Study on the Enhancement of a Square‐Core D‐Shaped Optical Fiber Sensor Based on Surface Plasmon Resonance Functionalized With Ag φ Au 1−φ Alloy and Amorphous Ge 2 Sb 2 Te 5 Nanolayer

ABSTRACT This study presents a numerical analysis of a D‐shaped fiber optic biosensor with a square core based on the surface plasmon resonance () phenomenon, incorporating a metal alloy layer composed of gold Au and silver Ag, then coated with a thin amorphous layer of (). The behavior and performance of the sensor are analyzed through simulations using the finite element method (). The flat surface of the D‐shaped fiber is functionalized with the alloy, followed by the deposition of the amorphous layer, allowing for a detailed evaluation of plasmonic interactions and biosensor efficiency. SPR‐based biosensors take advantage of variations in the surrounding refractive index to detect biomolecular interactions, pathogenic cells, or tissue anomalies, offering high sensitivity, excellent conductivity, and fast response times. In this study, the proposed biosensor operates in a biological environment with a wavelength‐dependent refractive index. Transmission, absorption, and dielectric loss spectra are obtained under electromagnetic excitation in the visible‐near infrared () range to determine the corresponding resonance wavelength (). By optimizing the structure geometry and material parameters, the sensor performance characteristics, such as sensitivity, full width at half maximum (), figure of merit (), and detection accuracy (), are significantly improved. The optimized biosensor exhibits a maximum sensitivity of nm/RIU, a narrow FWHM of , and a high Figure of Merit (FOM) of 190 RIU −1 , with a detection accuracy of . The results reveal that integrating the alloy with amSorphous and using a D‐shaped square core fiber allows for precise tuning of and increased sensitivity, surpassing conventional SPR biosensor technologies. This breakthrough paves the way for new next‐generation chemical and biological detection platforms.