Polarization- and wave-vector selective optical metasurface with near-field coupling

Metasurfaces are a powerful tool for manipulating light using small structures on the nanoscale. In most meta-surfaces, near-field couplings are treated as unfavorable perturbations. Here, we experimentally investigate a structure consisting of sinusoidally modulated silicon waveguides where near-field coupling of local resonances leads to negative coupling, i.e. a negative coupling constant. This gives rise to wave-vector dependent eigenstates of elliptical, linear and circular polarizations. In particular, fully circular polarization states are not only present at a single point in momentum-space (k-space), but along a line. This circular polarization line, as well as a linear polarization line, emanates from a polarization degeneracy at the Dirac point. We experimentally validate the existence of these eigenstates and demonstrate the energy-, polarization- and wave-vector-dependence of this metasurface. By tuning the incident k-vector, certain polarization-energy eigenstates are strongly reflected allowing for uses in angle-tunable polarization filters and light sources.

💡 Research Summary

This paper presents a novel optical metasurface that deliberately exploits near‑field coupling between local guided‑mode resonances to achieve a negative coupling constant, a concept that is usually regarded as a detrimental perturbation in conventional metasurface designs. The structure consists of sinusoidally modulated silicon waveguides: a single waveguide (SW) of width W₁ and a pair of double waveguides (DW) of width W₂ separated by a gap L₂. Both the SW and the inner edges of the DW are modulated with amplitudes A₁ and A₂, period b, and a relative phase Φ. In the unmodulated limit the SW supports a TE‑like mode (analogous to an s‑orbital) while the DW supports an odd mode (p‑orbital‑like). Because the field distribution of the p‑like mode is in phase with the s‑like mode on one side of the DW and out of phase on the other, the coupling constant κ between neighboring unit cells alternates in sign, producing an effective “alternating negative coupling”.

When this alternating coupling is inserted into a tight‑binding model, the eigenvectors become ⟨+| =