Zero-Index Material Enabled Hollow Core Optical Fiber

<br/>Conventional optical fibers, which guides light based on total internal reflection between a high-index core and low index cladding, have been studied extensively and used in a broad range of applications, including telecommunication, remote sensing, imaging, spectroscopy, etc. This type of fiber, though robust, still does not transmit information at the maximum theoretically possible speed, due to the high index of the medium that light is propagating through. Epsilon-near-zero (ENZ) materials, which have its real part of the permittivity reduced to zero at a certain wavelength, have been studied for its unique optical properties and multiple potential applications in photonics [1]. Recent studies have shown that ENZ material can be utilized in optical fiber designs to achieve novel properties such as enhanced optical confinement and optical sensing [2].<br/>In this work, we demonstrate a capillary optical fiber design, with a hollow air core where light propagates through, and cladding made of indium tin oxide (ITO) ENZ material. Near the tunable ENZ wavelength, chosen to be at telecommunication wavelength of 1550 nm in this work, the material has vanishingly small permittivity, leading to an index of refraction less than that of air. Thus, this structure creates a similar index profile as a conventional step-index fiber, with a high index core, and a lower index cladding, which allows for total internal reflection. Compared to a traditional optical fiber, a hollow core fiber design offers drastic improvement in speed of transmission, since light propagates faster in air than in glass. Our full-wave electromagnetic simulations show that the addition of a layer of ITO ENZ coating (with experimentally obtained dispersion) can reduce the loss of the waveguide by ~70% compared to the structure without ITO. Furthermore, the loss of the material itself has a large contribution to the overall loss of the hollow core fiber, thus further engineering of ENZ materials with lower imaginary part of the permittivity can further enhance the air-guiding performance of the fiber. In addition, we apply similar structure to existing hollow core fiber designs, such as hollow core photonic bandgap crystal fiber [3], as well as anti-resonant fiber [4]. The resulting spectra show similar behavior to those of the simple capillary fiber, and the performance can also be further improved as the loss of the ENZ material is reduced. This study opens up a new pathway for novel hollow core fiber designs, and may find potential applications in in-fiber imaging, sensing, etc.<br/><b>References</b><br/>1. Liberal, I.; Engheta, N. <i>Nature Photonics</i>, <b>11</b>(3), 149 (2017).<br/>2. Minn, K.; Anopchenko, A. <i>Scientific Reports </i><b>8</b>, (2018).<br/>3. Russell, P. <i>Science</i> <b>299</b>(5605), 358-362 (2003).<br/>4. Wei, C.; Weiblen, R.; Menyuk, C.; Hu, J. <i>Adv. Opt. Photon</i>. <b>9</b>, 504-561 (2017).