Thickness-Dependent Behavior of Phonon Polaritons in Hexagonal Boron Nitride Using Photo-Induced Force Microscopy

Hexagonal boron nitride (hBN) is a promising material for nanophotonic applications due to its ability to support phonon polaritons—strongly confined light-matter interactions within specific vibrational frequency bands. While some studies have examined the effect of thickness on phonon polaritons, no clear trend between thickness and resonance frequency has emerged in the literature. In this study, we investigated how the thickness of hBN impacts the resonance frequencies of these phonon polaritons. Using photo-induced force microscopy (PiFM), we performed nanoscale optical characterizations on hBN samples with thicknesses ranging from 3 nm to 30 nm. Our findings reveal a pronounced thickness-dependent shift in the resonance frequencies: as the hBN layer becomes thicker, the resonance frequency of the in-plane phonon polaritons increases, while that of the out-of-plane modes decreases. Additionally, we observed that in the out-of-plane band, the peak intensity increases with thickness, whereas in the in-plane band, the intensity decreases and the peak broadens. These results provide valuable insights into the fundamental behavior of phonon polaritons, emphasizing the critical role of thickness in tuning the optical properties of hBN. This investigation paves the way for more precise design and optimization of hBN-based nanophotonic devices.