Abstract
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.
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The data that support the findings of this study are available from the corresponding author upon request.
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Acknowledgements
The work described in this paper was substantially supported by grants from the NSFC/RGC Joint Research Scheme sponsored by the Research Grants Council of Hong Kong and the National Natural Science Foundation of China (Project No. N_PolyU597/24, Z.S.) and the National Natural Science Foundation of China (Grant No. 52505160, Y.H.). The research was also supported by the Research Grants Council of Hong Kong via General Research Funds (Nos. 15214323 and 15200922, Z.S.), and the Innovation and Technology Commission of Hong Kong via an Innovation and Technology Fund (ITF) project (ITS/005/24SC and K-BBY1, Z.S.). This work was also supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) [Grant No. 2019R1A3B3067987, H.S.], a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science, O.M., and a research grant from Insight K.K., O.M. We thank Alexei Maznev for critical comments on this work.
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Z.S., O.B.W., and O.M. proposed the research goals and supervised the project. Y.H. performed the experiments with the help of O.M. and G.L. The theoretical model and numerical model were developed by Y.H. and G.L. with the help of H.S., Z.S., and O.B.W. Data were analyzed by Y.H., Z.S., and O.B.W. Theoretical support was provided by Z.S., O.B.W., and O.M. All authors helped prepare, critically review, and revise the manuscript.
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He, Y., Luo, G., Sohn, H. et al. Revealing intrinsic 3D spin angular momentum of evanescent acoustic phonons on a single-crystal surface using ultrafast optoacoustics. Nat Commun (2026). https://doi.org/10.1038/s41467-026-70019-9
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DOI: https://doi.org/10.1038/s41467-026-70019-9
