Laser,domains,and more:an interview with Shining Zhu

Guoqing Chang spoke to Shining Zhu of Nanjing University,about his insights into laser science,applications,and future prospects.Chang:When did you first hear about laser?Zhu:As one of the greatest inventions of the 20th century,the laser has been applied to many fields,such as laser machining and processing,gravitational wave detection,precision metrology,laser spectroscopy and microscopy,5G\6G technology,microwave photonics,just to name a few.

Quantum photonics based on metasurfaces

From metamaterials to metasurfaces,optical nano-structure has been widely investigated for novel and high efficiency functionalities.Apart from the intrisinsic properties of composite material,rich capabilities can be derived from the judi-cious design of metasurfaces,which enable more excellent and highly integrated optical devices than traditional bulk op-tical elements.In the meantime,the abundant manipulation abilites of light in the classical domain can be carried over in-to quantum domain.In this review,we highlight recent development of quantum optics based on metasurfaces,ranging from quantum plasmonics,generation,manipulation and appplication of quantum light to quantum vaccum engineering etc.Finally,some promising avenues for quantum optics with the help of optical metasurface are presented.

Review:Chromatic Dispersion Manipulation Based on Optical Metasurfaces

Metasurfaces are densely arrayed two⁃dimensional(2D)artificial planar metamaterials,which can manipulate the polarization,distribution,and amplitude of light by accurately controlling the phase of the scattering light.The flat metasurface has the potential to substantially reduce the thickness and complexity of the structures and allows ease of fabrication and integration into devices.However,the inherent chromatic aberration of the metasurface originating from the resonant dispersion of the antennas and the intrinsic chromatic dispersion limit their quality.How to effectively suppress or manipulate the chromatic aberration of metalenses has attracted worldwide attention in the last few years,leading to a variety of excellent achievements.Furthermore,utilizing the chromatic dispersion of metasurface to realize special functionalities is also of significant importance.In this review,the most promising recent examples of chromatic dispersion manipulation based on optical metasurface materials are highlighted and put into perspective.

Localization and Steering of Light in One-Dimensional Parity-Time Symmetric Photonic Lattices

We theoretically study the propagation dynamics of input light in one-dimensional mixed linear-nonlinear photonic lattices with a complex parity-time symmetric potential. Numerical computation shows simultaneous localization and steering of the optical beam due to the asymmetric scatter and interplay between Kerr-type nonlinearity and PT symmetry. This may provide a feasible measure for manipulation light in optical communications, integrated optics and so on.

Gigahertz-rate-switchable wavefront shaping through integration of metasurfaces with photonic integrated circuit

Achieving spatiotemporal control of light at high speeds presents immense possibilities for various applications in communication,computation,metrology,and sensing.The integration of subwavelength metasurfaces and optical waveguides offers a promising approach to manipulate light across multiple degrees of freedom at high speed in compact photonic integrated circuit(PIC)devices.Here,we demonstrate a gigahertz-rate-switchable wavefront shaping by integrating metasurface,lithium niobate on insulator photonic waveguides,and electrodes within a PIC device.As proofs of concept,we showcase the generation of a focus beam with reconfigurable arbitrary polarizations,switchable focusing with lateral focal positions and focal length,orbital angular momentum light beams as well as Bessel beams.Our measurements indicate modulation speeds of up to the gigahertz rate.This integrated platform offers a versatile and efficient means of controlling the light field at high speed within a compact system,paving the way for potential applications in optical communication,computation,sensing,and imaging.

Revolutionary meta-imaging:from superlens to metalens

The refractive-lens technique has been well developed over a long period of evolution,offering powerful imaging functionalities,such as microscopes,telescopes,and spectroscopes.Nevertheless,the ever-growing requirements continue to urge further enhanced imaging capabilities and upgraded devices that are more compact for convenience.Metamaterial as a fascinating concept has inspired unprecedented new explorations in physics,material science,and optics,not only in fundamental researches but also novel applications.Along with the imaging topic,this paper reviews the progress of the flat lens as an important branch of metamaterials,covering the early superlens with super-diffraction capability and current hot topics of metalenses including a paralleled strategy of multilevel diffractive lenses.Numerous efforts and approaches have been dedicated to areas ranging from the new fascinating physics to feasible applications.This review provides a clear picture of the flat-lens evolution from the perspective of metamaterial design,elucidating the relation and comparison between a superlens and metalens,and addressing derivative designs.Finally,application scenarios that favor the ultrathin lens technique are emphasized with respect to possible revolutionary imaging devices,followed by conclusive remarks and prospects.

Neural network assisted high-spatial-resolution polarimetry with non-interleaved chiral metasurfaces

Polarimetry plays an indispensable role in modern optics.Nevertheless,the current strategies generally suffer from bulky system volume or spatial multiplexing scheme,resulting in limited performances when dealing with inhomogeneous polarizations.Here,we propose a non-interleaved,interferometric method to analyze the polarizations based on a tri-channel chiral metasurface.A deep convolutional neural network is also incorporated to enable fast,robust and accurate polarimetry.Spatially uniform and nonuniform polarizations are both measured through the metasurface experimentally.Distinction between two semblable glasses is also demonstrated.Our strategy features the merits of compactness and high spatial resolution,and would inspire more intriguing design for detecting and sensing.

Metamaterials:From fundamental physics to intelligent design

Metamaterials are artificial structures with the ability to efficiently control light-field,attracting intensive attention in the past few decades.People have studied the working principles,design strategies,and fabrication methods of metamaterials,making this field cross and combine with many disciplines,including physics,material science,electronics,and chemistry.In recent years,with the rapid development of high-efficiency and multifunctional metasurfaces,which are a two-dimensional version of metamaterials,great efforts have been made to push this material to practical applications.In particular,the introduction of artificial intelligent(AI)algorithms enables metamaterials-based photonic devices that exhibit excellent performances and intelligent functionalities.In this review,we first introduce the basic concepts,working principles,design methods,and applications of metamaterials,and then focus on the rapidly developing metamaterials research combined with AI algorithms.Finally,we conclude this review with personal perspectives on the current problems and future directions of metamaterials research and developments.

Spectral tomographic imaging with aplanatic metalens

Tomography is an informative imaging modality that is usually implemented by mechanical scanning,owing to the limited depth-of-field(DOF)in conventional systems.However,recent imaging systems are working towards more compact and stable architectures;therefore,developing nonmotion tomography is highly desirable.Here,we propose a metalens-based spectral imaging system with an aplanatic GaN metalens(NA=0.78),in which large chromatic dispersion is used to access spectral focus tuning and optical zooming in the visible spectrum.After the function of wavelength-switched tomography was confirmed on cascaded samples,this aplanatic metalens is utilized to image microscopic frog egg cells and shows excellent tomographic images with distinct DOF features of the cell membrane and nucleus.Our approach makes good use of the large diffractive dispersion of the metalens and develops a new imaging technique that advances recent informative optical devices.

Second-harmonic generation and manipulation in lithium niobate slab waveguides by grating metasurfaces

Nonlinear optical processes in waveguides play important roles in compact integrated photonics,while efficient coupling and manipulations inside the waveguides still remain challenging.In this work,we propose a new scheme for second-harmonic generation as well as beam shaping in lithium niobate slab waveguides with the assistance of well-designed grating metasurfaces atλ=1064 nm.By encoding the amplitude and phase into the holographic gratings,we further demonstrate strong functionalities of nonlinear beam shaping by the metasurface design,including dual focusing and Airy beam generation.Our approach would inspire new designs in the miniaturization and integration of compact multifunctional nonlinear light sources on chip.

Transformation optics based on metasurfaces

The development of transformation optics and metamaterials provides a new paradigm for efficiently controlling electromagnetic waves. It is well known that the metamaterial is a novel artificial material with periodic structure, and the unit geometric size and distance between the units are much smaller than the working wavelength. Therefore, for an electromagnetic wave, a macroscopic system comprising these sub-wavelength units can be regarded as a novelty artificial material with a different permittivity, permeability and refractive index compared to the original material. Furthermore, these electromagnetic parameters of the artificial material can be freely manipulated by modifying their unit structure, especially for a material that does not exist in nature. Among these materials, negative refractive index materials have been widely reported. For a negative refractive index material, when light is incident on its interface, the incident and refracted light are distributed on the same side of the surface normal, rather than on both sides as typical materials do. However, the field of transformation optics proposes the concept that the propagation of an electromagnetic wave in curved spacetime is identical in inhomogeneous and anisotropic optical mediums.

Quasi-phase-matching-division multiplexing holography in a three-dimensional nonlinear photonic crystal

Nonlinear holography has recently emerged as a novel tool to reconstruct the encoded information at a new wavelength,which has important applications in optical display and optical encryption.However,this scheme still struggles with low conversion efficiency and ineffective multiplexing.In this work,we demonstrate a quasi-phasematching(QPM)-division multiplexing holography in a three-dimensional(3D)nonlinear photonic crystal(NPC).3D NPC works as a nonlinear hologram,in which multiple images are distributed into different Ewald spheres in reciprocal space.The reciprocal vectors locating in a given Ewald sphere are capable of fulfilling the complete QPM conditions for the high-efficiency reconstruction of the target image at the second-harmonic(SH)wave.One can easily switch the reconstructed SH images by changing the QPM condition.The multiplexing capacity is scalable with the period number of 3D NPC.Our work provides a promising strategy to achieve highly efficient nonlinear multiplexing holography for high-security and high-density storage of optical information.

Electrically controllable laser frequency combs in graphene-fibre microresonators

Laser frequency combs emitting ultrafast pulses of light,at equidistantly discrete frequencies,are cornerstones of modern information networks.In recent years,the generation of soliton combs in microcavities with ultrahighquality factors has established microcombs as out-oflaboratory tools.However,the material and geometry of a laser cavity,which determine comb formation,are difficult to electrically tune.Such dynamic control can further enrich the diversity of comb outputs and help to actively stabilize them.

Optical studies of semiconductor perovskite nanocrystals for classical optoelectronic applications and quantum information technologies: a review

Semiconductor perovskite films are now being widely investigated as light harvesters in solar cells with ever-increasing power conversion efficiencies,which have motivated the fabrication of other optoelectronic devices,such as light-emitting diodes,lasers,and photodetectors.Their superior material and optical properties are shared by the counterpart colloidal nanocrystals(NCs),with the additional advantage of quantum confinement that can yield size-dependent optical emission ranging from the near-UV to near-infrared wavelengths.So far,intensive research efforts have been devoted to the optical characterization of perovskite NC ensembles,revealing not only fundamental exciton relaxation and recombination dynamics but also lowthreshold amplified spontaneous emission and novel superfluorescence effects.Meanwhile,the application of single-particle spectroscopy techniques to perovskite NCs has helped to resolve a variety of optical properties for which there are few equivalents in traditional colloidal NCs,mainly including nonblinking photoluminescence,suppressed spectral diffusion,stable exciton fine structures,and coherent singlephoton emission.While the main purpose of ensemble optical studies is to guide the smooth development of perovskite NCs in classical optoelectronic applications,the rich observations from single-particle optical studies mark the emergence of a potential platform that can be exploited for quantum information technologies.

Metalens-integrated compact imaging devices for wide-field microscopy

Metasurfaces have demonstrated unprecedented capabilities in manipulating light with ultrathin and flat architectures.Although great progress has been made in the metasurface designs and function demonstrations,most metalenses still only work as a substitution of conventional lenses in optical settings,whose integration advantage is rarely manifested.We propose a highly integrated imaging device with silicon metalenses directly mounted on a complementary metal oxide semiconductor image sensor,whose working distance is in hundreds of micrometers.The imaging performances including resolution,signal-to-noise ratio,and field of view(FOV)are investigated.Moreover,we develop a metalens array with polarization-multiplexed dual-phase design for a wide-field microscopic imaging.This approach remarkably expands the FOV without reducing the resolution,which promises a non-limited space-bandwidth product imaging for wide-field microscopy.As a result,we demonstrate a centimeter-scale prototype for microscopic imaging,showing uniqueness of meta-design for compact integration.

Heterogeneously integrated,superconducting silicon-photonic platform for measurementdevice-independent quantum key distribution

Integrated photonics provides a route to both miniaturization of quantum key distribution(QKD)devices and enhancing their performance.A key element for achieving discrete-variable QKD is a singlephoton detector.It is highly desirable to integrate detectors onto a photonic chip to enable the realization of practical and scalable quantum networks.We realize a heterogeneously integrated,superconducting silicon-photonic chip.Harnessing the unique high-speed feature of our optical waveguide-integrated superconducting detector,we perform the first optimal Bell-state measurement(BSM)of time-bin encoded qubits generated from two independent lasers.The optimal BSM enables an increased key rate of measurement-device-independent QKD(MDI-QKD),which is immune to all attacks against the detection system and hence provides the basis for a QKD network with untrusted relays.Together with the timemultiplexed technique,we have enhanced the sifted key rate by almost one order of magnitude.With a 125-MHz clock rate,we obtain a secure key rate of 6.166 kbps over 24.0 dB loss,which is comparable to the state-of-the-art MDI-QKD experimental results with a GHz clock rate.Combined with integrated QKD transmitters,a scalable,chip-based,and cost-effective QKD network should become realizable in the near future.

Metasurface empowered lithium niobate optical phased array with an enlarged field of view

Integrated optical phased arrays(OPAs) have attracted significant interest to steer laser beams for applications including free-space communications, holography, and light detection and ranging. Although many methods have been proposed to suppress grating lobes, OPAs have also been limited by the trade-off between field of view(FOV)and beamforming efficiency. Here, we propose a metasurface empowered port-selected OPA(POPA), an OPA steered by port selection, which is implemented by an aperiodic waveguide array with an average pitch less than the wavelength and phase controlled by coupling among waveguides. A metasurface layer above the POPA was designed to increase wide FOV steering, aliasing-free by polarization division. As a result, we experimentally demonstrate beam scanning over a ±41.04° × 7.06° FOV. The aliasing-free POPA with expanded FOV shows successful incorporation of the waveguide-based OPA technique with an emerging metasurface design, indicating much exploration in concepts for integrated photonic devices.

Light rays and waves on geodesic lenses

Starting from well-known absolute instruments that provide perfect imaging, we analyze a class of rotationally symmetric compact closed manifolds, namely, geodesic lenses. We demonstrate with a numerical method that light rays confined on geodesic lenses form closed trajectories, and that for optical waves, the spectrum of a geodesic lens is(at least approximately) degenerate and equidistant. Moreover, we fabricate two geodesic lenses in micrometer and millimeter scales and observe curved light rays along the geodesics. Our experimental setup may offer a new platform to investigate light propagation on curved surfaces.

Chip-scale metalens microscope for wide-field and depth-of-field imaging

Microscopy is very important in research and industry,yet traditional optical microscopy suffers from the limited field-of-view(FOV)and depth-of-field(DOF)in high-resolution imaging.We demonstrate a simultaneous large FOV and DOF microscope imaging technology based on a chip-scale metalens device that is implemented by a SiNxmetalens array with a co-and cross-polarization multiplexed dual-phase design and dispersive spectrum zoom effect.A 4-mm×4-mm FOV is obtained with a resolution of 1.74μm and DOF of200μm within a wavelength range of 450 to 510 nm,which definitely exceeds the performance of traditional microscopes with the same resolution.Moreover,it is realized in a miniaturized compact prototype,showing an overall advantage for portable and convenient microscope technology.

Large-scale achromatic flat lens by light frequency-domain coherence optimization

Flat lenses,including metalens and diffractive lens,have attracted increasing attention due to their ability to miniaturize the imaging devices.However,realizing a large scale achromatic flat lens with high performance still remains a big challenge.Here,we developed a new framework in designing achromatic multi-level diffractive lenses by light coherence optimization,which enables the implementation of large-scale flat lenses under non-ideal conditions.As results,a series achromatic polymer lenses with diameter from 1 to 10mm are successfully designed and fabricated.The subsequent optical characterizations substantially validate our theoretical framework and show relatively good performance of the centimeter-scale achromatic multi-level diffractive lenses with a super broad bandwidth in optical wavelengths(400-1100 nm).After comparing with conventional refractive lens,this achromatic lens shows significant advantages in white-light imaging performance,implying a new strategy in developing practical planar optical devices.