High- and ultrahigh-Q whispering-gallery mode resonators have the capability to trap photons by total internal reflection for a duration ranging from nanoseconds to milliseconds. These exceptionally long photon lifetimes enhance the light–matter interactions at all scales, namely at the electronic, molecular, and lattice levels. As a consequence, nonlinear photon scattering can be triggered with very low threshold powers, down to a few microwatts. The possibility to efficiently harness photon–photon interactions with a system optimizing size, weight, power, and cost constraints has created a new, quickly thriving research area in photonics science and technology. This topic is inherently cross-disciplinary, as it stands at the intersection of nonlinear and quantum optics, crystallography, optoelectronics, and microwave photonics. From a fundamental perspective, high-Q whispering-gallery mode resonators have emerged as an ideal platform to investigate light–matter interactions in nonlinear bulk materials. From an applied viewpoint, technological applications include time-metrology, aerospace engineering, coherent optical fiber communications, or nonclassical light generation, among others. The aim of this paper is to provide an overview of the most recent advances in this area, which is increasingly gaining importance in contemporary photonics.

Nonlinear photonics with high-Q whispering-gallery-mode resonators

Solitons occur in many physical systems when a nonlinearity compensates wave dispersion. Their recent formation in microresonators opens a new research direction for nonlinear optical physics and provides a platform for miniaturization of spectroscopy and frequency metrology systems. These microresonator solitons orbit around a closed waveguide path and produce a repetitive output pulse stream at a rate set by the round-trip time. In this work counter-propagating solitons that simultaneously orbit in an opposing sense (clockwise/counter-clockwise) are studied. Despite sharing the same spatial mode family, their round-trip times can be precisely and independently controlled. Furthermore, a state is possible in which both the relative optical phase and relative repetition rates of the distinct soliton streams are locked. This state allows a single resonator to produce dual-soliton frequency-comb streams having different repetition rates, but with high relative coherence useful in both spectroscopy and laser ranging systems.

Counter-propagating solitons in microresonators

The recent advancement in lithium-niobite-on-insulator (LNOI) technology is opening up new opportunities in optoelectronics, as devices with better performance, lower power consumption and a smaller footprint can be realised due to the high optical confinement in the structures. The LNOI platform offers both large χ(2) and χ(3) nonlinearities along with the power of dispersion engineering, enabling brand new nonlinear photonic devices and applications for the next generation of integrated photonic circuits. However, Raman scattering and its interaction with other nonlinear processes have not been extensively studied in dispersion-engineered LNOI nanodevices. In this work, we characterise the Raman radiation spectra in a monolithic lithium niobate (LN) microresonator via selective excitation of Raman-active phonon modes. The dominant mode for the Raman oscillation is observed in the backward direction for a continuous-wave pump threshold power of 20 mW with a high differential quantum efficiency of 46%. We explore the effects of Raman scattering on Kerr optical frequency comb generation. We achieve mode-locked states in an X-cut LNOI chip through sufficient suppression of the Raman effect via cavity geometry control. Our analysis of the Raman effect provides guidance for the development of future chip-based photonic devices on the LNOI platform. Better understanding, and thus control, of lithium niobate interactions with light could guide the development of an optical device that measures time even more precisely than atomic clocks. Marko Loncar of Harvard University and colleagues studied how light scatters when laser light is pumped into an optical cavity made from lithium niobate, a synthetic crystal widely used in optical materials. Their findings suggested that changing the shape of the lithium niobate cavity could help them suppress ‘Raman scattering’, a type of energy transfer that happens when light interacts with the material’s molecules. When laser light was shone through the specially tuned cavity, it exited in the form of light pulses of extremely short duration. The findings could guide the development of optical devices that can more precisely measure standard units such as distance and time.

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Raman lasing and soliton mode-locking in lithium niobate microresonators

Abstract Designing and engineering microresonator dispersion are essential for generating microresonator frequency comb. Microresonator frequency combs (microcombs, Kerr frequency combs) offer the potential for various attractive applications as a new type of coherent light source that is power efficient and compact and has a high repetition rate and a broad bandwidth. They are easily driven with a continuous-wave pump laser with adequate frequency tuning; however, the resonators must have a high quality (Q) factor and suitable dispersion. The emergence of cavity enhanced four-wave mixing, which is based on third-order susceptibility in the host material, results in the generation of broadband and coherent optical frequency combs in the frequency domain equivalent to an optical pulse in the time domain. The platforms on which Kerr frequency combs can be observed have been developed, thanks to intensive efforts by many researchers over a few decades. Ultrahigh-Q whispering gallery mode (WGM) microresonators are one of the major platforms since they can be made of a wide range of material including silica glass, fluoride crystals and semiconductors. In this review, we focus on the dispersion engineering of WGM microresonators by designing the geometry of the resonators based on numerical simulation. In addition, we discuss experimental methods for measuring resonator dispersion. Finally, we describe experimental results for Kerr frequency combs where second- and higher-order dispersions influence their optical spectra.

Dispersion engineering and measurement of whispering gallery mode microresonator for Kerr frequency comb generation

Development of planar-integrated microresonators with high quality factors (Q’s) is crucial for nonlinear photonics in a robust chip. Compared with silicon and silicon nitride, aluminum nitride (AlN) features intrinsic quadratic and cubic susceptibilities as well as an enormous band gap (∼6.2 eV), making it ideal for nonlinear optical interactions. However, sputtered polycrystalline AlN is susceptible to scattering and defect-related absorption losses, thereby inducing limited Q-factors. Here, we demonstrate single-crystalline AlN epitaxially grown on sapphire as a novel nonlinear platform for broadband chip-scale frequency comb generation. We fabricate an AlN-on-sapphire microring with a high loaded Q-factor of 1.1 × 106 and achieve a pure broadband Kerr comb with observable spectral lines ranging from ∼145 to 275 THz and a low parametric threshold of ∼25 mW. As crystalline AlN exhibits strong Raman gain, we further investigate the influence of stimulated Raman scattering (SRS) on four-wave mixing (FWM) ...

Integrated High-Q Crystalline AlN Microresonators for Broadband Kerr and Raman Frequency Combs

We demonstrated the deterministic generation of blue light emission (438 nm) via the third-harmonic process from an infrared pump by carefully engineering the dispersion of a high-quality-factor whispering gallery mode microcavity. We present two different approaches to obtaining broad bandwidth light. One is based on a clustered comb and the other employs a dispersive wave, and a broad Kerr comb spanning a half-octave is obtained. This allowed frequency conversion over a broad bandwidth ranging from 438 to 612 nm. This approach will enable the development of micro-scale light sources and frequency converters for future optical processing.

Third-harmonic blue light generation from Kerr clustered combs and dispersive waves.

This erratum presents the corrections to the Letter published in Opt. Lett.44, 3146 (2019)OPLEDP0146-959210.1364/OL.44.003146.

Octave-wide phase-matched four-wave mixing in dispersion-engineered crystalline microresonators

We generate broad bandwidth visible light ranging from 498 to 611 nm via third-harmonic generation in a silica toroid microcavity. The silica toroid microcavity is fed with a continuous-wave input at a telecom wavelength, where third-harmonic generation follows the generation of an infrared Kerr comb via cascaded four-wave-mixing and stimulated Raman scattering effects. Thanks to these cascaded effects (four-wave mixing, stimulated Raman scattering, and third-harmonic generation) in an ultrahigh-Q microcavity, a broad bandwidth visible light is obtained. The visible light couples with the whispering gallery mode of the cavity by demonstrating the evanescent coupling of the generated visible light with a tapered fiber based on an add-drop configuration.

Broad bandwidth third-harmonic generation via four-wave mixing and stimulated Raman scattering in a microcavity.

Microresonator-based optical frequency combs (known as microcombs or Kerr combs) have a large repetition frequency ranging typically from 10 to 1000 GHz, which is compatible with fast-scanning applications, including dual-comb spectroscopy and LiDAR. In this research, we numerically study dual-comb generation and soliton trapping in a single microresonator, whose two transverse modes are excited with orthogonally polarized dual pumping. The simulation model is described by using coupled Lugiato-Lefever equations (LLEs), which take account of cross-phase modulation and the difference in repetition frequencies. The numerical simulation calculates the dual-comb formation in a microresonator, whose microcombs propagate as soliton pulses and cause soliton trapping depending on the parameters. In the simulation, a trapped soliton is seeded by one of the original solitons in two transverse modes. In addition, we introduce an analytical solution for trapped solitons in coupled LLEs using a Lagrangian perturbation approach and clarify the relation between the parameters. Revealing the conditions of dual-comb soliton generation and soliton trapping is helpful in terms of optimizing the conditions for causing or avoiding these phenomena.

Shun Fujii

Papers

Dispersion engineering and measurement of whispering gallery mode microresonator for Kerr frequency comb generation

Third-harmonic blue light generation from Kerr clustered combs and dispersive waves.

Octave-wide phase-matched four-wave mixing in dispersion-engineered crystalline microresonators

Broad bandwidth third-harmonic generation via four-wave mixing and stimulated Raman scattering in a microcavity.

Theoretical Study on Dual-Comb Generation and Soliton Trapping in a Single Microresonator with Orthogonally Polarized Dual Pumping