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.

Single-sized multifunctional metasurfaces for simultaneous nanoprinting and holography inspired by tri-redundancy.

Multifunctional metasurfaces, where multiple functions can be integrated into a piece of metasurface, are preferably desired for compact systems with higher integration and subwavelength footprint. Particularly, metasurfaces for simultaneous nanoprinting and holography are one of the promising directions of development image display and information hiding in meta-devices. Here, inspired by tri-redundancy, a new, to the best of our knowledge, approach is proposed for generating a nanoprinting image in the near field and holographic image in the far field simultaneously, which can solve the extremum-mapping problem existing in single-sized scheme without increasing the complexity of the nanostructures. The tri-redundancy of image recognition, hologram designing and intensity modulation introduce an extra degree of freedom, which helps to find a balance between the two types of meta-images generated by utilizing the simulated annealing algorithm. A multifunctional metasurface composed of single-sized silver nanobricks with in-plane orientation has been fabricated to demonstrate the feasibility of encoding a binary image in the near field while reconstructing a 16-steps holographic image without twin-image in the far field. This multifunctional metasurface has flexible working modes, broadband working window and large robustness for fabrication errors, and it provides a simple design scheme for multifunctional integration. We expect it can empower advanced research and applications in high-end optical anticounterfeiting, image hiding and so on.

Tri-channel metasurface for watermarked structural-color nanoprinting and holographic imaging.

Structural-color nanoprinting, which can generate vivid colors with spatial resolution at subwavelength level, possesses potential market in optical anticounterfeiting and information encryption. Herein, we propose an ultracompact metasurface with a single-cell design strategy to establish three independent information channels for simultaneous watermarked structural-color nanoprinting and holographic imaging. Dual-channel spectrum manipulation and single-channel phase manipulation are combined together by elaborately introducing the orientation degeneracy into the design of variable dielectric nanobricks. Hence, a structural-color nanoprinting image covered with polarization-dependent watermarks and a holographic image can be respectively generated under different decoded environments. The proposed metasurface shows a flexible method for tri-channel image display with high information capacity, and exhibits dual-mode anticounterfeiting with double safeguards, i.e., polarization-controlled watermarks and a far-field holographic image. This study provides a feasible route to develop multifunctional metasurfaces for applications including optical anticounterfeiting, information encryption and security, information multiplexing, etc.

Non-orthogonal-polarization multiplexed metasurfaces for tri-channel gray-imaging.

Metasurface based polarization multiplexing is usually conducted in two orthogonal-polarization states, e.g., linearly polarized along x/y axes, left/right-handed circularly polarized states, etc. Herein, we show metasurfaces can be employed to implement tri-channel polarization multiplexing in three non-orthogonal-polarization states, merely with a single-size nanostructure design approach. Specifically, nanostructured metasurfaces acting as nano-polarizer arrays can modulate the incident light intensity pixel-by-pixel by controlling the orientation angles of nanostructures, governed by Malus's law. Hence, by inserting a metasurface between a bulk-optic polarizer and an analyzer, and elaborately controlling their polarization combinations, we show that the Malus-assisted metasurface can simultaneously record a continuous gray-image and two independent binary-patterns in three different information channels. We experimentally demonstrate this concept by recording three independent gray-images right at the metasurface surface. With the advantages of high information density, high security, high compatibility and ultracompactness, the proposed gray-imaging meta-device can play a significant role in the field of optical storage, anti-counterfeiting, and information multiplexing, etc.

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.

On-axis three-dimensional meta-holography enabled with continuous-amplitude modulation of light.

Conventional three-dimensional (3D) holography based on recording interference fringes on a photosensitive material usually has unavoidable zero-order light, which merges with the holographic image and blurs it. Off-axis design is an effective approach to avoid this problem; however, it in turn leads to the waste of at least half of the imaging space for holographic reconstruction. Herein, we propose an on-axis 3D holography based on Malus-assisted metasurfaces, which can eliminate the zero-order light and project the holographic image in the full transmission space. Specifically, each nanostructure in the metasurface acts as a nano-polarizer, which can modulate the polarization-assisted amplitude of incident light continuously, governed by Malus law. By carefully choosing the orientation angles of nano-polarizers, the amplitude can be both positive and negative, which can be employed to extinct zero-order light without affecting the intensity modulation for holographic recording. We experimentally demonstrate this concept by projecting an on-axis 3-layer holographic images in the imaging space and all experimental results agree well with our prediction. Our proposed metasurface carries unique characteristics such as ultracompactness, on-axis reconstruction, extinction of zero-order light and broadband response, which can find its market in ultracompact and high-density holographic recording for 3D objects.

Single-celled multifunctional metasurfaces merging structural-color nanoprinting and holography.

Nanostructured metasurfaces applied in structural-color nanoprinting and holography have been extensively investigated in the past several years. Recently, merging them together is becoming an emerging approach to improve the information capacity and functionality of metasurfaces. However, current approaches, e.g., segmenting, interleaving and stacking schemes for function merging, suffer from crosstalk, low information density, design and fabrication difficulties. Herein, we employ a single-celled approach to design and experimentally demonstrate a high-density multifunctional metasurface merging nanoprinting and holography, i.e., each nanostructure in the metasurface can simultaneously manipulate the spectra (enabled with varied dimensions of nanostructures) and geometric phase (enabled with varied orientation angles of nanostructures) of incident light. Hence, with different decoding strategies, a structural-color nanoprinting image emerges right at the metasurface plane under white light illumination, while a holographic image is reconstructed in the Fraunhofer diffraction zone under circularly polarized laser light incidence. And the two images have no crosstalk since they are independently designed and presented at different distances. Our proposal suggests a space-multiplexing scheme to develop advanced metasurfaces and one can find their markets in high-density information storage, optical information encryption, multi-channel image display, etc.

Asymmetric hologram with a single-size nanostructured metasurface.

Geometric metasurfaces, governed by PB phase, have shown their strong polarization sensitivity and can generate opposite phase delay when the handedness of incident circularly-polarized (CP) light is opposite. Here, we show this interesting characteristic can be employed to generate asymmetric forward and backward propagation with the same incident left- or right-handed CP light, which is hard to achieve with conventional optical elements and devices. Specifically, with the modified holographic design algorithm to consider both forward and backward CP light, an asymmetric meta-hologram is designed, which can project two different holographic images in the forward and backward directions, respectively. We demonstrate this concept by fabricating an asymmetric hologram with a single-size nanostructured metasurface, and the experimentally obtained holographic images in both directions have shown their advantages of high fidelity, broadband response and low crosstalk. The proposed asymmetric metasurface can play an important role in data storages, anti-counterfeitings, optical communications, displays and many other related fields.

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.

Structural-color nanoprinting with hidden watermarks.

Nanostructured metasurfaces can manipulate the spectrum and polarization of incident light at the nanoscale, which suggests a new integration of color nanoprints and polarizing-related components. Herein, we design and experimentally demonstrate a structural-color nanoprint carrying hidden watermarks, enabled with the polarization-assisted spectrum manipulation of light. Specifically, under unpolarized white light, the watermarks are concealed and a structural-color nanoprinting-image occupies the metasurface plane. Meanwhile, once linearly polarized white light is incident on the same metasurface, the hidden information can be decoded, and the same nanoprinting-image covered with watermarks appears. The proposed metasurface represents a paradigm for displaying color nanoprinting-images with or without watermarks, showing a flexible switch between the two operating modes and providing an easily camouflaged scheme for anticounterfeiting, encryption, information multiplexing, high-density optical storage, etc.

Multiplexed Anticounterfeiting Meta-image Displays with Single-Sized Nanostructures.

Metasurfaces have recently been used for multichannel image displays with pixel-size lower than a wavelength, which indicates the potential application in ultracompact anticounterfeiting with high-density and hidden information. However, current multichannel metasurfaces applied in anticounterfeiting are based on the sophisticated nanostructure design or at the cost of giving up some controls on the optical transmission matrix to encode multiple information channels. That is, the overall degrees of freedom offered by these metasurfaces are a "zero-sum game". Here, inspired by the orientation degeneracy indicated in Malus law, we propose a multiplexed anticounterfeiting metasurface consisting of single-sized nanostructures, which provide a new degree of freedom to increase the information capacity of anticounterfeiting without burdening the nanostructure design and fabrication. Specifically, the proposed metasurfaces can record a continuous grayscale image (channel 1) multiplexed with a totally/partially independent, interrelated, or watermarked anticounterfeiting pattern (channel 2). The two channels can be readily switched by polarization control. All experimental metasurface-images (meta-images) with high fidelity agree well with our design. With advantages such as ultracompactness, high-density information, multichannel displays, and strong concealment, the anticounterfeiting metasurfaces can empower advanced research and applications of metasurfaces in high-end optical anticounterfeiting and many other related fields.

Silicon-on-insulator based multifunctional metasurface with simultaneous polarization and geometric phase controls.

Enabled with both magnetic resonance and geometric phase, dielectric nanobrick based metasurfaces have shown their unusual abilities to produce high-definition and high-efficiency holographic images. Herein, we further show that such a metasurface can not only project a holographic image in far field but also record a grayscale image right at the metasurface plane simultaneously, merely with a single-celled nanostructure design approach. Specifically, each nanobrick in a unit-cell of the metasurface acts as a half-wave plate and it can continuously rotate the polarization direction of incident linearly polarized light. Governed by Malus law, light intensity modulation is available with the help of a bulk-optic analyzer and a continuous grayscale image appears right at the metasurface plane. At the same time, the concept of orientation degeneracy of nanostructures can be utilized to generate a 4-step geometric phase, with which a holographic image is reconstructed in far field. We experimentally demonstrate this multifunctional meta-device by employing the widely used silicon-on-insulator (SOI) material and all results agree well with our theoretical prediction. With the novel features of easiness in design, high efficiency, broadband spectrum response, strong robustness, high security and high information density, the proposed SOI-based metasurfaces will have extensive applications in optical information security and multiplexing.

2π-space uniform-backscattering metasurfaces enabled with geometric phase and magnetic resonance in visible light.

Metasurfaces have shown unusual abilities to modulate the phase, amplitude and polarization of an incident lightwave with spatial resolution at the subwavelength scale. Here, we experimentally demonstrate a dielectric metasurface enabled with both geometric phase and magnetic resonance that scatters an incident light beam filling the full reflective 2π-space with high-uniformity. Specifically, by delicately reconfiguring the orientations of dielectric nanobricks acting as nano-half-waveplates in a metasurface, the optical power of phase-modulated output light is almost equally allocated to all diffraction orders filling the full reflection space. The measured beam non-uniformity in the full hemispheric space, defined as the relative standard deviation (RSD) of all scattered optical power, is only around 0.25. More interestingly, since the target intensity distribution in a uniform design is rotationally centrosymmetric, the diffraction results are identical under arbitrary polarization states, e.g., circularly polarized, linearly polarized or even unpolarized light, which brings great convenience in practical applications. The proposed uniform-backscattering metasurface enjoys the advantages including polarization insensitivity, high-integration-density and high-stability, which has great potential in sensing, lighting, laser ranging, free-space optical communication and so on.

Continuous amplitude-modulated meta-fork gratings with zero-order extinction.

Metasurfaces, acting as arrays of perfect nano-polarizers, provide a promising approach to manipulate the amplitude of an incident light at the sub-wavelength scale. In this Letter, we design and demonstrate continuous amplitude-modulated meta-fork gratings to generate optical vortex beams. More importantly, benefiting from the unique negative amplitude modulation, the unavoidable zero-order light that conventional amplitude-only elements always suffer disappears by carefully adjusting the orientation of each nanobrick. The dramatically dropped zero-order light with only 3% leakage energy verifies our design. With the advantages of continuous amplitude modulation, zero-order extinction, and super-high resolution, the proposed meta-fork grating will have a widespread application in integrated optical vortex manipulation and promote the emergence of many other amplitude-modulated nano-optical devices.

Single-size nanostructured metasurface for dual-channel vortex beam generation.

Under the government of Malus's law, metasurfaces composed of anisotropic nanostructures acting as nano-polarizers have shown their precise optical manipulation of polarization profile of incident light at the nanoscale. The orientation degeneracy implied in Malus's law provides a new design degree of freedom for polarization multiplexing, which can be employed to design amplitude-modulated multiplexing meta-devices. Herein, we experimentally demonstrate this concept by encoding two independent amplitude profiles into a single metasurface under different polarization controls, merely with a single-size nanostructure design approach. Hence, the multiplexing metasurface functions as two independent fork gratings to generate two vortex beams with different topological charges, and the two channels can be readily switched by rotating the metasurface sample around its optical axis from 0° to 45° or vice versa. The proposed metasurface for vortex beam generation enjoys advantages including high resolution, ultracompactness, dual-channel information capacity, and ultrasimple nanostructures, and it can be extended to a variety of practical applications in information multiplexing, orbital angular momentum (OAM) multiplexing communication, quantum information processing, etc.

Ultracompact, high-resolution and continuous grayscale image display based on resonant dielectric metasurfaces.

Since the electromagnetic resonance that happens in dielectric nanobricks can be meticulously designed to control both amplitude and polarization of light, an ultracompact, high-resolution and continuous grayscale image display method based on resonant dielectric metasurfaces is proposed. Magnetic resonance occurs in dielectric nanobricks can yield unusual high reflectivity depending on the polarization state of incident light, which paves a new way for ultracompact image display when the resonant metasurfaces consisting of nano-polarizer arrays operate. Governed by Malus's law, nano-polarizer arrays featured with different orientations have been demonstrated to continuously manipulate the intensity of linearly polarized light cell-by-cell. Hence, it can practically enable recording a high fidelity grayscale image right at the sample surface with resolution as high as 84,667 dpi (dots per inch). This proposed resonant metasurface image (meta-image) display enjoys the advantages including continuous grayscale modulation, broadband working window, high-stability and high-density, which can easily find promising applications in ultracompact displays, high-end anti-counterfeiting, high-density optical information storage and information encryption, etc.

Depth perception based 3D holograms enabled with polarization-independent metasurfaces.

Metasurfaces consist of dielectric nanobrick arrays with different dimensions in the long and short axes can be used to generate different phase delays, predicting a new way to manipulate an incident beam in the two orthogonal directions separately. Here we demonstrate the concept of depth perception based three-dimensional (3D) holograms with polarization-independent metasurfaces. 4-step dielectric metasurfaces-based fan-out optical elements and holograms operating at 658 nm were designed and simulated. Two different holographic images with high fidelity were generated at the same plane in the far field for different polarization states. One can observe the 3D effect of target objects with polarized glasses. With the advantages of ultracompactness, flexibility and replicability, the polarization-independent metasurfaces open up depth perception based stereoscopic imaging in a holographic way.

The Effect of Height Error on Performance of Propagation Phase-Based Metalens.

Metalenses, as a new type of planar optical device with flexible design, play an important role in miniaturized and integrated optical devices. Propagation phase-based metalenses, known for their low loss and extensive design flexibility, are widely utilized in optical imaging and optical communication. However, fabrication errors introduced by thin-film deposition and etching processes inevitably result in variations in the height of the metalens structure, leading to the fabricated devices not performing as expected. Here, we introduce a reflective TiO2 metalens based on the propagation phase. Then, the relationship between the height variation and the performance of the metalens is explored by using the maximum phase error. Our results reveal that the height error of the unit structure affects the phase rather than the amplitude. The focusing efficiency of our metalens exhibits robustness to structural variations, with only a 5% decrease in focusing efficiency when the height varies within ±8% of the range. The contents discussed in this paper provide theoretical guidance for the unit design of the propagation phase-based metalens and the determination of its allowable fabrication error range, which is of great significance for low-cost and high-efficiency manufacturing.

Fabrication of Large-Area Nanostructures Using Cross-Nanoimprint Strategy.

Nanostructures with sufficiently large areas are necessary for the development of practical devices. Current efforts to fabricate large-area nanostructures using step-and-repeat nanoimprint lithography, however, result in either wide seams or low efficiency due to ultraviolet light leakage and the overflow of imprint resin. In this study, we propose an efficient method for large-area nanostructure fabrication using step-and-repeat nanoimprint lithography with a composite mold. The composite mold consists of a quartz support layer, a soft polydimethylsiloxane buffer layer, and multiple intermediate polymer stamps arranged in a cross pattern. The distance between the adjacent stamp pattern areas is equal to the width of the pattern area. This design combines the high imprinting precision of hard molds with the uniform large-area imprinting offered by soft molds. In this experiment, we utilized a composite mold consisting of three sub-molds combined with a cross-nanoimprint strategy to create large-area nanostructures measuring 5 mm × 30 mm on a silicon substrate, with the minimum linewidth of the structure being 100 nm. Compared with traditional step-and-flash nanoimprint lithography, the present method enhances manufacturing efficiency and generates large-area patterns with seam errors only at the micron level. This research could help advance micro-nano optics, flexible electronics, optical communication, and biomedicine studies.

Toward Water-Immersion Programmable Meta-Display.

Heading toward next-generation intelligent display, dynamic control capability for meta-devices is critical for real world applications. Beyond the conventional electrical/optical/mechanical/thermal tuning methods, liquid immersion recently has emerged as a facile tuning mechanism which is easily accessible (especially water) and practically implementable for large tuning area. However, due to the longstanding and critical drawback of lacking independent-encoding capability, the state-of-art immersion approach remains incapable of pixel-level programmable switching. Here a water-immersion tuning scheme with pixel-scale programmability for dynamic meta-displays is proposed. Tunable meta-pixels can be engineered to construct spectral selective patterns at prior-/post- immersion states, such that a metasurface enables pixel-level transforming animations for dynamic multifield meta-displays, including near-field dual-nanoprints and far-field dual-holographic displays. The proposed water-immersion programmable approach for meta-display, benefitting from its large tuning area, facile operation and strong repeatability, may find a revolutionary path toward next-generation intelligent display with practical applications in dynamic display/encryption, information anticounterfeit/storage, and optical sensors.