Transmission resonances in silicon subwavelength grating slot waveguide with functional host material for sensing applications

•A CO2 sensor based on subwavelength grating (SWG) slot waveguide is presented.•The slot gap of the SWG slot waveguide is filled with a functional material (PHMB).•The designed sensor offers a sensitivity of 12.9 pm/ppm to CO2 gas. A highly sensitive and selective CO2 gas sensor is presented based on a subwavelength grating (SWG) slot waveguide. Polyhexamethylene biguanide (PHMB) as a functional material fills the slot gap as well as the space between the silicon pillars of the SWG structure. Beyond the photonic bandgap of the SWG slot waveguide, there are transmission resonances sensitive to the refractive index changes of PHMB due to the infiltration of CO2 molecules into the functional material. The numerical simulations indicate that the sensitivity of the structure is S = 12.9 pm/ppm which is considerably higher than the previously designed gas sensors based on functional materials. The higher sensitivity of the proposed sensor is attributed to the strong confinement of the light in the slot gap filled with functional material while previous designs have limited light-matter interaction by placing the functional material in the cladding. The proposed structure may be used to design various sensors by utilizing different functional material sensitive to the desired analyte.