Type-II Dirac Photons at Metasurfaces

Abstract

Topological characteristics of energy bands, such as Dirac and Weyl nodes, have attracted substantial interest in condensed matter systems as well as in classical wave systems. Among these energy bands, the type-II Dirac point is a nodal degeneracy with tilted conical dispersion, leading to a peculiar crossing dispersion in the constant-energy plane. Such nodal points have recently been found in electronic materials. The analogous topological feature in photonic systems remains a theoretical curiosity, with experimental realization expected to be challenging. Here, we experimentally realize the type-II Dirac point using a planar metasurface architecture, where the band degeneracy point is protected by the underlying mirror symmetry of the metasurface. Gapless edge modes are found and measured at the boundary between the different domains of the symmetry-broken metasurface. Our Letter shows that metasurfaces are simple and practical platforms for realizing electromagnetic type-II Dirac points, and their planar structure is a distinct advantage that facilitates applications in two-dimensional topological photonics.

Robust Fano resonance in the photonic valley Hall states

“…The long and short side lengths are set to be l = 0.61a and d = 0.1a, respectively, where a is the lattice constant. For them the meta-atoms are composed of nonmagnetic material with relative dielectric constant ε r = 16 which can be achieved in microwave and light [118,119]. The magnetic and electric fields for the considered photonic topological states are perpendicular and parallel to the unit-cell plane, respectively.…”

Topological modes in radiofrequency resonator arrays

“…This discovery has not only led to observations of new properties of periodic systems observed in nature, but also to new materials in the quantum and classical regimes. Many of these systems are relatively simple, and include chains of massspring oscillators [3], magnetically coupled spinners [2], plasmonic nanoparticles [4,5], dielectric resonator chains [6,7], planar metasurface architectures [8], microwave arrays [9], and acoustic systems [10].…”

Nonlinear topological valley Hall edge states arising from type-II Dirac cones

“…Different from type-I Dirac cones in graphene where the corresponding Fermi surface is a point, there are also so-called tilted type-II Dirac cones with the Fermi surface being a pair of crossing lines. [53][54][55][56][57][58][59][60][61][62][63][64] Type-II Dirac cones violate the Lorentz symmetry, thus allowing for quasi-particle-mediated phenomena that do not exist in high-energy physics analogically investigated in condensed matter physics. In photonics, for example, type-II Dirac cones are expected to bring new features due to their nonisotropic transport properties arising from the distinctive dispersions.…”