Fragile topologically protected perfect reflection for acoustic waves

Abstract

Fragile topology is firstly demonstrated in acoustic crystals and then a realistic scheme is proposed to manipulate the transport of acoustic topological edge states (ATESs), i.e., by coupling them with side acoustic cavities. We find that single-mode cavities can completely flip the ATES pseudospin to form a perfect reflection, as long as their resonant frequencies fall into the topological band gap. The perfect reflection of the ATESs is protected by the fragile topology, which is proved by the one-dimensional topological waveguide-cavity transport theory. This fragile topologically protected perfect reflection is immune to the conventional defects (such as bending and disorder) and provides a realistic paradigm for manipulating the ATES transport. As examples, two potential applications, i.e., distance sensors and acoustic switches, are proposed based on the perfect reflection.

Exceptional points in a topological photonic system

“…Only when the frequencies of the perturbation-induced states fall into the bulk band gap, the perturbation would exert a strong influence on the transport of the TESs [63]. Consequently, the TESs constructed by the crystal-symmetry-protected topological insulators are immune to most of common perturbations, such as waveguide bending, rod missing, and local disorder [68]. In fact, this characteristic will be used to adjust the backscattering strength, g, between the clockwise and counter clockwise TWGMs.…”

“…The topological optical interfaces are often used to design the ideal waveguides [59][60][61][62][63] and the topological corners can work as the amazing optical cavities [45,46,64]. Other types of robust optical devices have also been attempted, for example, topological lasers [65][66][67] and perfect reflectors [68]. Accordingly, the PTIs provide an extraordinary platform for exploring and understanding the topology [62,63,[69][70][71][72].…”

Robust Fano resonance in the photonic valley Hall states

“…One remarkable application of the photonic topological insulators is the robust transport of the topological edge states (TESs), protected by the nontrivial topology of the systems. The robust transport has been widely studied in experiments and theories, while the effect of defect-based cavities on the robustness of TESs is still far less considered [9,103,104]. For the optical analogues of Chern topological insulators, the chiral TESs are indeed unprecedented robustness.…”

“…For the optical analogues of Chern topological insulators, the chiral TESs are indeed unprecedented robustness. However, the TES transport may show fragility in the systems with the time-reversal (TR) symmetry (TRS) due to a pair of timereversal modes can always be coupled together by a possible mechanism that breaks the unidirectional transport of the TESs, for example, recent works on the QSHE in the photonic and acoustic topological insulators [103,104]. The breaking of the unidirectional transport provides the tunability for the transport of the TESs, possessing significant potential applications, such as switches.…”

Pseudospin-dependent Acoustic Topological Insulator by Sonic Crystals With Same Hexagonal Rods

“…Moreover, to realize acoustic valley Hall (AVH) states, the nonzero Berry curvature located near the valley K/Kʹ was obtained by breaking mirror or inversion symmetry, and valley-projected topological edge states were observed at the interface between two sonic crystals (SCs) with distinct AVH phases [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. Furthermore, to mimic quantum spin Hall states in acoustics, the SCs with C 6 crystal symmetry were introduced to realize artificial pseudospins-1/2 states by hybridizing degenerate modes [29][30][31][32][33][34][35][36][37][38][39]. Generally, the observed double Dirac cones are accidental degeneracy in these pseudospin-dependent ATIs, and the sizes or shapes of rods in the SCs are different, which is still a challenge for practical applications.…”