Abstract
Epsilon-near-zero and epsilon near-pole materials enable reflective systems supporting a class of symmetry-protected and accidental embedded eigenstates (EEs) characterized by a diverging phase resonance. Here we show that pairs of topologically protected scattering singularities necessarily emerge from EEs when a non-Hermitian parameter is introduced, lifting the degeneracy between oppositely charged singularities. The underlying topological charges are characterized by an integer winding number and appear as phase vortices of the complex reflection coefficient. By creating and annihilating them, we show that these singularities obey charge conservation, and provide versatile control of amplitude, phase, and polarization in reflection, with potential applications for polarization control and sensing.
Highlights
Along with the progress in fabrication technologies observed in the last few decades, the concepts of metamaterials [1], metasurfaces [2] and photonic crystals [3] have brought forward a previously unattainable level of control of visible, infrared and microwave electromagnetic waves
The presented theory builds upon the topological features of EE-related phenomena, it is worth stressing the differences between the homogeneous planar systems analyzed here and EEs emerging due to the periodicity in photonic crystals (PCs)
EEs in PCs come as isolated points in k-space, whereas in the homogeneous planar systems studied here, EEs generally emerge in rings in k-space and isolated points in real space
Summary
Along with the progress in fabrication technologies observed in the last few decades, the concepts of metamaterials [1], metasurfaces [2] and photonic crystals [3] have brought forward a previously unattainable level of control of visible, infrared and microwave electromagnetic waves. The pursuit of wave control has been expanded with tools borrowed from the field of topology [4], enabling phenomena such as robustness to imperfections and immunity to back-scattering [5,6], as well as unidirectional transport [7]. Topological photonics has been rapidly growing in recent years, transferring established concepts from condensed-matter systems to electromagnetics research and metamaterials. Bound states in the continuum (BIC) or embedded eigenstates (EE), examples of peculiar features associated with electromagnetic radiation and scattering, provide topological features to the optical response rooted in modes that are non-radiative yet embedded within the radiation continuum [14,15,16,17,18,19,20,21]
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