Figure 5. Broadband topologically protected acoustic diode.

(a) The boundary between two crystal domains with opposite modulation handedness makes a reconfigurable, backscattering immune and broadband nonreciprocal waveguide, which perfectly transmits acoustic waves along the boundary, from the input port 1 to the output port 2, regardless of turns and defects. This path for the acoustic signal is a reconfigurable one-way channel, and transmission from 2 to 1 is zero, realizing an acoustic diode. A single trimer can also make a diode, transmitting sound from port 1 to 2 (b), but not from 2 to 1 (c). (d) Frequency dependence of the acoustic power transmission coefficient from 1 to 2 (T_1→2, blue curves) and from 2 to 1 (T_2→1, red curves) in the case of the lattice acoustic diode in a (solid lines), demonstrating broadband isolation. For comparison, the single trimer (b,c) behaves like an imperfect acoustic diode operating only in the vicinity of a single frequency (dashed lines).