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

## TL;DR

This paper demonstrates the preservation of orbital angular momentum in Dirac modes within a helically twisted hollow core photonic crystal fiber, combining FEM simulations and analytical theory to explore mode dynamics.

## Contribution

It introduces a novel twisted HC-PCF design that supports OAM-preserving Dirac modes, verified through combined numerical and analytical approaches.

## Key findings

- Dirac point can emerge in twisted systems for specific geometries
- OAM is preserved during axial translation of trapped modes
- Twist rate and defect geometry critically influence mode propagation

## Abstract

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.

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Source: https://tomesphere.com/paper/1902.09117