Ammonia Gas Optical Sensor Based on Lossy Mode Resonances

This letter presents the fabrication and characterization of an ammonia (NH
₃
) gas optical sensor based on lossy mode resonances (LMRs). A chromium (III) oxide (Cr
₂
O
₃
) thin film deposited onto a planar waveguide was used as LMR supporting coating. The obtained LMR shows a maximum attenuation wavelength or resonance wavelength centered at 673 nm. The optical properties of the coating can be modified as a function of the presence and concentration of NH
₃
in the external medium. Consequently, the refractive index of the Cr
₂
O
₃
thin film will change, producing a red-shift of the resonance wavelength. Obtained devices were tested for different concentrations of NH
₃
as well as repetitive cycles. Concentrations as low as 10 ppbv of NH
₃
were detected at room temperature. Machine learning regression models were used to mitigate the cross-sensitivity of the device under temperature and humidity fluctuations.