High-Quality Optical Resonators in Photonic Systems

High-quality optical resonators lie at the heart of modern photonics, offering exceptionally low optical loss and strong field confinement in compact footprints. These devices—including microdisks, microrings and whispering-gallery-mode cavities—exploit total internal reflection or engineered refractive‐index contrasts to sustain circulating light for many optical cycles. Advances in material platforms such as silicon‐on‐insulator, silicon nitride and crystalline fluorides have driven quality factors (Q) from 10^5 into excess of 10^8, enabling sensitive molecular detection, low-threshold nonlinear optics and robust frequency-comb generation. Precision fabrication techniques—from thermal smoothing and etchless processes to ultraprecision machining and annealing—have systematically reduced surface roughness and material absorption. Meanwhile, innovative coupling schemes, including integrated bus waveguides and tapered fibres, have achieved critical coupling and minimal insertion loss. The convergence of materials science, microfabrication and optical design now supports scalable resonator arrays with reproducible performance, offering a pathway towards on-chip quantum photonic circuits, compact gyroscopes and high-resolution spectroscopy across the visible to mid-infrared spectral regions.

Research from Nature Portfolio

Recent studies have extended ultra-high Q operation into the mid-infrared by polishing alkaline earth metal fluoride crystals and employing uncoated chalcogenide tapered fibres for efficient coupling. These resonators demonstrate Q factors surpassing 10^8 at 4.5 µm, driven by the intrinsically low multiphonon absorption of BaF2 and SrF2. Temperature-dependent cavity ring-down measurements reveal that MgF2 is limited by bulk phonon processes, whereas SrF2 and BaF2 maintain record optical finesse in the mid-infrared. In parallel, novel ring geometries wrapped by a secondary curved waveguide have been shown to suppress bending losses and optimise the propagation constant mismatch. By fine-tuning the concentric guide width, researchers have achieved intrinsic Q factors approaching 7 × 10^5, illustrating a straightforward route to enhance performance in planar microring platforms.

High-Quality Optical Resonators in Photonic Systems publication trend

The graph below shows the total number of articles in high-quality optical resonators in photonic systems across all publications each year (not limited to Nature Index journals).

Technical terms

Quality factor (Q): Dimensionless parameter quantifying the ratio of stored optical energy to energy lost per cycle, indicating resonator sharpness and energy retention.

Whispering‐gallery mode: Optical mode guided by continuous total internal reflection around a curved boundary, yielding high field confinement near the periphery of circular resonators.

Intrinsic Q: Quality factor determined solely by material absorption and scattering losses, excluding contributions from coupling to external waveguides or fibres.

Critical coupling: Condition in which external coupling losses exactly match intrinsic losses, resulting in maximal energy transfer into the resonator and complete extinction in the through port.

Optical finesse: Measure of a resonator’s spectral resolving power, defined as the ratio of the free spectral range to the resonance linewidth.

References

1. Ultra-High-Q Racetrack Resonators on Thick SOI Platform Through Hydrogen Annealing Smoothing. Journal of Lightwave Technology (2023).
2. Mid-infrared ultra-high-Q resonators based on fluoride crystalline materials. Nature Communications (2016).
3. Billion Q-factor in silicon WGM resonators. Optica (2018).
4. All-precision-machining fabrication of ultrahigh-Q crystalline optical microresonators. Optica (2020).
5. High Q-factor microring resonator wrapped by the curved waveguide. Scientific Reports (2015).