Measurement of small molecule diffusion with an optofluidic silicon chip.

In this work we explore the micro-ring resonator platform to study the diffusion-driven mass transport of small molecules within microfluidic channels. The micro-ring resonators are integrated on a silicon-on-insulator photonic chip and combined with microfluidics in poly(dimethylsiloxane) (PDMS). We apply a strong initial gradient in the solute concentration and use the micro-ring resonators to observe how this concentration evolves over time and space. This can be achieved by tracking the optical resonances of multiple micro-rings as they shift with changing solute concentration. Experiments are performed for both glucose and NaCl and at different temperatures. The measured concentration profiles are used to calculate the diffusion coefficient of both glucose and NaCl in water. The good agreement between measurement and theoretical prediction demonstrates the relevance of this method.

Integrated interferometric approach to solve microring resonance splitting in biosensor applications.

Silicon-on-insulator microring resonators have proven to be an excellent platform for label-free nanophotonic biosensors. The high index contrast of silicon-on-insulator allows for fabrication of micrometer-size sensors. However, it also limits the quality of the resonances by introducing an intrinsic mode-splitting. Backscattering of optical power at small waveguide variations lifts the degeneracy of the normal resonator modes. This severely deteriorates the quality of the output signal, which is of utmost importance to determine the performance of the microrings as a biosensor. We suggest an integrated interferometric approach to give access to the unsplit, high-quality normal modes of the microring resonator and experimentally show an improvement of the quality factor by a factor of 3.