Induced Directional Switching of Platicon Microcombs in Photonic Crystal Ring Resonators

Abstract: Microcombs in normal-dispersion photonic crystal ring resonators (PhCRs) are a versatile building block for next-generation integrated photonic circuits, yet they inherently suffer from a directional bias that favors backward-propagating states. This necessitates bulky, non-integrated optical circulators for comb extraction, creating a significant bottleneck for full on-chip integration. In this work, we demonstrate a deterministic method to control and reverse this directionality through Side-mode Induced Forward Forcing (SIFF). By engineering auxiliary mode splittings on resonances adjacent to the pump, we show that the nonlinear dynamics can be steered to favor stable, forward-propagating platicon states. We establish an optimal synchronization condition between the pump and side-mode coupling rates that ensures forward-comb dominance across a wide parameter range. Our findings, validated both numerically and experimentally, provide a critical pathway for circulator-free, integrated normal-dispersion microcombs, offering a scalable architecture for compact telecommunications and sensing systems.