanoresonators. However, expanding far-field quantities such as
the energy flux is an open problem because QNMs diverge with an increasing
distance to the resonant systems. We introduce a theory to compute modal
expansions of far-field quantities rigorously. The presented approach is based
on the complex eigenfrequencies of QNMs. The divergence problem is circumvented
by using contour integration with an analytical continuation of the far-field
quantity into the complex frequency plane. We demonstrate the approach by
computing the angular resolved modal energy flux in the far field of a
nanophotonic device.
