Multi-Pole Mode Confinement and Optomechanical Mode Rotation in Hollow-Core Hexagonal Terahertz Photonic Crystal Fiber for Multi-Gas Sensing

A hollow-core hexagonal lattice terahertz (THz) photonic crystal fiber (PCF) is designed. The proposed THz PCF supports multi-pole modes spanning monopole-like, dipole-like, quadrupole-like, and hexapole-like modes. A low confinement loss of 0.087 dB/cm with a high Q-factor on the order of 106 has been obtained for a hexapole-like (LP31) mode at 0.210 THz. Moreover, a dipole-like (LP11) mode at 0.126 THz yields a maximum relative sensitivity of 88.35% for an effective electromagnetic (EM) field-analyte interaction. By introducing a rotating dielectric septum inside the PCF, the dipole-mode conservation for various C6v symmetry rotational angles is demonstrated. Using the optomechanical mode rotation of the proposed PCF, a method to sense two different gases (methane and carbon dioxide) is presented.