Surface Phonon Coupling within Boron Nitride Resolved by a Novel Near-Field Infrared Pump-Probe Imaging Technique

The excitation of surface phonon-polariton (SPhP) modes in polar materials using scattering type near-field optical microscopy (s-SNOM) has recently become an area of interest because of its potential for application as naturally occurring meta-materials and in low-loss energy transfer. Within this area, hexagonal boron nitride (h-BN) and boron nitride nanotubes (BNNTs) are the primary structures under investigation. Here we present pump-probe continuous wave (CW) scattering-type scanning near-field optical microscopy (s-SNOM) - a novel two color pump-probe infrared technique which uses two continuous wave tunable light sources and is based on s-SNOM. The technique allows us to spatially resolve coupling of the longitudinal optical and surface phonon polariton modes in BNNTs. However, no similar coupling is observed in two-dimensional h-BN crystals.