Photon-pair generation in a quadratically nonlinear parity-time symmetric coupler

Integrated nonlinear waveguide structures enable generation of quantum entangled photons. We describe theoretically the effects of spatially inhomogeneous loss on the creation of photon pairs through spontaneous parametric down-conversion in quadratically nonlinear directional couplers, where photons experience effective parity-time(PT) symmetric potential due to the presence of optical loss in one of the waveguides. We show that for losses below the PT-breaking threshold, the quantum photon states can be flexibly tuned similarly to conservative couplers, whereas for stronger losses, the correlations between two waveguide modes are suppressed. We also formulate a quantum-classical correspondence with sum-frequency generation for fast evaluation of device performance.These results can be applied for the design of quantum plasmonic circuits.

Multidimensional synthetic chiral-tube lattices via nonlinear frequency conversion

Geometrical dimensionality plays a fundamentally important role in the topological effects arising in discrete lattices.Although direct experiments are limited by three spatial dimensions,the research topic of synthetic dimensions implemented by the frequency degree of freedom in photonics is rapidly advancing.The manipulation of light in these artificial lattices is typically realized through electro-optic modulation;yet,their operating bandwidth imposes practical constraints on the range of interactions between different frequency components.Here we propose and experimentally realize all-optical synthetic dimensions involving specially tailored simultaneous short-and long-range interactions between discrete spectral lines mediated by frequency conversion in a nonlinear waveguide.We realize triangular chiral-tube lattices in three-dimensional space and explore their four-dimensional generalization.We implement a synthetic gauge field with nonzero magnetic flux and observe the associated multidimensional dynamics of frequency combs,all within one physical spatial port.We anticipate that our method will provide a new means for the fundamental study of high-dimensional physics and act as an important step towards using topological effects in optical devices operating in the time and frequency domains.

Nontrivial coupling of light into a defect:the interplay of nonlinearity and topology

The flourishing of topological photonics in the last decade was achieved mainly due to developments in linear topological photonic structures.However,when nonlinearity is introduced,many intriguing questions arise.For example,are there universal fingerprints of the underlying topology when modes are coupled by nonlinearity,and what can happen to topological invariants during nonlinear propagation?To explore these questions,we experimentally demonstrate nonlinearity-induced coupling of light into topologically protected edge states using a photonic platform and develop a general theoretical framework for interpreting the mode-coupling dynamics in nonlinear topological systems.Performed on laser-written photonic Su-Schrieffer-Heeger lattices,our experiments show the nonlinear coupling of light into a nontrivial edge or interface defect channel that is otherwise not permissible due to topological protection.Our theory explains all the observations well.Furthermore,we introduce the concepts of inherited and emergent nonlinear topological phenomena as well as a protocol capable of revealing the interplay of nonlinearity and topology.These concepts are applicable to other nonlinear topological systems,both in higher dimensions and beyond our photonic platform.

Direct characterization of a nonlinear photonic circuit’s wave function with laser light

Integrated photonics is a leading platform for quantum technologies including nonclassical state generation1–4,demonstration of quantum computational complexity5 and secure quantum communications6.As photonic circuits grow in complexity,full quantum tomography becomes impractical,and therefore an efficient method for their characterization7,8 is essential.Here we propose and demonstrate a fast,reliable method for reconstructing the two-photon state produced by an arbitrary quadratically nonlinear optical circuit.By establishing a rigorous correspondence between the generated quantum state and classical sum-frequency generation measurements from laser light,we overcome the limitations of previous approaches for lossy multimode devices9,10.We applied this protocol to a multi-channel nonlinear waveguide network and measured a 99.28±0.31%fidelity between classical and quantum characterization.This technique enables fast and precise evaluation of nonlinear quantum photonic networks,a crucial step towards complex,largescale,device production.

Nonlinear integrated photonics

The field of nonlinear photonics is in full development. This special issue of Photonics Research takes you through the current issues of this fast-growing field of research, drawing on the current state of the art and seeking, through a selection of articles, to outline some trends for the future.

Dielectric Mie voids: confining light in air

Manipulating light on the nanoscale has become a central challenge in metadevices,resonant surfaces,nanoscale optical sensors,and many more,and it is largely based on resonant light confinement in dispersive and lossy metals and dielectrics.Here,we experimentally implement a novel strategy for dielectric nanophotonics:Resonant subwavelength localized confinement of light in air.We demonstrate that voids created in high-index dielectric host materials support localized resonant modes with exceptional optical properties.Due to the confinement in air,the modes do not suffer from the loss and dispersion of the dielectric host medium.We experimentally realize these resonant Mie voids by focused ion beam milling into bulk silicon wafers and experimentally demonstrate resonant light confinement down to the UV spectral range at 265 nm(4.68 eV).Furthermore,we utilize the bright,intense,and naturalistic colours for nanoscale colour printing.Mie voids will thus push the operation of functional high-index metasurfaces into the blue and UV spectral range.The combination of resonant dielectric Mie voids with dielectric nanoparticles will more than double the parameter space for the future design of metasurfaces and other micro-and nanoscale optical elements.In particular,this extension will enable novel antenna and structure designs which benefit from the full access to the modal field inside the void as well as the nearly free choice of the high-index material for novel sensing and active manipulation strategies.

Dual-band polarized upconversion photoluminescence enhanced by resonant dielectric metasurfaces

Lanthanide-doped upconversion nanoparticles emerged recently as an attractive material platform underpinning a broad range of innovative applications such as optical cryptography,luminescent probes,and lasing.However,the intricate 4f-associated electronic transition in upconversion nanoparticles leads only to a weak photoluminescence intensity and unpolarized emission,hindering many applications that demand ultrabright and polarized light sources.Here,we present an effective strategy for achieving ultrabright and dual-band polarized upconversion photoluminescence.We employ resonant dielectric metasurfaces supporting high-quality resonant modes at dual upconversion bands enabling two-order-of-magnitude amplification of upconversion emissions.We demonstrate that dual-band resonances can be selectively switched on polarization,endowing cross-polarization controlled upconversion luminescence with ultra-high degrees of polarization,reaching approximately 0.86 and 0.91 at dual emission wavelengths of 540 and 660 nm,respectively.Our strategy offers an effective approach for enhancing photon upconversion processes paving the way towards efficient low-threshold polarization upconversion lasers.

Unlocking the out-of-plane dimension for photonic bound states in the continuum to achieve maximum optical chirality

The realization of lossless metasurfaces with true chirality crucially requires the fabrication of three-dimensional structures,constraining experimental feasibility and hampering practical implementations.Even though the threedimensional assembly of metallic nanostructures has been demonstrated previously,the resulting plasmonic resonances suffer from high intrinsic and radiative losses.The concept of photonic bound states in the continuum(BICs)is instrumental for tailoring radiative losses in diverse geometries,especially when implemented using lossless dielectrics,but applications have so far been limited to planar structures.Here,we introduce a novel nanofabrication approach to unlock the height of individual resonators within all-dielectric metasurfaces as an accessible parameter for the efficient control of resonance features and nanophotonic functionalities.In particular,we realize out-of-plane symmetry breaking in quasi-BIC metasurfaces and leverage this design degree of freedom to demonstrate an optical all-dielectric quasi-BIC metasurface with maximum intrinsic chirality that responds selectively to light of a particular circular polarization depending on the structural handedness.Our experimental results not only open a new paradigm for all-dielectric BICs and chiral nanophotonics,but also promise advances in the realization of efficient generation of optical angular momentum,holographic metasurfaces,and parity-time symmetry-broken optical systems.

Meta-optics and bound states in the continuum

We discuss the recent advances in meta-optics and nanophotonics associated with the physics of bound states in the continuum(BICs). Such resonant states appear due to a strong coupling between leaky modes in optical guiding structures being supported by subwavelength high-index dielectric Mieresonant nanoantennas or all-dielectric metasurfaces. First, we review briefly very recent developments in the BIC physics in application to isolated subwavelength particles. We pay a special attention to novel opportunities for nonlinear nanophotonics due to the large field enhancement inside the particle volume creating the resonant states with high-quality(high-Q) factors, the so-called quasi-BIC, that can be supported by the subwavelength particles. Second, we discuss novel applications of the BIC physics to alldielectric optical metasurfaces with broken-symmetry meta-atoms when tuning to the BIC conditions allows to enhance substantially the Q factor of the flat-optics dielectric structures. We also present the original results on nonlinear high-Q metasurfaces and predict that the frequency conversion efficiency can be boosted dramatically by smart engineering of the asymmetry parameter of dielectric metasurfaces in the vicinity of the quasi-BIC regime.

Optical metasurfaces: new generation building blocks for multi-functional optics

Optical metasurfaces(OMs)have emerged as promising candidates to solve the bottleneck of bulky optical elements.OMs offer a fundamentally new method of light manipulation based on scattering from resonant nanostructures rather than conventional refraction and propagation,thus offering efficient phase,polarization,and emission control.This perspective highlights state of the art OMs and provides a roadmap for future applications,including active generation,manipulation and detection of light for quantum technologies,holography and sensing.

Enhanced second-harmonic generation from two-dimensional MoSe_(2) on a silicon waveguide

Two-dimensional transition-metal dichalcogenides(TMDCs)with intrinsically broken crystal inversion symmetry and large secondorder nonlinear responses have shown great promise for future nonlinear light sources.However,the sub-nanometer monolayer thickness of such materials limits the length of their nonlinear interaction with light.Here,we experimentally demonstrate the enhancement of the second-harmonic generation from monolayer MoSe_(2) by its integration onto a 220-nm-thick silicon waveguide.Such on-chip integration allows for a marked increase in the interaction length between the MoSe_(2) and the waveguide mode,further enabling phase matching of the nonlinear process.The demonstrated TMDC–silicon photonic hybrid integration opens the door to second-order nonlinear effects within the silicon photonic platform,including efficient frequency conversion,parametric amplification and the generation of entangled photon pairs.

Ultrathin tunable terahertz absorber based on MEMS-driven metamaterial

The realization of high-performance tunable absorbers for terahertz frequencies is crucial for advancing applications such as single-pixel imaging and spectroscopy.Based on the strong position sensitivity of metamaterials’electromagnetic response,we combine meta-atoms that support strongly localized modes with suspended flat membranes that can be driven electrostatically.This design maximizes the tunability range for small mechanical displacements of the membranes.We employ a micro-electromechanical system technology and successfully fabricate the devices.Our prototype devices are among the best-performing tunable THz absorbers demonstrated to date,with an ultrathin device thickness(~1/50 of the working wavelength),absorption varying between 60%and 80%in the initial state when the membranes remain suspended,and fast switching speed(~27μs).The absorption is tuned by an applied voltage,with the most marked results achieved when the structure reaches the snap-down state.In this case,the resonance shifts by >200% of the linewidth(14% of the initial resonance frequency),and the absolute absorption modulation measured at the initial resonance can reach 65%.The demonstrated approach can be further optimized and extended to benefit numerous applications in THz technology.

Boosting third-harmonic generation by a mirror-enhanced anapole resonator

We demonstrate that a dielectric anapole resonator on a metallic mirror can enhance the third harmonic emission by two orders of magnitude compared to a typical anapole resonator on an insulator substrate.By employing a gold mirror under a silicon nanodisk,we introduce a novel characteristic of the anapole mode through the spatial overlap of resonantly excited Cartesian electric and toroidal dipole modes.This is a remarkable improvement on the early demonstrations of the anapole mode in which the electric and toroidal modes interfere off-resonantly.Therefore,our system produces a significant near-field enhancement,facilitating the nonlinear process.Moreover,the mirror surface boosts the nonlinear emission via the free-charge oscillations within the interface,equivalent to producing a mirror image of the nonlinear source and the pump beneath the interface.We found that these improvements result in an extremely high experimentally obtained efficiency of 0.01%.

Enhanced light-matter interactions in dielectric nanostructures via machine-learning approach

A key concept underlying the specific functionalities of metasurfaces is the use of constituent components to shape the wavefront of the light on demand.Metasurfaces are versatile,novel platforms for manipulating the scattering,color,phase,or intensity of light.Currently,one of the typical approaches for designing a metasurface is to optimize one or two variables among a vast number of fixed parameters,such as various materials’properties and coupling effects,as well as the geometrical parameters.Ideally,this would require multidimensional space optimization through direct numerical simulations.Recently,an alternative,popular approach allows for reducing the computational cost significantly based on a deep-learning-assisted method.We utilize a deep-learning approach for obtaining high-quality factor(high-Q)resonances with desired characteristics,such as linewidth,amplitude,and spectral position.We exploit such high-Q resonances for enhancedlight–matter interaction in nonlinearoptical metasurfaces and optomechanical vibrations,simultaneously.We demonstrate that optimized metasurfaces achieve up to 400-fold enhancement of the third-harmonic generation;at the same time,they also contribute to 100-fold enhancement of the amplitude of optomechanical vibrations.This approach can be further used to realize structures with unconventional scattering responses.

Terahertz bound states in the continuum with incident angle robustness induced by a dual period metagrating

Metasurface-empowered bound state in the continuum(BIC)provides a unique route for fascinating functional devices with infinitely high quality factors.This method is particularly attractive to the terahertz community because it may essentially solve the deficiencies in terahertz filters,sensors,lasers,and nonlinear sources.However,most BIC metasurfaces are limited to specified incident angles that seriously dim their application prospects.Here,we propose that a dual-period dielectric metagrating can support multiple families of BICs that originate from guided mode resonances in the dielectric grating and exhibit infinite quality factors at arbitrarily tilted incidence.This robustness was analyzed based on the Bloch theory and verified at tilted incident angles.We also demonstrate that inducing geometric asymmetry is an efficient way to manipulate the leakage and coupling of these BICs,which can mimic the electromagnetically induced transparency(EIT)effect in our dual-period metagrating.In this demonstration,a slow-light effect with a measured group delay of 117 ps was achieved.The incidence-insensitive BICs proposed here may greatly extend the application scenarios of the BIC effect.The high Q factor and outstanding slow-light effect in the metagrating show exciting prospects in realizing high-performance filters,sensors,and modulators for prompting terahertz applications.

Moiré-driven electromagnetic responses and magic angles in a sandwiched hyperbolic metasurface

Recent moiréconfigurations provide a new platform for tunable and sensitive photonic responses,as their enhanced light–matter interactions originate from the relative displacement or rotation angle in a stacking bilayer or multilayer periodic array.However,previous findings are mostly focused on atomically thin condensed matter,with limitations on the fabrication of multilayer structures and the control of rotation angles.Structured microwave moiréconfigurations are still difficult to realize.Here,we design a novel moiréstructure,which presents unprecedented capability in the manipulation of light–matter interactions.Based on the effective medium theory and S-parameter retrieval process,the rotation matrix is introduced into the dispersion relation to analyze the underlying physical mechanism,where the permittivity tensor transforms from a diagonal matrix to a fully populated one,whereas the permeability tensor evolves from a unit matrix to a diagonal one and finally becomes fully filled,so that the electromagnetic responses change drastically as a result of stacking and rotation.Besides,the experiment and simulation results reveal hybridization of eigenmodes,drastic manipulation of surface states,and magic angle properties by controlling the mutual rotation angles between two isolated layers.Here,not only a more precisely controllable bilayer hyperbolic metasurface is introduced to moiréphysics,the findings also open up a new avenue to realize flat bands at arbitrary frequencies,which shows great potential in active engineering of surface waves and designing multifunctional plasmonic devices.

Bistable lasing in parity-time symmetric coupled fiber rings

We propose a parity-time(PT) symmetric fiber laser composed of two coupled ring cavities with gains and losses,which operates both in PT-symmetric and symmetry-broken regimes depending on the static phase shifts. We perform analytical and numerical analysis by the transfer matrix method taking into account gain saturation and predict laser bistability in the PT-symmetric regime in contrast to a symmetry-broken single-mode operation.In the PT-broken regime, the generation power counterintuitively increases with an increase of the cavity losses.

Disorder-protected quantum state transmission through helical coupled-resonator waveguides

We predict the preservation of temporal indistinguishability of photons propagating through helical coupled-resonator optical waveguides(H-CROWs).H-CROWs exhibit a pseudospin-momentum locked dispersion,which we show suppresscs on-site disorder-induced backscattering and group velocity fluctuations.We simulate numerically the propagation of two-photon wave packets,demonstrating that they exhibit almost perfect Hong-Ou-Mandel dip visibility and then can preserve their quantum coherence even in the presence of moderate disorder,in contrast with regular CROws,which are highly sensitive to disorder.As indistinguishability is the most fundamental resource of quantum information processing,H-CROWs may find applications for the implementation of robust optical links and delay lines in the emerging quantum photonic communication and computational platforms.

Topology-empowered membrane devices for terahertz photonics

Control of terahertz waves offers a profound platform for next-generation sensing,imaging,and information communications.However,all conventional terahertz components and systems suffer from bulky design,sensitivity to imperfections,and transmission loss.We propose and experimentally demonstrate onchip integration and miniaturization of topological devices,which may address many existing drawbacks of the terahertz technology.We design and fabricate topological devices based on valley-Hall photonic structures that can be employed for various integrated components of on-chip terahertz systems.We demonstrate valleylocked asymmetric energy flow and mode conversion with topological waveguide,multiport couplers,wave division,and whispering gallery mode resonators.Our devices are based on topological membrane metasurfaces,which are of great importance for developing on-chip photonics and bring many features into terahertz technology.

Surface that perceives depth:3D imaging with metasurfaces

When Galileo Galilei(1564-1642)was building his super bulky lenses for telescopes,he could not imagine that 400 years later,metasurfaces,which are hundred times thinner than a human hair,1–3 could reproduce the function of his lenses.Indeed,for a few centuries,conventional bulky optics,such as lenses,prisms,mirrors,etc.,have been the only tools for shaping the wave-front of light via engineering the optical path of light beams through media of given refractive indices.Recent advances in nanofabrication,characterization,and computational optics techniques have enabled the development of ultrathin metasurfaces,composed of a single layer or few-layer stacks of periodic subwavelength nanostructures,that can reproduce the functions of bulk optics with better performance,4–6 and occasionally offer new functionalities that are not possible with conventional diffractive optics.