Beyond the detection limit: A review of high-Q optical ring resonator sensors

High-quality factor (high-Q) optical ring resonators have become essential in modern photonic sensing systems due to their exceptional sensitivity, compact footprint, and compatibility with integrated platforms. This review provides a comprehensive overview of the fundamental principles, material platforms, fabrication techniques, and architectural advancements that govern the performance of high-Q ring resonator sensors. Key factors influencing the Q-factor, including waveguide geometry, surface roughness, and coupling conditions, are examined in detail, along with their impact on sensor sensitivity, limit of detection, and stability. Various resonator architectures such as microrings, racetracks, microtoroids, and photonic crystal rings are analyzed, highlighting strategies to achieve ultra-high Q-factors and enhanced light-matter interactions. Applications in biosensing, environmental monitoring, and physical parameter detection are discussed, emphasizing label-free single-molecule detection and integration into lab-on-chip systems. Challenges such as fabrication reproducibility, thermal drift, environmental noise, and scalability to large sensor arrays are critically evaluated. The review concludes by exploring emerging trends, including hybrid integration, nonlinear enhancements, and quantum sensing applications. Overall, this work underscores the pivotal role of high-Q ring resonators in advancing next-generation optical sensors and outlines future directions toward their widespread deployment in real-world environments.