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Silicon photonics lacks a second-order nonlinear optical (χ(2)) response in general, because the typical constituent materials are centrosymmetric and lack inversion symmetry, which prohibits χ(2) nonlinear processes such as second-harmonic generation (SHG). Here, we realize high SHG efficiency in silicon photonics by combining a photoinduced effective χ(2) nonlinearity with resonant enhancement and perfect phase matching. We show a conversion efficiency of (2,500 ± 100)% W−1 that is two to four orders of magnitude larger than previous field-induced SHG works. In particular, our devices realize milliwatt-level SHG output powers with up to (22 ± 1)% power conversion efficiency. This demonstration is a breakthrough in realizing efficient χ(2) processes in silicon photonics, and paves the way for further integration of self-referenced frequency combs and optical frequency references.By combining a photoinduced effective χ(2) nonlinearity with resonant enhancement and perfect phase matching in a silicon nitride microring resonator, second-harmonic generation with milliwatt-level output powers with up to 22 ± 1% power conversion efficiency is demonstrated.

Photon pair sources are fundamental building blocks for quantum entanglement and quantum communication. Recent studies in silicon photonics have documented promising characteristics for photon pair sources within the telecommunications band, including sub-milliwatt optical pump power, high spectral brightness and high photon purity. However, most quantum systems suitable for local operations, such as storage and computation, support optical transitions in the visible or short near-infrared bands. In comparison to telecommunications wavelengths, the higher optical attenuation in silica at such wavelengths limits the length scale over which optical-fibre-based quantum communication between such local nodes can take place. One approach to connect such systems over fibre is through a photon pair source that can bridge the visible and telecom bands, but an appropriate source, which should produce narrow-band photon pairs with a high signal-to-noise ratio, has not yet been developed in an integrated platform. Here, we demonstrate a nanophotonic visible–telecom photon pair source, using high quality factor silicon nitride resonators to generate narrow-band photon pairs with unprecedented purity and brightness, with a coincidence-to-accidental ratio up to 3,780 ± 140 and a detected photon pair flux up to (18,400 ± 1,000) pairs s−1. We further demonstrate visible–telecom time–energy entanglement and its distribution over a 20 km fibre, far exceeding the fibre length over which purely visible wavelength quantum light sources can be efficiently transmitted. Finally, we show how dispersion engineering of the microresonators enables the connections of different species of trapped atoms/ions, defect centres and quantum dots to the telecommunications bands for future quantum communication systems.Efficient photon pair sources connecting visible and telecommunication spectral regions are essential for viable long-distance fibre optic quantum communication architectures. A nanophotonic device is presented that allows kilometre-scale time–energy entanglement as an application.

Recent advances in nonlinear optics have revolutionized integrated photonics, providing on-chip solutions to a wide range of new applications. Currently, state of the art integrated nonlinear photonic devices are mainly based on dielectric material platforms, such as Si 3 N 4 and SiO 2 . While semiconductor materials feature much higher nonlinear coefficients and convenience in active integration, they have suffered from high waveguide losses that prevent the realization of efficient nonlinear processes on-chip. Here, we challenge this status quo and demonstrate a low loss AlGaAs-on-insulator platform with anomalous dispersion and quality ( Q ) factors beyond 1.5 × 10 6 . Such a high quality factor, combined with high nonlinear coefficient and small mode volume, enabled us to demonstrate a Kerr frequency comb threshold of only ∼36 µW in a resonator with a 1 THz free spectral range, ∼100 times lower compared to that in previous semiconductor platforms. Moreover, combs with broad spans (>250 nm) have been generated with a pump power of ∼300 µW, which is lower than the threshold power of state-of the-art dielectric micro combs. A soliton-step transition has also been observed for the first time in an AlGaAs resonator. Despite larger nonlinear coefficients, waveguide losses have prevented using semiconductors instead of dielectric materials for on-chip frequency-comb sources. By significantly reducing waveguide loss, ultra-low-threshold Kerr comb generation is demonstrated in a high- Q AlGaAs-on-insulator microresonator system.

Broadband and low-noise microresonator frequency combs (microcombs) are critical for deployable optical frequency measurements. Here we expand the bandwidth of a microcomb far beyond its anomalous dispersion region on both sides of its spectrum through spectral translation mediated by mixing of a dissipative Kerr soliton and a secondary pump. We introduce the concept of synthetic dispersion to qualitatively capture the system’s key physical behavior, in which the second pump enables spectral translation through four-wave mixing Bragg scattering. Experimentally, we pump a silicon nitride microring at 1063 nm and 1557 nm to enable soliton spectral translation, resulting in a total bandwidth of 1.6 octaves (137–407 THz). We examine the comb’s low-noise characteristics, through heterodyne beat note measurements across its spectrum, measurements of the comb tooth spacing in its primary and spectrally translated portions, and their relative noise. These ultra-broadband microcombs provide new opportunities for optical frequency synthesis, optical atomic clocks, and reaching previously unattainable wavelengths. Integrated optical frequency measurements, benefit from broadband on-chip frequency combs. Here the authors present a low-noise microcomb whose span extends from telecom to near-visible wavelengths. Here the authors present a dissipative Kerr soliton formation approximated by introducing the concept of synthetic dispersion.

Nonlinearity in complex systems leads to pattern formation through fundamental interactions between components. With integrated photonics, precision control of nonlinearity explores novel patterns and propels applications. In particular, Kerr-nonlinear resonators support stationary states—including Turing patterns—composed of a few interfering waves, and localized solitons composed of waves across a broad spectrum. Although Turing patterns emerge from an unstable Kerr resonator with sufficiently intense excitation, Kerr solitons do not form spontaneously under constant excitation, making this useful state challenging to access. Here we explore an edgeless photonic crystal resonator (PhCR) that enables spontaneous soliton formation in place of Turing patterns. We design the PhCR nanopattern for single-azimuthal-mode engineering of a group-velocity-dispersion defect that balances Kerr-nonlinear frequency shifts in favour of the soliton state. Our experiments establish PhCR solitons as modelocked pulses through ultraprecise optical-frequency measurements. We show that nanophotonics expand the palette for nonlinear engineering, enabling new phenomena and light sources.Researchers have demonstrated spontaneous soliton formation in an edgeless photonic crystal resonator.

Recent advances in realizing optical frequency combs using nonlinear parametric processes in integrated photonic resonators have revolutionized on-chip optical clocks, spectroscopy and multichannel optical communications. At the same time, the introduction of topological physics in photonic systems has allowed the design of photonic devices with novel functionalities and inherent robustness against fabrication disorders. Here we use topological design principles to theoretically propose the generation of optical frequency combs and temporal dissipative Kerr solitons in a two-dimensional array of coupled ring resonators that creates a synthetic magnetic field for photons and exhibits topological edge states. We show that these topological edge states constitute a travelling-wave super-ring resonator that leads to the generation of coherent nested optical frequency combs, as well as the self-formation of nested temporal solitons and Turing rolls that are remarkably phase-locked over more than 40 rings. Moreover, we show that the topological nested solitons are robust against defects in the lattice, and a single nested soliton achieves a mode efficiency of over 50%, an order of magnitude higher than single-ring frequency combs. Our topological frequency comb works in a parameter regime that can be readily accessed using existing low-loss integrated photonic platforms like silicon nitride.Optical frequency combs are a key technology in precision time keeping, spectroscopy and metrology. A theoretical proposal shows that introducing topological principles into their design makes on-chip combs more efficient and robust against fabrication defects.

The ability to spectrally translate lightwave signals in a compact, low-power platform is at the heart of the promise of nonlinear nanophotonic technologies. For example, a device to connect the telecommunications band with visible and short near-infrared wavelengths can enable a connection between high-performance chip-integrated lasers based on scalable nanofabrication technology with atomic systems used for time and frequency metrology. Although second-order nonlinear (χ(2)) systems are the natural approach for bridging such large spectral gaps, here we show that third-order nonlinear (χ(3)) systems, despite their typically much weaker nonlinear response, can realize spectral translation with unprecedented performance. By combining resonant enhancement with nanophotonic mode engineering in a silicon nitride microring resonator, we demonstrate efficient spectral translation of a continuous-wave signal from the telecom band (~1,550 nm) to the visible band (~650 nm) through cavity-enhanced four-wave mixing. We achieve such translation over a wide spectral range >250 THz with a translation efficiency of (30.1 ± 2.8)% and using an ultralow pump power of (329 ± 13) μW. The translation efficiency projects to (274 ± 28)% at 1 mW and is more than an order of magnitude larger than what has been achieved in current nanophotonic devices.

Microcombs—optical frequency combs generated in microresonators—have advanced tremendously in the past decade, and are advantageous for applications in frequency metrology, navigation, spectroscopy, telecommunications, and microwave photonics. Crucially, microcombs promise fully integrated miniaturized optical systems with unprecedented reductions in cost, size, weight, and power. However, the use of bulk free-space and fiber-optic components to process microcombs has restricted form factors to the table-top. Taking microcomb-based optical frequency synthesis around 1550 nm as our target application, here, we address this challenge by proposing an integrated photonics interposer architecture to replace discrete components by collecting, routing, and interfacing octave-wide microcomb-based optical signals between photonic chiplets and heterogeneously integrated devices. Experimentally, we confirm the requisite performance of the individual passive elements of the proposed interposer—octave-wide dichroics, multimode interferometers, and tunable ring filters, and implement the octave-spanning spectral filtering of a microcomb, central to the interposer, using silicon nitride photonics. Moreover, we show that the thick silicon nitride needed for bright dissipative Kerr soliton generation can be integrated with the comparatively thin silicon nitride interposer layer through octave-bandwidth adiabatic evanescent coupling, indicating a path towards future system-level consolidation. Finally, we numerically confirm the feasibility of operating the proposed interposer synthesizer as a fully assembled system. Our interposer architecture addresses the immediate need for on-chip microcomb processing to successfully miniaturize microcomb systems and can be readily adapted to other metrology-grade applications based on optical atomic clocks and high-precision navigation and spectroscopy.Key steps towards photonic interposers for integrated optical synthesizers are shown: filters, mixers, linear processing of an octave-spanning microcomb, integration of microcomb and interposer layers, and a system-level feasibility analysis.

Optical parametric oscillation (OPO) is distinguished by its wavelength access, that is, the ability to flexibly generate coherent light at wavelengths that are dramatically different from the pump laser, and in principle bounded solely by energy conservation between the input pump field and the output signal/idler fields. As society adopts advanced tools in quantum information science, metrology, and sensing, microchip OPO may provide an important path for accessing relevant wavelengths. However, a practical source of coherent light should additionally have high conversion efficiency and high output power. Here, we demonstrate a silicon photonics OPO device with unprecedented performance. Our OPO device, based on the third-order ( χ (3) ) nonlinearity in a silicon nitride microresonator, produces output signal and idler fields widely separated from each other in frequency ( > 150 THz), and exhibits a pump-to-idler conversion efficiency up to 29 % with a corresponding output idler power of > 18 mW on-chip. This performance is achieved by suppressing competitive processes and by strongly overcoupling the output light. This methodology can be readily applied to existing silicon photonics platforms with heterogeneously-integrated pump lasers, enabling flexible coherent light generation across a broad range of wavelengths with high output power and efficiency. Flexible and coherent light generation is of paramount importance to enable new functionalities in integrated silicon photonics. Here the authors, develop an optical parametric oscillator with high conversion efficiency and high output power, based on the third order nonlinearity in a silicon nitride microresonator

A Correction to this paper has been published: https://doi.org/10.1038/s41467-021-22031-4