Figure 5.

Scattering-type scanning near-field optical microscopy (s-SNOM) images of hyperbolic phonon polaritons in hexagonal boron nitride. (a) Experiment setup. An exfoliated microcrystal of hBN (thin film, tens of layers) was transferred onto the Si/SiO₂ substrate with an air trench such that part of the hBN was suspended. In the experiment, the AFM tip is illuminated (red solid arrow) by an infrared (IR) beam from a quantum cascade laser (QCL). The backscattered IR signal was then collected (red dashed arrow). (b) AFM image of the suspended and supported hBN studied in this experiment. (c, d) s-SNOM line profiles for hyperbolic phonon polaritons in suspended (blue) and supported (red) hBN. (c) s-SNOM line profiles taken along horizontal axes within supported and suspended hBN regions, the fringe periods are indicated with double arrows (blue and red arrows for Δ_sus and Δ_sup, respectively). (d) Fourier transform spectra of the s-SNOM line profile in (c). α and β indicate phonon polariton peaks in the FT spectra. (e) Line profiles of Δ = λ_p/2 (λ_p is the polariton wavelength and Δ is the typical polariton fringe period close to the hBN edge) and the corresponding damping factor γ. The line profiles were obtained by inverse Fourier transform of the spectra in (c). IR frequency ω = 1450 cm^–1. Reproduced with permission from ref (23). Copyright 2019 American Chemical Society.