Coherent perfect absorption in photonic structures

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 (Haus in Waves and fields in optoelectronics. Prentice-Hall, Englewood Cliffs, 1984; Yariv and Yeh in Optical Electronics in Modern Communication. Oxford University Press, New York, 2007). The latter detail the conditions under which the energy of the input field is fed in full to the absorbing element. In a multi-port system, the relative phase of the incoming fields can yield the ultimate control of the absorption, from CPA to complete transparency (coherent perfect transparency, CPT), and also beyond, in amplifying regimes, to laser threshold control (Longhi in Phys Rev A 82:031801, 2010; Sun et al. in Phys Rev Lett 112:143903, 2014). This interferometric control of absorption can be employed to reach perfect energy feeding into nanoscale systems such as plasmonic nanoparticles (Noh et al. in Phys Rev Lett 108(18):186805, 2012), and multi-port interference can be used to enhance the absorption when they are embedded in a strongly scattering system (Chong and Stone in Phys Rev Lett 107(16):163901, 2011), 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 (Zanotto et al. in Nature Phys 10(11):830-834, 2014), 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.