Coherent perfect absorption and transparency in lossy and loss/gain metasurface-embedding structures

In this paper we report about the possibility to control a strong signal beam with a much weaker control beam, resorting to the linear optical phenomena known as coherent perfect absorption and transparency (CPA and CPT). First, analytical formulas for CPA and CPT in a realistic yet prototypical configuration of a substrate-backed optically conducting surface (graphene, metallic metasurface...) are reported. Maximal optical control can be achieved by only imposing the conditions for CPA, since CPT is always present in this system. Secondly, we observe that the performance of passive two-port devices as optical switches is always limited by fundamental energy conservation constraints. Meanwhile, we outline how a hybrid gain/loss optical device reminiscent of parity-time symmetric objects can overcome such limitation, enabling the realization of ideal full optical control.