Mode control in SOI microring resonators through sub-wavelength modifications

Silicon-on-insulator (SOI) microring waveguiding structures show significant promise in telecommunications due to their narrowband resonances, and in biosensing due to their sensitivity to external perturbations. Furthermore,... [ view full abstract ]

Silicon-on-insulator (SOI) microring waveguiding structures show significant promise in telecommunications due to their narrowband resonances, and in biosensing due to their sensitivity to external perturbations. Furthermore, all applications can benefit from the strong confinement of optical energy, hence, small footprint, enabled by the large refractive index of silicon as well as compatibility with mature CMOS technology. However, especially in sensing, strong field confinement can be detrimental as it limits optical interaction with the surrounding media. Furthermore, since microrings support multiple resonances the free spectral range available for sensing is limited. These challenges illustrate a clear need to devise ways to control the energy confined within a microring resonator in both the spatial and spectral dimension.
The main methods of mode control involve structural modifications of the waveguide-coupled microring resonators on the subwavelength scale. Electron beam lithography (EBL), despite its relatively slow serial scanning mode of operation, offers resolution in excess of 10 nm. This makes it capable of control over minute structural features and also to independently tailor exposure conditions for different patterned components, hence, more flexibly compensate for distortive proximity effects. EBL, combined with finite-difference time domain photonic simulation methods, underlies rapid prototyping of novel designs.
In this work both FDTD simulation as well as EBL fabrication of novel SOI based waveguide-coupled microring resonator designs are discussed. Different complimentary approaches to augmenting the performance of microrings in the context of sensing applications are presented. Ring resonators with subwavelength-size annular gratings with finely controlled groove radii ranging from 10 nm to 80 nm have been fabricated using EBL and demonstrate Bragg selection of certain modes as well as slow-light derived dispersion related quality factor enhancement. Numerical simulations show how this principle can be applied to plasmonic nanodisk gratings on ring resonators which lead to a greatly extended free-spectral range and sensing performance. Lastly, EBL fabricated gradient effective refractive index resonators offer strong mode delocalization useful in bulk sensing and also exhibit enhanced Q-factor values. These examples illustrate ways to circumvent known limitations of SOI label-free optical sensors, leading to the increase in sensitivity and a broader utility of such devices.