Guiding light through modes embedded in the radiation continuum

Guiding light through modes embedded in the radiation continuum

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The concept of bound states in the continuum (BICs), introduced nearly 90 years ago in quantum physics as a theoretical construction, has become the subject of an intensive investigation in the realm of photonics. In this talk, we report on our recent studies on BICs which appear in the photonic waveguides. The propagating BICs are usually observed in waveguides that exhibit the effect of lateral leakage. However, the states have some limitations: they are localized in or in the vicinity of high-refractive-index materials, they are observed as quasi-TM polarized waves, and they appear only for certain "magic" sets of parameters. Here, we focus on two systems that enable alternative mechanisms of BIC formation. In contrast to the previously described configurations, the first geometry, consisting of a rectangular waveguide coupled to a planar waveguide, facilitates genuine guiding of quasi-TE modes in the core with the lower refractive index. The TE BICs appear when the coupling between the bound and radiation modes is suppressed due to accidental orthogonality of modes. The second system, proton exchanged waveguide in Z-cut thin-film lithium niobate, can support very low-loss TM-polarized quasi-BIC modes for a range of waveguide widths not limited to "magic widths". This is because the system exhibits a very small change of the ordinary refractive index between the core and substrate, which enables formation of polarization-protected quasi-BICs. We believe that the presented approaches well illustrate rich physics associated with BICs and offer new possibilities in their technical applications.

The concept of bound states in the continuum (BICs), introduced nearly 90 years ago in quantum physics as a theoretical construction, has become the subject of an intensive investigation in the realm of photonics. In this talk, we report on our recent studies on BICs which appear in the photonic waveguides. The propagating BICs are usually observed in waveguides that exhibit the effect of lateral leakage. However, the states have some limitations: they are localized in or in the vicinity of high-refractive-index materials, they are observed as quasi-TM polarized waves, and they appear only for certain "magic" sets of parameters. Here, we focus on two systems that enable alternative mechanisms of BIC formation. In contrast to the previously described configurations, the first geometry, consisting of a rectangular waveguide coupled to a planar waveguide, facilitates genuine guiding of quasi-TE modes in the core with the lower refractive index. The TE BICs appear when the coupling between the bound and radiation modes is suppressed due to accidental orthogonality of modes. The second system, proton exchanged waveguide in Z-cut thin-film lithium niobate, can support very low-loss TM-polarized quasi-BIC modes for a range of waveguide widths not limited to "magic widths". This is because the system exhibits a very small change of the ordinary refractive index between the core and substrate, which enables formation of polarization-protected quasi-BICs. We believe that the presented approaches well illustrate rich physics associated with BICs and offer new possibilities in their technical applications.

bound states in the continuum, integrated photonics, guided modes, leaky modes, anisotropic waveguides

bound states in the continuum, integrated photonics, guided modes, leaky modes, anisotropic waveguides

PETRÁČEK, J.; CTYROKY, J.; KUZMIAK, V.; RICHTER, I.

2026

05.06.2025

SPIE-INTERNATIONAL SOCIETY FOR OPTICAL ENGINEERING

BELLINGHAM

978-1-5106-8856-8

Proceedings of SPIE

Proceedings of SPIE

Spojené státy americké

6

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/13530/3056433/Guiding-light-through-modes-embedded-in-the-radiation-continuum/10.1117/12.3056433.short