Defect-mode ternary MgF2/SiON/TiO2 photonic crystal for theoretical NIR discrimination of literature-derived skin-cancer refractive-index scenarios

Abstract

One-dimensional photonic crystal (1D-PhC) defect-mode resonators offer a compact and tunable platform for optical biosensing, yet achieving reliable discrimination of closely spaced analytes within a single photonic bandgap (PBG) remains challenging because the resonance characteristics depend jointly on the mirror materials, defect-cavity thickness, and number of periods. We present a ternary 1D-PhC biosensor, Air/(MgF₂/SiON/TiO₂)\({}^{N}\)/D/(MgF₂/SiON/TiO₂)\({}^{N}\)/Glass, evaluated by the transfer matrix method against literature-derived refractive indices for melanoma (ME), basal cell carcinoma (BCC), and squamous cell carcinoma (SCC), yielding simulated \(S=646\)–652 nm RIU\({}^{-1}\), \(Q=31248\)–31644, and FOM = 19785–19966 RIU\({}^{-1}\) with the mirror pair lossless (\(k=0\)) to experimental precision; these are loss-consistent, not idealized, values, still simulated and unfabricated, with defect-layer and sample losses unmodeled. Within a 905.0–1342.3 nm PBG, the three subtypes’ defect modes separate by 5.83–46.93 nm, or 178–1654 times the mode linewidth. The primary discrimination claims are BCC\(\rightarrow \)ME and SCC\(\rightarrow \)ME (Monte Carlo reversal probability \(\le 12.6\%\)), spanning different modalities, not only biology; SCC\(\rightarrow \)BCC is excluded and reported only as an exploratory observation, since its oral-mucosa-proxy SCC index carries a \(20.7\%\) reversal probability exceeding the discrimination gap. The ternary architecture and TMM formalism are established in prior literature, not claimed as new; the contribution is a skin-specific triple-subtype application with an explicit two-tier validity framework separating supported claims from exploratory and calibration comparisons, within which the ternary-over-binary benefit is quantified (FOM 3.12–\(3.14\times \) higher at the ME mode) rather than assumed. The results demonstrate optical distinguishability of the literature-derived refractive indices used, not clinical distinguishability of skin lesions, requiring dedicated skin-specific measurement and validation.