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Nature Communications

Ultra high-Q tunable microring resonators enabled by slow light

Priyash Barya, Ashwith Varadaraj Prabhu, Laura Heller, Edmond K. C. Chow, Elizabeth A. Goldschmidt

Published Nov 25, 2025• 11 citations
Publisher

Abstract

High-Q nanophotonic resonators are crucial for many applications in classical and quantum optical processing, communication, and sensing. We achieve ultra-high quality factors via a method previously limited to bulk systems, preparing a highly transparent and strongly dispersive medium within the resonator that causes a reduction in the group velocity and a corresponding increase in the quality factor. We implement this via spectral hole burning in erbium-doped thin-film lithium niobate microring resonators, and show Q-factors enhanced by nearly three orders of magnitude to exceed 108. Additionally, we show dynamic control of the resonances via electro-optic tuning. Finally, we present a theoretical model for our experimentally observed resonator linewidths, which are not well-described by the standard Bloch equations. Our results show a dramatic reduction in the erbium dephasing rate under a strong optical drive, leading to much narrower linewidths than would otherwise be expected given the large circulating intensity in the resonator. Light can be slowed to extreme degrees in engineered media, giving rise to unique behavior. Here, the authors show that slow light enables the formation of ultra-narrow tunable mirroring resonators, introducing a new paradigm for controlling optical responses on a photonic chip.

DOI
10.1038/s41467-025-65533-1

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High-Q nanophotonic resonators are crucial for many applications in classical and quantum optical processing, communication, sensing, and more. We achieve ultra-high quality factors by preparing a highly transparent and strongly dispersive medium within a resonator, causing a reduction in the group velocity that leads to a nearly three order of magnitude increase in the quality factor. We implement this via spectral hole burning in erbium-doped thin-film lithium niobate microring resonators, and show Q-factors exceeding $10^8$. Additionally, we show that the interplay between the spectrally narrowed resonance and the broader bare resonance produces a Fano lineshape, which we dynamically control via electro-optic tuning. Finally, we present a theoretical model for our experimentally observed resonator linewidths, which are not well-described by the standard Bloch equations. Our results show a dramatic reduction in the erbium dephasing rate under a strong optical drive, leading to much narrower linewidths than would otherwise be expected given the large circulating intensity in the resonator.

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•Jun 2, 2017•0 citations
High-Q nanophotonic resonators are crucial for many applications in classical and quantum optical processing, communication, and sensing. We achieve ultra-high quality factors via a method previously limited to bulk systems, preparing a highly transparent and strongly dispersive medium within the resonator that causes a reduction in the group velocity and a corresponding increase in the quality factor. We implement this via spectral hole burning in erbium-doped thin-film lithium niobate microring resonators, and show Q-factors enhanced by nearly three orders of magnitude to exceed 108. Additionally, we show dynamic control of the resonances via electro-optic tuning. Finally, we present a theoretical model for our experimentally observed resonator linewidths, which are not well-described by the standard Bloch equations. Our results show a dramatic reduction in the erbium dephasing rate under a strong optical drive, leading to much narrower linewidths than would otherwise be expected given the large circulating intensity in the resonator. Light can be slowed to extreme degrees in engineered media, giving rise to unique behavior. Here, the authors show that slow light enables the formation of ultra-narrow tunable mirroring resonators, introducing a new paradigm for controlling optical responses on a photonic chip.
High-Q nanophotonic resonators are crucial for many applications in classical and quantum optical processing, communication, sensing, and more. We achieve ultra-high quality factors by preparing a highly transparent and strongly dispersive medium within a resonator, causing a reduction in the group velocity that leads to a nearly three order of magnitude increase in the quality factor. We implement this via spectral hole burning in erbium-doped thin-film lithium niobate microring resonators, and show Q-factors exceeding 10810^8108. Additionally, we show that the interplay between the spectrally narrowed resonance and the broader bare resonance produces a Fano lineshape, which we dynamically control via electro-optic tuning. Finally, we present a theoretical model for our experimentally observed resonator linewidths, which are not well-described by the standard Bloch equations. Our results show a dramatic reduction in the erbium dephasing rate under a strong optical drive, leading to much narrower linewidths than would otherwise be expected given the large circulating intensity in the resonator.
Efficiently generating photon pairs with high heralding efficiency and high single photon purity that are bandwidth matched to quantum emitters, quantum memories, and other matter-based qubits is critical for quantum networking applications. However, nonlinear optics-based sources require substantial spectral engineering to overcome the orders of magnitude bandwidth mismatch between those sources and qubit systems. A popular solution is cavity-enhanced spontaneous parametric down conversion (SPDC) where the cavity sets the photon bandwidth and simultaneously enhances the spectral brightness of the SPDC. Bulk, free-space configurations are generally required to achieve the MHz-scale bandwidths required to interface with most qubit systems. Replicating these in scalable integrated photonic architectures is an ongoing challenge due to the much higher propagation losses that limit the size and linewidth of chip-based resonators. We show here how an intra-cavity slow light medium, acting as an ultra-narrow filter, would enable narrowband photon pair generation in broadband cavities with high single photon purity and without compromising the heralding efficiency. We show that such metrics can be readily realized in erbium doped thin-film lithium niobate microrings using realistic design parameters.
Optical resonators with controllable Q factors are key components in many areas of optical physics and engineering. We propose and investigate a Q -factor controllable system composed of two directly coupled microring resonators, one of which is tunable and coupled to dual waveguides. By shifting the resonance of the controllable microring, the Q factor of the system as well as the other microring changes significantly. We have demonstrated wide-range controllable Q factors based on this structure in silicon-on-insulator, for example. The influences of spectral detuning and coupling strength between two resonators on the variation of Q factors are studied in detail experimentally. Then, we explore its applications in optical buffering. Tunable fast-to-slow/slow-to-fast light has been carried out by switching the system between the high- Q state and low- Q state. Moreover, optical pulse capture and release are also achievable using this structure with dynamic tuning, and the photon storage properties are investigated. The proposed Q -factor tunable system is simple, flexible, and realizable in various integrated photonic platforms, allowing for potential applications in on-chip optical communications and quantum information processing.
Summary form only given. Optical microcavities with high quality factors Q are of great interest in integrated micro- and nanophotonics. Their narrow resonance and long photon lifetime (τcav) can be used in fundamental physics, sensing applications, microwave signal optical processing or for optical delay line and memory miniaturization [1]. Considering a Lorentzian resonance profile, the quality factor and the photon lifetime of a microcavity with a resonant angular frequency ωο = 2·πε/λ0reads: Q = ω0τcab= nG(ω0)LF/λ0, (1) where ngis the group index of the material constituting the microcavity, F is the finesse and L is the cavity round-trip length. The usual method to obtain ultra-high quality factors consists in optimizing the manufacturing process to reach high finesse values. Following this approach record quality factors up to 1011have been measured in millimetric CaF2 whispering gallery mode (WGM) resonators [2]. Otherwise, Eq. (1) shows that for a given finesse, the use of resonators made with slow-light materials displaying high group indices can also increase the quality factor. This method has already been successfully implemented in bulky resonators including a highly dispersive slow-light medium [3]. We have recently demonstrated that this approach can be applied to solid-state optical micro-resonators [4]. The slow-light effect was introduced thanks to population oscillations in an Er3+doped WGM fluoride glass microsphere (diameter 100 μm). Since the lifetime of the4I13/2level is very long (T ≈ 10 ms) strong group index up to 106can be obtained. Using a cavity-ring-down method (see Fig. 1) in a pump probe configuration, we have measured a photon lifetime of 2.5 ms at 1530 nm corresponding to a quality factor of 3 × 1012[4].
We propose using thin-film lithium niobate on insulator (LNOI) doped with Erbium (Er3+) as a promising solution for implementing large-scale quantum memory. However, the transition from bulk crystals to thin films poses challenges, notably reduced optical depth, which is critical for broad atomic frequency comb memory. To address this, we plan to utilize impedance-matched cavities, increasing the effective optical depth. Furthermore, the cavity would boost the rate of spontaneous emission, increasing the efficiency of spectral hole burning. Our preliminary results reveal high-Q micro-ring resonators (Q≈190k) on Er3+: LNOI, demonstrating a nearly 3.5-fold reduction in the optical lifetime due to cavity resonance.
Rare earth quantum light-matter interfaces (QLMIs) are uniquely suited for various quantum communication applications, including quantum memories and quantum optical to microwave transducers. Among rare earths, erbium QLMIs are particularly appealing due to erbium’s long lived telecom wavelength resonance, allowing integration with existing optical communication technology and infrastructure. Micro-resonator QLMIs have various advantages over bulk rare earth crystal memories. They provide the opportunity for on-chip integration; for example, optical resonators can be integrated with microwave resonators for quantum optical-microwave transduction. For spectral hole-burning based quantum memories, coupling rare earth ions to a resonator can provide improved memory initialization via Purcell enhancement of optical lifetimes, while impedance matching the resonator to the ions can raise the theoretical memory efficiency to 100%. We present hybrid nanoscale quantum light matter interfaces in the form of amorphous silicon ring resonators on yttrium orthosilicate (YSO) substrate doped with erbium ions. While working with rare earth crystal hosts can be challenging, the fabrication process for these devices is simple and robust, using traditional thin film fabrication technologies. Our devices have measured quality factors of over 105 in the 11 µm diameter rings, and evanescent coupling to an ensemble of erbium ions characterized by a cooperativity of 0.54. We present simulation and experimental results of the optical properties of these cavities, and their coupling to erbium ions, including a demonstration of Purcell enhancement of the erbium telecom transition. We then analyze their potential as quantum memories and in optical to microwave transducers.