Advancing on-chip Kerr optical parametric oscillation towards coherent applications covering the green gap

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(<1 MHz).Our work represents an important step in the quest to bring nonlinear nanophotonics and its advantages to the visible spectrum.

Observation of Topological Links Associated with Hopf Insulators in a Solid-State Quantum Simulator

Hopf insulators are intriguing three-dimensional topological insulators characterized by an integer topological invariant. They originate from the mathematical theory of Hopf fibration and epitomize the deep connection between knot theory and topological phases of matter, which distinguishes them from other classes of topological insulators. Here, we implement a model Hamiltonian for Hopf insulators in a solid-state quantum simulator and report the first experimental observation of their topological properties, including nontrivial topological links associated with the Hopf fibration and the integer-valued topological invariant obtained from a direct tomographic measurement. Our observation of topological links and Hopf fibration in a quantum simulator opens the door to probe rich topological properties of Hopf insulators in experiments. The quantum simulation and probing methods are also applicable to the study of other intricate three-dimensional topological model Hamiltonians.

Probe Knots and Hopf Insulators with Ultracold Atoms

Knots and links are fascinating and intricate topological objects.Their influence spans from DNA and molecular chemistry to vortices in superfluid helium,defects in liquid crystals and cosmic strings in the early universe.Here we find that knotted structures also exist in a peculiar class of three-dimensional topological insulators—the Hopf insulators.In particular,we demonstrate that the momentum-space spin textures of Hopf insulators are twisted in a nontrivial way,which implies the presence of various knot and link structures.We further illustrate that the knots and nontrivial spin textures can be probed via standard time-of-flight images in cold atoms as preimage contours of spin orientations in stereographic coordinates.The extracted Hopf invariants,knots,and links are validated to be robust to typical experimental imperfections.Our work establishes the existence of knotted structures in Hopf insulators,which may have potential applications in spintronics and quantum information processing.

Multi-mode microcavity frequency engineering through a shifted grating in a photonic crystal ring

Frequency engineering of whispering-gallery resonances is essential in microcavity nonlinear optics.The key is to control the frequencies of the cavity modes involved in the underlying nonlinear optical process to satisfy its energy conservation criterion.Compared to the conventional method that tailors dispersion by cross-sectional geometry,thereby impacting all cavity mode frequencies,grating-assisted microring cavities,often termed as photonic crystal microrings,provide more enabling capabilities through mode-selective frequency control.For example,a simple single period grating added to a microring has been used for single frequency engineering in Kerr optical parametric oscillation(OPO)and frequency combs.Recently,this approach has been extended to multifrequency engineering by using multi-period grating functions,but at the cost of increasingly complex grating profiles that require challenging fabrication.Here,we demonstrate a simple approach,which we term as shifted grating multiple mode splitting(SGMMS),where spatial displacement of a single period grating imprinted on the inner boundary of the microring creates a rotational asymmetry that frequency splits multiple adjacent cavity modes.This approach is easy to implement and presents no additional fabrication challenges compared to an unshifted grating,and yet is very powerful in providing multi-frequency engineering functionality for nonlinear optics.We showcase an example where SGMMS enables OPO across a wide range of pump wavelengths in a normal-dispersion device that otherwise would not support OPO.

Optomechanical feedback cooling of a 5 mm long torsional mode

We report three orders of magnitude optical cooling of the fundamental torsional mode of a 5 mm long,550 nm diameter optical nanofiber.The rotation of the nanofiber couples to the polarization of guided laser fields.We use a weak laser probe to monitor the rotation and use feedback to modulate the polarization of an auxiliary drive laser providing torque.Our results present a tool for the optomechanical control of large-scale torsional resonators,with metrological applications and potential implications for studying macroscopic objects in quantum states.

Universal frequency engineering tool for microcavity nonlinear optics:multiple selective mode splitting of whispering-gallery resonances

Whispering-gallery microcavities have been used to realize a variety of efficient parametric nonlinear optical processes through the enhanced light–matter interaction brought about by supporting multiple high quality factor and small modal volume resonances.Critical to such studies is the ability to control the relative frequencies of the cavity modes,so that frequency matching is achieved to satisfy energy conservation.Typically this is done by tailoring the resonator cross section.Doing so modifies the frequencies of all of the cavity modes,that is,the global dispersion profile,which may be undesired,for example,in introducing competing nonlinear processes.Here,we demonstrate a frequency engineering tool,termed multiple selective mode splitting(MSMS),that is independent of the global dispersion and instead allows targeted and independent control of the frequencies of multiple cavity modes.In particular,we show controllable frequency shifts up to 0.8 nm,independent control of the splitting of up to five cavity modes with optical quality factors≳10^5,and strongly suppressed frequency shifts for untargeted modes.The MSMS technique can be broadly applied to a wide variety of nonlinear optical processes across different material platforms and can be used to both selectively enhance processes of interest and suppress competing unwanted processes.

Identification of advanced spin-driven thermoelectric materials via interpretable machine learning

Machine learning is becoming a valuable tool for scientific discovery.Particularly attractive is the application of machine learning methods to the field of materials development,which enables innovations by discovering new and better functional materials.To apply machine learning to actual materials development,close collaboration between scientists and machine learning tools is necessary.However,such collaboration has been so far impeded by the black box nature of many machine learning algorithms.It is often difficult for scientists to interpret the data-driven models from the viewpoint of material science and physics.Here,we demonstrate the development of spin-driven thermoelectric materials with anomalous Nernst effect by using an interpretable machine learning method called factorized asymptotic Bayesian inference hierarchical mixture of experts(FAB/HMEs).Based on prior knowledge of material science and physics,we were able to extract from the interpretable machine learning some surprising correlations and new knowledge about spin-driven thermoelectric materials.Guided by this,we carried out an actual material synthesis that led to the identification of a novel spin-driven thermoelectric material.This material shows the largest thermopower to date.

Unsupervised phase mapping of X-ray diffraction data by nonnegative matrix factorization integrated with custom clustering

Analyzing large X-ray diffraction(XRD)datasets is a key step in high-throughput mapping of the compositional phase diagrams of combinatorial materials libraries.Optimizing and automating this task can help accelerate the process of discovery of materials with novel and desirable properties.Here,we report a new method for pattern analysis and phase extraction of XRD datasets.The method expands the Nonnegative Matrix Factorization method,which has been used previously to analyze such datasets,by combining it with custom clustering and cross-correlation algorithms.This new method is capable of robust determination of the number of basis patterns present in the data which,in turn,enables straightforward identification of any possible peak-shifted patterns.Peak-shifting arises due to continuous change in the lattice constants as a function of composition and is ubiquitous in XRD datasets from composition spread libraries.Successful identification of the peak-shifted patterns allows proper quantification and classification of the basis XRD patterns,which is necessary in order to decipher the contribution of each unique single-phase structure to the multi-phase regions.The process can be utilized to determine accurately the compositional phase diagram of a system under study.The presented method is applied to one synthetic and one experimental dataset and demonstrates robust accuracy and identification abilities.

Torsional optomechanical cooling of a nanofiber

We demonstrate the optomechanical cooling of a tapered optical nanofiber by coupling the polarization of light to the mechanical angular momentum of the system. The coupling is enabled by birefringence in the fiber and does not make use of an optical resonator. We find evidence for cooling in the distribution of thermally driven amplitude fluctuations and the noise spectrum of the torsional modes. Our proof-of-principle demonstration shows cavity-less cooling of the torsional degree of freedom of a macroscopically extended nanofiber.

Towards integrated photonic interposers for processing octave-spanning microresonator frequency combs

Microcombs-optical frequency combs generated in microresonators-have advanced tremendously in the past decade,and are advantageous for applications in frequency metrology,navigation,spectroscopy,telecommunications,and microwave photonics.Crucially,microcombs promise fully integrated miniaturized optical systems with unprecedented reductions in cost,size,weight,and power.However,the use of bulk free-space and fiber-optic comp on ents to process microcombs has restricted form factors to the table-top.Taking microcomb-based optical frequency synthesis around 1550 nm as our target application,here,we address this challenge by proposing an integrated photonics interposer architecture to replace discrete components by collecting,routing,and interfacing octave-wide microcomb-based optical signals between photonic chiplets and heterogeneously integrated devices.Experimentally,we con firm the requisite performa nee of the individual passive elements of the proposed interposer一octave-wide dichroics,multimode interferometers,and tunable ring filters,and implement the octave-spanning spectral filteri ng of a microcomb,central to the in terposer,using silicon n itride phot onics.Moreover,we show that the thick silicon nitride needed for bright dissipative Kerr soliton generation can be integrated with the comparatively thin silicon nitride interposer layer through octave-bandwidth adiabatic evanescent coupling,indicating a path towards future system-level consolidation.Fin ally,we numerically confirm the feasibility of operating the proposed in terposer synthesizer as a fully assembled system.Our interposer architecture addresses the immediate need for on-chip microcomb processing to successfully miniaturize microcomb systems and can be readily adapted to other metrology-grade applications based on optical atomic clocks and high-precision navigation and spectroscopy.

Observing the spin Hall effect of pseudothermal light through weak measurement

The spin Hall effect of light （SHEL） can be observed by the dark strip resulting from weak measurement. We find that the SHEL of a partially coherent beam （PCB） has a similar phenomenon as well. However, the dark strip in the SHEL of a PCB cannot be explained by considering the beam as an assemblance of coherent speckles. Also, the dark strip in a PCB is not purely dark. By analyzing the autocorrelation, we show that the SHEL of a PCB is the result of overlapping coherent speckles＇ SHEL. We further prove our conclusion by adjusting convergence and incident angles. Finally, we develop a qualitative theory to clarify the SHEL of a PCB.