Toggleable transparency states in thermally-shifted multiMRR cascaded filters

We showcase and analyze a control scheme to leverage thermal-induced transparency in cascaded multistage microring resonator (MRR) structures. MRRs are one of the basic building blocks in integrated photonics, typically used to implement high Q-factor filters and frequency-dependent circuits. While traditionally single- order rings are reserved for straightforward and simple applications, higher-order cascaded structures can be arranged to introduce and implement advanced filters, showcasing larger flat-top bandwidth and allowing precise tailoring of the response. Thermally induced phase delay between the MRRs can be used to achieve precise control of the response, while also allowing a transparent switching state obtained through destructive interference of the different ring responses. In our work, we investigate how this effect can be leveraged to turn off the filtering structure, introducing a toggleable state, while validating the theoretical uses and limitations of the control scheme by implementing the structure through programmable integrated photonic circuits