Ultra-Low-Loss Silicon Nitride on Sapphire for Broad-Transparency Nonlinear and Quantum Photonics

The field of photonic integrated circuits (PIC) has flourished in the past two decades, fueling numerous cutting-edge applications across sensing, networking, data interconnect, and quantum information processing. As a guiding material for PIC, Si₃N₄ has seen extensive use for its ultra-low loss, broad transparency, and diversity in implementation across both thin and thick films. Although the standard, traditional silicon dioxide SiO$₂$ on silicon (Si) substrates that underpin the majority of Si₃N₄ photonics face drawbacks in the form of long-wavelength transparency limited by SiO₂, high-stress deposition for anomalous dispersion thick-film Si₃N₄, and leakage loss to the Si layer for low-confinement thin-film Si₃N₄. Featuring increased long-wavelength transparency into the mid-infrared, low-stress deposition of Si₃N₄, and a low index, this work investigates sapphire substrates as alternate hosts for Si₃N₄ photonics with greater spectral coverage and reduced fabrication complexity. This work presents a robust method of fabricating ultra-low loss photonic integrated circuits on a 500-nm-thick Si₃N₄-on-sapphire platform, exhibiting record-low losses below 0.1 rm dB/cm. Implemented using this process are high-Q microrings with intrinsic quality factors in excess of 4.5times10⁶ and coupled-ring photonic molecules to support nonlinear gain. Leveraging the achievable low loss and high-Q, this work further reports the first demonstration of Kerr-comb and soliton generation on the Si₃N₄-on-sapphire platform. These advances in loss, quality factor, and soliton generation on this versatile, broad-transparency platform pave the way for future work in spectroscopy and quantum-enhanced sensing across previously prohibited spectral regions for Si₃N₄ photonics with reduced fabrication complexity.