A Silicon Nitride Microring Modulator for High-Performance Photonic Integrated Circuits

The use of the Silicon-on-Insulator (SOI) platform has been prominent for realizing CMOS-compatible, high-performance photonic integrated circuits (PICs). But in recent years, the silicon-nitride-on-silicon-dioxide (SiN-on-SiO$_2$) platform has garnered increasing interest as an alternative, because of its several beneficial properties over the SOI platform, such as low optical losses, high thermo-optic stability, broader wavelength transparency range, and high tolerance to fabrication-process variations. However, SiN-on-SiO$_2$ based active devices, such as modulators, are scarce and lack in desired performance due to the absence of free-carrier-based activity in the SiN material and the complexity of integrating other active materials with SiN-on-SiO$_2$ platform. This shortcoming hinders the SiN-on-SiO$_2$ platform for realizing active PICs. To address this shortcoming, in this article, we demonstrate a SiN-on-SiO$_2$ microring resonator (MRR) based active modulator. Our designed MRR modulator employs an Indium-Tin-Oxide (ITO)-SiO$_2$-ITO thin-film stack as the active upper cladding and leverages the free-carrier assisted, high-amplitude refractive index change in the ITO films to affect a large electro-refractive optical modulation in the device. Based on the electrostatic, transient, and finite difference time domain (FDTD) simulations, conducted using photonics foundry-validated tools, we show that our modulator achieves 450 pm/V resonance modulation efficiency, $\sim$46.2 GHz 3-dB modulation bandwidth, 18 nm free-spectral range (FSR), 0.24 dB insertion loss, and 8.2 dB extinction ratio for optical on-off-keying (OOK) modulation at 30 Gb/s.