Visible-Frequency Metasurface for Structuring and Spatially Multiplexing Optical Vortices

Dynamic Spatial-Selective Metasurface with Multiple-Beam Interference

The emergence of the metasurface has provided a versatile platform for the manipulation of light at the nanoscale. Recent research in metasurfaces has explored a plethora of dynamic control and switching of multifunctionalities, paving the way for innovative applications in fields such as imaging, sensing, and communication. However, current dynamic multifunctional metasurfaces face challenges in terms of functional scalability and selective activation. In this work, we introduce and experimentally demonstrate a strategy that utilizes multiple plane waves to create arbitrary periodic patterns on the metasurface, thus enabling the dynamic and arbitrary spatial-selective activation of its embedded multiplexed functionalities. Furthermore, our strategy facilitates dynamic light control through mechanical translation, as demonstrated by a high-speed, dynamically switchable beam deflection scenario. Our method effectively overcomes the limitations associated with traditional spatially multiplexing techniques, offering greater flexibility and selectivity for dynamic control in multifunctional metasurfaces.

Inverse design of metasurfaces with customized transmission characteristics of frequency band based on generative adversarial networks

In recent years, deep learning (DL) has demonstrated significant potential in the inverse design of metasurfaces, and the generation of metasurfaces with customized transmission characteristics of frequency band remains a challenging and underexplored area. In this study, we propose a DL-assisted method for the inverse design of transmissive metasurfaces. The method consists of a generative adversarial network (GAN)-based graph generator, an electromagnetic response predictor, and a genetic algorithm optimizer. By integrating these components, we can obtain customized metasurfaces with desired transmission characteristics of frequency band. We demonstrate the effectiveness of the proposed method through examples of inverse-designed three-layer cascaded transmissive metasurfaces with wideband, dual-band, and stopband responses in the 8∼12 GHz frequency range. Specifically, we realize three different types of dual-band metasurfaces, namely double-wide, front-wide and rear-narrow, and front-narrow and rear-wide configurations. Additionally, we analyze the accuracy and reliability of the inverse design method by employing data from the training dataset, self-defined objectives, and bandwidth-reduced target responses scaled from the wideband type as design inputs. Quantitative evaluation is performed using metrics such as mean absolute error and average precision. The proposed method successfully achieves the desired effect as intended.

Functionalizing nanophotonic structures with 2D van der Waals materials

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Design of a Metasurface with Long Depth of Focus Using Superoscillation

Longitudinal optical field modulation is very important for applications such as optical imaging, spectroscopy, and optical manipulation. It can achieve high-resolution imaging or manipulation of the target object, but it is also limited by its depth of focus. The depth of focus determines whether the target object can be clearly imaged or manipulated at different distances, so extending the depth of focus can improve the adaptability and flexibility of the system. However, how to extend the depth of focus is still a significant challenge. In this paper, we use a super-oscillation phase modulation optimization method to design a polarization-independent metalens with extended focal depth, taking the axial focal depth length as the optimization objective. The optimized metalens has a focal depth of 13.07 μm (about 22.3 λ), and in the whole focal depth range, the transverse full width at half maximum values are close to the Rayleigh diffraction limit, and the focusing efficiency is above 10%. The results of this paper provide a new idea for the design of a metalens with a long focal depth and may have application value in imaging, lithography, and detection.

Transmission-Reflection-Integrated Multifunctional Passive Metasurface for Entire-Space Electromagnetic Wave Manipulation

In recent years, many intriguing electromagnetic (EM) phenomena have come into being utilizing metasurfaces (MSs). However, most of them operate in either transmission or reflection mode, leaving the other half of the EM space completely unmodulated. Here, a kind of transmission-reflection-integrated multifunctional passive MS is proposed for entire-space electromagnetic wave manipulation, which can transmit the x-polarized EM wave and reflect the y-polarized EM wave from the upper and lower space, respectively. By introducing an H-shaped chiral grating-like micro-structure and open square patches into the unit, the MS acts not only as an efficient converter of linear-to-left-hand circular (LP-to-LHCP), linear-to-orthogonal (LP-to-XP), and linear-to-right-hand circular (LP-to-RHCP) polarization within the frequency bands of 3.05-3.25, 3.45-3.8, and 6.45-6.85 GHz, respectively, under the x-polarized EM wave, but also as an artificial magnetic conductor (AMC) within the frequency band of 12.6-13.5 GHz under the y-polarized EM wave. Additionally, the LP-to-XP polarization conversion ratio (PCR) is up to -0.52 dB at 3.8 GHz. To discuss the multiple functions of the elements to manipulate EM waves, the MS operating in transmission and reflection modes is designed and simulated. Furthermore, the proposed multifunctional passive MS is fabricated and experimentally measured. Both measured and simulated results confirm the prominent properties of the proposed MS, which validates the design's viability. This design offers an efficient way to achieve multifunctional meta-devices, which may have latent applications in modern integrated systems.

Characterization of Orbital Angular Momentum Quantum States Empowered by Metasurfaces

Twisted photons can in principle carry a discrete unbounded amount of orbital angular momentum (OAM), which are of great significance for quantum communication and fundamental tests of quantum theory. However, the methods for characterization of the OAM quantum states present a fundamental limit for miniaturization. Metasurfaces can exploit new degrees of freedom to manipulate optical fields beyond the capabilities of bulk optics, opening a broad range of novel and superior applications in quantum photonics. Here we present a scheme to reconstruct the density matrix of the OAM quantum states of single photons with all-dielectric metasurfaces composed of birefringent meta-atoms. We have also measured the Schmidt number of the OAM entanglement by the multiplexing of multiple degrees of freedom. Our work represents a step toward the practical application of quantum metadevices for the measurement of OAM quantum states in free-space quantum imaging and communications.

Advances in Meta-Optics and Metasurfaces: Fundamentals and Applications

Meta-optics based on metasurfaces that interact strongly with light has been an active area of research in recent years. The development of meta-optics has always been driven by human's pursuits of the ultimate miniaturization of optical elements, on-demand design and control of light beams, and processing hidden modalities of light. Underpinned by meta-optical physics, meta-optical devices have produced potentially disruptive applications in light manipulation and ultra-light optics. Among them, optical metalens are most fundamental and prominent meta-devices, owing to their powerful abilities in advanced imaging and image processing, and their novel functionalities in light manipulation. This review focuses on recent advances in the fundamentals and applications of the field defined by excavating new optical physics and breaking the limitations of light manipulation. In addition, we have deeply explored the metalenses and metalens-based devices with novel functionalities, and their applications in computational imaging and image processing. We also provide an outlook on this active field in the end.

Polarization-switchable focal vortex beam by an Archimedean array

Focal position control of vortex beams has tremendous applications in optical field. Herein, non-classical Archimedean arrays were proposed for optical devices with bifocal length and polarization-switchable focal length. The Archimedean arrays were constructed by rotational elliptical holes in a silver film, which were followed by two one-turned Archimedean trajectories. The elliptical holes in this Archimedean array provide the freedom of polarization control for the optical performance by their rotation status. The rotation of elliptical hole can provide additional phase to affect the shape of vortex beam (converged or diverged) under the illumination of circular polarization. The geometric phase of Archimedes trajectory will also determine the focal position of vortex beam. This Archimedean array can produce a converged vortex beam at the specific focal plane according to the handedness of the incident circular polarization and geometrical arrangement of array. The Archimedean array was also demonstrated by experiment and numerical simulation for its exotic optical performance.

Complex-amplitude modulation of surface waves based on a metasurface coupler

Simultaneous and independent modulation of the amplitude and phase of surface waves (SWs) is critical in photonics and plasmonics. Here, we propose a method for flexible complex-amplitude modulation of SWs based on a metasurface coupler. Benefiting from the full range complex-amplitude modulation ability of the meta-atoms over the transmitted field, the coupler can convert the incident wave into a driven surface wave (DSW) with an arbitrary combination of amplitude and initial phase. By placing a dielectric waveguide that supports guided SWs below the coupler, the DSWs can resonantly couple to SWs while preserving complex-amplitude modulation. The proposed scheme provides a practical way for freely tailoring the phase and amplitude profiles of SWs wavefronts. As verification, meta-devices for normal and deflected SW Airy beam generation and SW dual focusing are designed and characterized in the microwave regime. Our findings may stimulate various advanced surface optical meta-devices.

Broadband Achromatic Metalens in the Visible Light Spectrum Based on Fresnel Zone Spatial Multiplexing

Metalenses composed of a large number of subwavelength nanostructures provide the possibility for the miniaturization and integration of the optical system. Broadband polarization-insensitive achromatic metalenses in the visible light spectrum have attracted researchers because of their wide applications in optical integrated imaging. This paper proposes a polarization-insensitive achromatic metalens operating over a continuous bandwidth from 470 nm to 700 nm. The silicon nitride nanopillars of 488 nm and 632.8 nm are interleaved by Fresnel zone spatial multiplexing method, and the particle swarm algorithm is used to optimize the phase compensation. The maximum time-bandwidth product in the phase library is 17.63. The designed focal length can be maintained in the visible light range from 470 nm to 700 nm. The average focusing efficiency reaches 31.71%. The metalens can achieve broadband achromatization using only one shape of nanopillar, which is simple in design and easy to fabricate. The proposed metalens is expected to play an important role in microscopic imaging, cameras, and other fields.

A unique physics-inspired deep-learning-based platform introducing a generalized tool for rapid optical-response prediction and parametric-optimization for all-dielectric metasurfaces

Metasurfaces are composed of a two-dimensional array of carefully engineered subwavelength structures. They provide a novel compact alternative to conventional voluminous optical components. However, their design involves a time-consuming hit and trial procedure, requiring many iterative electromagnetic simulations through expensive commercial solvers. To overcome this non-practical design strategy, recently, various deep-learning-based fast and low computational cost networks have been proposed to design and optimize individual meta-atoms and complete metasurfaces. Most of them focus on optimizing the amplitude response of nanostructures, whereas mapping the phase response is a much more challenging problem that needs to be addressed. Since the metaatom's optical response is entirely reliant on and vulnerable to its geometrical structure, underlying material, and operating wavelength, changing any of these parameters changes the entire physics of the problem in hand. Here, we propose novel deep-learning-based generalized forward and inverse design approaches to optimize all-dielectric transmissive metasurfaces. The proposed forward predicting neural networks take all the geometrical parameters and the physical properties of the bar-shaped dielectric nano-resonators as the input and predict the cross-polarized transmission amplitude and modulated phase at eight distinct rotation angles of the nano-bar. These networks are generalized to predict the electromagnetic (EM) response of different dielectric materials at different operating wavelengths. An inverse design neural network is also proposed that takes the target transmission amplitude and phase at eight discrete orientation angles of the nano-bar as the primary input. The underlying physics of the problem is also incorporated by feeding the intrinsic material properties and the operating wavelength of the nano-bar as a second input to the inverse neural network. It predicts the optimum set of geometrical parameters to achieve maximum cross-polarized transmission and complete Pancharatnam-Berry (PB) phase coverage from 0 to 2π. The average test data mean square error (MSE) achieved for the forward predicting neural network is 1.8 × 10^-3 and that of the inverse design neural network is 2.8 × 10^-1. The average MSEs for different material's test samples are demonstrated to validate the generalizability of the proposed models in terms of seen and unseen materials. A comparative analysis of the proposed approach with conventional EM software optimization tools is performed to prove that the proposed inverse design works much faster than the conventional methods, also it can handle a comparatively larger range of parameters and predicts the results in a single run with high accuracy.

Optimizing Metasurface-Component Performance by Improving Transmittance and Phase Match of the Nanopillars

In the propagation phase of a dielectric metasurface, there are two important problems. Firstly, the range of transmittance of the nanopillars for a building metasurface is usually between 60% and 100%, which reduces the metasurface's overall transmittance and affects the uniformity of the transmitted light. Secondly, the realistic phase provided by the nanopillar cannot be matched very well with the theoretical phase at each lattice location.The phase difference (between a realistic phase and theoretical phase) may reach tens of degrees. Here, we propose an interesting method to solve these problems. With this new method, a metalens is designed in this paper. The nanopillars for building the metalens have transmittance over 0.95, which increases the metalens transmittance and improves the light uniformity. In addition, with the new method, the phase differences of all elements in the metalens can also be reduced to be below 0.05°, decreasing the metalens spherical aberration dramatically. This method not only helps us to optimize the metalens but also provides a useful way for designing high-quality metasurfaces.

Arbitrary-Order and Multichannel Optical Vortices with Simultaneous Amplitude and Phase Modulation on Plasmonic Metasurfaces

The highly localized and uneven spatial distribution of the subwavelength light field in metal metasurfaces provides a promising means for the generation of optical vortices (OVs) with arbitrary topological charges. In this paper, a simple and reliable way for generating multichannel OVs on gold nanoporous metasurfaces is reported. The instantaneous field of arbitrary-order OVs can be regulated and concentrated on the same focal surface by adapting photonic spin-orbit interaction (SOI) and geometric phase. The focal ring energy distribution of OVs along the conical propagation path is accurately calculated, and the double phase of units induced by spin rotation is confirmed. Based on the parameter optimization of the nanohole arrangement, the simultaneous amplitude and phase modulation of multichannel OVs has been realized. Furthermore, the average multichannel signal-to-noise ratio exceeds 15 dB, which meets the requirements of high resolution and low crosstalk. Our study obtains broadband and efficient OVs, which can contribute to improving the capacity storage and security of optical information and possess great application prospects in beam shaping, optical tweezers, and communication coding.

Nickel-Based High-Bandwidth Nanostructured Metamaterial Absorber for Visible and Infrared Spectrum

The efficient control of optical light at the nanoscale level attracts marvelous applications, including thermal imaging, energy harvesting, thermal photovoltaics, etc. These applications demand a high-bandwidth, thermally robust, angularly stable, and miniaturized absorber, which is a key challenge to be addressed. So, in this study, the simple and cost-effective solution to attain a high-bandwidth nanostructured absorber is demonstrated. The designed nanoscale absorber is composed of a simple and plain circular ring of nickel metal, which possesses many interesting features, including a miniaturized geometry, easily fabricable design, large operational bandwidth, and polarization insensitivity, over the previously presented absorbers. The proposed nanoscale absorber manifests an average absorption of 93% over a broad optical window from 400 to 2800 nm. Moreover, the detailed analysis of the absorption characteristics is also performed by exciting the optical light's various incident and polarization angles. From the examined outcome, it is concluded that the nanostructured absorber maintains its average absorption of 80% at oblique incident angles in a broad wavelength range from 400 to 2800 nm. Owing to its appealing functionalities, such as the large bandwidth, simple geometry, low cost, polarization insensitivity, and thermal robustness of the constituting metal, nickel (Ni), this nano-absorber is made as an alternative for the applications of energy harvesting, thermal photovoltaics, and emission.

Doublet Metalens with Simultaneous Chromatic and Monochromatic Correction in the Mid-Infrared

Metalenses provide a powerful paradigm for mid-infrared (MIR) imaging and detection while keeping the optical system compact. However, the design of MIR metalenses simultaneously correcting chromatic aberration and off-axis monochromatic aberration remains challenging. Here, we propose an MIR doublet metalens composed of a silicon aperture metalens and a silicon focusing metalens separated by a fused silica substrate. By performing ray-tracing optimization and particle-swarm optimization, we optimized the required phase profiles as well as the sizes and spatial distributions of silicon nanopillars of the doublet metalens. Simulation results showed that the MIR doublet metalens simultaneously achieved chromatic and off-axis monochromatic aberration reduction, realizing a continuous 400 nm bandwidth and 20° field-of-view (FOV). Thanks to its planar configuration, this metalens is suitable for integration with CMOS image sensor to achieve MIR imaging and detection, which has potential application in troubleshooting and intelligent inspection of power grids. This work may facilitate the practical application of metalens-integrated micro/nanosensors in intelligent energy.

Multiwavelength achromatic super-resolution focusing via a metasurface-empowered controlled generation of focused cylindrically polarized vortex beams

Non-invasive imaging beyond the diffraction limit and free from fluorescent labels in the visible is highly desired for microscopy. It remains a challenge to obtain such super-resolution focusing along with multiwavelength achromatic performance in the far field using an integratable and easily designed system. In this work, we demonstrate a straightforward metasurface-based method to realize multiwavelength achromatic generation and focusing of cylindrically polarized vortex beams (CPVBs). Attributed to the extra degrees of freedom of CPVBs and multi-section design, we have realized multiwavelength achromatic super-resolution focusing in the air with focal size tighter than that of normally used schemes like immersion metalenses or focused radially polarized beams. It is expected that this metasurface-empowered ultra-compact design will benefit potential applications which call for high resolution, like optical microscopy, laser processing, etc.

Ultra-Thin, Short-Focus, and High-Aperture Metalens for Generating and Detecting Laser Optical Vortices

A combined high-aperture metalens in a thin silicon nitride film that consists of two tilted sectored metalenses is considered. Each sector of the metalens consists of a set of binary subwavelength gratings. The diameter of the metalens is 14 μm. Using a time-domain finite difference method, we show that the metalens can simultaneously detect optical vortices with two topological charges −1 and −2, almost over the entire spectrum of visible wavelengths. The metalens can distinguish several wavelengths that are focused at different points in the focal plane due to a 1-nm change in wavelength resulting in a focal spot shift of about 4 nm. When the metalens is illuminated by a Gaussian beam with left-handed circular polarization, two optical vortices with topological charges 1 and 2 are simultaneously formed 6-μm apart at the focal distance of 6 μm.

Single-Step Fabricable Flexible Metadisplays for Sensitive Chemical/Biomedical Packaging Security and Beyond

Secure packaging and transportation of light-sensitive chemical and biomedical test tubes are crucial for environmental protection and public health. Benefiting from the compact form factor and high efficiency of optical metasurfaces, we propose a broad-band polarization-insensitive flexible metasurface for the security of sensitive packages in the transport industry. We employ both the propagation and the geometric phase of novel TiO₂ resin-based anisotropic nanoresonators to demonstrate a flexible and broad-band polarization-insensitive metasurface in the visible domain. The ultraviolet nanoimprint lithographic technique (UV-NIL) is used to fabricate high-index TiO₂ nanoparticle-embedded-resin (nano-PER) structures that are patterned on a flexible substrate. This novel approach provides swift single-step fabrication without secondary fabrication steps such as deposition and etching. Moreover, replicating and transforming patterns over flexible substrates make the proposed technique highly suitable for large-throughput commercial manufacturing. As the proposed metahologram manifests high transmission efficiency in the visible domain, such flexible metaholographic platforms could find several exciting applications in bendable/curved displays, wearable devices, and holographic labeling for interactive displays.

Tunable bilayer dielectric metasurface via stacking magnetic mirrors

Functional tunability, environmental adaptability, and easy fabrication are highly desired properties in metasurfaces. Here we provide a tunable bilayer metasurface composed of two stacked identical dielectric magnetic mirrors. The magnetic mirrors are excited by the interaction between the interference of multipoles of each cylinder and the lattice resonance of the periodic array, which exhibits nonlocal electric field enhancement near the interface and high reflection. We achieve the reversible conversion between high reflection and high transmission by manipulating the interlayer coupling near the interface between the two magnetic mirrors. Controlling the interlayer spacing leads to the controllable interlayer coupling and scattering of meta-atom. The magnetic mirror effect boosts the interlayer coupling when the interlayer spacing is small. Furthermore, the high transmission of the bilayer metasurface has good robustness due to the meta-atom with interlayer coupling can maintain scattering suppression against positional perturbation. This work provides a straightforward method to design tunable metasurface and sheds new light on high-performance optical switches applied in communication and sensing.

In situ tunable circular dichroism of flexible chiral metasurfaces composed of plasmonic nanorod trimers

The circularly polarized light source is one of the keys to chiral photonic circuits and systems. However, it is difficult to integrate conventional light-emitting devices with circular polarization converters directly into compact chip-scale photonic systems partly because of their bulky structures. In this study, in situ optical chirality tunable nanorod trimer metasurfaces consisting of two types of nanorod dimers are demonstrated and integrated with a flexible polydimethylsiloxane (PDMS) substrate. The optical chirality variations originating from the tunable asymmetricity of nanorod trimers under different stretching scenarios are evaluated. Through the processes, the gap distances between nanorods are varied, and the degree of circular polarization of the transmitted wave is controlled through the manipulation of localized surface plasmon resonance (LSPR) coupling. The results reveal the circular dichroism tunability and durability of fabricated chiral metasurfaces which can be important elements for chip-scale flexible optoelectronic integrated circuits for sensing, display and communication applications.

Broadband polarization-insensitive metalens integrated with a charge-coupled device in the short-wave near-infrared range

The performance of a charge-coupled device is important in detection accuracy for terminal sensitivity of a short-wave near-infrared spectrometer. The sizes of pixel pitch and pixel itself are reducing with the development of CCD technologies. However, the fill factor of CCD pixels has not been significantly improved due to the limits of the shift registers, which makes a lower utilization rate of incident light of CCD. In recent years, the advance of metasurface optics provides a new solution for solving this problem. In this paper, a polarization-insensitive metalens is experimentally demonstrated to increase the fill factor of short-wave near-infrared CCD pixels by 4 times, and the simulated results show that the designed metalens has an excellent optical crosstalk (≤0.8%). It proves that the fill factor of CCD pixels can be further improved by the proposed approach which would pave the way for the overall integration of metalens array and photodetectors, as well as the development of CCD miniaturization and lightweight.

Broadband achromatic longitudinal bifocal metalens in the visible range based on a single nanofin unit cell

Metasurfaces provide a remarkable platform to manipulate over phase, amplitude, and polarization flexibly and precisely. Bifocal metalens draws great research interest due to its ability of converging wavefronts to different focal positions horizontally and longitudinally. However, as wavelength of incident light changes, chromatic aberration will cause the focal lengths reliance on the incident wavelength, which will affect the performance of metasurface, especially for longitudinal bifocal metalens. In this work, a broadband achromatic longitudinal bifocal metalens (BALBM) based on single nanofin unit cell is demonstrated. Pancharatnam-Berry (PB) phase is used to converge the incident light. Cross commixed sequence distribution (CCSD) is introduced to control the positions of focal points F_Land F_Rwhen left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) incident. Propagation phase is used to compensate the phase difference caused by chromatic aberration. Simulation results show that in the continuous wavelength range from 500 nm to 700 nm, the focal point shifts caused by chromatic dispersion are reduced 81% for F_L and 83% for F_R, respectively. The focal length variations are stabilized to 6.21% for F_Land 4.8% for F_Rcomparing with the focal lengths at the initial wavelength 500 nm. The proposed BALBM brings advances to bifocal metasurfaces in versatile application areas including machine vision, optical computed tomography and microimaging.

Advances on Solid-State Vortex Laser

Vortex beams (VBs) are structured beams with helical wavefronts carrying orbital angular momentum (OAM) and they have been widely used in lots of domains, such as optical data-transmission, optical tweezer, quantum entanglement, and super-resolution imaging. The ability to generate vortex beams with favorable performance is of great significance for these advanced applications. Compared with extra-cavity schemes, such as spatial light modulation, mode conversion, and others which transform other modes into vortex modes, solid-state vortex lasers can output vortex beams directly and show advantages including a compact structure, high robustness, easy to integrate, and low cost. In this review, we summarize intra-cavity generation approaches to vortex beams in solid-state lasers. Our work on 1.6μm eye-safe vector vortex lasers is also introduced.

Optical Vector Vortex Generation by Spherulites with Cylindrical Anisotropy

Materials with crystalline structures of circular symmetry are rare in nature; however, they are highly desired in optical applications with structured lights, whose characteristics are of cylindrical symmetry. In this work, using a naturally existing circular anisotropy from a spherulite formed by molecular self-assembly, we obtain a cylindrical vector optical vortex beam generation transformed from the spin angular momentum in the wide visible range. The proposed strategy provides promising and broad opportunities for the applications of spherulites in the generation of structured lights and modulations of both the polarization and the angular momentum.

Controlling Dispersion Characteristic of Focused Vortex Beam Generation

As an important structured beam, vortex beams have a wide range of applications in many fields. However, conventional vortex beam generators require complex optical systems, and this problem is particularly serious with regards to focused vortex beam generators. The emergence of metasurfaces provides a new idea for solving this problem; however, the accompanying chromatic dispersion limits its practical application. In this paper, we show that the dispersion characteristic of focused vortex beam generators based on metasurfaces can be controlled by simultaneously manipulating the geometric and propagative phases. The simulation results show that the transmission-type focused vortex beam generators exhibit positive dispersion, zero dispersion, and negative dispersion, respectively. This work paves the way for the practical application of focused vortex beam generators.

Broadband tunable metasurface platform enabled by dynamic phase compensation

Broadband metasurfaces have attracted significant attention for a variety of applications in imaging and communication systems. Here, a method to alleviate the chromatic aberrations issue is proposed in the microwave region using dynamic phase compensation enabled by a reconfigurable metasurface. The dispersion characteristic of the meta-atom implemented with varactor diodes can be flexibly manipulated electronically, such that the dispersion-induced phase distortions over a wide frequency band can be compensated dynamically to achieve broadband performances. Various aberration-free functionalities can be realized with the proposed active metasurface. Near-field measurements are performed on a fabricated prototype to demonstrate aberration-free beam bending and hologram imaging, showing good agreement with simulation results. Such an active metasurface platform paves the way to efficient devices for wireless power transfer, sensors, and communication and antenna systems at radio or much higher frequencies.

Quasi-perfect vortices generated by Pancharatnam-Berry phase metasurfaces for optical spanners and OAM communication

Optical vortex (OV) can be used in the fields of optical manipulation and optical communication because of its inherent orbital angular momentum (OAM). The size of the OV ring increases with the correlated topological charge (TC), making the OV with large TC not suitable for optical rotation and short-distance communication. Perfect vortex (PV) has attracted much attention due to that its optical transmission profile is almost independent of TC. In this manuscript, we proposed a method to generate quasi- perfect vortices (Q-PVs) by Pancharatnam–Berry (PB) phase metasurfaces, the so-called Q-PV can be regarded as an annularly focused optical vortex whose focal ring in the focal plane has an angular phase gradient. It has a similar property to PV in that its light profile hardly changes with TC in the focal plane. We demonstrated that the Q-PV can be used for optical spanners that particles are trapped and rotated on the specific orbit. Non-coaxial and coaxial Q-PV arrays were further generated for OAM communication applications. We believe that the proposed Q-PVs has potential applications in optical manipulation and optical communication.

Additively Manufactured Multi-Material Ultrathin Metasurfaces for Broadband Circular Polarization Decoupled Beams and Orbital Angular Momentum Generation

Controlling the wavefront and manipulating the polarization of the electromagnetic wave using an ultrathin flat device are highly desirable in many emerging fields. To shape the wavefront between two decoupled orthogonal circular polarization states, that is, the right-hand circular polarization (RCP) and the left-hand circular polarization (LCP), most state-of-the-art metasurfaces (MSs) combine the propagation phase and Pancharatnam-Berry phase into meta-atoms. This article proposes a different strategy to fully decouple the LCP and RCP and control their wavefronts independently. By taking advantage of the conductive and dielectric multi-material-integrated additive manufacturing technique, the proposed transmissive MS has an ultrathin thickness (0.11 free-space wavelength) and controls the LCP and RCP wavefronts independently under linearly polarized incidence illumination. The proposed meta-atom consists of a receiving antenna on the top, a transmitting antenna at the bottom with a strip-line connecting them. The strip-line introduces the same phase shifts for both RCP and LCP waves, while the transmitting antenna with in-plane rotation leads to the opposite phase shifts for RCP and LCP waves. Therefore, the phase delays from the strip-line and the angular rotation of the transmitting antenna provide two degrees of freedom, enabling independent beam shaping of LCP and RCP waves. Two MSs with different functionalities are printed for proof-of-concept, and the performances are experimentally verified.

A High Aspect Ratio Inverse-Designed Holey Metalens

Free-standing nanofins or pillar meta-atoms are the most common constituent building blocks in metalenses and metasurfaces in general. Here, we present an alternative metasurface geometry based on high aspect ratio via-holes. We design and characterize metalenses comprising ultradeep via-holes in 5 μm thick free-standing silicon membranes with hole aspect ratios approaching 30:1. These metalenses focus incident infrared light into a diffraction-limited spot. Instead of shaping the metasurface optical phase profile alone, we engineer both transmitted phase and amplitude profiles simultaneously by inverse-designing the lens effective index profile. This approach improves the impedance match between the incident and transmitted waves, thereby increasing the focusing efficiency. The holey platform increases the accessible aspect ratio of optical nanostructures without sacrificing mechanical robustness. The high nanostructure aspect ratio also increases the chromatic group delay range attainable, paving the way for a generation of high aspect ratio ruggedized flat optics, including large-area broadband achromatic metalenses.

Multidimensional phase singularities in nanophotonics

[Figure: see text].

Modern Types of Axicons: New Functions and Applications

Axicon is a versatile optical element for forming a zero-order Bessel beam, including high-power laser radiation schemes. Nevertheless, it has drawbacks such as the produced beam's parameters being dependent on a particular element, the output beam's intensity distribution being dependent on the quality of element manufacturing, and uneven axial intensity distribution. To address these issues, extensive research has been undertaken to develop nondiffracting beams using a variety of advanced techniques. We looked at four different and special approaches for creating nondiffracting beams in this article. Diffractive axicons, meta-axicons-flat optics, spatial light modulators, and photonic integrated circuit-based axicons are among these approaches. Lately, there has been noteworthy curiosity in reducing the thickness and weight of axicons by exploiting diffraction. Meta-axicons, which are ultrathin flat optical elements made up of metasurfaces built up of arrays of subwavelength optical antennas, are one way to address such needs. In addition, when compared to their traditional refractive and diffractive equivalents, meta-axicons have a number of distinguishing advantages, including aberration correction, active tunability, and semi-transparency. This paper is not intended to be a critique of any method. We have outlined the most recent advancements in this field and let readers determine which approach best meets their needs based on the ease of fabrication and utilization. Moreover, one section is devoted to applications of axicons utilized as sensors of optical properties of devices and elements as well as singular beams states and wavefront features.

Multiplexing meta-hologram with separate control of amplitude and phase

Metasurfaces have shown their unique capabilities to manipulate the phase and/or amplitude properties of incident light at the subwavelength scale, which provides an effective approach for constructing amplitude-only, phase-only or even complexed amplitude meta-devices with high resolution. Most of meta-devices control the amplitude and/or phase of the incident light with the same polarization state; however, separately controlling of amplitude and phase of the incident light with different polarization states can provide a new degree of freedom for improving the information capacity of metasurfaces and designing multifunctional meta-devices. Herein, we combine the amplitude manipulation and geometric phase manipulation by only reconfiguring the orientation angle of the nanostructure and present a single-sized design strategy for a multiplexing meta-hologram which plays the dual roles: a continuous amplitude-only meta-device and a two-step phase-only meta-device. Two different modulation types can be readily switched merely by polarization controls. Our approach opens up the possibilities for separately and independently controlling of amplitude and phase of light to construct a multiplexing meta-hologram with a single-sized metasurface, which can contribute to the advanced research and applications in multi-folded optical anti-counterfeiting, optical information hiding and optical information encoding.

Dual-color meta-image display with a silver nanopolarizer based metasurface

Plasmonic metallic nanostructures with anisotropic design have unusual polarization-selective characteristic which can be utilized to build nanopolarizers at the nanoscale. Herein, we propose a dual-color image display platform by reconfiguring two types of silver nanoblocks in a single-celled metasurface. Governed by Malus's law, the two types of silver nanoblocks both acting as nanopolarizers with different orientations can continuously modulate the intensity of incident linearly polarized red and green light pixel-by-pixel, respectively. As a result, an ultra-compact, high-resolution, and continuous-greyscale dual-color image can be recorded right at the surface of the meta-device. We demonstrate the dual-color Malus metasurface by successfully encoding and decoding a red-green continuously-grayscale image into a metasurface sample. The experimentally captured meta-image with high-fidelity and resolution as high as 63500 dots per inch (dpi) has verified our proposal. With the advantages such as continuous grayscale modulation, ultrathin, high stability and high density, the proposed dual-color encoded metasurfaces can be readily used in ultra-compact image displays, high-end anti-counterfeiting, high-density optical information storage and information encryption, etc.

Recent Advances in Generation and Detection of Orbital Angular Momentum Optical Beams-A Review

Herein, we have discussed three major methods which have been generally employed for the generation of optical beams with orbital angular momentum (OAM). These methods include the practice of diffractive optics elements (DOEs), metasurfaces (MSs), and photonic integrated circuits (PICs) for the production of in-plane and out-of-plane OAM. This topic has been significantly evolved as a result; these three methods have been further implemented efficiently by different novel approaches which are discussed as well. Furthermore, development in the OAM detection techniques has also been presented. We have tried our best to bring novel and up-to-date information to the readers on this interesting and widely investigated topic.

Fundamentals and applications of spin-decoupled Pancharatnam-Berry metasurfaces

Manipulating circularly polarized (CP) electromagnetic (EM) waves at will is significantly important for a wide range of applications ranging from chiral-molecule manipulations to optical communication. However, conventional EM devices based on natural materials suffer from limited functionalities, bulky configurations, and low efficiencies. Recently, Pancharatnam-Berry (PB) phase metasurfaces have shown excellent capabilities in controlling CP waves in different frequency domains, thereby allowing for multi-functional PB meta-devices that integrate distinct functionalities into single and flat devices. Nevertheless, the PB phase has intrinsically opposite signs for two spins, resulting in locked and mirrored functionalities for right CP and left CP beams. Here we review the fundamentals and applications of spin-decoupled metasurfaces that release the spin-locked limitation of PB metasurfaces by combining the orientation-dependent PB phase and the dimension-dependent propagation phase. This provides a general and practical guideline toward realizing spin-decoupled functionalities with a single metasurface for orthogonal circular polarizations. Finally, we conclude this review with a short conclusion and personal outlook on the future directions of this rapidly growing research area, hoping to stimulate new research outputs that can be useful in future applications.

Transmissive mid-infrared achromatic bifocal metalens with polarization sensitivity

Metasurfaces have shown great potential in versatile areas such as vortex-beam generators, metalenses, holograms and so on. However, chromatic error hinders metasurfaces, especially metalenses, from wider applications. In this paper, we demonstrate a novel design for a transmissive mid-infrared achromatic bifocal metalens with polarization sensitivity. The compensation phase is used to eliminate the chromatic aberration. Simulation results show that, over a continuous waveband from 3.9 to 4.6µm, the focal length only changes by 2.26% with an average focusing efficiency of about 18%. This work can push the practical application of mid-infrared metasurfaces.

Quad-channel independent wavefront encoding with dual-band multitasking metasurface

Achieving multiple electromagnetic (EM) functionalities on a shared aperture in dual frequency bands is crucial for many applications; however, existing dual-band metasurfaces are affected by limited channels or narrow bandwidths. Herein, we propose a reflective coding metasurface that empowers four independent EM functionalities in quad-polarization channels in two wide frequency bands. By integrating quasi-I-shaped and cross-shaped metastructures, the meta-atom can feature independent phase modulation for two orthogonally linear and two decoupled circular polarizations at low and high frequencies, respectively. To validate the proposed metasurface, a multifunctional metadevice is designed that integrates beam deflection, diffuse scattering, and vortex beam generation. Both experimental and simulation results indicate distinct wavefront tailoring in each channel. The proposed multi-functional metasurface with low cross-talk and independent phase modulation depending on frequencies and polarizations may unlock the metasurfaces' potentials for complete wavefront control in EM function integration, multiple channel communication, polarization optics, etc.

Efficient All-Dielectric Diatomic Metasurface for Linear Polarization Generation and 1-Bit Phase Control

Optical metasurface has exhibited unprecedented capabilities in the regulation of light properties at a subwavelength scale. In particular, a multifunctional polarization metasurface making use of light polarization to integrate distinct functionalities on a single platform can be greatly helpful in the miniaturization of photonic systems and has become a hot research topic in recent years. Here, we propose and demonstrate an efficient all-dielectric diatomic metasurface, the unit cell of which is composed of a pair of a-Si:H-based nanodisks and nanopillars that play the roles as polarization-maintaining and polarization-converting meta-atoms, respectively. Through rigorous theoretical analyses and numerical simulations, we show that a properly designed diatomic metasurface can work as a nanoscale linear polarizer for generating linearly polarized light with a controllable polarization angle and superior performances including a maximum transmission efficiency of 96.2% and an extinction ratio of 32.8 dB at an operation wavelength of 690 nm. Three metasurface samples are fabricated and experimentally characterized to verify our claims and their potential applications. Furthermore, unlike previously reported dielectric diatomic metasurfaces which merely manipulate the polarization state, the proposed diatomic metasurface can be easily modified to empower 1-bit phase modulation without altering the polarization angle and sacrificing the transmission efficiency. This salient feature further leads to the demonstration of a metasurface beam splitter that can be equivalently seen as the integration of a nonpolarizing beam splitter and a linear polarizer, which has never been reported before. We envision that various metadevices equipping with distinct wavefront shaping functionalities can be realized by further optimizing the diatomic metasurface to achieve an entire 2π phase control.

Fine manipulation of terahertz waves via all-silicon metasurfaces with an independent amplitude and phase

Integrating independent wavefront controls into one device can meet the increasing demand for high-capacity flat electromagnetic devices. Simultaneously and independently controlling the amplitude and phase is pivotal for completely manipulating the propagation of electromagnetic waves. Here, we propose several all-silicon metasurfaces to achieve multifunctional designs and simultaneous modulation of amplitude and phase profiles in the terahertz (THz) band. These metasurfaces integrate two degrees of freedom of the propagation phase and Pancharatnam-Berry (PB) phase. To illustrate the feasibility of this design, three schematic functions are shown below: a three-channel vortex beam generator, a controllable intensity ratio of co- and cross-polarizations corresponding to the incident circular polarization (CP), and a bifocal metasurface that is capable of generating two off-axis vortices with controllable power allocation. A sample is fabricated to specifically verify the amplitude and phase modulation of this design. The experimental results agree well with the simulations and validate the good performances of our proposals. This approach for directly generating an editable amplitude and phase may provide a new choice to design ultra-thin photonic devices.

Large second-harmonic vortex beam generation with quasi-nonlinear spin-orbit interaction

A harmonic vortex beam is a typical vector beam with a helical wavefront at harmonic frequencies (e.g., second and third harmonics). It provides an additional degree of freedom beyond spin- and orbital-angular momentum, which may greatly increase the capacity for communicating and encoding information. However, conventional harmonic vortex beam generators suffer from complex designs and a low nonlinear conversion efficiency. Here, we propose and experimentally demonstrate the generation of a large second-harmonic (SH) vortex beam with quasi-nonlinear spin-orbit interaction (SOI). High-quality SH vortex beams with large topological charges up to 28 are realized experimentally. This indicated that the quasi-angular-momentum of a plasmonic spiral phase plate at the excitation wavelength (topological charge, q) could be imprinted on the harmonic signals from the attached WS₂ monolayer. The generated harmonic vortex beam has a topological charge of l_n=2nq (n is the harmonic order). The results may open new avenues for generating harmonic optical vortices for optical communications and enables novel multi-functional hybrid metasurface devices to manipulate harmonic beams.

Optical spin-symmetry breaking for high-efficiency directional helicity-multiplexed metaholograms

Helicity-multiplexed metasurfaces based on symmetric spin-orbit interactions (SOIs) have practical limits because they cannot provide central-symmetric holographic imaging. Asymmetric SOIs can effectively address such limitations, with several exciting applications in various fields ranging from asymmetric data inscription in communications to dual side displays in smart mobile devices. Low-loss dielectric materials provide an excellent platform for realizing such exotic phenomena efficiently. In this paper, we demonstrate an asymmetric SOI-dependent transmission-type metasurface in the visible domain using hydrogenated amorphous silicon (a-Si:H) nanoresonators. The proposed design approach is equipped with an additional degree of freedom in designing bi-directional helicity-multiplexed metasurfaces by breaking the conventional limit imposed by the symmetric SOI in half employment of metasurfaces for one circular handedness. Two on-axis, distinct wavefronts are produced with high transmission efficiencies, demonstrating the concept of asymmetric wavefront generation in two antiparallel directions. Additionally, the CMOS compatibility of a-Si:H makes it a cost-effective alternative to gallium nitride (GaN) and titanium dioxide (TiO2) for visible light. The cost-effective fabrication and simplicity of the proposed design technique provide an excellent candidate for high-efficiency, multifunctional, and chip-integrated demonstration of various phenomena.

Metasurfaces with Planar Chiral Meta-Atoms for Spin Light Manipulation

Spin light (i.e., circularly polarized light) manipulation based on metasurfaces with a controlled geometric phase (i.e., Pancharatnam-Berry (PB) phase) has achieved great successes according to its convenient design and robust performances, by which the phase control is mainly determined by the rotation angle of each meta-atom. This PB phase can be regarded as a global effect for spin light; here, we propose a local phase manipulation for metasurfaces with planar chiral meta-atoms. Planar chiral meta-atoms break fundamental symmetry restrictions and do not need a rotation for these kinds of meta-atoms to manipulate the spin light, which significantly expands the functionality of metasurface as it is incorporated with other modulations (e.g., PB phase, propagation phase). As an example, spin-decoupled holographic imaging is demonstrated with robust and broadband properties. Our work definitely enriches the design of metasurfaces and may trigger more exciting chiral-optics applications.

Building Multifunctional Metasystems via Algorithmic Construction

Flat optics foresees a promising route to ultracompact optical devices, where metasurfaces serve as the foundation. Conventional designs of metasurfaces start with a certain structure as the prototype, followed by extensive parametric sweeps to accommodate the requirements of phase and amplitude of the emerging light. Regardless of how computation consuming the process is, a predefined structure can hardly realize the independent control over polarization, frequency, and spatial channels, which hinders the potential of metasurfaces to be multifunctional. Besides, achieving complicated and multiple functions calls for designing metasystems with multiple cascading layers of metasurfaces, which introduces exponential complexity. In this work, we present a hybrid deep learning framework for designing multilayer metasystems with multifunctional capabilities. We demonstrate examples of a polarization-multiplexed dual-functional beam generator, a second-order differentiator for all-optical computing, and a space-polarization-wavelength multiplexed hologram. These examples are barely achievable by single-layer metasurfaces and unattainable by traditional design processes.

Switchable Metasurface with VO2 Thin Film at Visible Light by Changing Temperature

We numerically demonstrated switchable metasurfaces using a phase change material, VO2 by temperature change. The Pancharatnam–Berry metasurface was realized by using an array of Au nanorods on top of a thin VO2 film above an Au film, where the optical property of the VO2 film is switched from the insulator phase at low temperature to the metal phase at high temperature. At the optimal structure, polarization conversion efficiency of the normal incident light is about 75% at low temperature while that is less than 0.5% at high temperature in the visible region (λ∼ 700 nm). Various functionalities of switchable metasurfaces were demonstrated such as polarization conversion, beam steering, Fourier hologram, and Fresnel hologram. The thin-VO2-film-based switchable metasurface can be a good candidate for various switchable metasurface devices, for example, temperature dependent optical sensors, beamforming antennas, and display.

Generation of vector beams of Bell-like states by manipulating vector vortex modes with plasmonic metasurfaces

Metasurfaces with orthogonal nano-slit pairs arranged on spirals are proposed to generate vector beams (VBs) of Bell-like states and slanted polarizations. The design of the metasurfaces is based on the theoretically derived parameter condition for manipulation of the two vector vortex modes, which is satisfied by matching the three parameters of rotation order m, the spiral order n, and incident polarization helicity σ. The linear polarization states of the VBs are controlled by the initial orientation angle φ₀ of slit pairs. VBs of satisfying quality are experimentally obtained, with the analytical and simulated results validated.

Polarization-controllable perfect vortex beam by a dielectric metasurface

A perfect vortex beam has been attracting tremendous attention due to the fact that its ring radius is independent of the topological charge. Taking advantage of the superposition principle of phase in Fourier space, we proposed to generate perfect vortex beam using propagation-phase-based dielectric metasurface, which exhibits production efficiency larger than 83.5%. Due to the sensitivity of propagation phase to the polarization of incident beam, two sets of phase profiles can be imposed on a single dielectric metasurface, enabling the simultaneous generation of dual perfect vortex beams. Based on this property, convenient control to the radius and/or topological charge of perfect vortex beam is achieved by switching the incident polarization between two orthogonal polarizations, without redesigning metasurface or changing optical path. What's more important, the crosstalk of these two channels is low, less than 4%. Thus, the propagation-phase method of producing perfect vortex beam will find significant applications in optical communication, particle trapping, particle manipulation and holographic display.

Single-layered meta-reflectarray for polarization retention and spin-encrypted phase-encoding

Broadband communication with high data rates is a dire need for state-of-the-art wireless technologies. For achieving efficient wireless communication (particularly in an indoor environment), the electromagnetic (EM) waves should maintain their state of polarization despite encountering multiple reflections. Metasurfaces provide a unique platform to design subwavelength-featured meta-reflectarrays which enable the desired retention of the polarization state of an EM wave upon reflection. We present a single-layered broadband meta-reflectarray, simultaneously breaking n-fold (n > 2) rotational and mirror symmetry, which exhibits an unprecedented control over the phase, amplitude, and polarization of a reflected EM wave. This unique control enables the retention of polarization state and recording of spin-encrypted information for the reflected EM waves. Such novel multifunctional meta-reflectarray can be crucial to building an indoor setup for high data rate wireless communications. Meanwhile, the meta-array's ability to encode phase information provides an extra degree of freedom to structure and control (via incident spin) the reflected EM beam in the desired way. For the proof of concept, we have experimentally demonstrated a spin-encrypted holographic display which reconstructs the recorded holographic image at an image plane for the left circularly polarized (LCP) illumination and exhibits circular dichroism for the right circularly polarized (RCP) incident waves. The proposed meta-array can find applications in 5G indoor wireless communication, chiral sensing, spin-selective imaging, holography, and encryption.

Full-space metasurface holograms in the visible range

Conventional metasurface holography is usually implemented in either transmission space or reflection space. Herein, we show a dielectric metasurface that can simultaneously project two independent holographic images in the transmission and reflection spaces, respectively, merely with a single-layer design approach. Specifically, two types of dielectric nanobricks in a nanostructured metasurface are employed to act as half-wave plates for geometric phase modulation. One type of nanobrick is designed to reflect most of incident circularly-polarized light into reflection space, enabled with magnetic resonance, while another type of nanobrick transmits it into transmission space, without resonance involved. By controlling the orientation angles and randomly interleaving the two types of nanobricks to form a metasurface, a full-space metasurface hologram can be established. We experimentally demonstrate this trans-reflective meta-holography by encoding the geometric phase information of two independent images into a single metasurface, and all observed holographic images agree well with our predictions. Our research expands the field-of-view of metasurface holography from half- to full-space, which can find its markets in optical sensing, image displays, optical storages and many other potential applications.

Generation of Concentric Space-Variant Linear Polarized Light by Dielectric Metalens

Miniaturized flat and ultrathin optical components with spatial and polarization degrees of freedom are important for optical communications. Here, we use nanostructures that act as tiny phase plates on a dielectric metalens to generate a concentric polarization beam with different orientations along the radial direction. The important discoveries are that (1) the circularly polarized light can be converted into linearly polarized states with a different orientation at near field and that (2) this orientation is strongly correlated to the rotation of the nanostructures on the metalens. Stokes parameters are utilized to investigate the comprehensive polarization states embedded in the optical intensity along the propagation direction. The variation of the spatial polarization states transformed by the dielectric metalens can be properly mapped onto the Poincaré sphere. We believe that the variety of spatial polarizations within a miniaturized configuration provides a new degree of freedom for diverse applications in the future.