Hybrid Spin and Anomalous Spin-Momentum Locking in Surface Elastic Waves

TL;DR

This paper reveals an anomalous transverse spin behavior in elastic surface waves due to their hybrid nature, enabling spin-controlled excitation of Lamb modes and advancing elastic wave manipulation.

Contribution

It demonstrates both theoretically and experimentally that elastic surface waves exhibit an anomalous spin sign, unlike electromagnetic or water waves, enabling new spin-controlled elastic wave applications.

Findings

Anomalous spin sign near the surface for elastic waves

Hybrid nature causes the spin sign anomaly

Spin-controlled excitation of Lamb modes

Abstract

Transverse spin of surface waves is a universal phenomenon which has recently attracted significant attention in optics and acoustics. It appears in gravity water waves, surface plasmon-polaritons, surface acoustic waves, and exhibits remarkable intrinsic spin-momentum locking, which has found useful applications for efficient spin-direction couplers. Here we demonstrate, both theoretically and experimentally, that the transverse spin of surface elastic (Rayleigh) waves has an anomalous sign near the surface, opposite to that in the case of electromagnetic, sound, or water surface waves. This anomalous sign appears due to the hybrid (neither transverse nor longitudinal) nature of elastic surface waves. Furthermore, we show that this sign anomaly can be employed for the selective spin-controlled excitation of symmetric and antisymmetric Lamb modes propagating in opposite directions in an elastic plate. Our results pave the way for spin-controlled manipulation of elastic waves and can be important for a variety of areas, from phononic spin-based devices to seismic waves.