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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, ; Yariv and Yeh in Optical Electronics in Modern Communication. Oxford University Press, New York, ). 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, ; Sun et al. in Phys Rev Lett 112:143903, ). 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, ), 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. [ABSTRACT FROM AUTHOR]

The ability to drive a system with an external input is a fundamental aspect of light-matter interaction. The key concept in many photonic applications is the 'critical coupling' condition: at criticality, all the energy fed to the system is dissipated within the system itself. Although this idea was crucial to enhance the efficiency of many devices, it was never considered in the context of systems operating in a non-perturbative regime. In this so-called strong-coupling regime, the matter and light degrees of freedom are mixed into dressed states, leading to new eigenstates called polaritons. Here we demonstrate that the strong-coupling regime and the critical coupling condition can coexist; in such a strong critical coupling situation, all the incoming energy is converted into polaritons. A general semiclassical theory reveals that such a situation corresponds to a special curve in the phase diagram of the coupled light-matter oscillators. In the case of a system with two radiating ports, the phenomenology shown is that of coherent perfect absorption (CPA; refs , ), which is then naturally understood in the framework of critical coupling. Most importantly, we experimentally verify polaritonic CPA in a semiconductor-based intersubband-polariton photonic crystal resonator. This result opens new avenues in polariton physics, making it possible to control the pumping efficiency of a system independent of the energy exchange rate between the electromagnetic field and the material transition. [ABSTRACT FROM AUTHOR]

Resonators and the way they couple to external radiation rely on very different concepts if one considers devices belonging to the photonic and electronic worlds. The terahertz frequency range, however, provides intriguing possibilities for the development of hybrid technologies that merge ideas from both fields in novel functional designs. In this paper, we show that high-quality, subwavelength, whispering-gallery lasers can be combined to form a linear dipole antenna, which creates a very efficient, low-threshold laser emission in a collimated beam pattern. For this purpose, we employ a terahertz quantum-cascade active region patterned into two 19-μm-radius microdisks coupled by a suspended metallic bridge, which simultaneously acts as an inductive antenna and produces the dipole symmetry of the lasing mode. Continuous-wave vertical emission is demonstrated at approximately 3.5 THz in a very regular, low-divergence (±10°) beam, with a high slope efficiency of at least 160 mW A -1 and a mere 6 mA of threshold current, which is ensured by the ultra-small resonator size (V RES / λ 3 ≈10 -2 ). The extremely low power consumption and the superior beam brightness make this concept very promising for the development of miniaturized and portable THz sources to be used in the field for imaging and sensing applications as well as for exploring novel optomechanical intracavity effects., Competing Interests: The authors declare no conflict of interest.