Perfect Absorption and Phase Singularities in Plasmon Antenna Array Etalons

Abstract

We present an interferometrically resolved study of the amplitude and phase response of plasmon array etalons composed of a reflective surface with a metasurface of resonant plasmonic dipole antennas in front of it. Above a minimum antenna oscillator strength (set by antenna size and density), such structures show conditions of perfect absorption. In the parameter space spanned by frequency and etalon spacing, these singular points unavoidably come in pairs and are associated with a phase singularity. The topologically oppositely charged point pairs occur around the geometric Fabry−Peŕot condition. We elucidate 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 "pixels" 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 arbitrary stacks of metasurfaces and dielectric layers.

Gain-induced scattering anomalies of diffractive metasurfaces

“…Furthermore, as opposed to simple metasurfaces in a symmetric host [53], the proposed asymmetric multilayer geometry can also support phase singularities in the lossy regime, which is shown in Figure 8 (analogous to Figure 5). In case of a lossy waveguide, phase singularities become points of perfect absorption (zero in R, and lack of T due to evanescence), similar to those reported in the study by Berkhout and Koenderink [42].…”

“…In contrast to these reports, the anomalies discussed in this work result from interference effects caused by gain-induced phase alteration, leading to trapping of light inside the gain medium. Furthermore, by considering the case of a resonant gain, we show that these anomalies must always emerge in pairs, and each of them is surrounded by a phase vortex, producing a scattered field of diverging amplitude and undefined phase, which makes them analogous to phase singularities observed in perfectly absorbing structures [42]. We find that similar anomalies already occur in the reflectivity of a prism-coupled gain medium slab, where the incident light couples evanescently to guided modes in a traditional attenuated total reflection setup (Figure 1, right).…”

“…This is a direct consequence of the fact that the eigenpolarizabilities in Figure 4e-h are complex functions that show pairs of simple zeros. The scattering phase inside each vortex is undefined, and the vortices can be regarded as counterpart to the singular phase at scattering zeros in perfectly absorbing systems [42]. It is the Lorentzian resonant frequency dependence of the gain that causes the topological charge of each vortex to be ±1 and whereby the singularities must always emerge in oppositely charged pairs (one singularity per each side of the gain band).…”

A New Signature for Strong Light–Matter Coupling Using Spectroscopic Ellipsometry

“…The combined study of the amplitude and phase response of an optical system can provide insight that is not possible from intensity measurements alone. By analyzing the amplitude and phase response of plasmon antenna array etalons, Berkhout and Koenderink 15 showed that points of perfect absorption in such structures are topologically protected. Kravets et al 16 showed that the phase response of plasmonic nanostructures around points of perfect absorption can be used in single molecule detection.…”

A simple transfer-matrix model for metasurface multilayer systems

“…Second, seminal early examples of metasurfaces use stacking to design function. An important family of examples is that of perfect absorbers like Salisbury screens, and patch-antenna reflective metasurfaces, in which arrays of resonant antennas are placed in front of a reflector [3,[17][18][19][20][21][22][23][24][25][26]. Such structures control reflected waves in amplitude, phase and polarization through constructive and destructive interference of scattering contributions from a ground plane and a scatterer array [3,27].…”