Compact optical waveguide grating RI sensor integrated with an Au–ZnO-coated plus-shaped cavity

In this work, a compact optical waveguide grating sensor integrated with a gold (Au)–zinc oxide (ZnO)-coated plus-shaped cavity (PSC) is proposed for ultra-sensitive refractive index (RI) sensing of cerebrospinal fluid (CSF). The engineered PSC configuration significantly enhances light–matter interaction by enabling efficient phase-matched coupling of guided modes to surface plasmon polaritons (SPPs) at the Au–ZnO interface. The resulting strong electromagnetic field confinement is exploited for precise RI-based interrogation of CSF, whose RI varies from 1.33487 to 1.33924. For the tested RI range, the proposed sensor exhibits an auto-correlation coefficient (R 2 ) of 1 and a sensitivity of 3128.2(W.m −1 )⋅RIU −1 . This performance is achieved through rigorous optimization of key excitation parameters, including the input optical power, beam half-width, and operating wavelength. Furthermore, the influence of the PSC vertical slot width on device performance is investigated to ensure robust and optimized sensing characteristics. The obtained results demonstrate that the proposed PSC-based SPR sensor offers a compact, highly reliable, and high-resolution sensing platform, indicating potential for rapid and accurate RI-based diagnosis of CSF samples.