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In this Letter, a bilayer elastic C6v structure is proposed and used to precisely manipulate two topological nontrivial valley pseudo-spin states. According to the theoretical simulation results, the two elastic pseudo-spin states have opposite valley polarizations along propagation directions. Owing to the interlayer polarization and mixing effects of the elastic valley pseudo-spin states in the bilayer elastic C6v structure, the transport and prohibition of elastic valley pseudo-spin states and even the tunneling between layers are flexibly manipulated. The transmission efficiency of elastic valley pseudo-spin states across layers reaches 93%, which has also been confirmed in experiments by the laser ultrasonic method. The in-depth exploration of valley pseudo-spin states with layer indices further expands and enriches the research of valley pseudo-spin states in the elastic environment and lays the foundation for the further design of elastic topological devices. [ABSTRACT FROM AUTHOR]

Early detection and monitoring of heavy load induced plastic deformation in the structure is crucial for timely intervention before cracking occurs and the material completely fails. Ultrasound can be used for detecting plastic deformation provided the mechanical modulus, which can be probed via changes in velocities. This work presents a measurement scheme that makes use of optically detected, laser-induced surface acoustic waves (SAWs) for remote, real-time, online monitoring of the wave velocity during the elastic–plastic transition occurring in an aluminum plate undergoing a uniaxial tensile test and a dynamic fatigue test. Monochromatic SAWs were photoacoustically generated in a wavelength-controlled way through a transient thermal grating based laser ultrasonics excitation scheme. The SAWs were detected by a home-built photorefractive interferometer. The results reveal both regions of acceleration and slowing down, indicating that the material first stiffens and then starts to form microcracks that make it behave effectively softer for propagating SAWs. [ABSTRACT FROM AUTHOR]