Over 100 million Q silicon nitride Fabry–Perot Bragg nanoresonators enabled by advanced tapered gratings

We report new, to the best of our knowledge, advances in Fabry-Perot Bragg grating nanoresonators on low-loss silicon nitride (Si₃N₄). Building on a previous demonstration of an intrinsic quality factor Q_in of 19.3 million with 1.4 dB/m propagation loss, we now achieve Q_in up to 121 million on a 100-nm Si₃N₄ platform with an ultralow propagation loss of 0.19 dB/m. Even on a 300-nm Si₃N₄ platform, Q_in of 58 million is obtained with 0.45 dB/m loss. Crucially, an advanced tapered grating design is introduced to minimize mode mismatch between the waveguide and Bragg mirror. These improvements pave the way for compact ultrahigh-Q photonic integrated resonators for applications in nonlinear optics and precision photonics.