Phonon Polaritons Launched by Natural Boron Nitride Wrinkles Probed with Nano‐Fourier Transform Infrared Spectroscopy

Nano‐fourier transform infrared spectroscopy (FTIR) is a powerful tool to measure optical and electronic properties of materials at the nanoscale. It is especially useful for visualizing plasmon and phonon polaritons launched from edges of a sample or structures on top of it. Herein, an exfoliated hexagonal boron nitride flake with a thickness of ≈16 nm is transferred onto a gold substrate. The flake is characterized by micro‐Raman and nano‐FTIR spectroscopy. The Raman spectra show no difference between points on the flat surface and points on the wrinkles of the flake. Nano‐FTIR spectra, while comparable to conventional infrared spectra on the flat surface, show a strong change in the form of a second absorption peak appearing near a wrinkle in the flake. This second absorption peak shifts to higher wavenumber and becomes more intense as the probed spot gets closer to the wrinkle. This is consistent with the behavior of phonon polaritons when approaching the scattering point that is reflecting them.