Angle-resolved AlN/SiO2 phoxonic microcavity for optical-acoustic readout of sweat cortisol perturbations toward future receptor-mediated chemical transduction.

Sweat cortisol is a chemically relevant biomarker associated with physiological stress responses, but its direct optical and acoustic perturbation in aqueous media is extremely weak. Here, we present a numerical design of an AlN/SiO2 phoxonic microcavity framework toward future receptor-mediated chemical transduction, based on alternating aluminium nitride and silicon dioxide layers and considering a cortisol-responsive aptagel layer as a prospective receptor-mediated transduction element. The structure was analysed using angle-resolved optical transfer-matrix modelling for TE and TM polarizations together with longitudinal acoustic transfer-matrix calculations. Direct bulk-cortisol modelling, governed by Langmuir binding with KD = 500 nM, produced a refractive-index perturbation of 9.802 × 10-9 RIU at 50 ng mL-1, corresponding to an optical wavelength shift of 0.01153 pm at normal incidence. Geometry optimization increased the optical readout response to 0.02711 pm at 75° incidence using a defect-thickness factor of 1.60, providing a 1.77-fold optical readout enhancement. Acoustic modelling identified a loaded defect resonance at 6.158 GHz within a Bloch stop band of 5.952-9.644 GHz. After numerical convergence validation, the optimized acoustic mirror configuration with N = 12 yielded a quality factor of 1.99 × 105, an interpolated FWHM of 0.0309 MHz, and an 82.51-fold acoustic readout-quality enhancement. These improvements represent enhanced optical-acoustic resonance tracking rather than biochemical amplification. The proposed AlN/SiO2 phoxonic microcavity therefore provides a physics-based readout framework for cortisol-containing sweat media, while practical stress-associated sensing requires experimentally calibrated receptor-mediated transduction.