Abstract
Boron nitride nanotubes (BNNTs) are an emerging mid-infrared optical material that exhibits significant potential in nanoscale resonators, lasers, detectors, and sensors due to the strong light-matter interactions induced by phonon polaritons (PhPs). However, the large-scale controllable synthesis and device applications of BNNTs still face huge challenges. In this study, a substrate-pretreated and boron source dual-catalyst infiltrated growth method, and a damage-free purification method toward the high-density and aligned BNNT (ABNNT) thin films is developed. The polarization sensitivity and linear dichroism characteristics of BNNTs with a maximum anisotropy ratio of 4.53 in the mid-infrared and near-infrared regions are comprehensively verified. The nanotube-alignment induced permittivity anisotropy outside RB and the near-isotropic PhP absorption within RB enable a wavelength-selective polarizer and absorber throughout a broad infrared range. Uniquely, the BNNT polarizer exhibits excellent stability of polarization dichroism under severe bending conditions, demonstrating great potential in flexible and wearable optical devices. This work can initiate the utilization of high-density BNNT thin films in quantum information processing and high-resolution infrared imaging applications.