Optical hot-spots in boron-nitride nanotubes at mid infrared frequencies: One-dimensional localization due to random-scattering

We report experimental observations of optical hot-spots associated with surface phonon polaritons in boron nitride nanotubes. As revealed by near-field optical microscopy, the hot-spots have mode volumes as small as ≃ 2.7 × 10⁻⁶λ³₀ (λ₀ is the wavelength of the exciting light in vacuum), which are in the deep subwavelength regime. Such strong light-trapping leads to ultrahigh field enhancement with a Purcell factor of ≃ 1.8 × 10⁶. Remarkably, the hot-spots are not induced by designed structures, but by random scatterings with the rough gold substrate. The ultrahigh field enhancement can be used to improve nonlinear infrared spectroscopy, thermal emitters and detectors, and label-free molecule sensing at nanoscales.