Fig. 4.
Spin-momentum locking in elastic waves. The strong directional excitation of a spin-selected elastic wave can be realized based on the spin–momentum locking. (A) Nonsymmetric elastic wave excitation in bulk can be achieved by exploiting two circularly polarized elastic loads to induce an effective opposite spin pair. The total displacement field is plotted. (B) Unidirectional elastic surface mode, Rayleigh wave, can be excited selectively by setting different circularly polarized elastic loads. The total displacement field is plotted. (C) The spin-controlled elastic wave routing in bulk can be achieved in anisotropic materials interfaced with isotropic media. The chiral elastic wave spins are excited and characterized in the isotropic side. Different spin excitation in the isotropic side selectively couples with the different directional wave routing in the anisotropic side through the selective local mode couplings of different spins at the interface. The area above the black line is anisotropic material, and the area below is isotropic material, with the black line as an interface. The divergence of the total displacement field that reflects the longitudinal component is plotted. (D) The experimental scheme to realize circularly polarized elastic loads. Four piezoelectric ceramics with different excitation phases are inserted into the surface of aluminum. By adjusting the excitation pattern , we can excite the elastic wave with nontrivial spin density. After placing this array on the boundary of the aluminum block, we will observe spin–momentum locking phenomena: unidirectional excitations associated with their phase patterns. The spin–momentum locking analysis and experimental scheme details can be found in SI Appendix.