In detecting presence of a gas, light passes through a photonic crystal cavity configured to sample a gas and receive light. The light has a wavelength that at least partially falls within a specific absorption wavelength of the gas. At least one parameter of a band gap spectrum is generated from at least a portion of the light passing through the photonic crystal cavity. In generating the at least one parameter, a numerical simulation is performed within a frequency range of the photonic crystal cavity to generate a set of spectral response data points, rational function interpolation is performed on the set of spectral response data points to generate a spectral response, and at least one parameter is determined from the spectral response. The at least one parameter is compared with stored parameters of band gap spectrums, wherein a match indicates a presence and/or concentration of the gas.