Synthetic Three-Dimensional Z×Z_{2} Topological Insulator in an Elastic Metacrystal

Realization of Merged Topological Corner States in the Continuum in Acoustic Crystals

Merging bound states in the continuum (BICs) has significant promise for wave manipulation since it can provide an ultrahigh Q factor when compared to the isolated BICs. However, the study of merging topological bound states in the continuum (TBICs) remains largely unexplored. In this Letter, we introduce a straightforward structure for crafting the merged higher order TBICs, i.e., the merged topological corner states in the continuum (MTCICs), via a synthetic way. Two identical topological insulators, supporting topological corner states within the bulk band, are mirror stacked. The eigenstates inherited from a single layer experience energy spectrum shifts without hybridization as interlayer couplings change, leading to the emergence of MTCICs. With acoustic crystals, we experimentally identify and characterize the MTCICs. The results evidently confirm the superior energy confinement capabilities of MTCICs over TBICs and gapped topological corner states (TCSs). Our Letter may deepen the understandings of both topological and BIC physics to inspire the development of devices with ultrahigh Q factor and enhanced robustness.

Realization of a Z-Classified Chiral-Symmetric Higher-Order Topological Insulator in a Coupling-Inverted Acoustic Crystal

Higher-order topological band theory has transformed the landscape of topological phases in quantum and classical systems. Here, we experimentally demonstrate a two-dimensional higher-order topological phase, referred to as the multiple chiral topological phase, which is protected by a multipole chiral number (MCN). Our realization differs from previous higher-order topological phases in that it possesses a larger-than-unity MCN, which arises when the nearest-neighbor couplings are weaker than long-range couplings. Our phase has an MCN of 4, protecting the existence of 4 midgap topological corner modes at each corner. The multiple topological corner modes demonstrated here could lead to enhanced quantum-inspired devices for sensing and computing. Our study also highlights the rich and untapped potential of long-range coupling manipulation for future research in topological phases.

Topological Metamaterials

The topological properties of an object, associated with an integer called the topological invariant, are global features that cannot change continuously but only through abrupt variations, hence granting them intrinsic robustness. Engineered metamaterials (MMs) can be tailored to support highly nontrivial topological properties of their band structure, relative to their electronic, electromagnetic, acoustic and mechanical response, representing one of the major breakthroughs in physics over the past decade. Here, we review the foundations and the latest advances of topological photonic and phononic MMs, whose nontrivial wave interactions have become of great interest to a broad range of science disciplines, such as classical and quantum chemistry. We first introduce the basic concepts, including the notion of topological charge and geometric phase. We then discuss the topology of natural electronic materials, before reviewing their photonic/phononic topological MM analogues, including 2D topological MMs with and without time-reversal symmetry, Floquet topological insulators, 3D, higher-order, non-Hermitian and nonlinear topological MMs. We also discuss the topological aspects of scattering anomalies, chemical reactions and polaritons. This work aims at connecting the recent advances of topological concepts throughout a broad range of scientific areas and it highlights opportunities offered by topological MMs for the chemistry community and beyond.

Photonic topological insulator induced by a dislocation in three dimensions

The hallmark of topological insulators (TIs) is the scatter-free propagation of waves in topologically protected edge channels¹. This transport is strictly chiral on the outer edge of the medium and therefore capable of bypassing sharp corners and imperfections, even in the presence of substantial disorder. In photonics, two-dimensional (2D) topological edge states have been demonstrated on several different platforms^2-4 and are emerging as a promising tool for robust lasers⁵, quantum devices^6-8 and other applications. More recently, 3D TIs were demonstrated in microwaves⁹ and  acoustic waves^10-13, where the topological protection in the latter  is induced by dislocations. However, at optical frequencies, 3D photonic TIs have so far remained out of experimental reach. Here we demonstrate a photonic TI with protected topological surface states in three dimensions. The topological protection is enabled by a screw dislocation. For this purpose, we use the concept of synthetic dimensions^14-17 in a 2D photonic waveguide array¹⁸ by introducing a further modal dimension to transform the system into a 3D topological system. The lattice dislocation endows the system with edge states propagating along 3D trajectories, with topological protection akin to strong photonic TIs^19,20. Our work paves the way for utilizing 3D topology in photonic science and technology.