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Integrated photonics enables signal synthesis, modulation and conversion using photonic integrated circuits (PICs). Many materials have been developed, among which silicon nitride (Si3N4) has emerged as a leading platform particularly for nonlinear photonics. Low-loss Si3N4 PICs have been widely used for frequency comb generation, narrow-linewidth lasers, microwave photonics and photonic computing networks. Yet, among all demonstrated functionalities for Si3N4 integrated photonics, optical non-reciprocal devices such as isolators and circulators have not been achieved. Conventionally, they are realized based on the Faraday effect of magneto-optic materials under an external magnetic field; however, it has been challenging to integrate magneto-optic materials that are not compatible with complementary metal-oxide-semiconductors and that require bulky external magnet. Here we demonstrate a magnetic-free optical isolator based on aluminium nitride (AlN) piezoelectric modulators monolithically integrated on low-loss Si3N4 PICs. The transmission reciprocity is broken by spatio-temporal modulation of a Si3N4 microring resonator with three AlN bulk acoustic wave resonators that are driven with a rotational phase. This design creates an effective rotating acoustic wave that allows indirect interband transition in only one direction among a pair of strongly coupled optical modes. A maximum of 10 dB isolation is achieved under 300 mW total radiofrequency power applied to three actuators, with minimum insertion loss of 0.1 dB. An isolation bandwidth of 700 MHz is obtained, determined by the optical resonance linewidth. The isolation remains constant over nearly 30 dB dynamic range of optical input power, showing excellent optical linearity. Our integrated, linear, magnetic-free, electrically driven optical isolator could be a key building block for integrated lasers and optical interfaces for superconducting circuits., An electrically driven, magnetic-free optical isolator is demonstrated. The device, based on aluminium nitride piezoelectric modulators and a silicon nitride microring resonator, may be useful for integrated lasers and other opto-electric systems.

Silicon photonics enables wafer-scale integration of optical functionalities on chip. Silicon-based laser frequency combs can provide integrated sources of mutually coherent laser lines for terabit-per-second transceivers, parallel coherent light detection and ranging, or photonics-assisted signal processing. We report heterogeneously integrated laser soliton microcombs combining both indium phospide/silicon (InP/Si) semiconductor lasers and ultralow-loss silicon nitride (Si3N4) microresonators on a monolithic silicon substrate. Thousands of devices can be produced from a single wafer by using complementary metal-oxide-semiconductor-compatible techniques. With on-chip electrical control of the laser-microresonator relative optical phase, these devices can output single-soliton microcombs with a 100-gigahertz repetition rate. Furthermore, we observe laser frequency noise reduction due to self-injection locking of the InP/Si laser to the Si3N4 microresonator. Our approach provides a route for large-volume, low-cost manufacturing of narrow-linewidth, chip-based frequency combs for next-generation high-capacity transceivers, data centers, space and mobile platforms.

Low-loss photonic integrated circuits and microresonators have enabled a wide range of applications, such as narrow-linewidth lasers and chip-scale frequency combs [1] , [2] . To translate these into a widespread technology, attaining ultralow optical losses with established foundry manufacturing is critical. Recent advances in integrated Si 3 N 4 photonics [1] , [3] have shown that ultralow-loss, dispersion-engineered microresonators with quality factors Q > 10×10 6 can be attained at die-level throughput. Yet, current fabrication techniques do not have sufficiently high yield and performance for existing and emerging applications.

Monolayer transition metal dichalcogenides with direct bandgaps are emerging candidates for microelectronics, nano-photonics, and optoelectronics. Transferred onto photonic integrated circuits (PICs), these semiconductor materials have enabled new classes of light-emitting diodes, modulators and photodetectors, that could be amenable to wafer-scale manufacturing. For integrated photonic devices, the optical losses of the PICs are critical. In contrast to silicon, silicon nitride (Si3N4) has emerged as a low-loss integrated platform with a wide transparency window from ultraviolet to mid-infrared and absence of two-photon absorption at telecommunication bands. Moreover, it is suitable for nonlinear integrated photonics due to its high Kerr nonlinearity and high-power handing capability. These features of Si3N4 are intrinsically beneficial for nanophotonics and optoelectronics applications. Here we report a low-loss integrated platform incorporating monolayer molybdenum ditelluride (1L-MoTe2) with Si3N4 photonic microresonators. We show that, with the 1L-MoTe2, microresonator quality factors exceeding 3 million in the telecommunication O-band to E-band are maintained. We further investigate the change of microresonator dispersion and resonance shift due to the presence of 1L-MoTe2, and extrapolate the optical loss introduced by 1L-MoTe2 in the telecommunication bands, out of the excitonic transition region. Our work presents a key step for low-loss, hybrid PICs with layered semiconductors without using heterogeneous wafer bonding.

We demonstrated a magnetic-free optical isolator for Si3N4 photonics using A1N piezoelectric actuators. Maximum of 10 dB isolation and

We study nonlinear dynamics emerging in a photonic microring dimer. It is shown that four wave mixing pathways between the dimer supermodes lead to novel nonlinear phenomena going beyond the single resonator physics. (C) 2020 The Author(s)

Optical frequency combs have a wide range of applications in science and technology(1). An important development for miniature and integrated comb systems is the formation of dissipative Kerr solitons in coherently pumped high-quality-factor optical microresonators(2-9). Such soliton microcombs(10) have been applied to spectroscopy(11-13), the search for exoplanets(14,15), optical frequency synthesis(16), time keeping(17) and other areas(10). In addition, the recent integration of microresonators with lasers has revealed the viability of fully chip-based soliton microcombs(18,19). However, the operation of microcombs requires complex startup and feedback protocols that necessitate difficult-to-integrate optical and electrical components, and microcombs operating at rates that are compatible with electronic circuits-as is required in nearly all comb systems-have not yet been integrated with pump lasers because of their high power requirements. Here we experimentally demonstrate and theoretically describe a turnkey operation regime for soliton microcombs co-integrated with a pump laser. We show the appearance of an operating point at which solitons are immediately generated by turning the pump laser on, thereby eliminating the need for photonic and electronic control circuitry. These features are combined with high-quality-factor Si3N4 resonators to provide microcombs with repetition frequencies as low as 15 gigahertz that are fully integrated into an industry standard (butterfly) package, thereby offering compelling advantages for high-volume production.

Low-loss photonic integrated circuits and microresonators have enabled a wide range of applications, such as narrow-linewidth lasers and chip-scale frequency combs. To translate these into a widespread technology, attaining ultralow optical losses with established foundry manufacturing is critical. Recent advances in integrated Si3N4 photonics have shown that ultralow-loss, dispersion-engineered microresonators with quality factors Q > 10 × 106 can be attained at die-level throughput. Yet, current fabrication techniques do not have sufficiently high yield and performance for existing and emerging applications, such as integrated travelling-wave parametric amplifiers that require meter-long photonic circuits. Here we demonstrate a fabrication technology that meets all requirements on wafer-level yield, performance and length scale. Photonic microresonators with a mean Q factor exceeding 30 × 106, corresponding to 1.0 dB m−1 optical loss, are obtained over full 4-inch wafers, as determined from a statistical analysis of tens of thousands of optical resonances, and confirmed via cavity ringdown with 19 ns photon storage time. The process operates over large areas with high yield, enabling 1-meter-long spiral waveguides with 2.4 dB m−1 loss in dies of only 5 × 5 mm2 size. Using a response measurement self-calibrated via the Kerr nonlinearity, we reveal that the intrinsic absorption-limited Q factor of our Si3N4 microresonators can exceed 2 × 108. This absorption loss is sufficiently low such that the Kerr nonlinearity dominates the microresonator’s response even in the audio frequency band. Transferring this Si3N4 technology to commercial foundries can significantly improve the performance and capabilities of integrated photonics., For widespread technological application of nonlinear photonic integrated circuits, ultralow optical losses and high fabrication throughput are required. Here, the authors present a CMOS fabrication technique that realizes integrate photonic microresonators on waver-level with mean quality factors exceeding 30 million and 1 dB/m optical losses.

Emergent phenomena are ubiquitous in nature and refer to spatial, temporal, or spatiotemporal pattern formation in complex nonlinear systems driven out of equilibrium that is not contained in the microscopic descriptions at the single-particle level. Examples range from novel phases of matter in both quantum and classical many-body systems, to galaxy formation or neural dynamics. Two characteristic phenomena are length scales that exceed the characteristic interaction length and spontaneous symmetry breaking. Recent advances in integrated photonics indicate that the study of emergent phenomena is possible in complex coupled nonlinear optical systems. Here we demonstrate that out-of-equilibrium driving of a strongly coupled ("dimer") pair of photonic integrated Kerr microresonators, which at the "single-particle" (i.e. individual resonator) level generate well understood dissipative Kerr solitons, exhibit emergent nonlinear phenomena. By exploring the dimer phase diagram, we find unexpected and therefore unpredicted regimes of soliton hopping, spontaneous symmetry breaking, and periodically emerging (in)commensurate dispersive waves. These phenomena are not included in the single-particle description and related to the parametric frequency conversion between hybridized supermodes. Moreover, by controlling supermode hybridization electrically, we achieve wide tunability of spectral interference patterns between dimer solitons and dispersive waves. Our findings provide the first critical step towards the study of emergent nonlinear phenomena in soliton networks and multimode lattices., 19 pages, 12 figures

Silicon nitride (Si3N4) has emerged as a promising material for integrated nonlinear photonics and has been used for broadband soliton microcombs and low-pulse-energy supercontinuum generation. Therefore understanding all nonlinear optical properties of Si3N4 is important. So far, only stimulated Brillouin scattering (SBS) has not been reported. Here we observe, for the first time, backward SBS in fully cladded Si3N4 waveguides. The Brillouin gain spectrum exhibits an unusual multi-peak structure resulting from hybridization with with high-overtone bulk acoustic resonances (HBARs) of the silica cladding. The reported intrinsic Si3N4 Brillouin gain at 25 GHz is estimated as 7x10^-13 m/W. Moreover, the magnitude of the Si3N4 photoelastic constant is estimated as |p12| = 0.047 +/- 0.004. Since SBS imposes an optical power limitation for waveguides, our results explain the capability of Si3N4 to handle high optical power, central for integrated nonlinear photonics., 7 pages, 3 figures

We demonstrate the integration of a Si3N4 Damascene photonic platform with thin-film lithium niobate on insulator (LNOI) via direct wafer bonding. This process enables fabrication of hybrid X(2)-X(3) microresonators with Q " 106, as well as integrated travelling wave EO modulators.

Quasi-phase-matching (QPM) has become one of the most common approaches for increasing the efficiency of nonlinear three-wave mixing processes in integrated photonic circuits. Here, we provide a study of dispersion engineering of QPM second-harmonic (SH) generation in stoichiometric silicon nitride ( Si 3 N 4 ) waveguides. We apply waveguide design and lithographic control in combination with the all-optical poling technique to study the QPM properties and shape the waveguide dispersion for broadband spectral conversion efficiency inside Si 3 N 4 waveguides. By meeting the requirements for maximal bandwidth of the conversion efficiency spectrum, we demonstrate that group-velocity matching of the pump and SH is simultaneously satisfied, resulting in efficient SH generation from ultrashort optical pulses. The latter is employed for retrieving a carrier-envelope-offset frequency of a frequency comb by using an f − 2 f interferometric technique, where supercontinuum and SH of a femtosecond pulse are generated in Si 3 N 4 waveguides. Finally, we show that the waveguide dispersion determines the QPM wavelength variation magnitude and sign due to the thermo-optic effect.

Recent advances in silicon nitride integrated photonics have achieved ultralow optical losses. Transferring this technology to standard commercial foundries, and merging it with silicon photonics via heterogeneous integration, will seed novel laser and quantum applications. (C) 2020 The Author(s)

Photonics chip-based soliton microcombs have been used in many applications including LIDAR, spectroscopy, coherent communication and astronomical spectrometer calibration [1]. Current-initiated soliton microcombs have been demonstrated [2, 3] recently, signifying improvements in the fabrication of high-Q Si 3 N 4 microresonators. However, both approaches suffer from limited input laser power, thus only demonstrated single-soliton at repetition rates above 149 GHz, which are challenging to detect with commercially available photodetectors. Here we demonstrate a single-soliton generation in 100-GHz-FSR Si 3 N 4 microresonators fabricated using the photonic Damascene reflow process [4], yielding the intrinsic Q-factor exceeding 15 million [5]. Using a compact hybrid laser with narrow linewidth, low relative intensity noise (−160 dBc/Hz at f offset =100 kHz) and high output power up to 100 mW [6], different comb states are observed by simply changing the current of the laser diode, without the need of complex tuning mechanism such as a single sideband modulator [7]. As the laser noise is directly transferred to the soliton comb line, this low-noise laser can be utilized in applications where the phase noise is a critical parameter, e.g. low-noise microwave generation or coherent communication. The experimental setup shown in Fig. 1 (a) consists of an ULN laser operated by a current source and temperature controllers to tune its frequency and power. After the light is coupled into the Si 3 N 4 photonic chip via double inverse nano tapers [8], the temperature of FBG/GC is changed to align the laser wavelength to the resonance of the microresonator. The laser diode current is increased (≈ 330 mA) until soliton existence range is sufficiently long. This is indicated by transmission signal directly observed after the chip on the photodetector (Fig. 1 c). Due to high-Q factor of the Si 3 N 4 , the soliton state can be accessed via simply frequency forward tuning [9], without the need of any complex soliton tuning mechanism. Further, different comb states are observed, i.e. modulation instability, multi-soliton state and single-soliton state, via laser diode current tuning. The coherence properties of the soliton-comb teeth is asserted by performing a heterodyne beatnote measurement using a reference laser with a short-time linewidth of 10 kHz (Fig. 1b). The soliton spectrum is fitted with a sech2 function corresponding to a 3 dB bandwidth of ≈19.3 nm and a ≈131.5 fs pulse.

Soliton microcmb [1] devices based on integrated waveguides are miniaturized optical frequency combs, which are ideal for portable applications for timing, spectroscopy and metrology. Chip-based soliton microcomb devices have already been employed for coherent telecommunications, dual-comb spectroscopy, ultrafast ranging, astrophysical spectrometer calibration, and for creating integrated frequency synthesizers and atomic clocks. A particularly promising application is to build soliton-microcomb-based photonic integrated microwave oscillators, whose center frequency can be freely designed and actively tuned by users. Yet, so far, soliton microcombs at microwave repetition rate have not been reported on integrated material platform, such as silicon nitride (Si 3 N 4 ) [2], mainly due to high optical loss in waveguides caused by the fabrication process. Therefore, sufficient power assisted by power amplifiers for soliton formation is required, which however is incompatible with current integrated lasers. Here, through improving the photonic Damascene reflow process [3, 4] to fabricate integrated Si 3 N 4 microresonators with quality factor exceeding 22 × 106 [5], we demonstrate, not only for the first time but also with an ultralow power of 35 mW, the single soliton formation at 19.6 GHz repetition rate, at microwave K-band. We further characterize the phase noise of the soliton repetition rate, and reveal the main issue limiting the phase noise performance, i.e. the chip device input coupling, which causes significant power fluctuation. Our results pave way to build low-noise, cost-effective, small-footprint, soliton-microcomb-based integrated microwave oscillator using low-loss Si 3 N 4 waveguides, promising for chip-based telecommunication, high-frequency arbitrary waveform generation, frequency synthesis and spectroscopy.

While soliton microcombs offer the potential for integration of powerful frequency metrology and precision spectroscopy systems, their operation requires complex startup and feedback protocols that necessitate difficult-to-integrate optical and electrical components. Moreover, CMOS-rate microcombs, required in nearly all comb systems, have resisted integration because of their power requirements. Here, a regime for turnkey operation of soliton microcombs co-integrated with a pump laser is demonstrated and theoretically explained. Significantly, a new operating point is shown to appear from which solitons are generated through binary turn-on and turn-off of the pump laser, thereby eliminating all photonic/electronic control circuitry. These features are combined with high-Q $Si_3N_4$ resonators to fully integrate into a butterfly package microcombs with CMOS frequencies as low as 15 GHz, offering compelling advantages for high-volume production., Comment: Boqiang Shen, Lin Chang, Junqiu Liu, Heming Wang and Qi-Fan Yang contributed equally to this work

Thermoelectricity is a thermorelated property that is of great importance in single-molecule junctions. The electrical conductance (σ), electron-derived thermal conductance (κel) and Seebeck coefficient (S) of B80-based single-molecule junctions are investigated by using density functional theory in combination with non-equilibrium Green’s function. When the distance between the left/right electrodes is 11.4 A, the relationship between σ and κel obeys the Wiedemann–Franz law very well because of the strong hybridization between B80 molecular orbitals and the surface states of Au electrodes. Furthermore, the calculated Lorenz number is close to the famous value in metal or degenerate semiconductors. In addition, S is only −19.09 μV/K at 300 K, thus leading to the smaller electron’s thermoelectric figure of merit (ZelT = S2σT/κel). Interestingly, the strain and chemical potential can modulate B80-based single-molecule junctions from n-type to p-type when the compressive strain reaches −0.6 A or the chemical potential shifts to −0.16 eV. This might be attributed that S reflects the asymmetry in the electrical conductance with respect to the chemical potential and is proportional to the slopes of the transmission spectrum.

Chalcogenide thin films are exciting candidates for electronic applications such as spintronic devices, nonvolatile memories and thermoelectric materials. To achieve such applications the understanding of their thin film growth is of paramount importance. In this work the epitaxy of exemplary chalcogenides Sb2Te3 and GeTe on different surfaces of Si(111) with atomically sharp interfaces is presented and compared using plan-view transmission electron microscopy and electron diffraction. It is shown that depending on the monolayer surface termination the resulting films present drastic differences in terms of film morphology and crystallinity. In particular, a profound difference is found between the films grown on H-passivated and Sb-passivated surfaces. In both cases, the out-of-plane texture is strongly c-axis oriented, but the case of Si(111)-H shows the frequent occurrence of random in-plane twist for both films, while for Si(111)-Sb this is strongly suppressed. The role of the substrate-film interface for the epitaxy is discussed and the consequences for the properties of the films are highlighted. In general, the insights of these results shed light on chalcogenide thin film growth for topological insulator, ferroelectric, thermoelectric and phase-change materials research.

GeTe has been predicted to be the father compound of a new class of multifunctional materials, ferroelectric Rashba semiconductors, displaying a coupling between spin-dependent k-splitting and ferroelectricity. In this paper, we report on epitaxial Fe/GeTe(111) heterostructures grown by molecular beam epitaxy. Spin-pumping experiments have been performed in a radio-frequency cavity by pumping a spin current from the Fe layer into GeTe at the Fe ferromagnetic resonance and detecting the transverse charge current originated in the slab due to spin-to-charge conversion. Preliminary experiments indicate that a clear spin to charge conversion exists, thus unveiling the potential of GeTe for spin-orbitronics.

The electronic properties of bilayer strained boronitrenes are investigated under an external electric field using density functional methods. Our result is just the same as the previous conclusion: ie, that the electric field will reduce their band gaps. Except for the decrease of their band gaps, the degeneracy of π valence bands at K points will be lifted and the degenerate gap will increase with the electric field increasing. Moreover, the widths of π valence bands are nearly robust and increase a little. In addition, a simple tight-binding model, where different electrostatic potentials are applied to boronitrene layers, can be sufficient to describe the variations of their band gaps. It is found that the interlayer hopping interaction increases while the intralayer hopping parameter changes little with increasing the electric field. Furthermore, a band gap phase diagram is determined within the in-plane strain [-0.2, 0.2] and the interlayer bias [0, 10] V nm(-1). The strain could make the bottom of conduction bands shift from K to M, then to Γ in the Brillouin zone, while the top of valence bands shifts from K to Γ. Thus, a direct-gap semiconductor at K points is changed into an indirect-gap semiconductor, and then a semiconductor with the direct band gap at Γ points. When bilayer boronitrene is a semiconductor with a direct gap at K points, the electric field and strain are inverse proportional relationships. Particularly, when the compressive strain exceeds -0.194, there is an insulator-metal transition and the system becomes metallic with sizable pocket Fermi surfaces.

Density functional calculations have been performed to analyze the electronic and mechanical properties of a number of 2D boroxine-linked covalent organic frameworks (COFs), which are experimentally fabricated from di-borate aromatic molecules. Furthermore, the band structures are surprising and show flat-band characteristics which are mainly attributed to the delocalized π-conjugated electrons around the phenyl rings and can be better understood within aromaticity theories. Next, the effects of branch sizes and hydrostatic strains on their band structures are systematically considered within generalized gradient approximations. It is found that their band gaps will start to saturate when the branch size reaches 9. For boroxine-linked COFs with only one benzene ring in the branch, the band gap is robust under compressive strain while it decreases with the tensile strain increasing. When the branch size is equal or greater than 2, their band gaps will monotonously increase with the strain increasing in the range of [-1.0, 2.0] A. All boroxine-linked COFs are semiconductors with controllable band gaps, depending on the branch length and the applied strain. In comparison with other 2D materials, such as graphene, hexagonal boron nitride, and even γ-graphyne, all boroxine-linked COFs are much softer and even more stable. That is, they can maintain the planar features under a larger compressive strain, which means that they are good candidates in flexible electronics.

Superlattices made of Sb2Te3/GeTe phase change materials have demonstrated outstanding performance with respect to GeSbTe alloys in memory applications. Recently, epitaxial Sb2Te3/GeTe superlattices were found to feature GexSb2Te3+x blocks as a result of intermixing between constituting layers. Herewe present the epitaxy and characterization of Sb2Te3/GexSb2Te3+x van derWaals superlattices, where GexSb2Te3+x was intentionally fabricated. X-ray diffraction, Raman spectroscopy, scanning transmission electron microscopy, and lateral electrical transport data are reported. The intrinsic 2D nature of both sublayers is found to mitigate the intermixing in the structures, significantly improving the interface sharpness and ultimately the superlattice structural and electrical properties. (C) 2017 Author(s).

Superlattices made of alternating layers of nominal GeTe and Sb2Te3 have been studied by micro-Raman spectroscopy. A structural irreversible transformation into ordered GeSbTe alloy is induced by high power laser light exposure. The intensity ratio of anti-Stokes and Stokes scattering under laser illumination gives a maximum average temperature in the sample of 177 °C. The latter is lower than the growth temperature and of 400 °C necessary by annealing to transform the structure in a GeSbTe alloy. The absence of this configuration after in situ annealing even up to 300 °C evidences an electronic excitation induced-transition which brings the system into a different and stable crystalline state.

Titanium sesquioxide (Ti2O3) nanopowders have been successfully synthesized using TiCl4 assisted hydrogen reduction method from P25 (TiO2) powders at 970 °C. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy reveal its high purity. The temperature dependent XRD, resistance, and infrared absorption investigations exhibit Z-shaped curves (cell parameter a, resistance, and infrared transmittance) indicating the semiconductor-metal transition (SMT) in the range of 135–220 °C. With prolonging the annealing time, the starting temperature of SMT is found to move toward the low-temperature side, and this was also discussed. The c/a ratio alteration is considered to result in the modulation of property. Besides, the results are verified comparing with the theoretical calculation of band structure.

We demonstrate piezoelectric actuators integrated on ultralow-loss Si3N4 waveguides with MHz bandwidth, ultra-low power, bi-polar. The voltage applied on the actuator is used to tune the microresonator, initiate, switch, stabilize, tightly phase lock soliton microcombs. © 2020 The Author(s)

We demonstrated an integrated magnetic-free optical isolator for Si 3 N 4 photonics using AlN piezoelectric actuators via spatio-temporal modulation. By entering into the strong mode coupling regime, maximum of 9dB isolation and

We report the first demonstration of microresonator dark pulse generation by self-injection locking of a DFB laser to a Si3N4 microresonator with normal GVD. The 26 GHz repetition rate beatnote is characterized. ©2020 The Author(s)

We demonstrate a photonic-electronic analog-to-digital converter (ADC) offering a record-high acquisition bandwidth of 320 GHz. The system combines a high-speed electro-optic modulator with a Kerr comb for spectrally sliced coherent detection and is used for digitizing ultra-broadband data signals.

Microwave photonic technologies, which upshift the carrier into the optical domain, have facilitated the generation and processing of ultra-wideband electronic signals at vastly reduced fractional bandwidths. For microwave photonic applications such as radars, optical communications and low-noise microwave generation, optical frequency combs are useful building blocks. By virtue of soliton microcombs, frequency combs can now be built using CMOS-compatible photonic integrated circuits. Yet, currently developed integrated soliton microcombs all operate with repetition rates significantly beyond those that conventional electronics can detect, preventing their use in microwave photonics. Access to this regime is challenging due to the required ultra-low waveguide loss and large dimensions of the nanophotonic resonators. Here, we demonstrate soliton microcombs operating in two widely employed microwave bands, the X-band (~10 GHz, for radar) and the K-band (~20 GHz, for 5G). Driven by a low-noise fibre laser, these devices produce more than 300 frequency lines within the 3 dB bandwidth, and generate microwave signals featuring phase noise levels comparable to modern electronic microwave oscillators. Our results establish integrated microcombs as viable low-noise microwave generators. Furthermore, the low soliton repetition rates are critical for future dense wavelength-division multiplexing channel generation schemes and could significantly reduce the system complexity of soliton-based integrated frequency synthesizers and atomic clocks. Nanophotonic microwave synthesizers in the X-band (10 GHz, for radar) and K-band (20 GHz, for 5G), based on integrated soliton microcombs driven by a low-noise fibre laser, link the fields of microwave photonics and integrated microcombs.

High-speed actuation of laser frequency(1)is critical in applications using lasers and frequency combs(2,3), and is a prerequisite for phase locking, frequency stabilization and stability transfer among optical carriers. For example, high-bandwidth feedback control of frequency combs is used in optical-frequency synthesis(4), frequency division(5)and optical clocks(6). Soliton microcombs(7,8)have emerged as chip-scale frequency comb sources, and have been used in system-level demonstrations(9,10). Yet integrated microcombs using thermal heaters have limited actuation bandwidths(11,12)of up to 10 kilohertz. Consequently, megahertz-bandwidth actuation and locking of microcombs have only been achieved with off-chip bulk component modulators. Here we demonstrate high-speed soliton microcomb actuation using integrated piezoelectric components(13). By monolithically integrating AlN actuators(14)on ultralow-loss Si(3)N(4)photonic circuits(15), we demonstrate voltage-controlled soliton initiation, tuning and stabilization with megahertz bandwidth. The AlN actuators use 300 nanowatts of power and feature bidirectional tuning, high linearity and low hysteresis. They exhibit a flat actuation response up to 1 megahertz-substantially exceeding bulk piezo tuning bandwidth-that is extendable to higher frequencies by overcoming coupling to acoustic contour modes of the chip. Via synchronous tuning of the laser and the microresonator, we exploit this ability to frequency-shift the optical comb spectrum (that is, to change the comb's carrier-envelope offset frequency) and make excursions beyond the soliton existence range. This enables a massively parallel frequency-modulated engine(16,17)for lidar (light detection and ranging), with increased frequency excursion, lower power and elimination of channel distortions resulting from the soliton Raman self-frequency shift. Moreover, by modulating at a rate matching the frequency of high-overtone bulk acoustic resonances(18), resonant build-up of bulk acoustic energy allows a 14-fold reduction of the required driving voltage, making it compatible with CMOS (complementary metal-oxide-semiconductor) electronics. Our approach endows soliton microcombs with integrated, ultralow-power and fast actuation, expanding the repertoire of technological applications of microcombs., By monolithically integrating piezoelectric actuators on ultralow-loss photonic circuits, soliton microcombs-a spectrum of sharp lines over a range of optical frequencies-can be modulated at high speeds with megahertz bandwidths.

Photonic chip-based soliton microcombs have shown rapid progress and have already been used in many system-level applications. There has been substantial progress in realizing soliton microcombs that rely on compact laser sources, culminating in devices that only utilize a semiconductor gain chip or a self-injection-locked laser diode as the pump source. However, generating single solitons with electronically detectable repetition rates from a compact laser module has remained challenging. Here we demonstrate a current-initiated, Si(3)N(4 )chip-based, 99-GHz soliton microcomb driven directly by a compact, semiconductor-based laser. This approach does not require any complex soliton tuning techniques, and single solitons can be accessed by tuning the laser current. Further, we demonstrate a generic, simple, yet reliable, packaging technique to facilitate the fiber-chip interface, which allows building a compact soliton microcomb package that can benefit from the fiber systems operating at high power (> 100 mW). Both techniques can exert immediate impact on chip-based nonlinear photonic applications that require high input power, high output power, and interfacing chip-based devices to mature fiber systems. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement