Highly sensitive refractive index sensing by fast detuning the critical coupling condition of slot waveguide ring resonators.

We experimentally investigate refractive index sensing in silicon slot waveguide ring resonators by the detection of the giant shift of the ring transmission spectrum envelope enabled by the following specific conditions: the slot waveguide cross section as well as the ring couplers have been designed to lead to a V-shaped microring resonator spectrum modulated by the classical frequency comb and exhibiting quality factor peaks of 2000-6000 around λ=1.5  μm. By tracking the spectrum envelope wavelength shift, sensitivity up to S=1,300  nm per refraction index unit (RIU) is reported when the slots are filled by liquids with refraction index values close to 1.33.