Whispering-Gallery Phonon Polaritons and Localized Field-Enhanced Coupling Control in Boron Nitride Nanotubes

Boron nitride nanotubes (BNNTs) exhibit various deep subwavelength polarization modes in the mid-infrared region, with unique optical characteristics particularly in their transverse and longitudinal optical responses. These deep subwavelength polarization modes have broad applications in fields such as photonics, sensing, and imaging. Here, we studied the hyperbolic whispering-gallery mode phonon-polaritons (WGM-PhPs) within the cross-section of a single bamboo-like segment of BNNTs. The study reveals that the electric field distribution is primarily concentrated in the sidewall region of the BNNT, propagating along the circumference of the nanotube and encircling the area between its inner and outer walls. Additionally, the larger the duty cycle, the stronger the localized electric field intensity of the WGM-PhPs. By calculating the excitation characteristics of WGM-PhPs across different wavelengths, we found that they are more readily excited when approaching the longitudinal optical (LO) mode. Additionally, we observed that a homojunction of BNNTs and h-BN can significantly enhance the localized electric field of WGM-PhPs, increasing the field intensity by approximately four times compared to intrinsic BNNT WGM-PhPs. In a heterojunction of BNNTs, graphene, and h-BN, the localized electric field intensity increases by approximately six times compared to intrinsic BNNT WGM-PhPs. This finding demonstrates the unique advantages of this structure in polariton coupling, offering potential applications in nanoscale light propagation, on-chip optical device integration, and optical communication devices.