Investigating the Metal-like Optical Behavior of Boron Nitride

Recent advances in scanning near-field optical microscopy (SNOM) have led to rapid growth of the field of phonon polaritonics. Due to strong vibrational resonances, which are responsible for negative permittivity in the mid-infrared (mid-IR) region, hexagonal boron nitride (hBN) is of high interest in the field. Here, we investigate the metal-like optical properties of hBN governed by phonons. We demonstrate one of the first experimental observations of surface phonon polaritons (SPhPs) in boron nitride nanotubes (BNNTs), which exhibits a very high effective index (neff ~70), yet a long propagation length. We show that the apparent periodicity can be controlled by varying the nanotube size and the supporting substrate. We also observed IR polaritonic interaction between graphene plasmons and surface phonon polaritons of BNNT, which are capable of coherently exchanging their energies at the nanoscale. 2D hBN films were also studied, and we report a metal-free method of launching phonon polaritons by making use of the intrinsic property of the material itself. To support these studies, we also developed and implemented an opto-mechanic stabilization mechanism for the SNOM and showed improved phase stability. Our reported modification of the SNOM setup to work with 2 IR continuous wave (CW) lasers opens further doors for studying inter-mode energy transductions by performing IR pump-probe CW spectroscopy.Ph.D