We report the design and multiphysics simulation of a monolithically integrated optoelectronic methane (CH\(_4\)) sensor that operates without external electrical bias or spectroscopic instrumentation. The device comprises a methane-selective cryptophane-A-infiltrated GaAs photonic crystal slab, engineered to support a sharp Fano resonance at \(\lambda \approx 1560\) nm, monolithically integrated with an InP/In\(_{0.53}\)Ga\(_{0.47}\)As/InP p-i-n photodiode. Methane adsorption reduces the refractive index of the cryptophane-A layer, inducing a blueshift of the Fano resonance that is directly transduced into a photocurrent change in the underlying detector. Simulations predict a sensitivity of \(0.8124\) (A/cm\(^2\))/%CH\(_4\) with excellent linearity (\(R^2 = 0.9996\)) in self-powered mode, and an ultrafast photoresponse with \(>10\) GHz bandwidth–orders of magnitude faster than the methane adsorption kinetics. The InP/InGaAs heterojunction design, employing a wide-bandgap p-InP anode rather than conventional p-InGaAs, suppresses dark current to \(1.04\times 10^{-7}\) A/cm\(^2\) at zero bias while maintaining \(>94\%\) absorption efficiency in the 3 \(\mu\)m intrinsic layer. Fabrication tolerance analysis demonstrates robust performance against \(\pm 5\) nm variations in hole radius and \(\pm 10\) nm variations in analyte thickness, with sensitivity degradation below \(3\%\). Unlike passive ultrahigh-Q resonators that require external spectroscopy, this fully integrated platform combines high optical sensitivity, efficient photocarrier generation, and zero static power consumption in a compact, scalable architecture suitable for distributed environmental monitoring, industrial safety, and IoT applications.
