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Phonon dispersion in crystals determines many important material properties, but its measurement usually requires large-scale facilities and is limited to bulk samples. Here, we demonstrate the measurement of full phonon dispersion in nanoscale systems by using picosecond acoustics. A heterostructure sample was prepared consisting of layers of hexagonal boron nitride (hBN) sandwiching a thin layer of black phosphorous (BP), within which a strain pulse was generated by photoexcitation and observed with an optical probe in the BP layer. The strain pulse traversed to the few nanometer thick hBN layers, where it propagated to the edge and echoed back, like acoustic waves in Newton’s cradle. The echoes returning to the BP layer provided information on the frequency-dependent time-of-flight and group velocity dispersion of the sample system. Phonon frequency combs observed in the Fourier transform spectrum confirmed the strain wave round trips and demonstrated the feasibility of determining group velocity dispersion through photoacoustics.

Rights:

• Creative Commons BY Attribution 4.0 International
  

Keyword: Phonon dispersion, picosecond acoustics, heterostructure

Date published: 2024-06-13

Publisher: Springer Science and Business Media LLC

Journal:

• npj 2D Materials and Applications (ISSN: 23977132) vol. 8 issue. 1 39

Funding:

• MEXT | Japan Society for the Promotion of Science 202115353
• MEXT | Japan Society for the Promotion of Science 23K04356

Manuscript type: Publisher's version (Version of record)

MDR DOI:

First published URL: https://doi.org/10.1038/s41699-024-00475-8

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Updated at: 2025-02-23 22:47:55 +0900

Published on MDR: 2025-02-23 22:47:56 +0900