Ultrasensitive topological interface modes for refractive index sensing via all-dielectric one-dimensional photonic...

Topological interface modes (TIMs) are robust, localized states that emerge at the interface between two topologically distinct materials. In this work, the TIMs are studied in a broader frequency spectrum for both TE and TM polarizations using all-dielectric photonic crystal (PC) layers of Si and SiO₂ in a one-dimensional setting, focusing on their localization, tunability, and robustness. By using the novel phase gradient cancellation (PGC) strategy we combine the two centrosymmetric PCs, P and Q, with the same optical thickness but different topological properties, and achieve highly localized TIMs in the working wavelength window around 1548.6 nm using two distinct odd gaps indicated as Gap-1 and Gap-2. Further extending the proposed design to two schemes for defecting the interface region, i.e., by inserting the defect layer directly at the interface, keeping the two PCs intact, and by removing the terminal layers from the interface of adjacent PCs we achieve highly tunable topological defect modes (TDMs) within the topological operating region. The refractive index (RI) sensing by using these two schemes demonstrate the superior performance under ideal conditions defined by the parameters such as the sensitivity (for the best values) of 397.7 THz/RIU or 3179.56 nm/RIU, the ultra-high-quality factor (Q) of 7.24⤫10⁹, ultra-low detection limit (DL) of 7.6⤫10^- 11 RIU, and a high figure of merit (FOM) of 1.32 ⤫10⁹ RIU^- 1. When realistic material absorption and interface roughness (σ = 0-3 nm, Nevot-Croce model) are incorporated, these metrics represent Q ~ 10⁶, DL ~ 10⁻⁷ RIU, and FOM ~ 10⁷ RIU⁻¹, consistent with experimental demonstrations in comparable Si/SiO₂ structures. This work not only enriches the understanding of TIM phenomena but also paves the way to realize multifunctional and high-performance devices in biomedical applications, optical communications, and integrated photonics.