Metasurface Hologram Research Articles

Optical secret sharing with cascaded metasurface holography.

Secret sharing is a well-established cryptographic primitive for storing highly sensitive information like encryption keys for encoded data. It describes the problem of splitting a secret into different shares, without revealing any information to its shareholders. Here, we demonstrate an all-optical solution for secret sharing based on metasurface holography. In our concept, metasurface holograms are used as spatially separable shares that carry encrypted messages in the form of holographic images. Two of these shares can be recombined by bringing them close together. Light passing through this stack of metasurfaces accumulates the phase shift of both holograms and optically reconstructs the secret with high fidelity. In addition, the hologram generated by each single metasurface can uniquely identify its shareholder. Furthermore, we demonstrate that the inherent translational alignment sensitivity between two stacked metasurface holograms can be used for spatial multiplexing, which can be further extended to realize optical rulers.

Intelligent coding metasurface holograms by physics-assisted unsupervised generative adversarial network

Intelligent coding metasurface is a kind of information-carrying metasurface that can manipulate electromagnetic waves and associate digital information simultaneously in a smart way. One of its widely explored applications is to develop advanced schemes of dynamic holographic imaging. By now, the controlling coding sequences of the metasurface are usually designed by performing iterative approaches, including the Gerchberg–Saxton (GS) algorithm and stochastic optimization algorithm, which set a large barrier on the deployment of the intelligent coding metasurface in many practical scenarios with strong demands on high efficiency and capability. Here, we propose an efficient non-iterative algorithm for designing intelligent coding metasurface holograms in the context of unsupervised conditional generative adversarial networks (cGANs), which is referred to as physics-driven variational auto-encoder (VAE) cGAN (VAE-cGAN). Sharply different from the conventional cGAN with a harsh requirement on a large amount of manual-marked training data, the proposed VAE-cGAN behaves in a physics-driving way and thus can fundamentally remove the difficulties in the conventional cGAN. Specifically, the physical operation mechanism between the electric-field distribution and metasurface is introduced to model the VAE decoding module of the developed VAE-cGAN. Selected simulation and experimental results have been provided to demonstrate the state-of-the-art reliability and high efficiency of our VAE-cGAN. It could be faithfully expected that smart holograms could be developed by deploying our VAE-cGAN on neural network chips, finding more valuable applications in communication, microscopy, and so on.

Light field on a chip: metasurface-based multicolor holograms

Multicolor holography can faithfully record the color, depth, parallax, and other properties of scenes and has thus found numerous applications, for example, in optical document security, nonvolatile data storage, and virtual or augmented reality systems. Nanophotonic metasurfaces present multiple degrees of freedom to manipulate the properties of optical fields at visible wavelengths. These in turn provide opportunities for metasurface-based multicolor holography. We describe recent developments in multicolor metasurface holograms. These are categorized based on their color-separating mechanisms rather than their structural properties, such as whether they are plasmonic or dielectric. We hope this review will provide readers with new insights and thus help extend applications of metasurface-based multicolor holography to other fields.

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.

High Efficiency Polarization‐Encoded Holograms with Ultrathin Bilayer Spin‐Decoupled Information Metasurfaces

AbstractInformation metasurfaces, including digital coding and programmable metasurfaces, have reformed the design theory of metasurfaces from effective medium to coding pattern, and bridged the physical and digital world. Metasurface hologram has shown great potential in imaging and manipulating electromagnetic waves. In this work, bilayer information metasurfaces are proposed to achieve polarization‐encoded holograms in microwave regime. The designed metasurfaces consist of 3‐bit spin‐decoupled meta‐atoms, which possess ultrathin profiles (0.14λ0) and high cross‐polarization transmittance (over 0.8). The incident wave, which is right‐handed circularly polarized or left‐handed circularly polarized, is encoded with code “0” or “1”. The spin‐decoupled design is combined with diffuse scattering and holographic technology. Based on this, a novel algorithm is proposed to generate coding sequences according to the code of incident wave, to achieve different functionalities for corresponding circularly polarized channels. In this way, polarization‐encoded holograms, including diffuse scattering holograms (code “00”), single channel holograms (code “01” or “10”), and spin‐selective holograms (code “11”), are achieved. Simulated and measured results show that the maximum imaging efficiency can reach up to 65.1%, verifying the high efficiency of this holographic technology. This work is expected to open a way of realizing more complicated and higher efficiency information metasurface holography.

Recent Progress on Ultrathin Metalenses for Flat Optics.

SummaryAs technology advances, electrical devices such as smartphones have become more and more compact, leading to a demand for the continuous miniaturization of optical components. Metalenses, ultrathin flat optical elements composed of metasurfaces consisting of arrays of subwavelength optical antennas, provide a method of meeting those requirements. Moreover, metalenses have many other distinctive advantages including aberration correction, active tunability, and semi-transparency, compared to their conventional refractive and diffractive counterparts. Therefore, over the last decade, great effort has been focused on developing metalenses to investigate and broaden the capabilities of metalenses for integration into future applications. Here, we discuss recent progress on metalenses including their basic design principles and notable characteristics such as aberration correction, tunability, and multifunctionality.

Dual‐Polarized Tri‐Channel Encrypted Holography Based on Geometric Phase Metasurface

Metasurface‐based encrypted holography has drawn much attention recently due to its excellent ability in storing/displaying information with good security. To enhance the encryption security of metasurface holograms, multiplexing techniques, for which a large number of parameters need to be scanned to achieve the desired meta‐atoms, are highly demanded. Herein, a metasurface design scheme, which utilizes solely geometric phase elements to manipulate both co‐ and cross‐polarized reflected fields independently, is proposed. Using an improved weighted Gerchberg–Saxton (GSW), a holographic algorithm is proposed for 1‐bit phase, dual‐polarized tri‐channel encrypted metamirrors. Proof‐of‐concept prototypes are fabricated and experimental demonstrations are performed at microwave frequencies. Simulations and measurements are carried out to validate the proposed design, and the results agree well with the theoretical design scheme. Such dual‐polarized and tri‐channel encrypted metamirrors are appealing for applications in polarimetric imaging, information encryption/storage and beam splitting, shaping and steering.

Metasurface holographic movie: a cinematographic approach

Animation for a metasurface hologram was achieved using a cinematographic approach. Time-lapsed images were reconstructed using sequentially arranged metasurface hologram frames. An Au rectangular nanoaperture was adopted as a meta-atom pixel and arrayed to reproduce the phase distribution based on the help of a Pancharatnam-Berry phase. We arrayed 48 hologram frames on a 2-cm2 substrate and measured and assessed the retardation of fabricated meta-atoms to reconstruct the holographic image, successfully demonstrating the movie with a frame rate of 30 frames per second.

Full‐State Synthesis of Electromagnetic Fields using High Efficiency Phase‐Only Metasurfaces

AbstractA metasurface is a thin array of subwavelength elements with designable scattering responses, and metasurface holography is a powerful tool for imaging and field control. The existing metasurface holograms are classified into two types: one is based on phase‐only metasurfaces (including the recently presented vectorial metasurface holography), which has high power efficiency but cannot control the phases of generated fields; while the other is based on phase‐amplitude‐modulated metasurfaces, which can control both field amplitudes and phases in the region of interest (ROI) but has very low efficiency. Here, for the first time, it is proposed to synthesize the field amplitudes and phases in ROI simultaneously and independently by using high‐efficiency phase‐only metasurfaces. All points in ROI may have independent values of field amplitudes and phases, and the requirements for X and Y components may be different in achieving spatially varied polarization states. To this end, an efficient design method based on equivalent electromagnetic model and gradient‐based nonlinear optimization is proposed. Full‐wave simulations and experimental results demonstrate that the phase‐only metasurface designed by the method has 10 times higher efficiency than the phase‐amplitude‐modulated metasurface. This work opens a way to realize more complicated and high‐efficiency metasurface holography.

Wide-spatial scattering and arbitrary-angular tunability based on hybridized phase gradients from metasurface hologram algorithm

Light steering and wavefront shaping by metasurfaces typically relies on the elaborate phase arrangement from discrete plasmonic or dielectric resonators. A linear phase gradient profile could simply enable the beam bending (anomalous reflection/transmission) functionality. Comparing to the single beam steering performance, the applicable wide-scattering property is also highly in demand for more practical applications of full-spatial imaging and detection, etc. However, it is still challenging to come up with an efficient design route to manipulate the arbitrary wide-spatial scattering and complex phase gradient hybridization. Here, we propose a metasurface design route that could arbitrarily expand the visible-frequency light scattering into wide outgoing directions. An inverse-design metasurface based on hologram algorithm is created to form the complex phase gradient hybridization and realize wide-spatial scattering covering the outgoing angle range between ±60°. Such design method can also tailor to the arbitrary angular scattering functionality to outgoing angular direction at 15°–30°, 30°–45°and 45°–75°, respectively. Additionally, the proposed architecture operates at broadband visible wavelength rather than a single wavelength, and it also enjoys the robust optical scattering performance which is irrelevant to the incident angle variation. Overall, the presented design route is universal and can easily find applications for optical diffuser, light coupling/de-coupling, directional optical emitter, full-spatial imaging, light trapping and spectrum splitting, etc.

Metasurface holographic image projection based on mathematical properties of Fourier transform

Fourier transform, mapping the information in one domain to its reciprocal space, is of fundamental significance in real-time and parallel processing of massive data for sound and image manipulation. As a powerful platform of high-efficiency wave control, Huygens’ metasurface may offer to bridge the electromagnetic signal processing and analog Fourier transform at the hardware level and with remarkably improved performance. We here demonstrate a Huygens’ metasurface hologram, where the image pattern can be self-rotated or projected in free space by modulating the phase distribution based on the rotational invariance, time-shifting and scaling properties of Fourier transform. Our proof-of-concept experiment shows high-efficiency imaging operation in accordance with theoretical predictions, validating the proposed scheme as an ideal way to perform largely parallel spatial-domain mathematical operations in the analog domain using electromagnetic fields.

Real-time terahertz meta-cryptography using polarization-multiplexed graphene-based computer-generated holograms

Abstract As one of the cutting-edge technologies in advanced information science, wave-based cryptography is a prerequisite to enable a plethora of secure encrypting platforms which can be realized by smart multiplexing techniques together with suitable metasurface holograms (meta-holograms). Here, relying on the polarization multiplicity and re-writability of a computer-generated meta-hologram, a fully secure communication protocol is elaborately developed at the terahertz spectrum to host unique merits for exploring real-time metasurface-based cryptography (meta-cryptography) where highly restricted access of information is imposed. The proposed meta-cryptography exploits two dynamic near-field channels of a meta-hologram whose information can be instantaneously re-written without any polarization rotation and with high contrast and acceptable frequency bandwidth. The computer-generated meta-hologram is constructed based on the weighted Gerchberg–Saxton algorithm via a two-dimensional array of vertical graphene strips whose anisotropic reflection is merely determined by external biasing conditions. Several illustrative examples have been presented to demonstrate the perfect secrecy and polarization cross-talk of the proposed meta-cryptography. Numerical simulations corroborate well our theoretical predictions. As the first demonstration of dynamic THz meta-cryptography, the meta-hologram information channels can be deciphered into manifold customized messages which would be instrumental in data storage systems offering far higher data rates than electronic encryption can deliver.

Metasurface Array Illuminator Based on Fresnel Holography

A kind of array illuminators is proposed based on optical metasurface consisting of rectangular nanoholes etched in silver film. The information of the metasurface hologram comes from the binarized Fresnel hologram of the target pattern. The light spot arrays are generated through the diffraction of the metasurface hologram under the right-handed circularly polarized light illumination. The shape and layout of the light spots can be adjusted through changing the arrangement of rectangular holes. The propagation distance of our array illuminator is set at 10μm. The advantages of the metasurface array illuminator including ultrathin thickness, compact structure, higher resolution and feasible fabrication are favorable to promote the applications of the metasurface holography in optical interconnection and optical integration.

Polarization Multiplexing Terahertz Metasurfaces through Spatial Femtosecond Laser‐Shaping Fabrication

AbstractTerahertz (THz) metasurface has emerged as a powerful technique to fully modulate the wavefronts of THz radiation. To improve the information storage capability and enhance the encryption security of THz metasurface holograms, smart metasurface designs, and fabrications for polarization multiplexing are essential. In this study, two kinds of polarization multiplexing THz metasurfaces are fabricated through the slit‐based spatial modulation of femtosecond laser. Controllable elliptical aperture arrays can easily and flexibly be fabricated on gold films by adjusting the width and position of the slit combined with the laser pulse energy. Based on the Pancharatnam–Berry phase and amplitude modulation, respectively, THz metasurfaces, with two independent target phases or amplitude profiles, are obtained. A variety of information can be processed under different polarization states through these THz metasurfaces. This method allows flexible and efficient customization of THz metasurfaces because of the high‐speed fabrication and reproducibility as well as precise control of elliptical apertures location on the surface.

Design of a reconfigurable broadband greyscale multiplexed metasurface hologram.

Holograms are a promising state-of-the-art technique that are able to reproduce fully three-dimensional images. However, most elementary holograms only recover a nonadjustable image restricted by a certain amplitude and phase. Recently, the concept of the reconfigurable metasurface has come into sight. The reconfigurable metasurfaces can be tuned dynamically through a physics control signal from outside. Here we design and realize a series of novel, to the best of our knowledge, reconfigurable metasurfaces. Using these devices, an advanced metasurface hologram effect could be easier to achieve. Moreover, the reconfigurable property of such tunable metasurface enables independent control of advanced multiplexed channel functionalities to exist in comparison with the conventional static metasurface holograms. Our method creates unprecedented display and virtual effects, and it opens a novel way to a more flexible methodology of dynamic control of electromagnetic waves.

Low‐Profile Electromagnetic Holography by Using Coding Fabry–Perot Type Metasurface with In‐Plane Feeding

AbstractMetasurface holography has been well studied due to its features that are superior to the traditional holographic technologies. However, high profiles of the reflection‐ and transmission‐type configurations in the metasurface holography restrict its applications. On the other hand, digital coding metasurfaces have received great attention recently because they build a link between the information theory and surface electromagnetic fields, which also bring new viewpoint and convenience for digital computational holography. Here, a real low‐profile holographic method is proposed using a leaky‐type 2‐bit coding Fabry–Perot metasurface with in‐plane feeding in the microwave frequency. For constructing the metasurface hologram, the weighted Gerchberg–Saxton algorithm is presented to modulate the energy proportion of focal points by adjusting the phase distribution. As proof of concept, some typical metasurface holograms are demonstrated in microwave experiments at the height of 35 mm (≈3 wavelengths), which show good agreement with theoretical analyses and full‐wave simulations. The design method can be easily extended to the millimeter wave and terahertz bands.

Composite nanostructured design for dynamic control of metasurface holograms

Metasurface holograms that can be dynamically controlled have attracted wide attention and been realized in many ways. In particular, the phase change materials (PCMs) that can be switched into amorphous and crystalline states at high speed by external thermal stimulation are one of the most promising candidates to break the limitations of material properties and fixed structures. Here we propose a metasurface hologram that aims to realize a dual display that can reconstruct different images by annealing thermal stimulation. The designed metasurface incorporates alternated chalcogenide PCMs cubes and sector columns that possess uniform amplitude and full 2 π phase coverage. Simplicity and compactness make this metadevice able to be easily integrated into various applications such as memory storage, augmented reality, anti-counterfeiting, and so on.

Acoustic holography using composite metasurfaces

Acoustic holography has emerged as a revolutionary technique to reconstruct the desired wavefronts of sound fields. Traditional acoustic holography typically relies on arrays of discrete sound emitters independently driven by sophisticated circuits to generate the required phase and amplitude profile. Recently, we witnessed growing efforts to conquer the realm of acoustic metasurfaces as a possible platform to reconstruct computer-generated holograms due to their advantages in manipulating sound waves with a well-controlled phase and/or amplitude. In this article, we review the latest efforts to explore an acoustic metasurface hologram in transmission-type and reflection-type systems, where we discuss how artificial meta-pixels appear as highly useful ingredients to tailor the flow of sound and transform a conventional incident wave into a desired arbitrary-complex wavefront in the far field. We summarize the latest advancements from their design principles to versatile functional applications, and some future research directions are also provided.

Surface Susceptibility Synthesis of Metasurface Holograms for Creating Electromagnetic Illusions

A systematic approach is presented to exploit the rich field transformation capabilities of Electromagnetic (EM) metasurfaces for creating a variety of illusions using the concept of metasurface holograms. A system level approach to metasurface hologram synthesis is presented here, in which the hologram is co-designed with the desired object to be projected. A structured approach for the classification of the creation of EM illusions is proposed for better organization and tractability of the overall synthesis problem. The delineation is in terms of the initial incident (reference) illumination of the object to be recreated (front/back-lit), the position of illusion (posterior/anterior), and the illumination used to create the illusion (front/back). Therefore the classification is based on the specific relationship between the reference object to be recreated, the observer measuring the object, the orientation and placement of the reference and illumination field, and the desired placement of the metasurface hologram creating a virtual image. In the paper a general design procedure to synthesize metasurface holograms is presented based on Integral Equations (IE) and Generalized Sheet Transition Conditions (GSTCs), where the metasurface hologram is described as zero thickness sheet with tensorial surface susceptibility densities. Several selected configurations are chosen to illustrate various aspects of the hologram creation in 2D, along with a novel numerical technique to artificially reverse-propagate the scattered fields, required in the synthesis process. Finally, the impact of the metasurface size and the illumination field strength on the quality of the reconstructed scattered fields is also discussed.

Correction: Engineering spin and antiferromagnetic resonances to realize an efficient direction-multiplexed visible meta-hologram

Correction for ‘Engineering spin and antiferromagnetic resonances to realize an efficient direction-multiplexed visible meta-hologram’ by Muhammad Afnan Ansari et al., Nanoscale Horiz., 2020, 5, 57–64.