Inner–outer dielectrics modulation of surface phonon polaritons in quasi-1D SiC nanotubes: Full Reststrahlen band opening and independent branch control

Surface phonon polaritons (SPhPs) enable strong field confinement and subwavelength light manipulation in the infrared region, but their spectral range is limited to a partial Reststrahlen band in conventional quasi-1D nanowires. A long-standing challenge remains to unlock the full Reststrahlen band and achieve independent control of multiple SPhP branches in polar nanostructures. Here, within the framework of the dielectric continuum model, we theoretically realize independent modulation of SPhP branches in quasi-1D cubic SiC nanotubes (NTs) by engineering inner and outer nonpolar dielectrics. In sharp contrast to nanowires, SiC NTs completely eliminate the forbidden band and enable SPhP propagation across the full Reststrahlen band from ωTO to ωLO. Two SPhP branches are identified: a low-frequency branch (LFB) corresponding to the bonding state and a high-frequency branch (HFB) corresponding to the antibonding state. The outer dielectric selectively modulates only the LFB, whereas the inner dielectric effectively tunes both LFB and HFB branches simultaneously, achieving truly independent control. The physical mechanism is revealed by distinct field localization at the positively curved outer and negatively curved inner surfaces. Dual-dielectric engineering also strongly modulates the wavelength compression ratio and group velocity. This work extends SPhP theory to quasi-1D NT systems and offers a flexible route toward reconfigurable infrared nanophotonic devices.