Angular momentum holography via a minimalist metasurface for optical nested encryption

Metasurfaces can perform high-performance multi-functional integration by manipulating the abundant physical dimensions of light,demonstrating great potential in high-capacity information technologies.The orbital angular momentum(OAM)and spin angular momentum(SAM)dimensions have been respectively explored as the independent carrier for information multiplexing.However,fully managing these two intrinsic properties in information multiplexing remains elusive.Here,we propose the concept of angular momentum(AM)holography which can fully synergize these two fundamental dimensions to act as the information carrier,via a single-layer,non-interleaved metasurface.The underlying mechanism relies on independently controlling the two spin eigenstates and arbitrary overlaying them in each operation channel,thereby spatially modulating the resulting waveform at will.As a proof of concept,we demonstrate an AM meta-hologram allowing the reconstruction of two sets of holographic images,i.e.,the spin-orbital locked and the spin-superimposed ones.Remarkably,leveraging the designed dual-functional AM meta-hologram,we demonstrate a novel optical nested encryption scheme,which is able to achieve parallel information transmission with ultra-high capacity and security.Our work opens a new avenue for optionally manipulating the AM,holding promising applications in the fields of optical communication,information security and quantum science.

Diffractive optical elements 75 years on:from micro-optics to metasurfaces

Diffractive optical elements(DOEs)are intricately designed devices with the purpose of manipulating light fields by precisely modifying their wavefronts.The concept of DOEs has its origins dating back to 1948 when D.Gabor first introduced holography.Subsequently,researchers introduced binary optical elements(BOEs),including computer-generated holograms(CGHs),as a distinct category within the realm of DOEs.This was the first revolution in optical devices.The next major breakthrough in light field manipulation occurred during the early 21st century,marked by the advent of metamaterials and metasurfaces.Metasurfaces are particularly appealing due to their ultra-thin,ultra-compact properties and their capacity to exert precise control over virtually every aspect of light fields,including amplitude,phase,polarization,wavelength/frequency,angular momentum,etc.The advancement of light field manipulation with micro/nano-structures has also enabled various applications in fields such as information acquisition,transmission,storage,processing,and display.In this review,we cover the fundamental science,cutting-edge technologies,and wide-ranging applications associated with micro/nano-scale optical devices for regulating light fields.We also delve into the prevailing challenges in the pursuit of developing viable technology for real-world applications.Furthermore,we offer insights into potential future research trends and directions within the realm of light field manipulation.

Optical spiral vortex from azimuthally increasing/decreasing exponential phase gradients

A new type of power-exponent-phase vortex-like beams with both quadratic and cubic azimuthal phase gradients is investigated in this work.The intensity and orbital angular momentum(OAM)density distributions are noticeably different when the phase gradient increases or decreases along the azimuth angle,while the orthogonality and total OAM remain constant.The characteristics of the optical field undergo a significant change when the phase shifts from linear to nonlinear,with the variation of the power index having little impact on the beam characteristics under nonlinear phase conditions.These characteristics provide new ideas for applications such as particle manipulation,optical communications,and OAM encryption.

Dynamic polarization rotation and vector field steering based on phase change metasurface

Polarization rotation and vector field steering of electromagnetic wave are of great significance in modern optical applications.However,conventional polarization devices are bulky,monofunctional and lack of tunability,which pose great challenges to the miniaturized and multifunctional applications.Herein,we propose a meta-device that is capable of multi-state polarization rotation and vector field steering based on phase change metasurface.The supercell of the meta-device consists of four Ge2Sb2Te5(GST)elliptic cylinders located on a SiO2 substrate.By independently controlling the phase state(amorphous or crystalline)of each GST elliptic cylinder,the meta-device can rotate the polarization plane of the linearly polarized incident light to different angles that cover from 19.8°to 154.9°at a wavelength of 1550 nm.Furthermore,by merely altering the phase transition state of GST elliptic cylinders,we successfully demonstrated a vector field steering by generating optical vortices carrying orbital angular momentums(OAMs)with topological charges of 0,1 and–1,respectively.The proposed method provides a new platform for investigating dynamically tunable optical devices and has potential applications in many fields such as optical communications and information processing.

Nondiffractive polarization feature of optical vortices

Optical vortices have been extensively researched in recent years for applications in optical manipulation,orbital angular momentum,opti-cal mode multiplexing,and quantum information.1 When the polariza-tions(spin angular momentum)encounter the phase singularity(orbital angular momentum)of an optical vortex,spin-orbit interaction occurs,leading to the discovery of even more fascinating features such as op-tical skyrmions 2 and polarization robustness.3 Optical vortices are like a rich goldmine with many more fascinating features yet to be revealed.

Optical vortices 30 years on:OAM manipulation from topological charge to multiple singularities

Thirty years ago,Coullet et al.proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex.Since then,optical vortices have been widely studied,inspired by the hydrodynamics sharing similar mathematics.Akin to a fluid vortex with a central flow singularity,an optical vortex beam has a phase singularity with a certain topological charge,giving rise to a hollow intensity distribution.Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics.These amazing properties provide a new understanding of a wide range of optical and physical phenomena,including twisting photons,spin–orbital interactions,Bose-Einstein condensates,etc.,while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible.Hitherto,owing to these salient properties and optical manipulation technologies,tunable vortex beams have engendered tremendous advanced applications such as optical tweezers,high-order quantum entanglement,and nonlinear optics.This article reviews the recent progress in tunable vortex technologies along with their advanced applications.

Integrated(de)multiplexer for orbital angular momentum fiber communication

The quickly increasing data transfer load requires an urgent revolution in current optical communication. Orbital angular momentum(OAM) multiplexing is a potential candidate with its ability to considerably enhance the capacity of communication. However, the lack of a compact, efficient, and integrated OAM(de)multiplexer prevents it from being widely applied. By attaching vortex gratings onto the facets of a few-mode fiber, we demonstrate an integrated fiber-based OAM(de)multiplexer. A vortex grating fabricated on the fiber facet enables the direct multiplexing of OAM states at one port and the demultiplexing of OAM states at the other port. The measured bit error rate of the carrier signal after propagating through a 5-km few-mode fiber confirms the validity and effectiveness of the proposed approach. The scheme offers advantages in future high-capacity OAM communication based on optical fiber.

Versatile on-chip light coupling and(de)multiplexing from arbitrary polarizations to controlled waveguide modes using an integrated dielectric metasurface

Metasurfaces have found broad applicability in free-space optics,while its potential to tailor guided waves remains barely explored.By synergizing the Jones matrix model with generalized Snell’s law under the phase-matching condition,we propose a universal design strategy for versatile on-chip mode-selective coupling with polarization sensitivity,multiple working wavelengths,and high efficiency concurrently.The coupling direction,operation frequency,and excited mode type can be designed at will for arbitrary incident polarizations,outperforming previous technology that only works for specific polarizations and lacks versatile mode controllability.Here,using silicon-nanoantenna-patterned silicon-nitride photonic waveguides,we numerically demonstrate a set of chip-scale optical couplers around 1.55μm,including mode-selective directional couplers with high coupling efficiency over 57%and directivity about 23 d B.Polarization and wavelength demultiplexer scenarios are also proposed with 67%maximum efficiency and an extinction ratio of 20 d B.Moreover,a chip-integrated twisted light generator,coupling free-space linear polarization into an optical vortex carrying 1 h orbital angular momentum(OAM),is also reported to validate the mode-control flexibility.This comprehensive method may motivate compact wavelength/polarization(de)multiplexers,multifunctional mode converters,on-chip OAM generators for photonic integrated circuits,and high-speed optical telecommunications.

Optical meta-waveguides for integrated photonics and beyond

The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip.The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits,giving rise to numerous metawaveguides with unprecedented strength in controlling guided electromagnetic waves.Here,we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms,such as dielectric or plasmonic waveguides and optical fibers.Foundational results and representative applications are comprehensively summarized.Brief physical models with explicit design tutorials,either physical intuition-based design methods or computer algorithms-based inverse designs,are cataloged as well.We highlight how meta-optics can infuse new degrees of freedom to waveguide-based devices and systems,by enhancing light-matter interaction strength to drastically boost device performance,or offering a versatile designer media for manipulating light in nanoscale to enable novel functionalities.We further discuss current challenges and outline emerging opportunities of this vibrant field for various applications in photonic integrated circuits,biomedical sensing,artificial intelligence and beyond.

Ultra-broadband on-chip twisted light emitter for optical communications

On-chip twisted light emitters are essential components of orbital angular momentum(OAM)communication devices1,2.These devices address the growing demand for high-capacity communication systems by providing an additional degree of freedom for wavelength/frequency division multiplexing(WDM/FDM).Although whispering-gallery-mode-enabled OAM emitters have been shown to possess some advantages3–5,such as compactness and phase accuracy,their inherent narrow bandwidths prevent them from being compatible with WDM/FDM techniques.Here,we demonstrate an ultra-broadband multiplexed OAM emitter that utilizes a novel joint path-resonance phase control concept.The emitter has a micron-sized radius and nanometer-sized features.Coaxial OAM beams are emitted across the entire telecommunication band from 1,450 to 1,650 nm.We applied the emitter to an OAM communication with a data rate of 1.2 Tbit/s assisted by 30-channel optical frequency combs(OFCs).The emitter provides a new solution to further increase capacity in the OFC communication scenario.

Optical orbital angular momentum multiplexing communication via inversely-designed multiphase plane light conversion

Multiplexing and demultiplexing of optical orbital angular momentum(OAM)are critical operations in modedivision multiplexing communications.Traditional Dammann gratings,spiral phase planes,and optical geometric transformations are regarded as convenient methods for OAM mode(de)multiplexing.However,crosstalk between the different modes and the difficulty of mode multiplexing greatly limit their application to modedivision multiplexing communications.Here,using a set of inversely-designed phase planes,we demonstrate an OAM(de)multiplexer based on multiphase plane light conversion that can enable perfect OAM multiplexing communication.The sorted patterns are Gaussian-like and can be coupled easily into single-mode fiber arrays.Inputs from the fiber array are turned into coaxial OAM modes after the phase planes.OAM mode crosstalk generated by the multiplexer is less than-20 d B,with insertion loss of less than 2.6 d B.OAM modes are sorted by the demultiplexer with mode crosstalk below-10 d B,and the sorting results are coupled to the fiber array.OAM modes carrying 10 Gbit/s on–off keying signals were transmitted in a 5 km few-mode fiber.The measured bit-error-rate curves have power penalties of less than 10 d B.The proposed configuration is highly efficient and convenient and will be beneficial for potential applications in quantum information,information processing,and optical communications.

Broadband on-chip photonic spin Hall element via inverse design

The photonic spin Hall effect plays an important role in photonic information technologies,especially in on-chip spin Hall devices.However,conventional devices suffer from low efficiency or narrow bandwidth,which prevents their practical application.Here,we introduce a spin Hall device using inverse design to achieve both high efficiency and broadband.Spin-dependent light separation is enabled by a 2.4μm circular device with 100 nm pixels.The photonic spin Hall element is fabricated on a silicon-on-insulator wafer compatible with a standard integrated photonic circuit.The spin light is detected and emitted with an efficiency of up to 22%and 35%,respectively,over a 200 nm bandwidth at optical wavelength.

Optical topologicallattices of Bloch-type skyrmion and meron topologies

Optical skyrmions, quasiparticles that are characterized by the topologically nontrivial vectorial textures of optical parameters such as the electromagnetic field, Stokes parameters, and spin angular momentum, have aroused great attention recently. New dimensions for optical information processing, transfer, and storage have become possible, and developing multiple schemes for manipulating the topological states of skyrmions, thus, is urgent.Here we propose an approach toward achieving dynamic modulation of skyrmions via changing the field symmetry and adding chirality. We demonstrate that field symmetry governs the skyrmionic transformation between skyrmions and merons, whereas material chirality modulates the twist degree of fields and spins and takes control of the Néel-type–Bloch-type skyrmionic transition. Remarkably, the enantioselective twist of skyrmions and merons results from the longitudinal spin arising from the chirality-induced splitting of the hyperboloid in the momentum space. Our investigation, therefore, acts to enrich the portfolio of optical quasiparticles. The chiral route to topological state transitions will deepen our understanding of light–matter interaction and pave the way for chiral sensing, optical tweezers, and topological phase transitions in quantum matter.

3D waveguide device for few-mode multi-core fiber optical communications

Space-division multiplexing based on few-mode multi-core fiber(FM-MCF)technology is expected to break the Shannon limit of a single-mode fiber.However,an FM-MCF is compact,and it is difficult to couple the beam to each fiber core.3D waveguide devices have the advantages of low insertion loss and low cross talk in separating various spatial paths of multi-core fibers.Designing a 3D waveguide device for an FM-MCF requires considering not only higher-order modes transmission,but also waveguide bending.We propose and demonstrate a 3D waveguide device fabricated by femtosecond laser direct writing for various spatial path separations in an FM-MCF.The 3D waveguide device couples the LP01 and LP11a modes to the FM-MCF with an insertion loss below 3 dB and cross talk between waveguides below-36 dB.To test the performance of the 3D waveguide device,we demonstrate four-channel multiplexing communication with two LP modes and two cores in a 1-km few-mode sevencore fiber.The bit error rate curves show that the different degrees of bending of the waveguides result in a difference of approximately 1 dB in the power penalty.Femtosecond laser direct writing fabrication enables 3D waveguide devices to support high-order LP modes transmission and further improves FM-MCF communication.

Control amplitude and phase of light by plasmonic meta-hologram with T-shaped nano-cavity

Controlling both amplitude and phase of light in the subwavelength scale is a challenge for traditional optical devices.Here,we propose and numerically investigate a novel plasmonic meta-hologram,demonstrating broadband manipulation of both phase and amplitude in the subwavelength scale.In the meta-hologram,phase modulation is achieved by the detour phase distribution of unit cells,and amplitude is continuously modulated by a T-shaped nano-cavity with tunable plasmonic resonance.Compared to phase-only holograms,such a metahologram could reconstruct three-dimensional(3D)images with higher signal-to-noise ratio and better image quality,thus offering great potential in applications such as 3D displays,optical communications,and beam shaping.

Inverse design of 1D color splitter for high-efficiency color imaging

We introduce a simple one-dimensional(1D)structure in the design of 1D color splitters(1D-CSs)with RGB unit cells for color imaging and propose a single-to-double-layer design in 1D-CSs.Based on inverse design metasurfaces,we demonstrate numerically a single-layer 1D-CS with a full-color efficiency of 46.2%and a double-layer 1D-CS with a full-color efficiency of48.2%;both of them are significantly higher than that of traditional color filters.Moreover,we demonstrate a 1D-CS that has application value by evaluating the double-layer 1D-CS’s performances in terms of incident angle sensitivity,polarization angle sensitivity,and assembly tolerance.

All-dielectric metasurface for fully resolving arbitrary beams on a higher-order Poincaré sphere

Characterizing the amplitude, phase profile, and polarization of optical beams is critical in modern optics. With a series of cascaded optical components, one can accurately resolve the optical singularity and polarization state in traditional polarimetry systems. However, complicated optical setups and bulky configurations inevitably hinder future applications for integration. Here, we demonstrate a metadevice that fully resolves arbitrary beams on a higher-order Poincaré sphere(HOPS) via a single-layer all-silicon metasurface. The device is compact and capable of detecting optical singularities and higher-order Stokes parameters simultaneously through a single intensity measurement. To verify the validity of the proposed metadevice, different beams on HOPS0,0 and HOPS1,-1 are illuminated on the metadevices. The beams are fully resolved, and the reconstructed higher-order Stokes parameters show good agreement with the original ones. Taking the signal-to-noise ratio into account, the numerical simulations indicate that the design strategy can be extended to fully resolve arbitrary beams on HOPS with order up to 4. Because of the advantages of compact configuration and compatibility with current semiconductor technology, the metadevice will facilitate potential applications in information processing and optical communications.