  Scattering-type scanning near-field optical microscopy (s-SNOM) is
instrumental in exploring polaritonic behaviors of two-dimensional (2D)
materials at the nanoscale. A sharp s-SNOM tip couples momenta into 2D
materials through phase matching to excite phonon polaritons, which manifest as
nanoscale interference fringes in raster images. However, s-SNOM lacks the
ability to detect the progression of near-field property along the
perpendicular axis to the surface. Here, we perform near-field analysis of a
micro-disk and a reflective edge made of isotopically pure hexagonal boron
nitride (h-11BN), by using three-dimensional near-field response cubes obtained
by peak force scattering-type near-field optical microscopy (PF-SNOM). Momentum
quantization of polaritons from the confinement of the circular structure is
revealed in situ. Moreover, tip-sample distance is found to be capable of
fine-tuning the momentum of polaritons and modifying the superposition of
quantized polaritonic modes. The PF-SNOM-based three-dimensional near-field
analysis provides detailed characterization capability with a high spatial
resolution to fully map three-dimensional near-fields of nano-photonics and
polaritonic structures.
