  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.
