To achieve advanced control of light polarization, researchers at the Max Planck Institute for the Science of Light (MPL) developed a hollow-core optical fiber that can selectively guide optical vortices, depending on the helicity of the vortex. The hollow-core fiber could be used to realize new polarizing elements in previously inaccessible spectral regions, and for chiral sensing, broadband generation of vortex beams, and optical communication. (From left) Francesco Tani, leader of the Ultrafast and Twisted Photonics research group at the Max Planck Institute for the Science of Light; researcher Christof Helfrich; and Michael Frosz, leader of the Fibre Fabrication and Glass Studio Technology Unit. Courtesy of MPL/Susanne Viezens. Pure polarization states are crucial for many applications and research areas, and several waveguides and structured materials have been developed to preserve linear and circular polarization over long distances and analyze these states. However, new optical elements that can distinguish the helicity of optical vortices are needed to enhance control of polarization states. To date, the few structured materials developed for this purpose have not demonstrated high levels of discrimination. The hollow-core fiber developed by the MPL team demonstrated strong, broadband helical dichroism, exhibiting a different transmission depending on the orbital angular momentum (OAM) of the launched light. The optical attenuation of the hollow-core fiber depends on the OAM of the guided light, which causes the fiber to transmit optical vortices with a specific helicity and largely attenuate vortices with the opposite helicity. In experiments using a 25-centimeter-long, twisted, single-ring, hollow-core photonic crystal fiber, the researchers measured a loss difference of at least 10 decibels (dB) over a spectral range spanning more than 60 terahertz (THz). In numerical simulations, the team showed that a higher differential loss could be achieved over a broader spectral range (about 180 THz). The researchers used an analytical model to investigate the origin of the dichroism and how it was influenced by the design and twist rate of the fiber. Figure 1 (left): A schematic representation of twisted, hollow-core fiber exhibiting helical dichroism. When launching vortex beams of topological charge l=±1 into the waveguide, the transmission strongly depends on the sign of l. Figure 2 (right): The measured transmission for excitation of the fundamental mode (FM, l=0) and for vortex beams coupled into the twisted, hollow-core fiber (square/star markers for s=±1). Courtesy of C. Helfrich et al, “Giant Helical Dichroism in Twisted Hollow-Core Photonic Crystal Fibers,” ACS Photonics 2025 12 (2), 564-569. DOI: 10.1021/acsphotonics.4c02019. The team designed and fabricated a fiber that can be filled with liquid or gaseous media and used to study light-matter interactions over extended lengths. Also, the hollow waveguide can be designed for spectral regions that cannot be accessed through other types of optical systems. The researchers believe that their observations represent a significant step forward for the ongoing development of high-discrimination power polarizing elements in hard-to-reach spectral regions, and in the development of hollow-core waveguides with advanced polarization properties. The hollow-core waveguide extends previous studies of circularly dichroic waveguides and could lead to new ways to control polarization states. It could be used for various applications, such as chiral sensing of pharmaceuticals in liquid solutions, optical communication, and the generation of broadband vortex beams. With the new hollow-core fiber, the ability to control and manipulate light moves beyond linear and circular polarization. Helically dichroic waveguides promise the realization of new devices with exceptional discrimination capabilities — comparable to and even exceeding those obtained for linear polarization using crystal-based polarizers. The research was published in ACS Photonics (www.doi.org/10.1021/acsphotonics.4c02019).