Probing Strain-Induced Phonon Polaritons in Hexagonal Boron Nitride via Photo-Induced Force Microscopy

Hexagonal boron nitride (hBN) is a natural hyperbolic material, making it an ideal material for measuring phonon polaritons. These unique properties, however, can be affected by structural changes in the material. Wrinkles, as natural strain-induced regions, can significantly alter the vibrational properties of hBN, specifically the phonon polariton modes. Although studies on the effects of structural changes on phonon polaritons in hBN have been conducted, research on strain’s impact on phonon polariton resonance in hBN remains limited. To address this, we studied the formation of wrinkles on hBN, which are primarily attributed to the strain introduced naturally during the exfoliation and transfer processes. With photo-induced force microscopy (PiFM), we identified and observed polaritonic waves caused by strain-induced deformations on hBN samples. Our findings reveal that wrinkles not only modify the local intensity of phonon polariton propagation, but also lead to the emergence of new peaks in the PiFM spectra. Notably, we observed localized interference within the wrinkle caused by strain, which is distinct from structural reflectance. Our results are to be crucial for understanding the role of strain in the design of nanophotonic devices that leverage the unique properties of hBN.