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Deeply sub-wavelength coherent absorption in optically thick ENZ films

One interesting challenge in bio-sensing, photovoltaics, imaging and other related fields is the identification of approaches for tuning the broadband absorption in thin slab materials. In recent years, interest has been focused on the study of the effects that two counter-propagating beams produce by interfering with each other. In this configuration both complete absorption or lossless transmission of the incidence radiation can be achieved in an absorbing material. This phenomenon, named Coherent Perfect Absorption (CPA), was first observed in a Fabry-Perot cavity, where the light is trapped due to the multiple reflections. Recently it has been theoretically shown that CPA, also known as time-reversed lasing, can be observed in a light-absorbing film with a deeply sub-wavelength thickness, e.g. typically λ/100 is required [1, 2]. In this case, the coherent interference between the two counter-propagating beams produces a standing wave. If the position of the thin film corresponds to the position of the node with zero-field (resp. antinode with maximum field) of the stationary wave, the material can act like a transparent (resp. perfect absorber) slab.