The conventional operation of optical waveguides based on total internal reflection requires that light is guided typically in solid-state materials, which exhibit higher refractive indices than air. This represents a fundamental limitation for certain applications, for example, when any nonlinear interaction between the wave and the material of the core is undesirable or when the presence of a gas needs to be detected with light. The solution is presented by the adoption of alternative guidance mechanisms, such as the photonic bandgap effect or anti-resonant guidance, both of which allow light to be guided in a low-refractive index material surrounded by a higher index (microstructured) cladding. These mechanisms have been widely explored and already revolutionized fiber optics by enabling glass-free guided wave applications that were previously not possible. Naturally, the prospect of implementing similar devices in miniaturized waveguides raises significant interest. The paper by Bappi et al. presents a numerical study on the potential of hollow-core waveguides for realizing low-loss functional devices. Bragg reflectors and cavities are explored in particular, highlighting the non-trivial challenge of developing practical and competitive solutions.
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