  The ability to drive a system with an external input is a fundamental aspect
of light-matter interaction. The coherent perfect absorption (CPA) phenomenon
extends to the general multibeam interference phenomenology the well known
critical coupling concepts. This interferometric control of absorption can be
employed to reach full delivery of optical energy to nanoscale systems such as
plasmonic nanoparticles, and multi-port interference can be used to enhance the
absorption of a nanoscale device when it is embedded in a strongly scattering
system, with potential applications to nanoscale sensing. Here we review the
two-port CPA in reference to photonic structures which can resonantly couple to
the external fields. A revised two-port theory of CPA is illustrated, which
relies on the Scattering Matrix formalism and is valid for all linear two-port
systems with reciprocity. Through a semiclassical approach, treating two-port
critical coupling conditions in a non-perturbative regime, it is demonstrated
that the strong coupling regime and the critical coupling condition can indeed
coexist; in this situation, termed strong critical coupling, all the incoming
energy is converted into polaritons. Experimental results are presented, which
clearly display the elliptical trace of absorption as function of input
unbalance in a thin metallo-dielectric metamaterial, and verify polaritonic CPA
in an intersubband-polariton photonic-crystal membrane resonator. Concluding
remarks discuss the future perspectives of CPA with photonic structures.
