Racetrack Ring Resonator Integrated with Multimode Interferometer Structure Based on Low-Cost Silica-Titania Platform for Refractive Index Sensing Application

In this work, a racetrack ring resonator (RTRR) integrated with a multimode interferometer (MMI) structure based on a silica-titania (SiO2:TiO2) platform is projected for refractive index sensing application. The typical ring resonator structure requires a gap of similar to 100 nm to 200 nm between the bus waveguide (WG) and the ring structure which makes it challenging to fabricate a precise device. Thus, the device proposed in this paper can be considered a "gapless" ring resonator structure in which the coupling of light between the ring and bus WG can be achieved via an MMI coupler. A minor change in the refractive index in the vicinity of the MMI structure can trigger a shift in the resonance wavelength of the device. Thus, this simple and fascinating structure can be employed as a refractive index sensor. The device's sensitivity is similar to 142.5 nm/RIU in the refractive index range of 1.33 to 1.36 with a figure of merit (FOM) of 78.3. This simple device structure can potentially be fabricated via a low-cost and highly efficient sol-gel process and dip-coating method combined with the nanoimprint lithography (NIL) method.