Metalens for Generating a Customized Vectorial Focal Curve

Multiple Multicolored 3D Polarization Knots Arranged along Light Propagation

Polarization and color play essential roles in understanding optical phenomena and practical applications. Customized three-dimensional (3D) light fields, characterized by specific polarization and color distributions, have garnered growing interest owing to their unique optical attributes and expanded capacity for information encoding. To align with the ongoing trend of compactness and integration, it is desirable to develop lightweight optical elements that can simultaneously control polarization and color in 3D space. Although engineering longitudinally variable 3D optical structures with predesigned color and polarization information can add more degrees of freedom and additional capacity for information encoding, it has not been reported. We propose a metasurface approach to generating multiple 3D polarization knots along the light propagation direction. Each knot features two colors and an engineered 3D polarization profile. Different multicolored 3D polarization knots are obtained by controlling the observation region along the light propagation. Our approach simultaneously combines polarization, color, and longitudinal control in 3D environment, offering extra degrees of freedom for engineering complex vector beams. The unique properties of the developed metadevices, together with the design flexibility and compactness of metasurface, pave the way for polarization systems with small volumes applicable to some areas such as complex structured beams and encryption.

Multifaceted control of focal points along an arbitrary 3D curved trajectory

Metalenses can integrate the functionalities of multiple optical components thanks to the unprecedented capability of optical metasurfaces in light control. With the rapid development of optical metasurfaces, metalenses continue to evolve. Polarization and color play a very important role in understanding optics and serve as valuable tools for gaining insights into our world. Benefiting from the design flexibility of metasurfaces, we propose and experimentally demonstrate a super metalens that can realize multifaceted control of focal points along any 3D curved trajectory. The wavelengths and polarization states of all focal points are engineered in a desirable manner. The super metalens can simultaneously realize customized 3D positioning, polarization states, and wavelengths of focal points, which are experimentally demonstrated with incident wavelengths ranging from 501 to 700 nm. We further showcase the application of the developed super metalenses in 3D optical distance measurement. The compact nature of metasurfaces and unique properties of the proposed super metalenses hold promise to dramatically miniaturize and simplify the optical architecture for applications in optical metrology, imaging, detection, and security.

Metasurface for Engineering Superimposed Ince-Gaussian Beams

Ince-Gaussian beams (IGBs) are the third complete family of exact and orthogonal solutions of the paraxial wave equation and have been applied in many fields ranging from particle trapping to quantum optics. IGBs play a very important role in optics as they represent the exact and continuous transition modes connecting Laguerre-Gaussian and Hermite-Gaussian beams. The method currently in use suffers from the high cost, complexity, and large volume of the optical system. The superposition of IGBs can generate complicated structured beams with multiple phase and polarization singularities. A metasurface approach is proposed to realizing various superpositions of IGBs without relying on a complicated optical setup. By superimposing IGBs with even and odd modes, multiple phase, and polarization singularities are observed in the resultant beams. The phase and polarization singularities are modulated by setting the initial phase in the design and controlling the incident linear polarization. The compactness of the developed metasurface devices and the unique properties of the generated beams have the potential to impact many practical applications such as particle manipulation, orbital angular momentum spectrum manipulation, and optical communications.

Terahertz metalens for generating multi-polarized focal points and images with uniform intensity distributions

Metasurfaces have provided a flexible platform for designing ultracompact metalenses with unusual functionalities. However, traditional multi-foci metalenses are limited to generating circularly polarized (CP) or linearly polarized (LP) focal points, and the intensity distributions are always inhomogeneous/chaotical between the multiple focal points. Here, an inverse design approach is proposed to optimize the in-plane orientation of each meta-atom in a terahertz (THz) multi-foci metalens that can generate multi-polarized focal points with nearly uniform intensity distributions. As a proof-of-principle example, we numerically and experimentally demonstrate an inversely designed metalens for simultaneously generating multiple CP- and LP-based focal points with homogeneous intensity distributions, leading to a multi-polarized image (rather than the holography). Furthermore, the multi-channel and multi-polarized images consisting of multiple focal points with homogeneous intensity distributions are also numerically demonstrated. The unique approach for inversely designing multi-foci metalens that can generate multi-polarized focal points and images with uniform intensity distributions will enable potential applications in imaging and sensing.

All-dielectric geometric metasurfaces for the generation and manipulation of perfect high-order Poincaré sphere beams

A high-order Poincaré sphere (HOPS) can be used to describe high-order modes of waveguides and vector beams, since it generalizes the feature of spin and the orbital angular momentum of light. HOPS beams are such beams with polarization states on the HOPS, which have potential applications in optical manipulation and optical communication. In general, the intensity distribution of this kind of beam changes with the topological charge, which limits their practical applications. Based on the concept of perfect vortex beams (PVBs), perfect HOPS beams have been proposed to solve this problem. Here, a flexible and compact scheme based on all-dielectric metasurfaces for realizing and manipulating perfect HOPS beams at near-infrared wavelength was demonstrated. Geometric-phase-only manipulation was employed for simultaneously controlling the phase and polarization of the incident light. By varying the incident polarization, several selected polarization states on the HOPS could be realized by the proposed metasurface. Further, the single ultra-thin metasurface can also realize high quality multiplexing perfect HOPS beams that carry different topological charges. Finally, a cascaded metasurface system has been proposed for generating and manipulating multiple HOPS beams. This compact flat-optics-based scheme for perfect HOPS beam generation and manipulation demonstrated here can be used for on-chip optical manipulation and integrated optical communication in the future.

Multifunctional metalens optical tweezers for optical information recognition

Traditional optical information recognition (OIR), particle capture and manipulation require many optical devices or mechanical moving system components to achieve a specific function, which is difficult to achieve integration. This paper proposes a new method to realize these functions by using multi-focus metalens combining spectrum and polarization selection. The design incorporates three spectral bands, namely 500 nm, 580 nm, and 660 nm, within the visible light range. Additionally, it utilizes either left-handed or right-handed circularly polarized (LCP/RCP) light, resulting in six distinct focus focusing effects on a single focal plane. By analyzing the normalized light intensity (NLI) at the corresponding focus position, the OIR of any wavelength and polarization detection in the design can be realized, and the particle capture at different focusing positions can be realized. Our work can provide a new idea for the high integration of on-chip light recognition and operation and inspire the design of a highly integrated optical system with a smaller size and more substantial function.

Longitudinally variable 3D optical polarization structures

Generation and manipulation of three-dimensional (3D) optical polarization structures have received considerable interest because of their distinctive optical features and potential applications. However, the realization of multiple 3D polarization structures in a queue along the light propagation direction has not yet been reported. We propose and experimentally demonstrate a metalens to create longitudinally variable 3D polarization knots. A single metalens can simultaneously generate three distinct 3D polarization knots, which are indirectly validated with a rotating polarizer. The 3D polarization profiles are dynamically modulated by manipulating the linear polarization direction of the incident light. We further showcase the 3D image steganography with the generated 3D polarization structures. The ultrathin nature of metasurfaces and unique properties of the developed metalenses hold promise for lightweight polarization systems applicable to areas such as 3D image steganography and virtual reality.

Dynamic control of hybrid grafted perfect vector vortex beams

Perfect vector vortex beams (PVVBs) have attracted considerable interest due to their peculiar optical features. PVVBs are typically generated through the superposition of perfect vortex beams, which suffer from the limited number of topological charges (TCs). Furthermore, dynamic control of PVVBs is desirable and has not been reported. We propose and experimentally demonstrate hybrid grafted perfect vector vortex beams (GPVVBs) and their dynamic control. Hybrid GPVVBs are generated through the superposition of grafted perfect vortex beams with a multifunctional metasurface. The generated hybrid GPVVBs possess spatially variant rates of polarization change due to the involvement of more TCs. Each hybrid GPVVB includes different GPVVBs in the same beam, adding more design flexibility. Moreover, these beams are dynamically controlled with a rotating half waveplate. The generated dynamic GPVVBs may find applications in the fields where dynamic control is in high demand, including optical encryption, dense data communication, and multiple particle manipulation.

Compact multi-foci metalens spectrometer

A lightweight and portable spectrometer is desirable for miniaturization and integration. The unprecedented capability of optical metasurfaces has shown much promise to perform such a task. We propose and experimentally demonstrate a compact high-resolution spectrometer with a multi-foci metalens. The novel metalens is designed based on wavelength and phase multiplexing, which can accurately map the wavelength information into its focal points located on the same plane. The measured wavelengths in the light spectra agree with simulation results upon the illumination of various incident light spectra. The uniqueness of this technique lies in the novel metalens that can simultaneously realize wavelength splitting and light focusing. The compactness and ultrathin nature of the metalens spectrometer render this technology have potential applications in on-chip integrated photonics where spectral analysis and information processing can be performed in a compact platform.

Multiplexing Optical Images for Steganography by Single Metasurfaces

Image steganography based on intelligent devices is one of the effective routes for safely and quickly transferring secret information. However, optical image steganography has attracted far less attention than digital one due to the state-of-the-art technology limitations of high-resolution optical imaging in integrated devices. Optical metasurfaces, composed of ultrathin subwavelength meta-atoms, are extensively considered for flat optical-imaging nano-components with high-resolutions as competitive candidates for next-generation miniaturized devices. Here, multiplex imaging metasurfaces composed of single nanorods are proposed under a detailed strategy to realize optical image steganography. The simulation and experimental results demonstrate that an optical steganographic metasurface can simultaneously transfer independent secret image information to two receivers with special keys, without raising suspicions for the general public under the cloak of a cover image. The proposed optical steganographic strategy by metasurfaces can arbitrarily distribute a continuous grayscale image together with a black-and-white image in separate channels, implying the distinguishing feature of high-density information capacity for integration and miniaturization in optical meta-devices.

Dynamic adjustable metalens based on a stretchable substrate with a double-layer metal microstructure

Based on the impedance-matching theory, a double-layer metal structure dynamical focusing cylindrical metalens with a stretchable substrate was designed at the operation frequency of 0.1 THz. The diameter, initial focal length, and NA of the metalens were 80 mm, 40 mm, and 0.7, respectively. The transmission phase of the unit cell structures could cover 0-2π by changing the size of the metal bars, and then the different unit cells were spatially arranged as the designed phase profile for the metalens. When the stretching range of the substrate was about 100%-140%, the focal length changed from 39.3 mm to 85.5 mm, the dynamic focusing range was about 117.6% of the minimum focal length, and the focusing efficiency decreases from 49.2% to 27.9%. Then, by rearranging the unit cell structures, a dynamically adjustable bifocal metalens was numerically realized. Using the same stretching ratio, compared to a single focus metalens, the bifocal metalens can provide a larger focal length control range.

Phase-assisted Bessel-metasurface: a single-sized approach for simultaneous printing and holography

Due to the unprecedented wavefront shaping capability, the metasurface has demonstrated state-of-the-art performances in various applications, especially in printing and holography. Recently, these two functions have been combined into a single metasurface chip to achieve a capability expansion. Despite the progress, current dual-mode metasurfaces are realized at the expense of an increase in the difficulty of the fabrication, reduction of the pixel resolution, or strict limitation in the illumination conditions. Inspired by the Jacobi-Anger expansion, a phase-assisted paradigm, called Bessel metasurface, has been proposed for simultaneous printing and holography. By elaborately arranging the orientations of the single-sized nanostructures with geometric phase modulation, the Bessel metasurface can not only encode a greyscale printing image in real space but can reconstruct a holographic image in k-space. With the merits of compactness, easy fabrication, convenient observation, and liberation of the illumination conditions, the design paradigm of the Bessel metasurface would have promising prospects in practical applications, including optical information storage, 3D stereoscopic displays, multifunctional optical devices, etc.

Metasurface-Enabled 3-in-1 Microscopy

Edge enhancement and polarization detection are critical to image transparent or low-contrast samples. However, currently available systems are limited to performing only a single functionality. To meet the requirement of system integration, there is a pressing need for a microscope with multiple functionalities. Here, we propose and develop a microscope with three different functionalities based on spatial multiplexing and polarization splitting. A novel geometric metasurface (MS) is used to realize a spiral phase profile and two phase gradient profiles along two vertical directions, which can perform such an extremely challenging optical task. This is the first demonstration of a 3-in-1 microscope that can simultaneously obtain five images with different optical properties in an imaging plane for the same sample. Imaging experiments with different samples verify its capability to simultaneously perform edge imaging, polarimetric imaging, and conventional microscope imaging. Benefiting from the compactness and multifunctionality of the optical MS device, the integration does not increase the volume of the microscope. This approach can enable users to visualize the multiple facets of samples in real-time.

Metalens for generating multi-channel polarization-wavelength multiplexing metasurface holograms

We demonstrate multi-channel metasurface holograms, where the pixels of holographic images are represented by the focal points of metalens, leading to the nanoscale resolution. The required phase profiles are implemented by elaborately arranging the hybrid all-dielectric meta-atoms with specific orientation angles. For verification, two-channel single-color images are reconstructed on the focal plane of the metalens by polarization control. Alternatively, three-channel color holograms are exhibited by manipulating the incident wavelengths. More uniquely, the metalens can be further engineered to generate polarization-wavelength multiplexing color holograms in six channels. Our work provides an effective approach to reconstructing holographic images and enables potential applications including color display, information engineering, and optical encryption.

Immersion-Triggered Active Switch for Spin-Decoupled Meta-Optics Multi-Display

Meta-optics exhibits many promising applications in various fields of optical displays, imaging, and information encryption. However, heading towards next-generation intelligent displays, its broad implementation is critically restricted by the lack of practical active tuning capability. Beyond the conventional electrical/optical/mechanical/thermal tuning methods, liquid immersion has recently emerged as a facile mechanism for active spectral tuning. To further conquer the challenge in achieving active complicated optical-field manipulation, here, an environment-compliant switch for meta-optics multi-display is originally proposed and experimentally realized via the liquid immersion tuning scheme. By designing the spin-decoupled phase array for left-/right-handed circular polarizations, it flexibly presents quad-fold independent-encoded phase distributions for different medium-relevant and polarization-controlled channels, thus enabling four switchable holographic images through immersion tuning. Such a proposed immersion tuning design is quite a straightforward approach for meta-optics holographic displays, enjoying full-spatial usage, design flexibility, and large-scale facile implementation. Overall, the proposed liquid immersion tuning strategy for a meta-optics multi-display would strongly benefit the practical applications in biochemical sensing, environment-adaptive displays, and information encryption.

Creating perfect composite vortex beams with a single all-dielectric geometric metasurface

Optical vortex beam carrying orbit angular momentum has been extensively researched and applied recently. Among which a perfect vortex beam (PVB) has attracted much attention owing to its topological charge (TC)-irrelevant intensity profile. However, the morphology singularity, as well as implementation complexity of the PVB tie the degree of freedom for multiplexing. Herein, by introducing the concept of a composite vortex beam, we originally propose a novel kind of PVB - perfect composite vortex beam (PCVB) - which possesses a rosette-like intensity pattern that is exactly correlated with the TC and can be directly generated using a single all-dielectric geometric metasurface rather than bulky optical systems. We numerically simulate the broadband generation of the proposed PCVB with various TCs, sizes, and rotation angles. To further explore the potential of our design in practical applications, we demonstrated the coaxial array of the PCVBs and detected their optical angular force for manipulating nanoparticles. We believe that our fruitage may pave a desirable avenue for optical communication, information processing, and optical manipulation.

Color-selective three-dimensional polarization structures

Polarization as an important degree of freedom for light plays a key role in optics. Structured beams with controlled polarization profiles have diverse applications, such as information encoding, display, medical and biological imaging, and manipulation of microparticles. However, conventional polarization optics can only realize two-dimensional polarization structures in a transverse plane. The emergent ultrathin optical devices consisting of planar nanostructures, so-called metasurfaces, have shown much promise for polarization manipulation. Here we propose and experimentally demonstrate color-selective three-dimensional (3D) polarization structures with a single metasurface. The geometric metasurfaces are designed based on color and phase multiplexing and polarization rotation, creating various 3D polarization knots. Remarkably, different 3D polarization knots in the same observation region can be achieved by controlling the incident wavelengths, providing unprecedented polarization control with color information in 3D space. Our research findings may be of interest to many practical applications such as vector beam generation, virtual reality, volumetric displays, security, and anti-counterfeiting.

All-dielectric metasurface for polarization-selective full-space complex amplitude modulations

Metasurfaces have exhibited powerful capabilities in the modulation of electromagnetic waves. Here, we demonstrate the polarization-selective full-space complex amplitude modulations of incident electromagnetic waves using all-dielectric metasurfaces. This is done via ingeniously designed subwavelength-scale super-pixels. As a proof of concept, we design two metasurfaces working in transmission and reflection spaces: one generates two independent vortex beams and the other generates two pairs of foci of arbitrary intensity ratios. The proposed full-space complex amplitude modulation provides more choices for the manipulation of electromagnetic waves.

Dielectric metalens for miniaturized imaging systems: progress and challenges

Lightweight, miniaturized optical imaging systems are vastly anticipated in these fields of aerospace exploration, industrial vision, consumer electronics, and medical imaging. However, conventional optical techniques are intricate to downscale as refractive lenses mostly rely on phase accumulation. Metalens, composed of subwavelength nanostructures that locally control light waves, offers a disruptive path for small-scale imaging systems. Recent advances in the design and nanofabrication of dielectric metalenses have led to some high-performance practical optical systems. This review outlines the exciting developments in the aforementioned area whilst highlighting the challenges of using dielectric metalenses to replace conventional optics in miniature optical systems. After a brief introduction to the fundamental physics of dielectric metalenses, the progress and challenges in terms of the typical performances are introduced. The supplementary discussion on the common challenges hindering further development is also presented, including the limitations of the conventional design methods, difficulties in scaling up, and device integration. Furthermore, the potential approaches to address the existing challenges are also deliberated.

Creating Composite Vortex Beams with a Single Geometric Metasurface

Composite vortex beams (CVBs) have attracted considerable interest recently due to the unique optical properties and potential applications. However, these beams are mainly generated using spatial light modulators, which suffer from large volume, high cost, and limited resolution. Benefiting from the ultrathin nature and unprecedented capability in light manipulation, optical metasurfaces provide a compact platform to perform this task. A metasurface approach to creating these CVBs is proposed and experimentally demonstrated. The design is based on the superposition of multiple circularly polarized vortex beams with different topological charges, which is realized based on a geometric metasurface consisting of metallic nanorods with spatially variant orientations. The effects of the initial phases, amplitude coefficients, incident polarization state, and propagation distance on the generated CVBs, which are in good agreement with the theoretical prediction, are experimentally analyzed. This work has opened a new avenue for engineering CVBs with a minimal footprint, which has promising applications ranging from multiple optical traps to quantum science.

Recent Advances in Planar Optics-Based Glasses-Free 3D Displays

Glasses-free three-dimensional (3D) displays are one of the technologies that will redefine human-computer interfaces. However, many geometric optics-based 3D displays suffer from a limited field of view (FOV), severe resolution degradation, and visual fatigue. Recently, planar optical elements (e.g., diffraction gratings, diffractive lenses and metasurfaces) have shown superior light manipulating capability in terms of light intensity, phase, and polarization. As a result, planar optics hold great promise to tackle the critical challenges for glasses-free 3D displays, especially for portable electronics and transparent display applications. In this review, the limitations of geometric optics-based glasses-free 3D displays are analyzed. The promising solutions offered by planar optics for glasses-free 3D displays are introduced in detail. As a specific application and an appealing feature, augmented reality (AR) 3D displays enabled by planar optics are comprehensively discussed. Fabrication technologies are important challenges that hinder the development of 3D displays. Therefore, multiple micro/nanofabrication methods used in 3D displays are highlighted. Finally, the current status, future direction and potential applications for glasses-free 3D displays and glasses-free AR 3D displays are summarized.

Reconstructing subwavelength resolution terahertz holographic images

Computer-generated holography typically generates terahertz (THz) holographic images with a pixel size larger than wavelength. We propose a multi-foci metalens model to reconstruct THz holographic images with subwavelength resolution. The designed devices are realized based on dielectric metasurfaces consisting of silicon micropillars with spatially variant orientations. By exploiting quasi-continuous profile of focal points as the pixels of a holographic image, a metalens can reconstruct a high-resolution target image on its focal plane. The effects of size and pitch of each sub-diffraction focal point on imaging quality and pixel resolution are discussed. The intensity distribution at each focal point indicates that the reconstructed images have subwavelength resolution. In comparison with conventional hologram designs, this design method can be used to reconstruct THz holographic images with subwavelength resolution, which have potential applications in THz communication, information security and anti-counterfeiting.