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Contact: Michael Grad

The objective of this project is to demonstrate that optical ring resonators can be used as time-resolved refractive index sensors embedded in microfluidic channels. The optical racetrack resonators (figure a) consist of silicon waveguides (nSi=3.4) fabricated on top of a silica lower cladding layer (nSiO2=1.5). Light is confined within the waveguide due to total internal reflection at the interface between the silicon and its surrounding cladding layers. Perturbations in the refractive index of the upper cladding layer from different fluids in a microchannel (i.e. nwater=1.33 and noil=1.43) will cause changes in the effective index of refraction of the waveguides, altering the spectral location of its resonant peak. By measuring these resonance shifts we exploit these resonators as refractive index sensors.

The steady-state sensitivity, resolution, and detection limit of the sensors are characterized with different salt concentrations (figure b). Time-resolved measurements are performed by sensing liquid films associated with oil/water segmented flow in microfluidic channels. The influence of the interrogation wavelength is investigated, and the optimal wavelength is determined (figure c). Finally, the film thickness between the droplet and the resonator is measured for different capillary numbers and channel diameters, and compared with existing results (figure d).