Photonic Type-III Nodal Loop and Topological Phase Transitions at Bilayer Metasurfaces

Abstract

In momentum space, the nodal loop is regarded as a ring-shaped band degeneracy and is classified into type-I and type-II configurations depending on the positive/negative dispersions of the degenerating bands. Here, we experimentally observe a new class of nodal loop in the photonic band structure, employing an artificially designed bilayer metasurface. Such degeneracy, termed type-III nodal loop, is formed by the crossing between a resonant flat band and a positively dispersive band and is protected by mirror symmetry Mz, which manifests in the metasurface’s bilayer architecture. Furthermore, the sequential topological transitions of band degeneracy from the nodal loop via Dirac point to gapped phase are demonstrated at the metasurfaces. Such transitions are enabled by the bilayer design and end with a pair of edge states on the domain wall of gapped systems. Our work reveals that properly engineered bilayer metasurfaces offer rich physics and vast flexibility in two-dimensional topological photonic research through assembling and tunning more symmetries and degrees of freedom along the stacking direction.

Nodal degeneracy of guided modes in uniaxial crystal slabs

“…The gapping reason is that the two modes display the like parity in the mirror symmetry M x : (x, y, z) → (−x, y, z) (the symmetry classifies all modes as even or odd parity with respect to the mirror plane x = 0, see Section B in Supplementary Materials) and simultaneously M y is broken upon θ ≠ 0. Such 2D type-II DP has been observed in the artificially designed metasurfaces with Frontiers in Physics frontiersin.org periodic metallic patterns at microwave frequencies [38][39][40]. In contrast, our system is of no discrete translational symmetry, but continuous in space.…”

Investigation of Multiple High Quality-Factor Fano Resonances in Asymmetric Nanopillar Arrays for Optical Sensing

“…Zhou et al designed a guided-mode resonance sensor with a shallow subwavelength structure with a Q-factor up to 8000 [12]. All-dielectric metasurfaces offer greater freedom and design flexibility, allowing for more diversified functionality [13]. Different optical functions can be achieved by precisely designing nanostructures that allow precise modulation of several parameters of light, such as polarization, wavelength, phase, and amplitude [14].…”