Heterogeneous sapphire-supported low-loss photonic platform.

Sapphire is a promising wideband substrate material for visible photonics. It is a common growth substrate for III-nitride light-emitting diodes and laser structures. Doped sapphires are important gain media foundational to the development of titanium-sapphire and ruby lasers. For lasers operating at visible and near-infrared wavelengths, a photonic platform that minimizes loss while maximizing gain material overlap is crucial. Here, we introduce a novel low-loss waveguiding strategy that establishes high-performance integrated photonics on sapphire substrates. This platform achieves a high intrinsic quality factor of 5.6 million near 780 nm and features direct compatibility with a range of solid-state laser gain media.

Ultra-high Q alumina optical microresonators in the UV and blue bands.

UV and visible photonics enable applications ranging from spectroscopic sensing to communication and quantum information processing. Photonics structures in these wavelength regimes, however, tend to experience higher loss than their IR counterpart. Particularly in the near-UV band, on-chip optical microresonators have not yet achieved a quality factor beyond 1 million. Here, we report ultra-low-loss photonic waveguides and resonators patterned from alumina thin films prepared by a highly scalable atomic layer deposition process. We demonstrate ultra high Q factor of 1.5×106 at 390 nm, a record value at UV bands, and 1.9×106 at 488.5 nm.

Quadratic strong coupling in AlN Kerr cavity solitons.

Photonic platforms with χ(2) nonlinearity offer new degrees of freedom for Kerr frequency comb development. Here, we demonstrate Kerr soliton generation at 1550 nm with phase-matched quadratic coupling to the 775 nm harmonic band in a single AlN microring and thus the formation of dual-band mode-locked combs. In the strong quadratic coupling regime where the χ(2) phase-matching window overlaps the pump mode, the pump-to-harmonic-comb conversion efficiency is optimized. However, the strong quadratic coupling also drastically modifies the Kerr comb generation dynamics and decreases the probability of soliton generation. By engineering the χ(2) phase-matching wavelength, we are able to achieve a balance between high conversion efficiency and high soliton formation rate under the available pump power and microring quality factors. Our numerical simulations confirm the experimental observations. These findings provide guidance on tailoring single-cavity dual-band coherent comb sources.

Superconducting Cavity Electromechanics: The Realization of an Acoustic Frequency Comb at Microwave Frequencies.

We present a nonlinear multimode superconducting electroacoustic system, where the interplay between superconducting kinetic inductance and piezoelectric strong coupling establishes an effective Kerr nonlinearity among multiple acoustic modes at 10 GHz that could hardly be achieved via intrinsic mechanical nonlinearity. By exciting this multimode Kerr system with a single microwave tone, we further demonstrate a coherent electroacoustic frequency comb and provide theoretical understanding of multimode nonlinear interaction in the superstrong coupling limit. This nonlinear superconducting electroacoustic system sheds light on the active control of multimode resonator systems and offers an enabling platform for the dynamic study of microcombs at microwave frequencies.

Photonic integration of Er3+:Y2SiO5 with thin-film lithium niobate by flip chip bonding.

Rare earth ions are known as promising candidates for building quantum light-matter interface. However, tunable photonic cavity access to rare earth ions in their desired host crystal remains challenging. Here, we demonstrate the integration of erbium doped yttrium orthosilicate (Er3+:Y2SiO5) with thin-film lithium niobate photonic circuit by plasma-activated direct flip chip bonding. Resonant coupling to erbium ions is realized by on-chip electro-optically tuned high Q lithium niobate micro-ring resonators. Fluorescence and absorption of erbium ions at 1536.48 nm are measured in the waveguides, while the collective ion-cavity cooperativity with micro-ring resonators is assessed to be 0.36. This work presents a versatile scheme for future rare earth ion integrated quantum devices.

Mitigating photorefractive effect in thin-film lithium niobate microring resonators.

Thin-film lithium niobate is an attractive integrated photonics platform due to its low optical loss and favorable optical nonlinear and electro-optic properties. However, in applications such as second harmonic generation, frequency comb generation, and microwave-to-optics conversion, the device performance is strongly impeded by the photorefractive effect inherent in thin-film lithium niobate. In this paper, we show that the dielectric cladding on a lithium niobate microring resonator has a significant influence on the photorefractive effect. By removing the dielectric cladding layer, the photorefractive effect in lithium niobate ring resonators can be effectively mitigated. Our work presents a reliable approach to control the photorefractive effect on thin-film lithium niobate and will further advance the performance of integrated classical and quantum photonic devices based on thin-film lithium niobate.

Stable tuning of photorefractive microcavities using an auxiliary laser.

Cavity nonlinear optics enables intriguing physical phenomena to occur at micro- or nano-scales with modest input powers. While this enhances capabilities in applications such as comb generation, frequency conversion, and quantum optics, undesired nonlinear effects including photorefraction and thermal bistability are exacerbated. In this Letter, we propose and demonstrate a highly effective method of achieving cavity stabilization using an auxiliary laser for controlling photorefraction in a z-cut periodically poled lithium niobate (LN) microcavity system. Our numerical study accurately models the photorefractive effect under high input powers, guiding future analyses and development of LN microcavity systems.

Photorefraction-induced Bragg scattering in cryogenic lithium niobate ring resonators.

We report intracavity Bragg scattering induced by the photorefractive (PR) effect in high-Q lithium niobate ring resonators at cryogenic temperatures. We show that when a cavity mode is strongly excited, the PR effect imprints a long-lived periodic space-charge field. This residual field in turn creates a refractive index modulation pattern that dramatically enhances the back scattering of an incoming probe light, and results in selective and reconfigurable mode splittings. This PR-induced Bragg scattering effect, despite being undesired for many applications, could be utilized to enable optically programmable photonic components.

Quantum Microwave Radiometry with a Superconducting Qubit.

The interaction of photons and coherent quantum systems can be employed to detect electromagnetic radiation with remarkable sensitivity. We introduce a quantum radiometer based on the photon-induced dephasing process of a superconducting qubit for sensing microwave radiation at the subunit photon level. Using this radiometer, we demonstrate the radiative cooling of a 1 K microwave resonator and measure its mode temperature with an uncertainty ∼0.01 K. We thus develop a precise tool for studying the thermodynamics of quantum microwave circuits, which provides new solutions for calibrating hybrid quantum systems and detecting candidate particles for dark matter.

Efficient Frequency Conversion in a Degenerate χ

Microresonators on a photonic chip could enhance nonlinear optics effects and thus are promising for realizing scalable high-efficiency frequency conversion devices. However, fulfilling phase matching conditions among multiple wavelengths remains a significant challenge. Here, we present a feasible scheme for degenerate sum-frequency conversion that only requires the two-mode phase matching condition. When the drive and the signal are both near resonance to the same telecom mode, an on-chip photon-number conversion efficiency up to 42% is achieved, showing a broad tuning bandwidth over 250 GHz. Furthermore, cascaded Pockels and Kerr nonlinear optical effects are observed, enabling the parametric amplification of the optical signal to distinct wavelengths in a single device. The scheme demonstrated in this Letter provides an alternative approach to realizing high-efficiency frequency conversion and is promising for future studies on communications, atom clocks, sensing, and imaging.

All-optical thermal control for second-harmonic generation in an integrated microcavity.

Nonlinear optical effects in integrated microcavities have been studied extensively with the advantages of strong light-matter interaction, great scalability, and stability due to the small mode volume. However, the pump lasers stimulating nonlinear effects impose obstacles for practical applications, since the material absorption causes thermal resonance drift and instability. Here we experimentally demonstrate an all-optical control of the thermal behavior in optical microcavities for tunable doubly-resonant second-harmonic (SH) generation on an integrated photonic chip. Through an auxiliary control laser, the temperature of a selected microring can be efficiently changed, thus allowing precise frequency tuning of the doubly-resonant wavelength while eliminating the distortion of the lineshape induced by the thermo-optic effect. Although the phase-matching conditions will limit the tuning range of 55GHz, the technique is still potential to achieve a larger tuning range in combination with temperature regulation. Additionally, this approach has the advantage of quick reconfiguration, showing a fast modulation rate up to about 256 kHz. The theoretical model behind our experimental scheme is universal and applicable to other microcavity-enhanced nonlinear optical processes, and our work paves the way for controlling and utilizing the thermal effect in the applications of microcavities.

Widely separated optical Kerr parametric oscillation in AlN microrings.

Here, we report $ {\chi ^{(3)}} $χ(3)-based optical parametric oscillation (OPO) with widely separated signal-idler frequencies from crystalline aluminum nitride microrings pumped at $ 2\,\,\unicode{x00B5}{\rm m} $2µm. By tailoring the width of the microring, OPO reaching toward the telecom and mid-infrared bands with a frequency separation of 64.2 THz is achieved. While dispersion engineering through changing the microring width is capable of shifting the OPO sideband by $ \gt {9}\;{\rm THz}$>9THz, the OPO frequency can also be agilely tuned in the ranges of 1 and 0.1 THz, respectively, by shifting the pump wavelength and controlling the chip's temperature. At high pump powers, the OPO sidebands further evolve into localized frequency comb lines. Such large-frequency-shift OPO with flexible wavelength tunability will lead to enhanced chip-scale light sources.

Ultraviolet to mid-infrared supercontinuum generation in single-crystalline aluminum nitride waveguides.

We demonstrate ultrabroadband supercontinuum generation from ultraviolet to mid-infrared wavelengths in single-crystalline aluminum nitride waveguides. Tunable dispersive waves are observed at the mid-infrared regime by precisely controlling the waveguide widths. In addition, ultraviolet light is generated through cascaded second-harmonic generation in the modal phase-matched waveguides. Numerical simulation indicates a high degree of coherence of the generated spectrum at around the telecom pump and two dispersive waves. Our results establish a reliable path for multiple octave supercontinuum comb generation in single-crystalline aluminum nitride to enable applications including precision frequency metrology and spectroscopy.

Photonic Dissipation Control for Kerr Soliton Generation in Strongly Raman-Active Media.

Microcavity solitons enable miniaturized coherent frequency comb sources. However, the formation of microcavity solitons can be disrupted by stimulated Raman scattering, particularly in the emerging crystalline microcomb materials with high Raman gain. Here, we propose and implement dissipation control-tailoring the energy dissipation of selected cavity modes-to purposely raise or lower the threshold of Raman lasing in a strongly Raman-active lithium niobate microring resonator and realize on-demand soliton mode locking or Raman lasing. Numerical simulations are carried out to confirm our analyses and agree well with experiment results. Our work demonstrates an effective approach to address strong stimulated Raman scattering for microcavity soliton generation.

Radiative Cooling of a Superconducting Resonator.

Cooling microwave resonators to near the quantum ground state, crucial for their operation in the quantum regime, is typically achieved by direct device refrigeration to a few tens of millikelvin. However, in quantum experiments that require high operation power such as microwave-to-optics quantum transduction, it is desirable to operate at higher temperatures with non-negligible environmental thermal excitations, where larger cooling power is available. In this Letter, we present a radiative cooling protocol to prepare a superconducting microwave mode near its quantum ground state in spite of warm environment temperatures for the resonator. In this proof-of-concept experiment, the mode occupancy of a 10 GHz superconducting resonator thermally anchored at 1.02 K is reduced to 0.44±0.05 from 1.56 by radiatively coupling to a 70 mK cold load. This radiative cooling scheme allows high-operation-power microwave experiments to work in the quantum regime, and opens possibilities for routing microwave quantum states to elevated temperatures.

Proposal for Heralded Generation and Detection of Entangled Microwave-Optical-Photon Pairs.

Quantum state transfer between microwave and optical frequencies is essential for connecting superconducting quantum circuits to optical systems and extending microwave quantum networks over long distances. However, establishing such a quantum interface is extremely challenging because the standard direct quantum transduction requires both high coupling efficiency and small added noise. We propose an entanglement-based scheme-generating microwave-optical entanglement and using it to transfer quantum states via quantum teleportation-which can bypass the stringent requirements in direct quantum transduction and is robust against loss errors. In addition, we propose and analyze a counterintuitive design-suppress the added noise by placing the device at a higher temperature environment-which can improve both the device quality factor and power handling capability. We systematically analyze the generation and verification of entangled microwave-optical-photon pairs. The parameter for entanglement verification favors the regime of cooperativity mismatch and can tolerate certain thermal noises. Our scheme is feasible given the latest advances on electro-optomechanics, and can be generalized to various physical systems.

Phonon Coupling between a Nanomechanical Resonator and a Quantum Fluid.

Owing to their extraordinary sensitivity to external forces, nanomechanical systems have become an important tool for studying mesoscopic physics and realizing hybrid quantum systems. While nanomechanics has been widely applied in solid-state systems, its use in liquid receives less attention. There it finds unique applications such as biosensing, rheological sensing, and studying both classical and quantum fluid dynamics in unexplored regimes. In this work, we demonstrate efficient coupling of a nano-optomechanical resonator to a bosonic quantum fluid, superfluid 4He, through ultrahigh-frequency phonons (i.e., sound waves) approaching gigahertz frequencies. A high phonon exchange efficiency >92% and minimum excitation rate of 0.25 phonons per oscillations period, or equivalently kB T/ hfm Qm = 0.044 ≪ 1, are achieved. Based on our experimental results, we further predict that strong coupling between a nanomechanical resonator and superfluid cavity phonons with cooperativity up to 880 can be achieved. Our study opens new opportunities in controlling and manipulating superfluid at the nanoscale and low-excitation level.

Stokes and anti-Stokes Raman scatterings from frequency comb lines in poly-crystalline aluminum nitride microring resonators.

In optical microresonators, stimulated Raman scattering (SRS) competes with four-wave mixing process and impact Kerr comb generation. Here, we demonstrate Raman frequency combs in poly-crystalline aluminum nitride (AlN) microring resonators. The Raman shifts at transverse-electric (TE) and transverse-magnetic (TM) polarizations are characterized from AlN straight waveguides using backscattering geometries. In poly-crystalline AlN microring resonators, the frequency matching of cavity resonances with broad Raman gain enhances the SRS and leads to Raman-assisted frequency combs. As a result, comb lines near Raman scattering regions of AlN are generated.

Cavity-enhanced optical controlling based on three-wave mixing in cavity-atom ensemble system.

Cavity-enhanced optical controlling is experimentally observed with a low-control laser power in a cavity-atom ensemble system. Here, the three-level atoms are coupled with two optical modes of a Fabry-Perot cavity, where a new theoretical model is developed to describe the effective three-wave mixing process between spin-wave and optical modes. By adjusting either temperature or cavity length, we demonstrate the precise frequency tuning of the hybrid optical-atomic resonances. When the doubly-resonant condition is satisfied, the probe laser can be easily modulated by a control laser. In addition, interesting non-Hermitian physics are predicted theoretically and demonstrated experimentally, and all-optical switching is also achieved. Such a doubly-resonant cavity-atom ensemble system without a specially designed cavity can be used for future applications, such as optical signal storage and microwave-to-optical frequency conversion.

Octave-spanning supercontinuum generation in nanoscale lithium niobate waveguides.

We demonstrate octave-spanning supercontinuum generation in unpoled lithium niobate waveguides, which are engineered to possess anomalous dispersion and pumped by a turn-key femtosecond laser centered at 1560 nm. Tunable dispersive waves and strong phase-matched second-harmonic generation are both observed by controlling the widths of the waveguides. The major features of the experimental spectra are reproduced by numerical modeling of the generalized nonlinear Schrödinger equation, which can be used to guide waveguide designs for tailoring the supercontinuum spectrum. Our results identify a path to a simple and integrable supercontinuum source in lithium niobate nanophotonic platform and will enable new capabilities in precision frequency metrology.