Engineering the spectral properties of Fano-type resonances in active racetrack-waveguide coupled structures

Racetrack resonators present an interesting alternative to implement waveguide/resonator coupling systems enabling accurate control of the power coupling. The intra-cavity reflections arising at the interfaces between the straight and the curved waveguides of the racetrack, provide an additional degree of freedom in the design of the spectral properties of the device and under certain conditions Fano-type resonances appear which can be useful in a number of applications including optical sensing. In this paper we present a comprehensive framework for engineering the spectral properties of these resonances. Suitable metrics that can be used to optimize the design are identified along with a set of conditions that the transfer function must fulfill from an application point-of-view. We show how the design parameters involved in the transfer function calculation, can be used to determine the required structural and material parameters of the structure and provide examples of structure designs that possess favorable spectral characteristics.