Advancing on-chip Kerr optical parametric oscillation towards coherent applications covering the green gap

Optical parametric oscillation(OPO)in Kerr microresonators can efficiently transfer near-infrared laser light into the visible spectrum.To date,however,chromatic dispersion has mostly limited output wavelengths to>560 nm,and robust access to the whole green light spectrum has not been demonstrated.In fact,wavelengths between 532 nm and 633 nm,commonly referred to as the“green gap”,are especially challenging to produce with conventional laser gain.Hence,there is motivation to extend the Kerr OPO wavelength range and develop reliable device designs.Here,we experimentally show how to robustly access the entire green gap with Kerr OPO in silicon nitride microrings pumped near 780 nm.Our microring geometries are optimized for green-gap emission;in particular,we introduce a dispersion engineering technique,based on partially undercutting the microring,which not only expands wavelength access but also proves robust to variations in resonator dimensions.Using just four devices,we generate>150 wavelengths evenly distributed throughout the green gap,as predicted by our dispersion simulations.Moreover,we establish the usefulness of Kerr OPO to coherent applications by demonstrating continuous frequency tuning(>50 GHz)and narrow optical linewidths(<1 MHz).Our work represents an important step in the quest to bring nonlinear nanophotonics and its advantages to the visible spectrum.

Breaking the in-coupling efficiency limit in waveguide-based AR displays with polarization volume gratings

Augmented reality(AR)displays,heralded as the next-generation platform for spatial computing,metaverse,and digital twins,empower users to perceive digital images overlaid with real-world environment,fostering a deeper level of human-digital interactions.With the rapid evolution of couplers,waveguide-based AR displays have streamlined the entire system,boasting a slim form factor and high optical performance.However,challenges persist in the waveguide combiner,including low optical efficiency and poor image uniformity,significantly hindering the long-term usage and user experience.In this paper,we first analyze the root causes of the low optical efficiency and poor uniformity in waveguide-based AR displays.We then discover and elucidate an anomalous polarization conversion phenomenon inherent to polarization volume gratings(PVGs)when the incident light direction does not satisfy the Bragg condition.This new property is effectively leveraged to circumvent the tradeoff between in-coupling efficiency and eyebox uniformity.Through feasibility demonstration experiments,we measure the light leakage in multiple PVGs with varying thicknesses using a laser source and a liquid-crystal-on-silicon light engine.The experiment corroborates the polarization conversion phenomenon,and the results align with simulation well.To explore the potential of such a polarization conversion phenomenon further,we design and simulate a waveguide display with a 50°field of view.Through achieving first-order polarization conversion in a PVG,the in-coupling efficiency and uniformity are improved by 2 times and 2.3 times,respectively,compared to conventional couplers.This groundbreaking discovery holds immense potential for revolutionizing next-generation waveguide-based AR displays,promising a higher efficiency and superior image uniformity.