Label-free biosensing by means of optical micro-ring resonator

Micro-ring resonators have been widely employed, in recent years, as wavelength filters, switches and frequency converters in optical communication circuits, but can also be successfully used as transducing elements in optical sensing and biosensing. Their operation is based on the optical coupling between a ring-shaped waveguide and one or more linear waveguides patterned on a planar surface, typically an input and an output waveguide. When incoming light has a wavelength which satisfies the resonance conditions, it couples into the micro-ring and continuously re-circulates within it. A fraction of this resonant light escapes the micro-ring structure and couples into the output waveguide. The presence of a target analyte over the top surface of the micro-ring (i.e. within the evanescent field) changes the effective refractive index of the mode propagating into the structure, thus causing a shift in resonance wavelength which can be determined by monitoring the spectrum at the output port. Proper functionalization of the micro-ring surface allows to add selectivity to the sensing system and to detect specific interaction between a bioprobe and its proper target (e.g. protein-ligand, DNA-cDNA interactions). We present our preliminary results on the design of micro-ring resonators on silicon-on-insulator substrate, aimed at selective detection of several biomolecules. The design of the structure has been accomplished with the help of FDTD 2D numerical simulations of the distribution of the electromagnetic fields inside the waveguides, the micro-ring and near the micro-ring surface. Furthermore, all the functionalization reactions and the bio/non-bio interfaces have been studied and modelled by means of spectroscopic ellipsometry.