Magnetoplasmonic core–shell nanowire integrated in D-shaped fibers for enhanced optical sensing

Optical fiber sensors have become essential tools in modern photonics due to their compactness, flexibility, and capability for real-time and remote detection. Among the various sensing approaches, surface plasmon resonance (SPR) techniques stand out for their high refractive index (RI) sensitivity and compatibility with nanostructured materials. In this context, conventional in-fiber SPR sensors operate purely on passive plasmonic effects, whereas the integration of magneto-optical (MO) materials introduces an additional degree of control, enabling active tunability and sharper optical responses. However, the realization of in-fiber MO-SPR configurations that simultaneously provide magnetic tunability, strong field confinement, and a broad RI operating range remains largely underexplored. Herein, a MO-SPR sensor based on a D-shaped optical fiber side-coupled to a core–shell nanowire (NW) structure is presented. The proposed approach uniquely combines core–shell nanowire-based SPR, MO tunability, and an extended RI operating range within a single in-fiber platform. The NW comprises a metallic core coated with a ferrimagnetic shell and is positioned on the polished surface of a single-mode fiber (SMF), where the guided mode HE \(_{11}^y\) couples to the magnetoplasmonic mode in the NW. This interaction yields strong electromagnetic field confinement and tunable dispersion at the NW–fiber interface. Numerical analysis demonstrates a pronounced confinement loss peak of 3345 dB/m and a high RI sensitivity of S = 3240 nm/RIU over a wide analyte RI range, spanning the optical S-, C-, L-, and U-bands. When an external magnetic field is applied, the MO effect induces distinct resonance shifts depending on the magnetization direction, resulting in a MO-SPR sensor with an enhanced figure of merit (FoM) of 1912 RIU⁻¹, approximately four times greater than that achieved with conventional SPR configurations. These results demonstrate that the proposed D-shaped MO-SPR fiber architecture provides a robust and highly sensitive platform for next-generation in-fiber sensing, combining the advantages of plasmonic field confinement and magnetically tunable optical response.