# Experimental observation of topological Dirac vortex mode in terahertz photonic crystal fibers

## TL;DR

Researchers observed a new type of light mode in terahertz fibers that enables high-quality single-mode transmission over a broad bandwidth.

## Contribution

First experimental observation of a topologically protected Dirac vortex mode in terahertz photonic crystal fibers.

## Key findings

- A single linearly dispersed Dirac vortex mode with single vortex polarization was demonstrated.
- The mode supports a broad 85.7% fractional bandwidth for single-polarization single-mode propagation.
- The study provides insights into the mode's dispersion, effective area, and numerical aperture.

## Abstract

Photonic crystal fibers have significantly advanced optoelectronics, enabling a wide range of applications from communications to sensing and imaging. A long-standing challenge in these areas has been achieving pure single-polarization single-mode (SPSM) waveguiding for high-quality information transmission. Traditional approaches, however, inevitably introduce polarization dispersion and operate within a narrow bandwidth. Recent advancements in topological phases offer a promising opportunity to access previously unattainable mode properties, though experimental demonstrations remain scarce. In this work, we present the first experimental observation of a topologically protected photonic Dirac vortex mode that supports pure SPSM propagation in terahertz fibers. Utilizing terahertz scanning near-field microscopic spectroscopy, we map the temporal, spectral, and spatial characteristics of the topological mode, providing insights into its mode profile, dispersion, effective area, and numerical aperture. We demonstrate a single linearly dispersed Dirac vortex mode with a single vortex polarization and a broad 85.7% fractional bandwidth. This breakthrough fills a crucial gap in the development of SPSM fibers and introduces a comprehensive methodology for exploring mode properties, paving the way for advancements in terahertz optoelectronics, topological photonics, and specialty optical fibers.

We report the first experimental demonstration of a topological photonic Dirac vortex mode in terahertz fiber, enabling unprecedented broadband single-mode single-polarization transmission.