Topological acoustic polaritons: robust sound manipulation at the subwavelength scale

Abstract

Topological insulators, a hallmark of condensed matter physics, have recently reached the classical realm of acoustic waves. A remarkable property of time-reversal invariant topological insulators is the presence of unidirectional spin-polarized propagation along their edges, a property that could lead to a wealth of new opportunities in the ability to guide and manipulate sound. Here, we demonstrate and study the possibility to induce topologically non-trivial acoustic states at the deep subwavelength scale, in a structured two-dimensional metamaterial composed of Helmholtz resonators. Radically different from previous designs based on non-resonant sonic crystals, our proposal enables robust sound manipulation on a surface along predefined, subwavelength pathways of arbitrary shapes.

Reconfigurable topological photonic crystal

“…Despite that, these edge states support scatter-free propagation along interfaces with sharp turns, as it will be shown in figure 4. In the following, we will refer to the edges states in our structure as 'topological edge states' as they owe their existence to different topology of the two surrounding bulks [18,20,[68][69][70].…”

“…There is a small frequency range where neither the edge states nor the bulk states exist. This global gap is a result of the avoided crossing of the edge states caused by their interaction due to the broken C 6 symmetry at the conducting interface [18,20,[68][69][70]. The position and size of the bandgap is affected by variations of the background refractive index.…”

“…We choose to analyze the behavior of the structure at a normalized frequency of 0.433 that is located outside of the bandgap for n bg =1.51, and inside of the bandgap for n bg =1. 69. Figures 3(a) and (d) show the propagation of light along an interface with a rhombus-shaped defect at ωa 0 /(2πc)=0.433 for background refractive indices of 1.51 and 1.69, respectively.…”

Experimental demonstration of topological waveguiding in elastic plates with local resonators

“…This can pose challenges especially under stringent size limitations of wave media. Recent studies in photonics [28] and acoustics [29] have proposed the use of resonating elements within a topological medium to lower the operating frequencies. In elastic systems, recent numerical investigations have provided novel solutions to these issues [26,30,31].…”

Switching between deterministic and accidental Dirac degeneracy by rotating scatterers and the multi-channel topological transport of sound

“…Recently, with the advent of topology mechanisms, the exploration of the topological states in a classical wave system has been an active research area [16][17][18][19][20][21][22][23][24][25][26][27]. One of the necessary conditions for the topological transition is the band inversion process of the opening, closing, and reopening of the band gap.…”