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1. Directly accessing the single-soliton state of a Kerr microcomb and its universal scaling law [Invited]

   https://doi.org/10.1364/OME.541224  

   Sun, Wenhan; Li, Jingwei; Wang, Ruixuan; Li, Qing (January 2024, Optical Materials Express)

   Soliton microcombs have attracted considerable research interest due to their unique properties. Being able to directly access the single-soliton state in a Kerr microresonator simplifies the device operation and may inspire new applications. However, the general conditions leading to such operations are not well understood. In this work, we aim to elucidate the key factors enabling the direct access of the single-soliton state in a Kerr microresonator by combining the experimental results in an integrated silicon carbide platform and a comprehensive analysis based on the normalized Lugiato-Lefever equation. A general criterion linking the Kerr nonlinearity, dispersion, and thermo-optic properties has been derived, which is applicable to Kerr microresonators with varied materials, sizes, optical quality factors, and dispersion. 

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2. Reduction of thermal instability of soliton states in coupled Kerr-microresonators

   https://doi.org/10.1063/5.0257045  

   Stone, Brandon D; Rukh, Lala; Colación, Gabriel M; Drake, Tara E (May 2025, APL Photonics)

   Kerr-microresonator frequency combs in integrated photonics waveguides are promising technologies for next-generation positioning, navigation, and timing applications, with advantages that include platforms that are mass-producible and CMOS-compatible and spectra that are phase-coherent and octave-spanning. Fundamental thermal noise in the resonator material typically limits the timing and frequency stability of a microcomb. The small optical mode volume of the microresonators exaggerates this effect, as it both increases the magnitude and shortens the timescale of thermodynamic fluctuations. In this work, we investigate thermal instability in silicon nitride microring resonators as well as techniques for reducing their effects on the microcomb light. We characterize the time-dependent thermal response in silicon nitride microring resonators through experimental measurements and finite element method simulations. Through fast control of the pump laser frequency, we reduce thermal recoil due to heating. Finally, we demonstrate the utility of a coupled microresonator system with tunable mode interactions to increase the stability of a soliton against thermal shifts. 

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3. A perspective on nonlinear, microwave, and quantum photonics with Kerr microcombs

   https://doi.org/10.1063/5.0181707  

   Chembo, Yanne K; Heidari, Elham; Menyuk, Curtis R (December 2023, Applied Physics Letters)

   Microresonator Kerr optical frequency combs currently constitute a well-established research area in integrated, nonlinear, and quantum photonics. These systems have found a plethora of technological applications, while serving as an excellent platform to investigate fundamental scientific topics such as light–matter interactions, pattern formation in driven-dissipative systems, or entangled twin-photon generation. We here provide a brief overview of the topic, highlight some of the most recent advances, and discuss a few of the main challenges ahead in this field. 

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4. Nanopatterned parametric oscillators for laser conversion beyond an octave

   https://doi.org/10.1364/OPTICA.545158  

   Brodnik, Grant M; Liu, Haixin; Carlson, David R; Black, Jennifer A; Papp, Scott B (January 2025, Optica)

   Many uses of lasers place the highest importance on access to specific wavelength bands. For example, mobilizing optical-atomic clocks for a leap in sensing requires compact lasers at frequencies spread across the visible and near-infrared. Integrated photonics enables high-performance, scalable laser platforms. However, customizing laser-gain media to support wholly new bands is challenging and often prohibitively mismatched in scalability to early quantum-based sensing and information systems. Here, we demonstrate a tantalum pentoxide microresonator optical-parametric oscillator (OPO) that converts a pump laser to an output wave within a frequency span exceeding an octave. We control phase matching for oscillation by nanopatterning the microresonator to open a photonic-crystal bandgap on the mode of the pump laser. The photonic crystal splits only the pump mode and preserves the broader mode structure of the resonator, thus affording a single parameter to control output waves across the octave span using a nearly fixed frequency pump laser. We also demonstrate tuning the oscillator in free-spectral-range steps, more finely with temperature, and minimal additive frequency noise of the laser-conversion process. Our work shows that nanophotonic structures offer control of laser conversion in microresonators, bridging phase-matching of nonlinear optics and application requirements for laser designs. 

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5. A squeezed quantum microcomb on a chip

   https://doi.org/10.1038/s41467-021-25054-z  

   Yang, Zijiao; Jahanbozorgi, Mandana; Jeong, Dongin; Sun, Shuman; Pfister, Olivier; Lee, Hansuek; Yi, Xu (December 2021, Nature Communications)

   Abstract The optical microresonator-based frequency comb (microcomb) provides a versatile platform for nonlinear physics studies and has wide applications ranging from metrology to spectroscopy. The deterministic quantum regime is an unexplored aspect of microcombs, in which unconditional entanglements among hundreds of equidistant frequency modes can serve as critical ingredients to scalable universal quantum computing and quantum networking. Here, we demonstrate a deterministic quantum microcomb in a silica microresonator on a silicon chip. 40 continuous-variable quantum modes, in the form of 20 simultaneously two-mode squeezed comb pairs, are observed within 1 THz optical span at telecommunication wavelengths. A maximum raw squeezing of 1.6 dB is attained. A high-resolution spectroscopy measurement is developed to characterize the frequency equidistance of quantum microcombs. Our demonstration offers the possibility to leverage deterministically generated, frequency multiplexed quantum states and integrated photonics to open up new avenues in fields of spectroscopy, quantum metrology, and scalable, continuous-variable-based quantum information processing. 

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