Achieving Fano resonance with an ultra-high slope rate by silicon nitride CROW embedded in a Mach-Zehnder interferometer.

Abstract

Fano resonance with asymmetric line shape is very promising in many applications such as optical switching, sensing, slow light, laser. Fano resonances based on some integrated structures have been demonstrated on the silicon on insulator platform. However, the extinction ratios and slope rates of the most proposed integrated Fano resonances are relatively low, which limits their applications. In this paper, a tunable silicon nitride coupled resonator optical waveguide (CROW) embedded in a Mach-Zehnder interferometer (MZI) is proposed to achieve Fano resonance. Benefiting from fine tuning supported by the low thermo-optic coefficient of the silicon nitride optical waveguide, the optical amplitudes and phases in the two arms of the MZI were accurately adjusted to achieve destructive interference, which gives an ultra-high extinction ratio. Furthermore, high quality factor CROW, supported by the native low loss silicon nitride optical waveguide, greatly shrinks the resonance bandwidth. Combining the above two superiorities, a Fano resonance with a very high extinction ratio of up to 57 dB and slope rate as high as 8.1 × 104 dB/nm was obtained, which is an order of magnitude larger than the reported integrated Fano resonances. We believe that the proposed structure would be a promising candidate for high-performance switching and high-sensitivity sensing.