Perfect Absorption and Phase Singularities in Plasmon Antenna Array Etalons

We present an interferometrically resolved study of the amplitude and phase response of plasmon array etalons composed of a reflective surface with in front of it a metasurface of resonant plasmonic dipole antennas. We find that above a minimum antenna oscillator strength (set by antenna size and density), such structures show conditions of perfect absorption. Contrary to earlier findings on perfect absorption, we find that these singular points unavoidably come in pairs, and are associated with a phase singularity in the parameter space spanned by frequency and etalon spacing. The topologically oppositely charged point pairs occur around the geometric Fabry Perot condition. We elucidates the origin of these singularities, and their continuous evolution with oscillator strength in the 2D plane spanned by optical frequency and mirror-antenna spacing. Our findings extend the understanding of Salisbury screens and of \2\80\98pixels\2\80\99 in reflective metasurfaces for full control of amplitude and phase. Finally, our data demonstrates the limits of transfer-matrix approaches to predicting the response of arbitray stacks of metasurfaces and dielectric layers.