Planar optical microring resonators used as biosensors: guidelines for designing polymer compared to semiconductor-based waveguides

Abstract

Due to their small footprint and high sensitivity to biological molecule binding, planar optical microring resonators gained high interest for use as optical biosensors. Typically, microring resonators are made of semiconductor based materials, and are manufactured by time-consuming lithography and etching steps. Semiconductor based waveguides have high refractive indices, and thus, a high refractive index contrast between core and cladding. In this case, due to strong mode confinement, bending loss is a comparably minor issue and becomes relevant only at small bending radii of less than 5 μm. The main loss is determined by surface scattering, and thus, semiconductor based curved waveguides need to be designed and manufactured to have very smooth sidewalls. If polymer materials are used, microring resonators can be cost-efficiently manufactured by nanoimprint lithography. The resulting larger polymer waveguide dimensions facilitate in- and out-coupling, and polymer surfaces allow using established surface biofunctionalization techniques. For polymer waveguides, due to the small refractive index contrast, surface scattering loss is a minor issue, but bending loss becomes dominant for radii of less than 80 μm due to the low mode confinement to the core. In this work, design guidelines for polymer microring resonator waveguides are given and compared to semiconductor based waveguides. Waveguide losses due to bending and surface roughness are determined analytically or numerically by finite element methods. Coupling coefficients are calculated by finite element methods and coupled-mode theory. Resulting conclusions for designing polymer waveguides and semiconductor waveguides are derived.