A planar chiral meta-surface for optical vortex generation and focusing

High efficiency focusing vortex generation and multichannel multiplexing based on terahertz metasurfaces

Single-function metasurfaces can only perform one task and thus cannot satisfy the demands of many advanced applications. Employing multifunctional and multiplexing metasurfaces considerably increases the integration density of functional devices. Therefore, orbital angular momentum (OAM) multiplexing metasurfaces have been designed to increase transmission capacity. In this study, a bilayer Pancharatnam-Berry phase unit cell operating at two frequencies in the terahertz (THz) band is proposed. The unit cell has a sub-wavelength thickness that can be used as a functional element for constructing ultrathin and compact metasurfaces for wavefront manipulation. Cross-polarization conversion ratio of 86.3% and 88.5% are obtained at the two frequencies, respectively. We design three multifunction metasurfaces to verify the method. First, two vortex metalenses are designed to focus vortex beams in the sub-wavelength scale at various frequencies. Furthermore, multichannel OAMs with various modes and arbitrary shape of OAM beam arrays can be realized using segmented metasurfaces. Finally, multichannel OAM multiplexing with the frequency selection and polarization dependence were simultaneously realized on a metasurface. Such metasurfaces can be used to realize large-capacity and high-spectrum-efficiency OAM communication. This paper contributes to the disciplines of photonic integration, multiplexing, and multifunctional metasurfaces.

Uniform dipole resonance and suppressed quadrupole resonance for constant transmittivity full phase control plasmonic metasurfaces

Transmission-type plasmonic phase metasurfaces utilizing the Pancharatnam-Berry (PB) phase require constant transmittivity with complete phase variation from 0 to 2π. Usually, this is achieved by rotating metallic nanoparticles in an otherwise uniform lattice arrangement. However, this rotation and the chosen lattice structure cause a significant change in the transmittivity, resulting in a lower intensity of light with certain phases and a higher intensity for other phases. Even though they are called full phase metasurfaces, their intensities can be near maximum or near minimum depending on the rotation and the lattice structure. We show that it is possible to achieve full phase constant transmittivity metasurfaces using the PB phase and the most elementary metallic anisotropic nanoparticles (elliptical) by inserting a thin metal sheet between the nanoparticles and the substrate. Without this metal sheet, while full phase control could be achieved by merely rotating the particles, the transmittivity varies by about 50% depending on the nanoparticles' orientation. With the metal sheet, full phase control from 0-2π with a transmittivity variation of only 13%, even in a square lattice, is demonstrated with simulations and experiments. We show that this is due to the annihilation of quadrupole resonances along with broader uniform dipole resonance in the case of the nanoparticles with the metal sheet below. We also show that precise phase control is possible by generating varieties of orbital angular momentum beams and complex beams in the visible spectrum using nanofabricated metasurfaces.

High-efficiency and broadband asymmetric spin-orbit interaction based on high-order composite phase modulation

Asymmetric spin-orbit interaction (ASOI) breaks the limitations in conjugate symmetry of traditional geometric phase metasurfaces, bringing new opportunities for various applications such as spin-decoupled holography, imaging, and complex light field manipulation. Since anisotropy is a requirement for spin-orbit interactions, existing ASOI mainly relies on meta-atom with C1 and C2 symmetries, which usually suffer from an efficiency decrease caused by the propagation phase control through the structural size. Here, we demonstrate for the first time that ASOI can be realized in meta-atoms with rotational symmetry ≥3 by combining the generalized geometric phase with the propagation phase. Utilizing an all-metallic configuration, the average diffraction efficiency of the spin-decoupled beam deflector based on C3 meta-atoms reaches ∼84 % in the wavelength range of 9.3-10.6 μm, which is much higher than that of the commonly used C2 meta-atoms with the same period and height. This is because the anisotropy of the C3 metasurface originates from the lattice coupling effect, which is relatively insensitive to the propagation phase control through the meta-atom size. A spin-decoupled beam deflector and hologram meta-device were experimentally demonstrated and performed well over a broadband wavelength range. This work opens a new route for ASOI, which is significant for realizing high-efficiency and broadband spin-decoupled meta-devices.

Giant chiral amplification of chiral 2D perovskites via dynamic crystal reconstruction

Chiral hybrid perovskites show promise for advanced spin-resolved optoelectronics due to their excellent polarization-sensitive properties. However, chiral perovskites developed to date rely solely on the interaction between chiral organic ligand cations exhibiting point chirality and an inorganic framework, leading to a poorly ordered short-range chiral system. Here, we report a powerful method to overcome this limitation using dynamic long-range organization of chiral perovskites guided by the incorporation of chiral dopants, which induces strong interactions between chiral dopants and chiral cations. The additional interplay of chiral cations with chiral dopants reorganizes the morphological and crystallographic properties of chiral perovskites, notably enhancing the asymmetric behavior of chiral 2D perovskites by more than 10-fold, along with the highest dissymmetry factor of photocurrent (gPh) of ~1.16 reported to date. Our findings present a pioneering approach to efficiently amplify the chiroptical response in chiral perovskites, opening avenues for exploring their potential in cutting-edge optoelectronic applications.

Generating first-order optical vortex beams by photonic crystal slabs

Optical vortices, which are beams with helical wavefronts and spiral phase mismatches, have garnered considerable attention in various fields. In this study, we theoretically proposed and experimentally implemented a simple method for generating first-order optical vortices. To generate first-order vortex beams using the polarization field in the momentum space of photonic crystal slabs, topological half charges are required. We propose a method to divide the polarization vortex in the momentum space by breaking symmetry, which results in Dirac points or circularly polarized points. This approach enables the transformation of topological integer charges into topological half-integer charges, thereby facilitating the generation of first-order vortex beams. This approach extends the application of bound states in continuum and topological photonics.

Metasurface-based optical system for miniaturization of atomic magnetometers

Recent research has focused on miniaturizing atomic devices like magnetometers and gyroscopes for quantum precision measurements, leading to energy savings and broader application. This paper presents the design and validation of metasurface-based optical elements for atomic magnetometers' optical paths. These include highly efficient half-wave plates, polarizers, circular polarization generators, polarization-preserving reflectors, and polarizing beam splitters. These components, compatible with semiconductor manufacturing, offer a promising solution for creating ultra-thin, compact atomic devices.

Perovskite-Supramolecular Co-Assembly for Chiral Optoelectronics

Hybrid inorganic-organic perovskites with chiral response and outstanding optoelectronic characteristics are promising materials for next-generation spin-optoelectronics. In particular, two-dimensional (2D) perovskites are promising chiroptical candidates due to their unique ability to incorporate chiral organic cations into their crystal structure, which imparts chirality. To enable their practical applications in chiral optoelectronic devices, it is essential to achieve an anisotropy factor (gCD ∼ 2) in chiral 2D perovskites. Currently, chiral 2D perovskites exhibit a relatively low gCD of 3.1 × 10-3. Several approaches have been explored to improve the chiral response of chiral 2D perovskites, including tailoring the molecular structure of chiral cations and increasing the degree of octahedral tilting in the perovskite lattice. However, current methods for chiral amplification have only achieved a moderate enhancement of gCD by 2-fold and are often accompanied by undesirable shifts or inversion in the circular dichroism spectra. There is a need for a more efficient approach to enhancing the chirality in 2D perovskites. Here, we report an innovative coassembly process that allows us to seamlessly grow chiral 2D perovskites on supramolecular helical structures. We discover that the interactions between perovskites and chiral supramolecular structures promote crystal lattice distortion in perovskites, which improves the chirality of 2D perovskites. Additionally, the obtained hierarchical coassembly can effectively harness the structural chirality of the supramolecular helices. The multilevel chiral enhancement leads to an enhancement in gCD by 2.7-fold without compromising the circular dichroism spectra of 2D perovskites.

Full-Space Wavefront Shaping of Broadband Vortex Beam with Switchable Terahertz Metasurface Based on Vanadium Dioxide

Currently, vortex beams are extensively utilized in the information transmission and storage of communication systems due to their additional degree of freedom. However, traditional terahertz metasurfaces only focus on the generation of narrowband vortex beams in reflection or transmission mode, which is unbeneficial for practical applications. Here, we propose and design terahertz metasurface unit cells composed of anisotropic Z-shaped metal structures, two dielectric layers, and a VO2 film layer. By utilizing the Pancharatnam-Berry phase theory, independent control of a full 2π phase over a wide frequency range can be achieved by rotating the unit cell. Moreover, the full-space mode (transmission and reflection) can also be implemented by utilizing the phase transition of VO2 film. Based on the convolution operation, three different terahertz metasurfaces are created to generate vortex beams with different wavefronts in full-space, such as deflected vortex beams, focused vortex beams, and non-diffraction vortex beams. Additionally, the divergences of these vortex beams are also analyzed. Therefore, our designed metasurfaces are capable of efficiently shaping the wavefronts of broadband vortex beams in full-space, making them promising applications for long-distance transmission, high integration, and large capacity in 6G terahertz communications.

Versatile generation and manipulation of phase-structured light beams using on-chip subwavelength holographic surface gratings

Phase-structured light beams carrying orbital angular momentum (OAM) have a wide range of applications ranging from particle trapping to optical communication. Many techniques exist to generate and manipulate such beams but most suffer from bulky configurations. In contrast, silicon photonics enables the integration of various functional components on a monolithic platform, providing a way to miniaturize optical systems to chip level. Here, we propose a series of on-chip subwavelength holographic waveguide structures that can convert the in-plane guided modes into desired wavefronts and realize complex free-space functions, including the generation of complex phase-structured light beams, arbitrarily directed vortex beam emission and vortex beam focusing. We use a holographic approach to design subwavelength holographic surface gratings, and demonstrate broadband generation of Laguerre-Gaussian (LG) and linearly polarized (LP) modes. Moreover, by assigning appropriate geometric phase profiles to the spiral phase distribution, the off-chip vortex beam manipulation including arbitrarily directed emission and beam focusing scenarios can be realized. In the experiment, directed vortex beam emission is realized by using a fabricated tilt subwavelength holographic fork grating. The proposed waveguide structures enrich the functionalities of dielectric meta-waveguide structures, which can find potential applications in optical communication, optical trapping, nonlinear interaction and imaging.

Millimeter-scale focal length tuning with MEMS-integrated meta-optics employing high-throughput fabrication

Miniature varifocal lenses are crucial for many applications requiring compact optical systems. Here, utilizing electro-mechanically actuated 0.5-mm aperture infrared Alvarez meta-optics, we demonstrate 3.1 mm (200 diopters) focal length tuning with an actuation voltage below 40 V. This constitutes the largest focal length tuning in any low-power electro-mechanically actuated meta-optic, enabled by the high energy density in comb-drive actuators producing large displacements at relatively low voltage. The demonstrated device is produced by a novel nanofabrication process that accommodates meta-optics with a larger aperture and has improved alignment between meta-optics via flip-chip bonding. The whole fabrication process is CMOS compatible and amenable to high-throughput manufacturing.

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.

Theoretical Study on Generation of Multidimensional Focused and Vector Vortex Beams via All-Dielectric Spin-Multiplexed Metasurface

The optical vortex (OV) beams characterized by orbital angular momentum (OAM) possess ubiquitous applications in optical communication and nanoparticle manipulation. Particularly, the vortex vector beams are important in classical physics and quantum sciences. Here, based on an all-dielectric transmission metasurface platform, we demonstrate a spin-multiplexed metadevice combining propagation phase and Pancharatnam–Berry (PB) phase. By utilizing a phase-only modulation method, the metadevice can generate spin-dependent and multidimensional focused optical vortex (FOV) under the orthogonally circularly polarized incident light, and it can successfully realize the multiplexed of the above-mentioned FOVs for linearly polarized light. Meanwhile, the superposition of multiple OAM states can also produce vector vortex beams with different modes. Additionally, the evolution process of the electric field intensity profile is presented after the resultant vector vortex beams through a horizontal linear polarization. This work paves an innovative way for generating structured beams, and it provides promising opportunities for advanced applications in optical data storage, optical micromanipulation, and data communication.

Multi-beam and multi-mode orbital angular momentum by utilizing a single metasurface

This paper proposes a novel metasurface that can simultaneously generate orbital angular momentum (OAM) beams with pre-designed different reflection directions, multi-beam and multi-mode under x-(y-) polarized terahertz wave incidence. The configuration of unit cell is made up of a hollow cross of Jesus structure as top layer, a PTFE substrate layer and a gold metal bottom plate. Theory of phase gradient distribution is derived and used to design multifunctional OAM metasurface. The proposed metasurface generates two OAM beams with OAM mode l = 1 and four OAM beams with l = -1 at frequency of 1 THz, respectively. Similarly, at frequency of 1.3 THz, the designed metasurface produces two OAM beams with l = -2 and an OAM beam with l = 2 for x-(y-) polarized wave incidence, respectively. Since each OAM mode can be used as an independent digital information coding channel, the designed multifunctional OAM metasurface has a wide application prospect in future terahertz communication.

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.

Nonlinear Bicolor Holography Using Plasmonic Metasurfaces

Nonlinear metasurface holography shows the great potential of metasurfaces to control the phase, amplitude, and polarization of light while simultaneously converting the frequency of the light. The possibility of tailoring the scattering properties of a coherent beam, as well as the scattering properties of nonlinear signals originating from the meta-atoms, facilitates a huge degree of freedom in beam shaping application. Recently, several approaches showed that virtual objects or any kind of optical information can be generated at a wavelength different from the laser input beam. Here, we demonstrate a single-layer nonlinear geometric-phase metasurface made of plasmonic nanostructures for a simultaneous second- and third-harmonic generation. Different from previous works, we demonstrate a two-color hologram with dissimilar types of nanostructures that generate the color information by different nonlinear optical processes. The amplitude ratio of both harmonic signals can be adapted depending on the nanostructures' resonance as well as the power and the wavelength of the incident laser beam. The two-color holographic image is reconstructed in the Fourier space at visible wavelengths with equal amplitudes using a single near-infrared wavelength. Nonlinear holography using multiple nonlinear processes simultaneously provides an alternative path to holographic color display applications, enhanced optical encryption schemes, and multiplexed optical data storage.

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.

Overcome chromatism of metasurface via Greedy Algorithm empowered by self-organizing map neural network

Chromatism generally exists in most metasurfaces. Because of this, the deflected angle of metasurface reflectors usually varies with frequency. This inevitably hinders wide applications of metasurfaces to broadband signal scenarios. Therefore, it is of great significance to overcome chromatism of metasurfaces. With this aim, we firstly analyze necessary conditions for achromatic metasurface deflectors (AMD) and deduce the ideal dispersions of meta-atoms. Then, we establish a Self-Organizing Map (SOM) Neural Network as a prepositive model to obtain a diversified searching map, which is then applied to Greedy Algorithm to search meta-atoms with the required dispersions. Using these meta-atoms, an AMD was designed and simulated, with a thickness about 1/15 the central wavelength. A prototype was fabricated and measured. Both the simulation and measurement show that the proposed AMD can achieve an almost constant deflected angle of 22° under normal incidence within 9.5-10.5GHz. This method may find wide applications in designing functional metasurfaces for satellite communications, mobile wireless communications and others.

Compact folded dipole metasurface for high anomalous reflection angles with low harmonic levels

A dense unit cell array used in a metasurface for a high reflection angle (θ_r > 50°) leads to high coupling among the unit cells; thus, parasitic reflections are unavoidable. The up-do-date patch-based metasurfaces for high reflection angles were electrically large (> 80 λ²), but for a practical point of view, a more compact metasurface design is needed. As a solution for these issues, we use the folded dipole-based unit cells with closed-loop currents for low near-field coupling and design compact metasurfaces (~ 40 λ²) for high reflection angles (θ_r = 56° and 70°) at 10 GHz. The folded dipole unit cells are arranged according to the recently developed non-linear phase boundary condition for low harmonic reflections. As a counterpart, we also designed a metasurface using conventional patch-shaped unit cells with the same reflection phases (θ_r = 70°). In experiments, the folded dipole metasurface shows lower harmonic levels (θ_r = 0° and - 70°) and a comparable anomalous reflection (θ_r = 70°) versus the patch-shaped metasurface. The time-domain analysis demonstrates that the low harmonic levels from the folded dipole metasurface are due to low scattering from the guided waves and the edge scattering. The proposed compact folded dipole-based metasurface with low undesired harmonics can be used as a practical reflect-array for millimeter-wave communication links.

Metasurface Spiral Focusing Generators with Tunable Orbital Angular Momentum Based on Slab Silicon Nitride Waveguide and Vanadium Dioxide (VO2)

The metasurface spiral focusing (MSF) generator has gained attention in high-speed optical communications due to its spatial orthogonality. However, previous MSF generators only can generate a single orbital angular momentum (OAM) mode for one polarized light. Here, a MSF generator with tunable OAM is proposed and it has the ability to transform linearly polarized light (LPL), circularly polarized light or Gaussian beams into vortex beams which can carry tunable OAM at near-infrared wavelength by controlling the phase transition of vanadium dioxide (VO₂). Utilizing this MSF generator, the beams can be focused on several wavelength-sized rings with efficiency as high as 76%, 32% when VO₂ are in the insulating phase and in the metallic phase, respectively. Moreover, we reveal the relationship between the reflective focal length and transmissive focal length, and the latter is 2.3 times of the former. We further demonstrate the impact of Gaussian beams with different waist sizes on MSF generators: the increase in waist size produces the enhancement in spiral focusing efficiency and the decrease in size of focal ring. The MSF generator we proposed will be applicable to a variety of integrated compact optical systems, such as optical communication systems and optical trapping systems.

Ultrafast reprogrammable multifunctional vanadium-dioxide-assisted metasurface for dynamic THz wavefront engineering

In this paper, for the first time, a new generation of ultrafast reprogrammable multi-mission bias encoded metasurface is proposed for dynamic terahertz wavefront engineering by employing VO2 reversible and fast monoclinic to tetragonal phase transition. The multi-functionality of our designed VO2 based coding metasurface (VBCM) was guaranteed by elaborately designed meta-atom comprising three-patterned VO2 thin films whose operational statuses can be dynamically tuned among four states of "00"-"11" by merely changing the biasing voltage controlled by an external Field-programmable gate array platform. Capitalizing on such meta-atom design and by driving VBCM with different spiral-like and spiral-parabola-like coding sequences, single vortex beam and focused vortex beam with interchangeable orbital angular momentum modes were satisfactorily generated respectively. Additionally, by adopting superposition theorem and convolution operation, symmetric/asymmetric multiple beams and arbitrarily-oriented multiple vortex beams in pre-demined directions with different topological charges are realized. Several illustrative examples successfully have clarified that the proposed VBCM is a promising candidate for solving crucial terahertz challenges such as high data rate wireless communication where ultrafast switching between several missions is required.

Polarization and direction-controlled asymmetric multifunctional metadevice for focusing, vortex and Bessel beam generation

Integrating multiple independent functionalities into one single photonic device has been an important part in optoelectronic system. In this paper, we here propose a kind of asymmetric multifunctional metadevice operating at 1550 nm (in optical communication band), which can manipulate the light with four different functions, depending on the polarization and illumination direction of incident light. As a proof of our concept, we design this metadevice composed of the upper metasurface layer, middle grating layer and lower metasurface layer. For x-polarized incident light, the metadevice under forward illumination works as transmissive focusing lens and vortex beam generator of y-polarized light, while under backward illumination it acts as a reflective vortex beam generator. In contrast, for y-polarized incident light, the metadevice under forward illumination behaves as a reflective Bessel beam generator, while a combination of transmissive vortex beam generator and focusing lens of x-polarized light under backward illumination. Our findings may motivate the realization of high-performance multifunctional metadevices and extend the application in complex integrated optical system.

Plasmonic lithography for the fabrication of surface nanostructures with a feature size down to 9 nm

Aiming to further improve the resolution and quality of plasmonic lithography, a self-aligned patterning technique is introduced to it to obtain ultrafine nanopatterns with high contrast and low line edge error (LER). By improving the line edge roughness of the initial plasmonic lithography patterns, 16 nm half-pitch surface nanostructures with a height of 70 nm can be obtained. Moreover, with the help of plasma etching and atomic layer deposition in this process flow, the LER of the 16 nm half-pitch surface nanostructure can achieve 1.3 nm. Further application indicates that this process can also be used to fabricate nanoholes with the feature size as small as 9 nm. This approach provides a new perspective on the manufacture of surface nanostructures with ultrahigh resolution and high aspect ratios, which would find potentially promising applications in metal-air transistors, biosensors, DNA sequencing, etc.

Efficient point-by-point manipulated visible meta-vortex-lenses with arbitrary orbital angular momentum

The ability of light to carry and deliver orbital angular momentum (OAM) in the form of optical vortices has attracted much interest. Conventional optical vortices are usually generated by bulky or expensive devices, which would sharply decrease the integration of optical communication systems. Here we demonstrate efficient large-area wavelength-thick metasurfaces that have the ability to produce high-quality optical vortexes with arbitrary OAM and to focus the beams into wavelength-scale rings with efficiency as high as 80%. Moreover, we reveal the relationship between size and energy distribution of focal rings (FR) with different OAMs: as the number of OAM increases, the size of the FR is linearly increasing, the peak focusing intensity (FFI) is decreasing in inverse proportional type, while the total energy on the FR remain almost unchanged. Rigorous quantitative analysis about the coupling effect among nanoantennas and the chromatic aberrations of the proposed metasurfaces are further discussed. We envision such highly efficient metasurfaces for spiral focusing will have potential applications in optical tweezers and communications.

High-Efficiency and Broadband Near-Infrared Bi-Functional Metasurface Based on Rotary Different-Size Silicon Nanobricks

Several novel spin-dependent bi-functional metasurfaces consisting of different-sized rotary silicon nanobricks have been proposed and numerically investigated based on the Pancharatnam-Berry phase and structural phase simultaneously. Here, a transmission mechanism is strictly deduced, which can avoid crosstalk from the multiplexed bi-functional metasurface. Four kinds of high-efficiency bi-functional devices have been designed successfully at infrared wavelengths, including a spin-dependent bi-functional beam deflector, a spin-dependent bi-functional metalens, a bi-functional metasurface with spin-dependent focusing and deflection function, and a spin-dependent bi-functional vortex phase plate. All of the results demonstrate the superior performances of our designed devices. Our work opens up new doors toward building novel spin-dependent bi-functional metasurfaces, and promotes the development of bi-functional devices and spin-controlled photonics.

Generation of wideband vortex beam with different OAM modes using third-order meta-frequency selective surface

Orbital angular momentum (OAM) beam generators have attracted tremendous interests recently due to their excellent performance and potential applications in wireless communication. However, the existing transmissive OAM generators suffer from several limitations, such as narrow bandwidth, high profile and low efficiency. In this study, a new wideband third-order meta-frequency selective surface (meta-FSS) for generating focusing vortex beam is developed. The proposed meta-FSS element is designed at X- band with a third-order band-pass filter property, which exhibits the merits of low profile, high transmissivity, and large angular stability. By employing the proposed meta-FSS element, prototypes of OAM generators for + 1 and -2 modes are designed, fabricated, and measured. Experimental results verify the ability of the proposed design to convert an incident left/right-handed circularly polarized (L/RHCP) spherical wave into a transmitted R/LHCP vortex carrying OAM wave from 9.0 GHz to 11.0 GHz with high mode purity. A good agreement is achieved between the experimental and numerical results, which demonstrates that the proposed structure paves the wave for generating desired OAM modes, and provides new possibility for designing novel low-profile wireless communication devices.

A Small-Divergence-Angle Orbital Angular Momentum Metasurface Antenna

Electromagnetic waves carrying an orbital angular momentum (OAM) are of great interest. However, most OAM antennas present disadvantages such as a complicated structure, low efficiency, and large divergence angle, which prevents their practical applications. So far, there are few papers and research focuses on the problem of the divergence angle. Herein, a metasurface antenna is proposed to obtain the OAM beams with a small divergence angle. The circular arrangement and phase gradient were used to simplify the structure of the metasurface and obtain the small divergence angle, respectively. The proposed metasurface antenna presents a high transmission coefficient and effectively decreases the divergence angle of the OAM beam. All the theoretical analyses and derivation calculations were validated by both simulations and experiments. This compact structure paves the way to generate OAM beams with a small divergence angle.

Mechanically tunable focusing metamirror in the visible

A compact, flat lens with dynamically tunable focal length will be an essential component in advanced reconfigurable optical systems. One approach to realize a flat tunable lens is by utilizing metasurfaces, which are two-dimensional nanostructures capable of tailoring the wavefront of incident light. When a metasurface with a hyperboloidal phase profile, i.e., a metalens, is fabricated on a substrate that can be actuated, its focal length can be adjusted dynamically. Here, we design and realize the first reflection type, tunable metalens (i.e., metamirror) operating in the visible regime (670 nm). It is shown that the focal length can be continuously adjusted by up to 45% with a 0% to 20% lateral stretching of the substrate, while maintaining diffraction-limited focusing and high focusing efficiency. Our design as a flat optics element has potential in widespread applications, such as wearable mixed reality systems, biomedical instruments and integrated optics devices.

Anomalous refraction and reflection characteristics of bend V-shaped antenna metasurfaces

Stabilization issue of anomalous refraction and reflection in V-shaped antenna metasurfaces are investigated. Specifically, when a V-shaped metasurface is artificially tilted, the induced refraction and reflection are theoretically analyzed. Detailed numerical and experimental study is then performed for the upward and downward bending metasurfaces. Our results show that although the anomalous reflection is sensitive to the deformation of metasurface geometry; the anomalous refraction is, surprisingly, barely affected by relatively small-angle tilting and able to support perfect beam orienting. Since in real-world applications, the optical objects are often affected by multiple uncertain factors, such as deformation, vibration, non-standard surface, non-perfect planar, etc., the stabilization of optical functionality has therefore been a long-standing design challenge for optical engineering. We believe our findings can shed new light on this stability issue.

Polarization-controlled unidirectional excitation of surface plasmon polaritons utilizing catenary apertures

Controlling the propagation direction of surface plasmon polaritons (SPPs) at will using planar structures has been investigated in recent years. However the realization of a high extinction ratio of a SPP directional launcher in a densely integrated and miniaturized way, especially at the wavelength scale, still remains a challenge. To the best of our knowledge, the maximum value of the extinction ratio of a unidirectional SPP launcher based on the planar metasurface in experiment is nearly 250, which relies on the combined effect of several gap-plasmon resonator blocks with a lateral dimension much larger than the incident wavelength. Here, we design and experimentally demonstrate a polarization-controlled unidirectional SPP launcher based on a single column catenary aperture array with a lateral dimension as small as 552 nm, which is even smaller than the working wavelength. Under the illumination of circularly polarized light, our designed SPP launcher exhibits a simulated extinction ratio reaching up to 495 at a wavelength of 618 nm and 283 in the experiment. The compact size and distinctive extinction ratio may pave a new way for the directional excitation of SPPs and can be useful in compact plasmonic circuits and other photonic integrated devices.

Dual-wavelength multifunctional metadevices based on modularization design by using indium-tin-oxide

Combining two or several functionalities into a single metadevice is of significant importance and attracts growing interest in recent years. We here introduce the concept of modularization design in dual-wavelength multifunctional metadevice, which is composed of a lower metasurface and an upper metasurface with an indium-tin-oxide (ITO) layer. Benefiting from the fact that ITO holds high infrared (IR) reflection while transparence at visible wavelengths, the metadevice can work in reflection and transmission modes at two very distinct wavelengths, one is 2365 nm in the IR band and the other 650 nm in the visible range. More interestingly and importantly, the two metasurface layers with different functionalities are easy to flexibly integrate into a series of dual-wavelength multifunctional metadevices, with negligible interaction between them and no need of re-designing or re-optimizing their structure parameters. Based on modularization design and functional integration, four kinds of dual-wavelength multifunctional metadevices are demonstrated, which can perform reflective deflection/focusing at 2365 nm and transmissive deflection/focusing at 650 nm. We believe our work may open a straight-forward and flexible way in designing multi-wavelength multifunctional metadevices and photonic integrated devices.

Dual-band vortex beam generation with different OAM modes using single-layer metasurface

Recently, considerable attention has been focused on orbital angular momentum (OAM) vortex wave, owing to its prospect of increasing communication capacity. Here, a single-layer metasurface is proposed to realize vortex beams with different OAM modes and polarizations carried at two distinctive bands. Both the resonant and geometric (Pancharatnam-Berry) phase cells are adopted to construct this metasurface for generating the desired phase profile, and each type of phase modulation cell can independently control the vortex beam at different frequencies. When a linearly-polarized wave is incident onto our metasurface, the resonant phase cells with spiral phase distribution can achieve OAM beam with topological charge of + 1 at 5.2 GHz. And under illumination of left-handed circular polarized (LHCP) wave, the rotated geometric phase cells assist the metasurface to generate the deflected OAM beam with topological charge of + 2 at 10.5~12 GHz. Both simulated and experimental results demonstrate good performance of the proposed single-layer metasurface at the above two frequency bands.

Subwavelength Artificial Structures: Opening a New Era for Engineering Optics

In the past centuries, the scale of engineering optics has evolved toward two opposite directions: one is represented by giant telescopes with apertures larger than tens of meters and the other is the rapidly developing micro/nano-optics and nanophotonics. At the nanoscale, subwavelength light-matter interaction is blended with classic and quantum effects in various functional materials such as noble metals, semiconductors, phase-change materials, and 2D materials, which provides unprecedented opportunities to upgrade the performance of classic optical devices and overcome the fundamental and engineering difficulties faced by traditional optical engineers. Here, the research motivations and recent advances in subwavelength artificial structures are summarized, with a particular emphasis on their practical applications in super-resolution and large-aperture imaging systems, as well as highly efficient and spectrally selective absorbers and emitters. The role of dispersion engineering and near-field coupling in the form of catenary optical fields is highlighted, which reveals a methodology to engineer the electromagnetic response of complex subwavelength structures. Challenges and tentative solutions are presented regarding multiscale design, optimization, fabrication, and system integration, with the hope of providing recipes to transform the theoretical and technological breakthroughs on subwavelength hierarchical structures to the next generation of engineering optics, namely Engineering Optics 2.0.

Electrically tunable multifunctional metasurface for integrating phase and amplitude modulation based on hyperbolic metamaterial substrate

Active metasurfaces, which are tunable and reconfigurable nanophotonic structures with active materials, have been in spotlight as a versatile platform to control the profiles of scattered light. These nanoscale structures show surpassing functionalities compared to the conventional metasurfaces. They also play an important role in a wide range of applications for imaging, sensing, and data storage. Hence, the expansion of functionalities has been highly desired, in order to overcome the limited space constraints and realize the integration of several optical devices on a single compact platform. In this context, an electrically tunable metasurface that enables respective modulation of the phase and amplitude of reflected light, depending on the angle of incidence at the targeted wavelength, is proposed. This resonance-based device with hyperbolic metamaterial substrate excites different kinds of highly confined modes, according to the incident angle. Indium tin oxide is employed to offer electrically tunable optical properties in the near-infrared regime. At the wavelength of 1450 nm, the proposed device modulates the phase of reflected light with ~207 degrees of modulation depth for normal incidence, whereas it shows ~86% of relative reflectance change for oblique incidence of 60 degrees. In principle, the proposed scheme might provide a path to applications for the next-generation ultracompact integrated systems.

Dynamic multilevel spiral phase plate generator

The design and characterisation of a reconfigurable multi-level spiral phase plate is shown. The device is based on a pie-shape liquid-crystal structure with 24 slices driven by custom electronics that allow independent excitation control of each electrode. The electrooptical cell was manufactured using maskless laser ablation lithography and has shown an unprecedented high fill factor. The topological charge can be dynamically changed between 1, 2, 3, 4, 6, 8 and 12. The device has been calibrated and characterised at 632.8 nm but can be employed at any wavelength in the visible and near infrared spectrum, just modifying the driving parameters of the electrodes. The experimental results have been compared to predictions derived from simulations. An excellent correspondence between theoretical and experimental result has been found in all cases.

Polarization-insensitive beam splitters using all-dielectric phase gradient metasurfaces at visible wavelengths

Beam splitters play important roles in several optical applications, such as interferometers, spectroscopy, and optical communications. In this study, we propose and numerically examine polarization-insensitive beam splitters utilizing two-step phase gradient all-dielectric metasurfaces in the visible spectrum. The metasurface is made of periodically arranged binary unit cells, and phase difference between neighboring unit cells on the surface is 180 deg. The metasurface is shown to have a special phase gradient whose sign changes periodically. The angle of the split beams on both sides and the corresponding total transmission value at 532 nm wavelength are found to be ±46.8° and 0.90, respectively.

Metalenses Based on Symmetric Slab Waveguide and c-TiO₂: Efficient Polarization-Insensitive Focusing at Visible Wavelengths

A key goal of metalens research is to achieve wavefront shaping of light using optical elements with thicknesses on the order of the wavelength. Here we demonstrate ultrathin highly efficient crystalline titanium dioxide metalenses at blue, green, and red wavelengths (λ₀ = 453 nm, 532 nm, and 633 nm, respectively) based on symmetric slab waveguide theory. These metalenses are less than 488 nm-thick and capable of focusing incident light into very symmetric diffraction-limited spots with strehl ratio and efficiency as high as 0.96 and 83%, respectively. Further quantitative characterizations about metalenses' peak focusing intensities and focal spot sizes show good agreement with theoretical calculation. Besides, the metalenses suffer only about 10% chromatic deviation from the ideal spots in visible spectrum. In contrast with Pancharatnam⁻Berry phase mechanism, which limit their incident light at circular polarization, the proposed method enables metalenses polarization-insensitive to incident light.

Reconstructing a plasmonic metasurface for a broadband high-efficiency optical vortex in the visible frequency

Metasurfaces consisting of a two-dimensional metallic nano-antenna array are capable of transferring a Gaussian beam into an optical vortex with a helical phase front and a phase singularity by manipulating the polarization/phase status of light. This miniaturizes a laboratory scaled optical system into a wafer scale component, opening up a new area for broad applications in optics. However, the low conversion efficiency to generate a vortex beam from circularly polarized light hinders further development. This paper reports our recent success in improving the efficiency over a broad waveband at the visible frequency compared with the existing work. The choice of material, the geometry and the spatial organization of meta-atoms, and the fabrication fidelity are theoretically investigated by the Jones matrix method. The theoretical conversion efficiency over 40% in the visible wavelength range is worked out by systematic calculation using the finite difference time domain (FDTD) method. The fabricated metasurface based on the parameters by theoretical optimization demonstrates a high quality vortex in optical frequencies with a significantly enhanced efficiency of over 20% in a broad waveband.

Advances in optical metasurfaces: fabrication and applications [Invited]

The research and development of optical metasurfaces has been primarily driven by the curiosity for novel optical phenomena that are unattainable from materials that exist in nature and by the desire for miniaturization of optical devices. Metasurfaces constructed of artificial patterns of subwavelength depth make it possible to achieve flat, ultrathin optical devices of high performance. A wide variety of fabrication techniques have been developed to explore their unconventional functionalities which in many ways have revolutionized the means with which we control and manipulate electromagnetic waves. The relevant research community could benefit from an overview on recent progress in the fabrication and applications of the metasurfaces. This review article is intended to serve that purpose by reviewing the state-of-the-art fabrication methods and surveying their cutting-edge applications.

Broadband Control of Topological Nodes in Electromagnetic Fields

We study topological nodes (phase singularities) in electromagnetic wave interactions with structures. We show that, when the nodes exist, it is possible to bind certain nodes to a specific plane in the structure by a combination of mirror and time-reversal symmetry. Such binding does not rely on any resonances in the structure. As a result, the nodes persist on the plane over a wide wavelength range. As an implication of such broadband binding, we demonstrate that the topological nodes can be used for hiding of metallic objects over a broad wavelength range.

High-Efficiency, Near-Diffraction Limited, Dielectric Metasurface Lenses Based on Crystalline Titanium Dioxide at Visible Wavelengths

Metasurfaces are planar optical elements that hold promise for overcoming the limitations of refractive and conventional diffractive optics. Previous metasurfaces have been limited to transparency windows at infrared wavelengths because of significant optical absorption and loss at visible wavelengths. Here we report a polarization-insensitive, high-contrast transmissive metasurface composed of crystalline titanium dioxide pillars in the form of metalens at the wavelength of 633 nm. The focal spots are as small as 0.54 λ d , which is very close to the optical diffraction limit of 0.5 λ d . The simulation focusing efficiency is up to 88.5%. A rigorous method for metalens design, the phase realization mechanism and the trade-off between high efficiency and small spot size (or large numerical aperture) are discussed. Besides, the metalenses can work well with an imaging point source up to ±15° off axis. The proposed design is relatively systematic and can be applied to various applications such as visible imaging, ranging and sensing systems.

High-efficiency dual-modes vortex beam generator with polarization-dependent transmission and reflection properties

Vortex beam is believed to be an effective way to extend communication capacity, but available efforts suffer from the issues of complex configurations, fixed operation mode as well as low efficiency. Here, we propose a general strategy to design dual-modes vortex beam generator by using metasurfaces with polarization-dependent transmission and reflection properties. Combining the focusing and vortex functionalities, we design/fabricate a type of compact dual-modes vortex beam generator operating at both reflection/transmission sides of the system. Experimental results demonstrate that the designed metadevice can switch freely and independently between the reflective vortex with topological charge m₁ = 2 and transmissive vortex with m₂ = 1. Moreover, the metadevice exhibits very high efficiencies of 91% and 85% for the reflective and transmissive case respectively. Our findings open a door for multifunctional metadevices with high performances, which indicate wide applications in modern integration-optics and wireless communication systems.

Multifunctional Metasurfaces Based on the “Merging” Concept and Anisotropic Single-Structure Meta-Atoms

Metasurfaces offer great opportunities to control electromagnetic (EM) waves, attracting intensive attention in science and engineering communities. Recently, many efforts were devoted to multifunctional metasurfaces integrating different functionalities into single flat devices. In this article, we present a concise review on the development of multifunctional metasurfaces, focusing on the design strategies proposed and functional devices realized. We first briefly review the early efforts on designing such systems, which simply combine multiple meta-structures with distinct functionalities to form multifunctional devices. To overcome the low-efficiency and functionality cross-talking issues, a new strategy was proposed, in which the meta-atoms are carefully designed single structures exhibiting polarization-controlled transmission/reflection amplitude/phase responses. Based on this new scheme, various types of multifunctional devices were realized in different frequency domains, which exhibit diversified functionalities (e.g., focusing, deflection, surface wave conversion, multi-beam emissions, etc.), for both pure-reflection and pure-transmission geometries or even in the full EM space. We conclude this review by presenting our perspectives on this fast-developing new sub-field, hoping to stimulate new research outputs that are useful in future applications.

All-dielectric planar chiral metasurface with gradient geometric phase

Planar optical chirality of a metasurface measures its differential response between left and right circularly polarized (CP) lights and governs the asymmetric transmission of CP lights. In 2D ultra-thin plasmonic structures the circular dichroism is limited to 25% in theory and it requires high absorption loss. Here we propose and numerically demonstrate a planar chiral all-dielectric metasurface that exhibits giant circular dichroism and transmission asymmetry over 0.8 for circularly polarized lights with negligible loss, without bringing in bianisotropy or violating reciprocity. The metasurface consists of arrays of high refractive index germanium Z-shape resonators that break the in-plane mirror symmetry and induce cross-polarization conversion. Furthermore, at the transmission peak of one handedness, the transmitted light is efficiently converted into the opposite circular polarization state, with a designated geometric phase depending on the orientation angle of the optical element. In this way, the optical component sets before and after the metasurface to filter the light of certain circular polarization states are not needed and the metasurface can function under any linear polarization, in contrast to the conventional setup for geometry phase based metasurfaces. Anomalous transmission and two-dimensional holography based on the geometric phase chiral metasurface are numerically demonstrate as proofs of concept.

Design of bifunctional metasurface based on independent control of transmission and reflection

Multifunctional metasurface integrating different functions can significantly save the occupied space, although most of bifunctional metasurfaces reported to date only control the wave in either reflection or transmission regime. In this paper, we propose a scheme that allows one to independently control the reflection and transmission wavefront under orthogonal polarizations. For demonstration, we design a bifunctional metasurface that simultaneously realizes a diffusion reflection and a focusing transmission. The diffusion reflection is realized using a random phase distribution, which was implemented by randomly arranging two basic coding unit cells with the aid of an ergodic algorithm. Meanwhile, the hyperbolic phase distribution was designed to realize the focusing functionality in the transmission regime. To further show the potential applications, a high-gain lens antenna was designed by assembling the proposed metasurface with a proper feed. Both simulation and measurement results have been carried out, and the agreement between the two results demonstrates the validity of the performance as expected. The backward scattering can be reduced more than 5 dB within 6.4-10 GHz compared with the metallic plate. Moreover, the lens antenna has a gain of 20 dB (with around 13 dB enhancement in comparison with the bare feeding antenna) and an efficiency of 32.5%.

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.

Polarization-switchable and wavelength-controllable multi-functional metasurface for focusing and surface-plasmon-polariton wave excitation

Realizing versatile functionalities in a single photonic device is crucial for photonic integration. We here propose a polarization-switchable and wavelength-controllable multi-functional metasurface. By changing the polarization state of incident light, its functionality can be switched between the flat focusing lens and exciting surface-plasmon-polariton (SPP) wave. Interestingly, by tuning the wavelength of incident light, the generated SPP waves can also be controlled at desired interfaces, traveling along the upper or lower interface of the metasurface, or along both of them, depending on whether the incident light satisfies the first or second Kerker condition. This polarization-switchable and wavelength-controllable multifunctional metasurface may provide flexibility in designing tunable or multifunctional metasurfaces and may find potential applications in highly integrated photonic systems.

On-chip generation of broadband high-order Laguerre-Gaussian modes in a metasurface

With experimental results, we demonstrate the generation of high-order Laguerre-Gaussian modes with non-zero radial indices using a metal meta-surface, which is composed of a series of rectangle nanoholes with different orientation angles. The phase shift after transmission through the metasurface is determined by the orientation angle of the nanohole. This device works over a broad wavelength band ranging from 700 to 1000 nm. Moreover, we achieve a LG mode with a radial mode index of 10. Our results provide an integrated method to obtain high-order LG modes, which can be used to enhance the capacity in optical communication and manipulation.

Broadband spin Hall effect of light in single nanoapertures

With properties not previously available, optical metamaterials and metasurfaces have shown their great potential in the precise control of light waves at the nanoscale. However, the use of current metamaterials and metasurfaces is limited by the collective response of the meta-atoms/molecules, which means that a single element cannot provide the functionalities required by most applications. Here, we demonstrate for the first time that a single achiral nanoaperture can be utilized as a meta-macromolecule to achieve giant angular spin Hall effect of light. By controlling the spin-related momenta, we show that these nanoapertures can enable full control of the phase gradient at a deep-subwavelength level, thus forming unique building blocks for optical metasurfaces. As a proof-of-concept demonstration, a miniaturized Bessel-like beam generator and flat lens are designed and experimentally characterized. The results presented here may open a door for the development of meta-macromolecule-based metasurfaces for integrated optical systems and nanophotonics.

Meta-Chirality: Fundamentals, Construction and Applications

Chiral metamaterials represent a special type of artificial structures that cannot be superposed to their mirror images. Due to the lack of mirror symmetry, cross-coupling between electric and magnetic fields exist in chiral mediums and present unique electromagnetic characters of circular dichroism and optical activity, which provide a new opportunity to tune polarization and realize negative refractive index. Chiral metamaterials have attracted great attentions in recent years and have given rise to a series of applications in polarization manipulation, imaging, chemical and biological detection, and nonlinear optics. Here we review the fundamental theory of chiral media and analyze the construction principles of some typical chiral metamaterials. Then, the progress in extrinsic chiral metamaterials, absorbing chiral metamaterials, and reconfigurable chiral metamaterials are summarized. In the last section, future trends in chiral metamaterials and application in nonlinear optics are introduced.

Power Flow in a Large-Core Multimode Fiber under External Perturbation and its Applications

Large core optical multimode fiber provides benefits such as a large light-coupling tolerance, easy handling, and delivery of higher light power without undesirable nonlinear effects. In this research, we exploit the effects of external perturbation on the power flow within the large core fiber and present two relevant applications, namely a perturbation sensor and a doughnut beam tuner. Since conventional multimode fiber power flow model does not take into consideration the perturbation effect, we modify the power flow model so that the influence of time varying perturbation can be theoretically analyzed. Based on our theory, we further conduct the numerical simulation and experiments on these two applications. For the fiber vibration sensor, the proposed numerical model shows that the sensor sensitivity depends on the intensity profile of the launched beam and also the higher-order harmonics that were not reported previously can become interferences to affect the signal. For the beam tuner application, we prove both theoretically and experimentally that the doughnut intensity profile at the fiber output can be tuned in real-time by applying external perturbations to the fiber. We expect that the results can be useful to further exploit the external perturbation on large core fiber in various applications.