Conformational analysis of proteins with a dual polarisation silicon microring.

Optical microresonator biosensors have proven to be a valid tool to perform affinity analysis of a biological binding event. However, when these microresonators are excited with a single optical mode they can not distinguish between a thin dense layer of biomolecules or a thick sparse layer. This means the sensor is "blind" to changes in shape of bound biomolecules. We succeeded in exciting a Silicon-on-Insulator (SOI) microring with TE and TM polarisations simultaneously by using an asymmetrical directional coupler and as such were able to separately determine the thickness and the density (or refractive index) of a bound biolayer. A proof-of-concept is given by determining both parameters of deposited dielectric layers and by analysing the conformational changes of Bovine Serum Albumin (BSA) proteins due to a change in pH of the buffer.