Orbital Angular Momentum preserving guided mode in helically twisted hollow core photonic crystal fiber at Dirac point.

We report trapping and propagation of photonic Dirac mode in a helically twisted hollow core photonic crystal fiber (HC-PCF) where the trapped light in the hollow (air) defect can preserve the orbital angular momentum (OAM). We show that a photonic Dirac point can emerge even in a twisted system for a suitable choice of curvilinear coordinate and the related waveguide defect modes defined in the new basis can preserve the associated OAM during axial translation. The effect of twist rate, defect geometry and crystal dimension on the propagation of OAM carrying trapped Dirac modes is critically analyzed. The results derived by FEM simulation are verified with an analytical theory based on dynamics of Bloch modes in twisted photonic crystals which are in good agreement. The proposed HC-PCF can play an important role in exciting and guiding of OAM carrying modes that help particle trapping and quantum communication.