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

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.