Direct Observation of Switching-Wave Dynamics in 20 GHz Normal Dispersion Spiral Microresonators

While traditional architectures rely on the anomalous-dispersion regime, this typically requires thick, highly strained silicon nitride layers that complicate standard CMOS-foundry fabrication. Thinner layers circumvent these fabrication constraints but yield normal-dispersion microcombs that generally require specific trigger mechanisms, such as deterministic seeding, due to the absence of spontaneous modulational instability.

Here, we demonstrate the generation of a 20 GHz microcomb in the normal-dispersion regime, driven by a synchronized dual-pump scheme via electro-optic sidebands modulation. The platform leverages an Archimedean spiral geometry on a foundry-compatible, 350 nm-thin SiN platform. By employing adiabatic curvature engineering, the resonator balances a compact footprint with an intrinsic quality factor exceeding $7 \times 10^6$, while effectively eliminating avoided mode crossings with higher order modes.