Abstract
Abstract Photonic bound states in the continuum (BICs) are eigenmodes with an infinite lifetime, which coexist with a continuous spectrum of radiating waves. BICs are not only of great theoretical interest but also have a wide range of practical applications, e.g. in the design of optical resonators. Here, we study this phenomenon in a new integrated nanophotonic element consisting of a single dielectric ridge terminating an abruptly ended slab waveguide. This structure can be considered as an on-chip analog of the Gires-Tournois interferometer (GTI). We demonstrate that the proposed integrated structure supports high-Q phase resonances and robust BICs. We develop a simple but extremely accurate coupled-wave model that clarifies the physics of BIC formation and enables predicting BIC locations. The developed model shows that the studied BICs are topologically protected and describes the strong phase resonance effect that occurs when two BICs with opposite topological charges annihilate.
Highlights
BICs are of great theoretical interest and have a wide range of practical applications, e.g. in the design of optical resonators. We study this phenomenon in a new integrated nanophotonic element consisting of a single dielectric ridge terminating an abruptly ended slab waveguide
Resonances are a key concept in photonics since they lie behind many intriguing optical effects arising in various photonic structures
We propose a new on-chip photonic structure – the integrated Gires-Tournois interferometer (GTI), which consists of a single dielectric ridge terminating an abruptly ended slab waveguide and operates in the total internal reflection geometry
Summary
Resonances are a key concept in photonics since they lie behind many intriguing optical effects arising in various photonic structures. Most studies on BICs are dedicated to periodic structures, in particular, photonic crystal slabs [1, 7, 12, 17,18,19], guided-mode resonant gratings [8, 15, 20], interfaces of photonic crystals [21], and infinite arrays of dielectric rods or spheres [19, 22,23,24] In all these structures, the open scattering channels are the “free-space” diffraction orders. We show that the considered structure supports BICs only when the the coupling between the TE and TM guided modes at the second edge of the ridge is relatively weak We formulate this “weak polarization coupling” condition analytically and show that when the structure is altered so that this condition becomes violated, BICs having opposite topological charges group in pairs, coalesce, and annihilate each other, leading to the strong phase resonance effect.
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