Realization of photonic charge-2 Dirac point by engineering super-modes in topological superlattices

Mengying Hu,Kun Ding,Qiang Wang,Xi Jiang,Shining Zhu,Tong Qiao,Hui Liu

doi:10.1038/s42005-020-00395-1

Abstract

Quite recently, an unconventional variety of fourfold linear band degeneracy points has been discovered in certain condensed-matter systems. Contrary to standard 3-D Dirac monopoles, these quadruple points known as the charge-2 Dirac points are characterized by nonzero topological charges, which can be exploited to delve into hitherto unknown realms of topological physics. Here, we report on the experimental realization of a charge-2 Dirac point by deliberately engineering hybrid topological states, called super-modes, in a 1-D optical superlattice system with synthetic dimensions. Utilizing direct reflection and transmission measurements, we propose the existence of the synthetic charge-2 Dirac point in the visible region. We also show an experimental approach to manipulating two spawned Weyl points possessing equal charge. Topological end modes resulting from the charge-2 Dirac point can be delicately controlled within truncated superlattices, opening a pathway to rationally engineer local fields with intense enhancement.

Highlights

Quite recently, an unconventional variety of fourfold linear band degeneracy points has been discovered in certain condensed-matter systems
We stack together two kinds of photonic crystals (PCs) belonging to different class of topology to create such topological interface modes (TIMs), and on this basis topological states of photons associated with charge-2 Dirac point (CDP) can be fully investigated under the introduction of synthetic space, facilitating the experimental realization, which is otherwise elusive at such frequencies
Our lattice consists of these two PCs stacked alternatively, in which each interface supports a TIM that hybridizes with each other to form a novel variety of artificial collective modes, resulting in a 1-D superlattice band structure where a single TIM serves as the photonic orbital

Summary

Introduction

An unconventional variety of fourfold linear band degeneracy points has been discovered in certain condensed-matter systems. It has been recently demonstrated that unconventional topological points appear in certain crystal structures[10,11,12,13,14,15,16,17,18], which cannot be described in accordance with an emergent relativistic field theory On such candidate is the charge-2 Dirac point (CDP), existing as a double-Weyl phonon in transition metal monosilicides[14,15]. These end modes uniquely result from the bulk-edge correspondence[44] for each of the two WPs producing the CDP in synthetic space, which in turn could be tuned independently Such topological end modes resemble surface states in Weyl semimetals[2,3,6], holding great potential for applications in nonlinear optics[45], quantum optics[46], and lasers[47] owing to strongly enhanced localized fields

Methods

Results

Conclusion