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Visible-light photonic integrated circuits (PICs) promise scalability for technologies such as quantum information, biosensing, and scanning displays, yet extending large-scale silicon photonics to shorter wavelengths has been challenging due to the higher losses. Silicon nitride (SiN) has stood out as the leading platform for visible photonics, but the propagation losses strongly depend on the film’s deposition and fabrication processes. Current loss measurement techniques cannot accurately distinguish between absorption and surface scattering, making it difficult to identify the dominant loss source and reach the platform’s fundamental limit. Here we demonstrate an ultra-low loss, high-confinement SiN platform that approaches the limits of absorption and scattering across the visible spectrum. Leveraging the sensitivity of microresonators to loss, we probe and discriminate each loss contribution with unparalleled sensitivity, and derive their fundamental limits and scaling laws as a function of wavelength, film properties and waveguide parameters. Through the design of the waveguide cross-section, we show how to approach the absorption limit of the platform, and demonstrate the lowest propagation losses in high-confinement SiN to date across the visible spectrum. We envision that our techniques for loss characterization and minimization will contribute to the development of large-scale, dense PICs that redefine the loss limits of integrated platforms across the electromagnetic spectrum.

We demonstrate a novel approach to actively and continuously tune the coupling condition of microresonators. Our approach allows for wavelength-dependent coupling and dispersion modification after fabrication.

We show the ability to control the degree of mode coupling in high quality factor silicon nitride (Si3N4) microresonators by controlling the atomic nature of the resonators interface.

We demonstrate the loading of very short optical pulses into a high-Q cavity with linewidth much narrower than the pulse frequency envelope. We show that loading into the cavity is significantly enhanced if the pulse is combined with a cw-field, thus altering the pulse frequency profile to better match the cavity profile.

Using silicon-nitride microresonators with integrated Moiré-Bragg gratings to suppress parasitic nonlinear processes, we demonstrate on-chip frequency conversion to a single idler tone with a record-high 71% efficiency using Bragg scattering four-wave-mixing.

In a passive cavity geometry, there exists a trade-off between resonant enhancement and response time, which is inherently limited by the cavity photon lifetime. We demonstrate frequency-selective, dynamic control of the photon lifetime using a silicon-nitride coupled-ring resonator. The photon lifetime is tuned by controlling an avoided mode crossing using thermo-optic tuning of the cavity resonance with integrated heaters. Using this effect, we achieve fast turn-on/off of a${\mathrm{\chi <\#comment/>}}^{(3)}$degenerate optical parametric oscillator (DOPO) and on-chip true random number generation. Our approach allows us to overcome the$Q$-limited generation rate of a single-ring-based DOPO and offers a path toward the development of a scalable integrated high-quality entropy source for modern cryptographic systems.

We experimentally demonstrate waveguiding at the critical angle in a dielectric multi-layered structure. At this exceptional point, the waveguide becomes scale invariant and the field is confined to the low-index region, with a spatially-uniform transverse profile