Tunable Fano and EIT-like resonances based on two cascaded micro-ring resonators

Microring resonators (MRRs) are key building blocks in integrated photonics, but their performances usually suffer from limitations in extinction ratio and slope rate (SR), primarily due to their symmetrical Lorentzian resonance line shape. To overcome these limitations, special transmission spectra (STS), such as Fano resonance and electromagnetically induced transparency (EIT)-like resonance, offer asymmetric line shapes that exhibit a higher extinction ratio (ER) and a steeper SR, thereby enhancing the sensitivity and selectivity of optical switches, filters, and high-sensitivity sensors. However, the current reported works typically achieved only one type of STS and had limited tunability. In this Letter, we experimentally demonstrate a novel, to the best of our knowledge, device using two cascaded MRRs with different quality factors to generate Fano and EIT-like resonances at distinct ports. The ER can reach up to 31.2 dB with a sharp SR of 130.1 dB/ nm, for Fano resonance, and the EIT-like resonance is 12.4 dB. By adjusting the resonator wavelength of the MRRs and the phase shifter, the STS can be working at any desired wavelength. The device is demonstrated on a silicon-on-insulator platform, offering versatile application potential in optical sensors, slow and fast light generation, and microwave photonics.