Beyond the lossless idealization: disturbance, losses, and real-air detection limits of a topological photonic crystal gas sensor

The reported performance of topological photonic crystal sensors is usually obtained under idealized assumptions (lossless and dispersionless materials, perfectly periodic structures, isothermal operation and pure analytes), leaving open whether the headline figures of merit survive contact with a real device. This work quantifies the practical operating envelope of a one-dimensional topological photonic crystal heterostructure gas sensor comprising silicon and air layers. The sensor exploits a topological edge state at the interface between two photonic crystals whose overlapping band gaps have opposite topological signs and, in the customary lossless limit, attains a sensitivity of 858 nm per refractive index unit (nm/RIU), a quality factor of 3.15 × 10 6 , and a detection limit of 5 × 10 -7 RIU. The envelope analysis shows that the interface state survives every disorder realization with fully preserved sensitivity and slower quality-factor degradation than a parameter-matched defect cavity, despite its wavelength is not topologically protected; that realistic silicon losses limit the quality factor to 10 4 –10 5 and define an optimal number of periods; that the thermal drift (27.3 pm/K) and the pressure and humidity cross-sensitivities can be compensated; and that methane is detectable in ambient air at ≈ 15% of its lower explosive limit with a 1 pm readout. A silicon-oninsulator fabrication route with direct gas access is also proposed. These results delimit combustible and industrial gas leak detection as the realistic application domain and provide a design-to-device template for evaluating topological photonic sensors. Trace-level monitoring and gas selectivity require functional coatings.