image: (a) Schematic of the paired Dirac cones whose positions can be changed by modulating the geometrical parameters. (b) Topological antenna with tunable emission angle, and (c) Klein tunneling from any direction.
Credit: ©Science China Press
Most topological acoustic devices behave like a train fixed on rails: the direction of sound transmission or radiation is locked due to the pinned position of Dirac points. Researchers have now broken that rail, developing a platform that can move Dirac points—the critical features that govern wave behavior—anywhere within the momentum space, enabling sound to be steered freely.
They designed a two-dimensional acoustic topological insulator in which the horizontal and vertical coupling strengths between cavities can be independently tuned. By adjusting the widths of connecting tubes, the Dirac points can be moved to arbitrary locations inside the Brillouin zone.
Using this principle, the group built two fully passive, angle-tunable devices. The first is a full-space scanning topological antenna: sound waves traveling along a topological interface are radiated into free space at an angle dictated by the Dirac point’s position. Merely changing the tube widths shifts the Dirac point, causing the emitted beam to sweep continuously from negative to positive angles. The second is an acoustic heterostructure that exhibits fully angle-tunable Klein tunneling, where the narrow angle of nearly perfect transmission through a barrier can be controlled.
By removing the angular constraint that has long limited topological acoustic systems, the work provides a versatile design approach for the sound-wave devices.