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Abstract Optical parametric oscillation (OPO) in Kerr microresonators can efficiently transfer near-infrared laser light into the visible spectrum. To date, however, chromatic dispersion has mostly limited output wavelengths to >560 nm, and robust access to the whole green light spectrum has not been demonstrated. In fact, wavelengths between 532 nm and 633 nm, commonly referred to as the “green gap”, are especially challenging to produce with conventional laser gain. Hence, there is motivation to extend the Kerr OPO wavelength range and develop reliable device designs. Here, we experimentally show how to robustly access the entire green gap with Kerr OPO in silicon nitride microrings pumped near 780 nm. Our microring geometries are optimized for green-gap emission; in particular, we introduce a dispersion engineering technique, based on partially undercutting the microring, which not only expands wavelength access but also proves robust to variations in resonator dimensions. Using just four devices, we generate >150 wavelengths evenly distributed throughout the green gap, as predicted by our dispersion simulations. Moreover, we establish the usefulness of Kerr OPO to coherent applications by demonstrating continuous frequency tuning (>50 GHz) and narrow optical linewidths (

Abstract Dispersion engineering of microring resonators is crucial for optical frequency comb applications, to achieve targeted bandwidths and powers of individual comb teeth. However, conventional microrings only present two geometric degrees of freedom – width and thickness – which limits the degree to which dispersion can be controlled. We present a technique where we tune individual resonance frequencies for arbitrary dispersion tailoring. Using a photonic crystal microring resonator that induces coupling to both directions of propagation within the ring, we investigate an intuitive design based on Fourier synthesis. Here, the desired photonic crystal spatial profile is obtained through a Fourier relationship with the targeted modal frequency shifts, where each modal shift is determined based on the corresponding effective index modulation of the ring. Experimentally, we demonstrate several distinct dispersion profiles over dozens of modes in transverse magnetic polarization. In contrast, we find that the transverse electric polarization requires a more advanced model that accounts for the discontinuity of the field at the modulated interface. Finally, we present simulations showing arbitrary frequency comb spectral envelope tailoring using our Fourier synthesis approach.

Twisted light in the form of orbital angular momentum (OAM) can provide an additional spatial dimension for information transmission. Here, the authors demonstrate a prescription for OAM generation using photonic crystal ring resonators, in which high cavity quality factors (up to 106) are retained.

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

Applications of ultra-low-loss photonic circuitry in quantum photonics, in particular including triggered single photon sources, are rare. Here, the authors show how InAs quantum dot single photon sources can be integrated onto wafer-scale, CMOS compatible ultra-low loss silicon nitride photonic circuits.

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.

Integrated heaters are essential in the photonics toolbox, particularly for microresonator frequency tuning through the thermo-refractive effect. Resonators that are fully embedded in a solid cladding (typically SiO2) allow for straightforward lossless integration of heater elements. However, air-clad resonators, which are of great interest for short wavelength dispersion engineering and direct interfacing with atomic/molecular systems, do not usually have similar low loss and efficient heater integration through standard fabrication. Here, we develop a new approach in which the integrated heater is embedded in SiO2 below the waveguiding layer, enabling more efficient heating and more arbitrary routing of the heater traces than possible in a lateral configuration. We incorporate these buried heaters within a stoichiometric Si3N4 process flow that includes high-temperature (>1000 °C) annealing. Microring resonators with a 1 THz free spectral range and quality factors near 106 are demonstrated, and the resonant modes are tuned by nearly 1.5 THz, a 5× improvement compared to equivalent devices with lateral heaters. Finally, we demonstrate broadband dissipative Kerr soliton generation in this platform and show how the heaters can be utilized to aid in bringing relevant lock frequencies within a detectable range.

Optical parametric oscillators are widely used to generate coherent light at frequencies not accessible by conventional laser gain. However, chip-based parametric oscillators operating in the visible spectrum have suffered from pump-to-signal conversion efficiencies typically less than 0.1%. Here, we demonstrate efficient optical parametric oscillators based on silicon nitride photonics that address frequencies between 260 (1150 nm) and 510 THz (590 nm). Pumping silicon nitride microrings near 385 THz (780 nm) yields monochromatic signal and idler waves with unprecedented output powers in this wavelength range. We estimate on-chip output powers (separately for the signal and idler) between 1 and 5 mW and conversion efficiencies reaching ≈15%. Underlying this improved performance is our development of pulley waveguides for broadband near-critical coupling, which exploits a fundamental connection between the waveguide-resonator coupling rate and conversion efficiency. Finally, we find that mode competition reduces conversion efficiency at high pump powers, thereby constraining the maximum realizable output power. Our work proves that optical parametric oscillators built with integrated photonics can produce useful amounts of visible laser light with high efficiency.

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.

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.

We present operation of a photonically integrated wavelength reference at 780 nm using a dual beam geometry. The dual beams are generated by a pair of extreme mode converting grating beam expanders that generate a pair of overlapped, counter-propagating beams inside the volume of a microfabricated vapor cell. We perform spectroscopy of rubidium and a characterize the stability of laser locked to a sub-Doppler transition at 780 nm. By stabilizing the laser to a known transition in an atomic vapor, this device can serve as a quantum wavelength reference. The dual beam geometry also enables investigation of two-photon absorption in a three-level ladder configuration in rubidium using excitation at 780 nm + 776 nm.

Implementing optical-frequency combs with integrated photonics will enable wider use of precision timing signals. Here, we explore the generation of an octave-span, Kerr-microresonator frequency comb using hybrid integration of an InP distributed-feedback laser and a SiN photonic-integrated circuit. We demonstrate electrically pumped and fiber-packaged prototype systems, enabled by self-injection locking. This direct integration of a laser and a microresonator circuit without previously used intervening elements, such as optical modulators and isolators, necessitates understanding self-injection-locking dynamics with octave-span Kerr solitons. In particular, system architectures must adjust to the strong coupling of microresonator backscattering and laser-microresonator frequency detuning that we uncover here. Our work illustrates critical considerations toward realizing a self-referenced frequency comb with integrated photonics.

We demonstrate a polarization rotator integrated at the output of a GaAs waveguide producing type I second harmonic generation (SHG). Form-birefringent phase matching between the pump fundamental transverse electric (TE) mode near 2.0 µm wavelength and the signal fundamental transverse magnetic (TM) mode efficiently generates light at 1.0 µm wavelength. A SiN waveguide layer is integrated with the SHG device to form a multifunctional photonic integrated circuit. The polarization rotator couples light between the two layers and rotates the polarization from TM to TE or from TE to TM. With a TE-polarized 2.0 µm pump, type I SHG is demonstrated with the signal rotated to TE polarization. Passive transmission near 1.0 µm wavelength shows ∼80% polarization rotation across a broad bandwidth of ∼100 nm. By rotating the signal polarization to match that of the pump, this SHG device demonstrates a critical component of an integrated self-referenced octave-spanning frequency comb. This device is expected to provide crucial functionality as part of a fully integrated optical frequency synthesizer with resolution of less than one part in 1014.

Effective use of single emitters in quantum photonics requires coherent emission, strong light-matter coupling, low losses and scalable fabrication. Here, Davanco et al. stride toward this goal by hybrid on-chip integration of Si3N4 waveguides and GaAs nanophotonic geometries with InAs quantum dots.

Abstract We report on a combined photoluminescence imaging and atomic force microscopy study of single, isolated self-assembled InAs quantum dots. The motivation of this work is to determine an approach that allows to assess single quantum dots as candidates for quantum nanophotonic devices. By combining optical and scanning probe characterization techniques, we find that single quantum dots often appear in the vicinity of comparatively large topographic features. Despite this, the quantum dots generally do not exhibit significant differences in their non-resonantly pumped emission spectra in comparison to quantum dots appearing in defect-free regions, and this behavior is observed across multiple wafers produced in different growth chambers. Such large surface features are nevertheless a detriment to applications in which single quantum dots are embedded within nanofabricated photonic devices: they are likely to cause large spectral shifts in the wavelength of cavity modes designed to resonantly enhance the quantum dot emission, thereby resulting in a nominally perfectly-fabricated single quantum dot device failing to behave in accordance with design. We anticipate that the approach of screening quantum dots not only based on their optical properties, but also their surrounding surface topographies, will be necessary to improve the yield of single quantum dot nanophotonic devices.

Atomically thin transition metal dichalcogenides constitute an ideal platform to investigate solid state excitonic effects. Here, the authors provide experimental evidence of a localized biexciton in a monolayer of WSe2, which induces an emission cascade of single photons.

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

Just as “classical” information technology rests on a foundation built of interconnected information-processing systems, quantum information technology (QIT) must do the same. A critical component of such systems is the “interconnect,” a device or process that allows transfer of information between disparate physical media, for example, semiconductor electronics, individual atoms, light pulses in optical fiber, or microwave fields. While interconnects have been well engineered for decades in the realm of classical information technology, quantum interconnects (QuICs) present special challenges, as they must allow the transfer of fragile quantum states between different physical parts or degrees of freedom of the system. The diversity of QIT platforms (superconducting, atomic, solid-state color center, optical, etc.) that will form a “quantum internet” poses additional challenges. As quantum systems scale to larger size, the quantum interconnect bottleneck is imminent, and is emerging as a grand challenge for QIT. For these reasons, it is the position of the community represented by participants of the NSF workshop on “Quantum Interconnects” that accelerating QuIC research is crucial for sustained development of a national quantum science and technology program. Given the diversity of QIT platforms, materials used, applications, and infrastructure required, a convergent research program including partnership between academia, industry, and national laboratories is required.

Nanobeam optomechanical crystals, in which localized GHz frequency mechanical modes are coupled to wavelength-scale optical modes, are being employed in a variety of experiments across different material platforms. Here, we demonstrate the electrostatic tuning and stabilization of such devices, by integrating a Si3N4 slot-mode optomechanical crystal cavity with a nanoelectromechanical systems element, which controls the displacement of an additional “tuning” beam within the optical near-field of the optomechanical cavity. Under DC operation, tuning of the optical cavity wavelength across several optical linewidths with little degradation of the optical quality factor (Q ≈ 105) is observed. The AC response of the tuning mechanism is measured, revealing actuator resonance frequencies in the 10 MHz–20 MHz range, consistent with the predictions from simulations. Feedback control of the optical mode resonance frequency is demonstrated, and alternative actuator geometries are presented.

Kerr microresonators generate interesting and useful fundamental states of electromagnetic radiation through nonlinear interactions of continuous-wave (CW) laser light. With photonic-integration techniques, functional devices with low noise, small size, low-power consumption, scalable fabrication, and heterogeneous combinations of photonics and electronics can be realized. Kerr solitons, which stably circulate in a Kerr microresonator, have emerged as a source of coherent, ultrafast pulse trains and ultra-broadband optical-frequency combs. Using the f-2f technique, Kerr combs can support carrier-envelope-offset phase stabilization to enable optical synthesis and metrology. Here, we introduce a Kerr-microresonator optical clockwork, which is a foundational device that distributes optical-clock signals to the mode-difference frequency of a comb. Our clockwork is based on a silicon-nitride (Si_{3}N_{4}) microresonator that generates a Kerr-soliton frequency comb with a repetition frequency of 1 THz. We measure our terahertz clockwork by electro-optic modulation with a microwave signal, enabling optical-based timing experiments in this wideband and high-speed frequency range. Moreover, by EO phase modulation of our entire Kerr-soliton comb, we arbitrarily generate additional CW modes between the 1-THz modes to reduce the repetition frequency and increase the resolution of the comb. Our experiments characterize the absolute frequency noise of this Kerr-microresonator clockwork to one part in 10^{17}, which is the highest accuracy and precision ever reported with this technology and opens the possibility of measuring high-performance optical clocks with Kerr combs.