Unraveling mode-mode interaction drives spectral performance in multi-mode photonic molecules

Multi-mode photonic molecules (MMPMs) based on cavity-coupled waveguide systems exhibit remarkable potential for diverse applications owing to their enhanced light-matter interactions. However, the fundamental mechanisms underlying mode-mode coupling remain inadequately understood, significantly impeding the development of advanced functional optical devices. To address this issue, we designed a theoretical framework based on the time coupled mode theory (TCMT) to elucidate the mode-mode interaction relationships within multimode optical molecules. To validate the universality of our model, we designed a plasmonic configuration featuring an aperture side-coupled slot cavity and a ring cavity. By introducing symmetrical breaking into the structure, we achieved a substantial enhancement in spectral properties, including a 44-fold improvement in the figure of merit (FOM) and a sensitivity of 1200 nm/RIU. These results pave the way for understanding the underlying physics of optical mode interactions and for achieving high-performance photonic devices.