Revealing intrinsic 3D spin angular momentum of evanescent acoustic phonons on a single-crystal surface using ultrafast optoacoustics.

Recent advances in spin angular momentum (SAM) of acoustic and elastic waves have deepened our understanding of phonons across classical and quantum regimes. Here, we investigate evanescent acoustic phonons (EAPs) on a single-crystal surface and reveal an intrinsic three-dimensional (3D) spin angular momentum (SAM) with nonzero components along all axes-distinct from isotropic media. EAPs exhibit spin-momentum locking along specific crystallographic directions, while other directions yield unconstrained SAM and fully 3D spin textures. Lattice-dynamics calculations demonstrate that 3D SAM is fundamental to EAP eigenstates, arising from crystal anisotropy. Experiments using an ultrafast Sagnac interferometer generate GHz EAP wave packets and image out-of-plane atomic velocity fields with microscale spatial and femtosecond temporal resolution, which are integrated with simulated in-plane velocity fields to verify findings. The structured 3D SAM distribution over the surface is governed by angular momentum conservation and crystal symmetry. Our findings facilitate engineering spin-orbit interactions for sensing, information encoding, and hybrid spintronic-photonic systems.