High-Sensitivity NIR Blood Analyte Sensing Using a Double-Defect 1D Photonic Crystal: A Comparative TMM and FEM Investigation

<title>Abstract</title> <p> In the present work, a symmetric one-dimensional (1D) Silicon/Silicon Dioxide double-defect photonic crystal biosensor for high-resolution blood component screening in the near-infrared (NIR) regime has been proposed. The proposed structure, optimized for a central design wavelength of 1550 nm, is represented as \(\:\text{Air}/\left(\text{BA}{)}^{N}\text{-B-D-B-D-}\right(\text{BA}{)}^{N}\text{-B/Air}\), where A and B represent Silicon Dioxide and Silicon layers, respectively, and D denotes identical microfluidic defect cavities containing blood analytes. The transmission spectra and localized electric field confinement profiles are rigorously modelled and validated using the Transfer Matrix Method (TMM) and a 1D Finite Element Method (FEM) Helmholtz solver. Due to electromagnetic state hybridization in the coupled defect cavities, the transmission spectrum exhibits split resonance modes (Peak 1 and Peak 2) inside the photonic bandgap. Under normal incidence θ = 0 ⁰ with a period N = 7, the sensor achieves a high refractive index sensitivity of up to \(\:544.14\text{nm/RIU}\) for Hemoglobin, extremely narrow linewidths (FWHM down to 5.08 pm), and Quality Factors (Q) up to 2.76×10 ⁵ for Plasma, resulting in a Limit of Detection (LoD) in the order of 10 ^− 6 RIU. The transition to extreme oblique angles of incidence (up to 80 ⁰ ) significantly elongates the optical path length through the cavity defects, enhancing the peak sensitivity to 784.18 nm/RIU and narrowing the resonance FWHM to sub-picometer levels 0.25 pm, which yields an ultra-high Quality Factor of 5.32×10 ⁶ and a Figure of Merit (\(\:\text{FoM}\)) of 3.12×106 RIU ^− 1 for the Plasma component. The comparative analysis shows perfect agreement between TMM and FEM calculations, with minor resonance peak shifts (+ 50 to + 90 pm) arising from finite-element discretization grid rounding, and peak shift discrepancies of less than 5 pm, validating the numerical consistency of the model. These results demonstrate that the proposed symmetric double-defect configuration represents a highly sensitive, and stable optical platform for rapid NIR blood diagnostics. </p>