Kerr-induced synchronization of a cavity soliton to an optical reference.

The phase-coherent frequency division of a stabilized optical reference laser to the microwave domain is made possible by optical-frequency combs (OFCs)1,2. OFC-based clockworks3-6 lock one comb tooth to a reference laser, which probes a stable atomic transition, usually through an active servo that increases the complexity of the OFC photonic and electronic integration for fieldable clock applications. Here, we demonstrate that the Kerr nonlinearity enables passive, electronics-free synchronization of a microresonator-based dissipative Kerr soliton (DKS) OFC7 to an externally injected reference laser. We present a theoretical model explaining this Kerr-induced synchronization (KIS), which closely matches experimental results based on a chip-integrated, silicon nitride, micro-ring resonator. Once synchronized, the reference laser captures an OFC tooth, so that tuning its frequency provides direct external control of the OFC repetition rate. We also show that the stability of the repetition rate is linked to that of the reference laser through the expected frequency division factor. Finally, KIS of an octave-spanning DKS exhibits enhancement of the opposite dispersive wave, consistent with the theoretical model, and enables improved self-referencing and access to the OFC carrier-envelope offset frequency. The KIS-mediated enhancements we demonstrate can be directly implemented in integrated optical clocks and chip-scale low-noise microwave generators.

Band flipping and bandgap closing in a photonic crystal ring and its applications.

The size of the bandgap in a photonic crystal ring is typically intuitively considered to monotonically grow as the modulation amplitude of the grating increases, causing increasingly large frequency splittings between the "dielectric" and "air" bands. In contrast, here we report that as the modulation amplitude in a photonic crystal ring increases, the bandgap does not simply increase monotonically. Instead, after the initial increase, the bandgap closes and then reopens again with the two bands flipped in energy. The air and dielectric band edges are degenerate at the bandgap closing point. We demonstrate this behavior experimentally in silicon nitride photonic crystal microrings, where we show that the bandgap is closed to within the linewidth of the optical cavity mode, whose intrinsic quality factor remains unperturbed with a value ≈ 1×106. Moreover, through finite-element simulations, we show that such bandgap closing and band flipping phenomena exist in a variety of photonic crystal rings with varying unit cell geometries and cladding layers. At the bandgap closing point, the two standing wave modes with a degenerate frequency are particularly promising for single-frequency lasing applications. Along this line, we propose a compact self-injection locking scheme that integrates many core functionalities in one photonic crystal ring. Additionally, the single-frequency lasing might be applicable to distributed-feedback (DFB) lasers to increase their manufacturing yield.

On-chip Kerr parametric oscillation with integrated heating for enhanced frequency tuning and control.

Nonlinear microresonators can convert light from chip-integrated sources into new wavelengths within the visible and near-infrared spectrum. For most applications, such as the interrogation of quantum systems with specific transition wavelengths, tuning the frequency of converted light is critical. Nonetheless, demonstrations of wavelength conversion have mostly overlooked this metric. Here, we apply efficient integrated heaters to tune the idler frequency produced by the Kerr optical parametric oscillation in a silicon nitride microring across a continuous 1.5 terahertz range. Finally, we suppress idler frequency noise between DC and 5 kHz by several orders of magnitude using feedback to the heater drive.

An optical-frequency synthesizer using integrated photonics.

Optical-frequency synthesizers, which generate frequency-stable light from a single microwave-frequency reference, are revolutionizing ultrafast science and metrology, but their size, power requirement and cost need to be reduced if they are to be more widely used. Integrated-photonics microchips can be used in high-coherence applications, such as data transmission 1 , highly optimized physical sensors 2 and harnessing quantum states 3 , to lower cost and increase efficiency and portability. Here we describe a method for synthesizing the absolute frequency of a lightwave signal, using integrated photonics to create a phase-coherent microwave-to-optical link. We use a heterogeneously integrated III-V/silicon tunable laser, which is guided by nonlinear frequency combs fabricated on separate silicon chips and pumped by off-chip lasers. The laser frequency output of our optical-frequency synthesizer can be programmed by a microwave clock across 4 terahertz near 1,550 nanometres (the telecommunications C-band) with 1 hertz resolution. Our measurements verify that the output of the synthesizer is exceptionally stable across this region (synthesis error of 7.7 × 10-15 or below). Any application of an optical-frequency source could benefit from the high-precision optical synthesis presented here. Leveraging high-volume semiconductor processing built around advanced materials could allow such low-cost, low-power and compact integrated-photonics devices to be widely used.

Kerr optical parametric oscillation in a photonic crystal microring for accessing the infrared.

Continuous wave optical parametric oscillation (OPO) provides a flexible approach for accessing mid-infrared wavelengths between 2 µm and 5 µm, but operation at these wavelengths has not yet been integrated into silicon nanophotonics. Typically, a Kerr OPO uses a single transverse mode family for pump, signal, and idler modes, and relies on a delicate balance to achieve normal (but close-to-zero) dispersion near the pump and the requisite higher-order dispersion needed for phase- and frequency-matching. Within integrated photonics platforms, this approach results in two major problems. First, the dispersion is very sensitive to geometry, so that small fabrication errors can have a large impact. Second, the device is susceptible to competing nonlinear processes near the pump. In this Letter, we propose a flexible solution to infrared OPO that addresses these two problems by using a silicon nitride photonic crystal microring (PhCR). The frequency shifts created by the PhCR bandgap enable OPO that would otherwise be forbidden. We report an intrinsic optical quality factor up to (1.2 ± 0.1)×106 in the 2-µm band, and use a PhC ring to demonstrated an OPO with a threshold dropped power in the cavity of (90 ± 20) mW, with the pump wavelength at 1998 nm, and the signal and idler wavelengths at 1937 nm and 2063 nm, respectively. We further discuss how to extend the OPO spectral coverage in the mid-infrared. These results establish the PhCR OPO as a promising route for integrated laser sources in the infrared.

Fractional Optical Angular Momentum and Multi-Defect-Mediated Mode Renormalization and Orientation Control in Photonic Crystal Microring Resonators.

Whispering gallery modes (WGMs) in circularly symmetric optical microresonators exhibit integer quantized angular momentum numbers due to the boundary condition imposed by the geometry. Here, we show that incorporating a photonic crystal pattern in an integrated microring can result in WGMs with fractional optical angular momentum. By choosing the photonic crystal periodicity to open a photonic band gap with a band-edge momentum lying between that of two WGMs of the unperturbed ring, we observe hybridized WGMs with half-integer quantized angular momentum numbers (m∈Z+1/2). Moreover, we show that these modes with fractional angular momenta exhibit high optical quality factors with good cavity-waveguide coupling and an order of magnitude reduced group velocity. Additionally, by introducing multiple artificial defects, multiple modes can be localized to small volumes within the ring, while the relative orientation of the delocalized band-edge states can be well controlled. Our Letter unveils the renormalization of WGMs by the photonic crystal, demonstrating novel fractional angular momentum states and nontrivial multimode orientation control arising from continuous rotational symmetry breaking. The findings are expected to be useful for sensing and metrology, nonlinear optics, and cavity quantum electrodynamics.

Purcell-Enhanced Single Photon Source Based on a Deterministically Placed WSe2 Monolayer Quantum Dot in a Circular Bragg Grating Cavity.

We demonstrate a deterministic Purcell-enhanced single photon source realized by integrating an atomically thin WSe2 layer with a circular Bragg grating cavity. The cavity significantly enhances the photoluminescence from the atomically thin layer and supports single photon generation with g(2)(0) < 0.25. We observe a consistent increase of the spontaneous emission rate for WSe2 emitters located in the center of the Bragg grating cavity. These WSe2 emitters are self-aligned and deterministically coupled to such a broadband cavity, configuring a new generation of deterministic single photon sources, characterized by their simple and low-cost production and intrinsic scalability.

Proposal for noise-free visible-telecom quantum frequency conversion through third-order sum and difference frequency generation.

Quantum frequency conversion (QFC) between the visible and telecom is a key to connect quantum memories in fiber-based quantum networks. Current methods for linking such widely separated frequencies, such as sum/difference frequency generation and four-wave mixing Bragg scattering, are prone to broadband noise generated by the pump laser(s). To address this issue, we propose to use third-order sum/difference frequency generation (TSFG/TDFG) for an upconversion/downconversion QFC interface. In this process, two long wavelength pump photons combine their energy and momentum to mediate frequency conversion across the large spectral gap between the visible and telecom bands, which is particularly beneficial from the noise perspective. We show that waveguide-coupled silicon nitride microring resonators can be designed for efficient QFC between 606 and 1550 nm via a 1990 nm pump through TSFG/TDFG. We simulate the device dispersion and coupling, and from the simulated parameters, estimate that the frequency conversion can be efficient (${\gt}80 \%$) at 50 mW pump power. Our results suggest that microresonator TSFG/TDFG is promising for compact, scalable, and low-power QFC across large spectral gaps.

Hybridization of circular and rectangular transverse profiles of nanophotonic modes for nonlinear optics.

Nanophotonic modes within rectangular cross sections are typically considered to have transverse rectangular field profiles. In this work, we show that, despite the rectangular cross section of most integrated waveguides and microring resonators, there exists considerable hybridization of transverse rectangular modes and transverse circular modes. These hybridized modes can be advantageous in nonlinear wave mixing processes. We use third-harmonic generation as an example to confirm that such a hybridized mode is advantageous in combining reasonable mode overlap and waveguide coupling to a fundamental mode in a silicon nitride microring. Our work illuminates the potential of using transverse circular modes in nanophotonic applications.

Impact of the precursor gas ratio on dispersion engineering of broadband silicon nitride microresonator frequency combs.

Microresonator frequency combs, or microcombs, have gained wide appeal for their rich nonlinear physics and wide range of applications. Stoichiometric silicon nitride films grown via low-pressure chemical vapor deposition (LPCVD), in particular, are widely used in chip-integrated Kerr microcombs. Critical to such devices is the ability to control the microresonator dispersion, which has contributions from both material refractive index dispersion and geometric confinement. Here, we show that modifications to the ratio of the gaseous precursors in LPCVD growth have a significant impact on material dispersion and hence the overall microresonator dispersion. In contrast to the many efforts focused on comparisons between Si-rich films and stoichiometric (Si3N4) films, here, we focus on films whose precursor gas ratios should nominally place them in the stoichiometric regime. We further show that microresonator geometric dispersion can be tuned to compensate for changes in the material dispersion.

Automated on-axis direct laser writing of coupling elements for photonic chips.

Direct laser writing (DLW) has recently been used to create versatile micro-optic structures that facilitate photonic-chip coupling, like free-form lenses, free-form mirrors, and photonic wirebonds. However, at the edges of photonic chips, the top-down/off-axis printing orientation typically used limits the size and complexity of structures and the range of materials compatible with the DLW process. To avoid these issues, we develop a DLW method in which the photonic chip's optical input/output (IO) ports are co-linear with the axis of the lithography beam (on-axis printing). Alignment automation and port identification are enabled by a 1-dimensional barcode-like pattern that is fabricated within the chip's device layer and surrounds the IO waveguides to increase their visibility. We demonstrate passive alignment to these markers using standard machine vision techniques, and print single-element elliptical lenses along an array of 42 ports with a 100 % fabrication yield. These lenses improve fiber-to-chip misalignment tolerance relative to other fiber-based coupling techniques. The 1 dB excess loss diameter increases from ≈ 2.3 µm when using a lensed fiber to ≈ 9.9 µm when using the DLW printed micro-optic and a cleaved fiber. The insertion loss penalty introduced by moving to this misalignment-tolerant coupling approach is limited, with an additional loss (in comparison to the lensed fiber) as small as ≈1 dB and ≈2 dB on average. Going forward, on-axis printing can accommodate a variety of multi-element free-space and guided wave coupling elements, without requiring calibration of printing dose specific to the geometry of the 3D printed structure or to the materials comprising the photonic chip. It also enables novel methods for interconnection between chips. To that end, we fabricate a proof-of-concept 3D photonic wire bond between two vertically stacked photonic chips.

Heterogeneous photodiodes on silicon nitride waveguides.

Heterogeneous integration through low-temperature die bonding is a promising technique to enable high-performance III-V photodetectors on the silicon nitride (Si3N4) photonic platform. Here we demonstrate InGaAs/InP modified uni-traveling carrier photodiodes on Si3N4 waveguides with 20 nA dark current, 20 GHz bandwidth, and record-high external (internal) responsivities of 0.8 A/W (0.94 A/W) and 0.33 A/W (0.83 A/W) at 1550 nm and 1064 nm, respectively. Open eye diagrams at 40 Gbit/s are demonstrated. Balanced photodiodes of this type reach 10 GHz bandwidth with over 40 dB common mode rejection ratio.

Broadband resonator-waveguide coupling for efficient extraction of octave-spanning microcombs: publisher's note.

This publisher's note contains corrections to Opt. Lett.44, 4737 (2019) OPLEDP0146-959210.1364/OL.44.004737.

Indistinguishable Photons from Deterministically Integrated Single Quantum Dots in Heterogeneous GaAs/Si3N4 Quantum Photonic Circuits.

Silicon photonics enables scaling of quantum photonic systems by allowing the creation of extensive, low-loss, reconfigurable networks linking various functional on-chip elements. Inclusion of single quantum emitters onto photonic circuits, acting as on-demand sources of indistinguishable photons or single-photon nonlinearities, may enable large-scale chip-based quantum photonic circuits and networks. Toward this, we use low-temperature in situ electron-beam lithography to deterministically produce hybrid GaAs/Si3N4 photonic devices containing single InAs quantum dots precisely located inside nanophotonic structures, which act as efficient, Si3N4 waveguide-coupled on-chip, on-demand single-photon sources. The precise positioning afforded by our scalable fabrication method furthermore allows observation of postselected indistinguishable photons. This indicates a promising path toward significant scaling of chip-based quantum photonics, enabled by large fluxes of indistinguishable single-photons produced on-demand, directly on-chip.

Dual-comb spectroscopy with tailored spectral broadening in Si3N4 nanophotonics.

Si3N4 waveguides, pumped at 1550 nm, can provide spectrally smooth, broadband light for gas spectroscopy in the important 2 µm to 2.5 µm atmospheric water window, which is only partially accessible with silica-fiber based systems. By combining Er+ fiber frequency combs and supercontinuum generation in tailored Si3N4 waveguides, high signal-to-noise dual-comb spectroscopy spanning 2 µm to 2.5 µm is demonstrated. Acquired broadband dual-comb spectra of CO and CO2 agree well with database line shape models and have a spectral-signal-to-noise as high as 48/√s, showing that the high coherence between the two combs is retained in the Si3N4 supercontinuum generation. The dual-comb spectroscopy figure of merit is 6 × 106/√s, equivalent to that of all-fiber dual-comb spectroscopy systems in the 1.6 µm band. based on these results, future dual-comb spectroscopy can combine fiber comb technology with Si3N4 waveguides to access new spectral windows in a robust non-laboratory platform.

Broadband resonator-waveguide coupling for efficient extraction of octave-spanning microcombs.

Octave-spanning frequency combs have been successfully demonstrated in Kerr nonlinear microresonators. These microcombs rely on both engineered dispersion, to enable generation of frequency components across the octave, and on engineered coupling, to efficiently extract the generated light into an access waveguide while maintaining a close to critically coupled pump. The latter is challenging, as the spatial overlap between the access waveguide and the ring modes decays with frequency. This leads to strong coupling variation across the octave, with poor extraction at short wavelengths. Here, we investigate how a waveguide wrapped around a portion of the resonator, in a pulley scheme, can improve the extraction of octave-spanning microcombs, in particular at short wavelengths. We use the coupled-mode theory to predict the performance of the pulley couplers and demonstrate good agreement with experimental measurements. Using an optimal pulley coupling design, we demonstrate a 20 dB improvement in extraction at short wavelengths compared to straight waveguide coupling.

Protein Binding Kinetics in Multimodal Systems: Implications for Protein Separations.

In this work, quartz crystal microbalance with dissipation (QCM-D) was employed to study the kinetic processes involved in the interaction of proteins with self-assembled monolayers (SAMs) of multimodal (MM) ligands. SAMs were fabricated to mimic two chromatographic multimodal resins with varying accessibility of the aromatic moiety to provide a well-defined model system. Kinetic parameters were determined for two different proteins in the presence of the arginine and guanidine and a comparison was made with chromatographic retention data. The results indicated that the accessibility of the ligand's aromatic moiety can have an important impact on the kinetics and chromatographic retention behavior. Interestingly, arginine and guanidine had very different effects on the protein adsorption and desorption kinetics in these MM systems. For cytochrome C, arginine resulted in a significant decrease and increase in the adsorption and desorption rates, respectively, while guanidine produced a dramatic increase in the desorption rate, with minimal effect on the adsorption rate. In addition, at different concentrations of arginine, two distinct kinetic scenarios were observed. For α-chymotrypsin, the presence of 0.1 M guanidine in the aromatic exposed ligand system produced an increase in the adsorption rate and only a moderate increase in the desorption rate, which helped to explain the surprising increase in the chromatographic salt elution concentration. These results demonstrate that protein adsorption kinetics in the presence of different mobile phase modifiers and MM ligand chemistries can play an important role in contributing to selectivity in MM chromatography.

Phased-locked two-color single soliton microcombs in dispersion-engineered Si3N4 resonators.

We propose and theoretically investigate a dispersion-engineered Si3N4 microring resonator, based on a cross section containing a partially-etched trench, that supports phase-locked, two-color soliton microcomb states. These soliton states consist of a single circulating intracavity pulse with a modulated envelope that sits on a continuous wave background. Such temporal waveforms produce a frequency comb whose spectrum is spread over two widely-spaced spectral windows, each exhibiting a squared hyperbolic secant envelope, with the two windows phase-locked to each other via Cherenkov radiation. The first spectral window is centered near the 1550 nm pump, while the second spectral window is tailored based on straightforward geometric control, and can be centered as short as 750 nm and as long as 3000 nm. We numerically analyze the robustness of the design to parameter variation, and consider its implications to self-referencing and visible wavelength comb generation.

Tunable mid-infrared generation via wide-band four-wave mixing in silicon nitride waveguides.

We demonstrate wide-band frequency down-conversion to the mid-infrared (MIR) using four-wave mixing (FWM) of near-infrared (NIR) femtosecond-duration pulses from an Er:fiber laser, corresponding to 100 THz spectral translation. Photonic-chip-based silicon nitride waveguides provide the FWM medium. Engineered dispersion in the nanophotonic geometry and the wide transparency range of silicon nitride enable large-detuning FWM phase-matching and results in tunable MIR from 2.6 to 3.6 µm on a single chip with 100-pJ-scale pump-pulse energies. Additionally, we observe up to 25 dB broadband parametric gain for NIR pulses when the FWM process is operated in a frequency up-conversion configuration. Our results demonstrate how integrated photonic circuits pumped with fiber lasers could realize multiple nonlinear optical phenomena on the same chip and lead to engineered synthesis of broadband, tunable, and coherent light across the NIR and MIR wavelength bands.

Interlocking Kerr-microresonator frequency combs for microwave to optical synthesis.

We report accurate phase stabilization of an interlocking pair of Kerr-microresonator frequency combs. The two combs, one based on silicon nitride and one on silica, feature nearly harmonic repetition frequencies and can be generated with one laser. The silicon-nitride comb supports an ultrafast-laser regime with three-optical-cycle, 1-picosecond-period soliton pulses and a total dispersive-wave-enhanced bandwidth of 170 THz, while providing a stable phase-link between optical and microwave frequencies. We demonstrate nanofabrication control of the silicon-nitride comb's carrier-envelope offset frequency and spectral profile. The phase-locked combs coherently reproduce their clock with a fractional precision of <6×10-13/τ, a behavior we verified through 2 h of measurement to reach <3×10-16. Our work establishes Kerr combs as a viable technology for applications like optical-atomic timekeeping and optical synchronization.