Layer-Dependent Phonon Polaritons in hBN Resolved by Photo-Induced Force Spectroscopy.

Hexagonal boron nitride (hBN) is a widely studied van der Waals material that supports highly confined phonon polaritons-hybrid light-matter quasiparticles arising from the coupling of infrared photons with optical phonons. While prior works-primarily using scattering-type near-field optical microscopy (s-SNOM)-have explored the effect of hBN layer number on the wavelength and propagation characteristics of phonon polaritons, the direct spectroscopic investigation of their resonance energy as a function of layer number has remained largely unaddressed. In this study, we present the first systematic spectroscopic analysis of layer-dependent phonon polariton resonance frequencies in hBN, using photo-induced force microscopy (PiFM). By probing hBN with layer numbers ranging from approximately 10 to 60, we uncover clear and opposing trends in the resonance behavior of the two principal Reststrahlen bands. Specifically, we observe that the in-plane phonon polariton resonance exhibits a blue shift with increasing layer number, while the out-of-plane resonance shows a red shift. Additionally, the peak intensity in the out-of-plane band increases with layer number, whereas the in-plane resonance becomes weaker and broader. These findings reveal critical insights into the optical response of hBN across different layer regimes and underscore the importance of layer number as a tuning parameter in nanophotonic applications.