Type-III and Tilted Dirac Cones Emerging from Flat Bands in Photonic Orbital Graphene

Abstract

The extraordinary electronic properties of Dirac materials, the two-dimensional partners of Weyl semimetals, arise from the linear crossings in their band structure. When the dispersion around the Dirac points is tilted, one can predict the emergence of intricate transport phenomena such as modified Klein tunneling, intrinsic anomalous Hall effects, and ferrimagnetism. However, Dirac materials are rare, particularly with tilted Dirac cones. Recently, artificial materials whose building blocks present orbital degrees of freedom have appeared as promising candidates for the engineering of exotic Dirac dispersions. Here we take advantage of the orbital structure of photonic resonators arranged in a honeycomb lattice to implement photonic lattices with semi-Dirac, tilted, and, most interestingly, type-III Dirac cones that combine flat and linear dispersions. Type-III Dirac cones emerge from the touching of a flat and a parabolic band when synthetic photonic strain is introduced in the lattice, and they possess a nontrivial topological charge. This photonic realization provides a recipe for the synthesis of orbital Dirac matter with unconventional transport properties and, in combination with polariton nonlinearities, opens the way to study Dirac superfluids in topological landscapes.

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Plain Language Summary

In a wide range of 2D materials such as graphene, electrons behave as massless “Dirac particles” that propagate at remarkable speeds with very low resistance. These properties are very promising for the fabrication of ultralow-power microelectronic devices. However, these “Dirac materials” are rare and difficult to synthesize, and so there is a need for new strategies to engineer them. Here, we arrange photonic resonators in a honeycomb lattice to create a photonic material with many properties of Dirac materials, including the discovery of Dirac quasiparticles that are massless when propagating in one direction yet have infinite mass when moving in the orthogonal direction.

The properties of Dirac materials arise from crossings in the band structure at the Dirac points, where the valence band transitions to the conduction band. When the dispersion around these points is asymmetric, many striking phenomena have been predicted, including superconductivity at high temperatures, yet they are difficult to find in nature. Recently, artificial photonic materials have appeared as promising candidates for the engineering of such exotic dispersions.

We fabricate an asymmetric honeycomb lattice of photonic micron-sized resonators to implement a photonic material with exotic Dirac crossings in its band structure, and we study the localization and transport properties of photons in such materials.

Our photonic lattices provide a recipe for the synthesis of Dirac materials with remarkable transport properties.