Twisted Anti-Resonant Hollow-Core Fiber as a Broadband Filter for the Sign of the Topological Charge

In analogy to circular dichroism (CD), the term helical dichroism (HD) describes the differential loss between opposite signs of the topological charge <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\ell=\pm 1,\ \pm 2,\ldots$</tex> of annular vortex beams. HD has been observed in different systems, including molecules [1] and nanostructured materials [2]. In this work, we present a waveguide exhibiting significantly stronger HD in comparison to previous reports about other platforms [2]. We measure 10 dB loss difference in a twisted anti-resonant hollow-core fiber for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\ell=\pm 1$</tex> (25 cm fiber length) in the spectral range 520 nm to 580 nm. Our numerical calculations predict even higher differential loss over a much broader spectral range.