Experimental study of subwavelength grating bimodal waveguides as ultrasensitive interferometric sensors.

Over the recent years, subwavelength grating (SWG) structures have increasingly attracted attention in the area of evanescent-field photonic sensors. In this Letter, for the first time to the best of our knowledge, we demonstrate experimentally the real-time refractive index (RI) sensing using the SWG bimodal interferometric structures. Two different configurations are considered to compare the effect of the nonlinear phase shift, obtained between the two first transverse electromagnetic propagating modes, in the measured bulk sensitivity. Very high experimental values up to 2270 nm/RIU are reached, which perfectly match the numerical simulations and significantly enhance other existing SWG and spectral-based sensors. By measuring the spectral shift, the obtained experimental sensitivity does not depend on the sensor length. As a result, a highly sensitive and compact single-channel interferometer is experimentally validated for refractive index sensing, thus opening new paths in the field of optical integrated sensors.