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The digital and programmable metasurfaces, as opposed to conventional metasurfaces, offer a more sophisticated method of collaborating information and physics, showcasing several real-time controls to electromagnetic (EM) ways in succinct ways. In this work, we propose a multi-frequency amplitude-programmable (MFAP) metasurface with multiple frequency channels to enhance the presentation and manipulation of EM data. With this metasurface, the reflected amplitudes can be simultaneously and independently encoded between high (digit “1”) and low (digit “0”) levels. The amplitude code is unique, which exhibits both reflection coefficients and radiation patterns to allow for flexible multi-functional EM operations with frequency. For instance, the MFAP metasurface can be used to design innovative communication systems by transmitting various EM signals individually across the channels in time domain. It is also possible to carry out multi-bit transmissions by mixing these frequency channels. By introducing complex coding patterns in space domain, it is possible to manipulate EM powers with greater precision. A square-split ring meta-atom that can achieve stable single-frequency amplitude control and multi-frequency 1 bit amplitude-programmable features is described as a proof-of-concept. Varactors loaded on metallic structures of various sizes are switched between operating states to modify the amplitude codes at each frequency channel. The suggested MFAP metasurface’s validity is confirmed by simulations and measurements from a dual-channel MFAP metasurface prototype.

Full-space metasurfaces demonstrate powerful abilities in manipulating electromagnetic (EM) waves, but most of them are usually single band. Here, a multiband bidirectional digital coding metasurface is proposed for multiple tasks, which can achieve completely different functions in up to six frequency bands when the EM waves are incident from the front and back of the metasurface. As a proof-of-concept, we design and fabricate a dual-band full-space metasurface with integrated functions of reflection, transmission, holographic imaging, and vortex-beam forming, and a six-band full-space metasurface with completely independent holographic imaging functions at different frequency bands. Simulated and measured results are in good agreements with the theoretical predictions, verifying the good performance of the proposed multitask bidirectional digital coding metasurface.

The integration of different functionalities into a real‐time reconfigurable metasurface can give it more powerful ability in manipulation of electromagnetic (EM) waves. Here, a reflection–transmission (R‐T) reconfigurable digital coding metasurface, which can be flexibly switched between the reflection and transmission working states with the completely different functions, is proposed and demonstrated. As a proof of concept, an R‐T reconfigurable metasurface with integrated functions of EM diffusion in reflection state and encryption holography in transmission state is designed and demonstrated. The results have been validated by both numerical simulations and experimental measurements, showing that the proposed R‐T reconfigurable metasurface can be real‐timely controlled to work in reflection state or transmission state by adjusting bias voltage, in which the incident EM waves are totally reflected and randomly diffused to achieve low radar cross section (RCS) in reflection state and efficiently transmitted to realize encrypted holographic images in transmission state. [ABSTRACT FROM AUTHOR]

Over the past decades, metasurfaces have experienced rapid development controlling the propagation of electromagnetic waves. Seamlessly combining metasurfaces with semiconductor devices enhance the device's performance and expand functional diversity. Currently, vertical‐cavity surface‐emitting lasers integrated with metasurfaces (MS‐VCSELs) employed to generate various wavefronts. However, the existing MS‐VCSELs that use only phase modulation still face limitations and challenges in generating customized fields, especially in 3D space. Here, amplitude‐phase metasurfaces are proposed and joint amplitude‐phase modulations are employed to increase the degree of freedom in controlling laser fields. By using electron beam lithography (EBL) and inductively coupled plasma reaction ion etching (ICP‐RIE), metasurfaces are directly fabricated on the back‐emitting surface of VCSELs, enabling samples to realize diverse functions such as multi‐focus multi‐plane generation and intensity control, multi‐plane holographic images, and laser beam deflections with different intensities. Compared to traditional discrete metasurface configurations, the proposed scheme achieves on‐chip integration of amplitude‐phase metasurface with the light source, enhancing the multidimensional laser field modulation capabilities of MS‐VCSELs in the 3D space while further promoting the cross‐integration of metasurfaces with optoelectronic devices. It is believed that MS‐VCSELs with amplitude‐phase modulations hold significant promise for applications in 3D imaging, facial recognition, and laser communications. [ABSTRACT FROM AUTHOR]

Recently, programmable metamaterials or metasurfaces have been developed to dynamically edit electromagnetic waves for realizing different functions in the same platform. However, the proposed programmable metasurfaces can only control reflected or transmitted wavefronts in half‐space. Here, a "Janus" digital coding metasurface with the capabilities to program various electromagnetic functions in the reflected (with R‐codes) and transmitted (with T‐codes) waves simultaneously is presented. Three PIN diodes are employed to design the metaparticle, and the state of the PIN diodes can be switched to change the reflected and transmitted phases independently. Three schemes achieved by the proposed programmable metasurface are provided as illustrative examples, including anomalous deflections, beam focusing, and scattering reduction in the full space. As a proof‐of‐concept, a prototype composed of 10 × 20 metaparticles is fabricated and the measured results are in good agreement with the designs and numerical results, validating the full‐space modulations enabled by the programmable metasurface. It is expected that the new programmable metasurface can broaden the applications in stealth technologies, imaging systems, and the next generation of wireless communications. [ABSTRACT FROM AUTHOR]

In recent years, metasurfaces have not only provided new mechanisms to effectively control the electromagnetic waves but also maintained lower profiles and easy fabrication. Based on metasurfaces, many novel devices and applications have been reported and realized successively. Here, a dual‐band metasurface operating at microwave frequencies to achieve independent manipulations of reflected and transmitted waves in two half‐spaces at two frequencies is proposed. The designed C‐shaped metaparticle consists of two C‐shaped patterns and a C‐shaped slot on three layers, which can modulate both amplitude and phase responses of the reflected and transmitted waves simultaneously by altering the opening and orientation angles at 7 and 17 GHz. The transmission mode works at 7 GHz, in which multifocal points can be formed on focal planes with different distances and intensities using the amplitude and phase modulations. While the reflection mode works at 17 GHz, the reflected beams can be generated and controlled with different intensities in far‐field. All experimental results have good agreements with numerical simulations. This proposed metasurface paves the way for attaining multifunctional metadevices with amplitude and phase modulations in the full space, which have potential applications in imaging and communication systems. [ABSTRACT FROM AUTHOR]

Digital coding metasurface, which provides a new approach to link the physical world and information science, has been quickly developed in recent years. However, all previously reported metasurfaces cannot achieve independent controls of different polarizations in both transmission and reflection spaces at the same time. In this work, a reconfigurable anisotropic digital coding metasurface loaded with electronically controlled PIN diodes is proposed that can independently manipulate not only the near/far‐field pattern but also the transmission and reflection modes of the electromagnetic waves under different polarizations. As a validation example, a multifunctional holographic imaging metasurface is designed, fabricated, and measured. Both simulated and measured results show that orthogonally polarized waves (vertical and horizontal polarizations) can be manipulated to achieve different images, and the transmission and reflection modes of the differently‐polarized images can be independently controlled in real time by changing the state of the loaded PIN diodes. [ABSTRACT FROM AUTHOR]

A class of anisotropic, transmissive electromagnetic metasurfaces is presented, which enable independent and simultaneous control of copolarized phase and amplitude responses to two linear, orthogonal polarizations. By varying the geometrical parameters, the transmission response of the proposed structure can yield a full phase coverage, accompanied by broadly adjustable amplitude and negligible cross‐polarized components. The full amplitude‐phase control together with the novel anisotropic character allows efficient implementation of complicated field manipulations. As representative application examples, which cannot be realized via conventional (phase‐only) metasurfaces, it is presented here: (1) the radiation of multiple equal‐power vortex beams (along arbitrarily predesigned directions, with designable orbital angular momentum modes under different polarizations), and (2) the realization of polarization‐reconfigurable multifocal metalenses. Full‐wave numerical simulations and experimental results demonstrate good agreement and confirm the versatility and effectiveness of the proposed approach to design advanced field‐manipulation systems. [ABSTRACT FROM AUTHOR]

Metasurfaces show great potential due to their powerful ability to manipulate electromagnetic waves as desired, which has been widely investigated in the microwave, terahertz, infrared, and visible regimes. Here, it is proposed to control near‐field distributions through use of a metasurface with both amplitude and phase modulations. A C‐shaped particle is designed to provide stable and continuous amplitude and phase profiles independently of transmitted waves by varying its opening and orientation angles. Benefiting from both amplitude and phase modulations on the metasurface, differentiation and integration on electric‐field distributions can be achieved by changing the arrangements of complex coefficients on the metasurface. Besides, the differential operation can be utilized to show the edges of predesigned electric‐field distributions. It is believed that the proposed method will accelerate the pace of metasurfaces towards many applications by engineering and operating the near‐field distributions. [ABSTRACT FROM AUTHOR]

Owing to the capability of providing a certain phase gradient on the interface between two media, metasurfaces have shown great promise for altering the directions of outgoing electromagnetic (EM) waves arbitrarily. With the suitable arrangement of particles on metasurfaces, anomalous reflection and refraction have been observed in wide frequency ranges. To completely control the propagation of EM waves, both phase and amplitude profiles are required in some applications. Herein, we propose a new type of metasurface with both phase and amplitude modulations, which is composed of C-shaped particles and can generate and control multiple beams using amplitude and phase responses simultaneously. An addition theorem of complex reflection coefficients is presented to acquire various states of multiple beams reflected from designed metasurfaces. Meanwhile, the intensities of multiple beams can be separately modulated as desired benefitting from the independent controls of phase and amplitude profiles. All the experimental results have good agreements with the numerical simulations. The presented method opens a new way to form and manipulate multiple beams using metasurfaces, which can find potential applications in beam shaping, radar detection systems, and high-quality holography. [ABSTRACT FROM AUTHOR]

Antennas and metasurfaces have enabled a number of far-field manipulation functions. According to the addition theorem of multipoles, near fields have richer spatial spectra than far fields; hence, in the near-field region, it is easier to achieve complex manipulations on field distributions. In this paper, an algorithm is presented to synthesize the near fields of source arrays. Given target distributions of near-field intensities and also predefined source magnitudes, the algorithm will find the needed source phases, which can then be applied on active antenna arrays and passive metasurfaces to induce the target distribution of fields. The algorithm adopts the dyadic Green’s function as the propagator, and hence, it naturally takes account of the near-field and vector properties of electromagnetic fields. As a typical application of the algorithm, an example is given to obtain the excitation phases of a dipole array, which is then physically imitated by a coding metasurface. Experimental measurement is performed and the result proves the validity of the algorithm. From the perspective of information metasurface, the algorithm finds the phase-coding pattern of metasurface to achieve specific functions. [ABSTRACT FROM AUTHOR]

Digital coding metasurfaces have attracted attention due to their advantages compared to metamaterials. Many devices have been proposed by encoding the phase responses on metasurfaces such as orbit angular momentum generation, prefect absorbers, and holography. For complete manipulation for propagation of electromagnetic waves, it would be beneficial to control both phase and amplitude responses. Here, we propose a metasurface with independent control of phase and amplitude profiles, which is composed of four unit cells, and the amplitude responses of all unit cells range from 0.3 to 0.7. The four units have phase responses of 0, π/2, π, and 3π/2 separately to mimic the “00,” “01,” “10,” and “11” digital elements. The direction of the reflected beam from the metasurface can be manipulated by different sequences of digital elements, and meanwhile, the intensity of the reflected beam can be modulated through changing different amplitude responses in the same direction. We show that the distributions of both phase and amplitude responses of the metasurface will contribute to scattering reduction. We have used indium tin oxide to design the patterns of four units. The experiments and simulations confirm the physical phenomena mentioned above. The separate control of phase and amplitude responses suggests potential applications in high quality holography and mathematical operations of metasurfaces. [ABSTRACT FROM AUTHOR]

Abstract: Coding representation of metamaterials builds up a bridge between the physical world and the digital world, making it possible to manipulate electromagnetic (EM) waves by digital coding sequences and reach field‐programmable metamaterials. Here, the coding space is extended to complex domain and proposed complex digital codes to provide closer essence of EM‐wave propagation. Based on the analytic geometry and complex variable functions, an addition theorem on complex coding is established, which reveals inherent connections among digital codes with different bits and enables all higher‐bit digital codes to be represented by the 1‐bit complex codes. According to the complex coding and addition theorem, multifunctional metamaterials can be directly designed and realized without considering mutual coupling. When two different coding patterns with different functions are added together via the addition theorem in complex form, the combined coding pattern will directly generate the two functions simultaneously without any perturbations. A series of realistic coding metasurfaces is presented to demonstrate the powerful and flexible performance of the complex coding and addition theorem for independent controls of EM waves to reach multiple functions. Good agreements between numerical simulations and experimental results prove the feasibility of the proposed concept and theorem in practical applications. [ABSTRACT FROM AUTHOR]

We propose a novel linearly polarized, dual-band, and high-gain transmitarray based on multilayer frequency-selective surface structures. This is the first time for actualizing a dual-band transmitarray with more than 50% aperture efficiency. The unit cell is composed of four metallic layers without dielectric substrates between every two layers, which results in good match, low insertion loss, and high radiation efficiency. We introduce two kinds of rectangular slots in the design to control the magnitude and phase range of transmission coefficients in the two designed frequency bands through changing the slot length. The isolation between two bands is excellent and the interference can be ignored. The induced electric fields of the cell are also investigated in order to elucidate the working principle intuitionally. The proposed dual-band transmitarray with a square aperture ( $240 \times 240$ mm ^{2} ) is designed, fabricated, and measured. A linearly polarized corrugated horn antenna is adopted to improve the aperture efficiency and ensure the similarity of the radiation patterns on E-plane and H-plane. Full-wave simulation and experimental results have very good agreements. It is demonstrated that the dual-band transmitarray works in two frequency bands 11.8–12.2 and 17.5–18.1 GHz simultaneously, and the peak gains in the designed central frequencies 12 and 18 GHz are 27.8 and 31.4 dB, respectively, which correspond to the aperture efficiencies (global efficiencies) of 52% and 53%. The proposed design has advantages of simple structure, small weight, low cost, and high performance, making it possible for real applications. [ABSTRACT FROM AUTHOR]

A band-notched UWB antenna is presented, which can switch between two notch bands and tune the central frequency simultaneously. It is the first time that the switchable and tunable behaviours are combined together in band-notched UWB antennas. In the band-notched structure, PIN diodes are used to switch the lower and upper frequency bands, while varactors could vary the central frequency of each notch band continuously. Measurement results show that the notch bands could switch between 4.2 GHz and 5.8 GHz when the state of varactors is fixed, and the ranges of tuning are 4.2–4.8 GHz and 5.8–6.5 GHz when the state of PIN diodes is ON and OFF, respectively. [ABSTRACT FROM AUTHOR]

Introducing certain surface susceptibilities on the interface of boundaries is a well‐known strategy to steer wave behaviors in different physical domains. However, most designs are thus far limited to engineer specific functionalities in a given physical application scenario. Herein, the approach manipulating electromagnetic (EM) and acoustic waves simultaneously on a shared digital coding metallic metasurface is described. The physical mechanism to realize the multiphysics digital coding is the equivalent governing equations behind wave behaviors as well as the boundary conditions with an identical mathematic model. Using split‐ring resonators, 3‐bit multiphysics digital coding elements are created, from which flexible wave‐control devices are realized by simply designing coding sequences. As examples, multiple‐beam generation, scattering reduction, beam steering, and beam forming are numerically demonstrated for both EM and acoustic fields. The anomalous reflection phenomena based on 3‐bit multiphysics coding elements are experimentally performed to validate the design concept. The design strategy provides possibilities to manipulate multiwave behaviors in the shared metasurface, with a wide range of potential applications in the future. [ABSTRACT FROM AUTHOR]

Digital coding metasurfaces have been introduced to describe the properties of metasurfaces digitally and provide a new approach to link the physical world and information science. However, most of the current digital metasurfaces are focused on phase code, limiting their functions and applications. Here, the concept of reflection–transmission (R–T) amplitude code is established, and novel digital metasurfaces for full‐space electromagnetic (EM) manipulations are proposed, in which the digital states of the R–T amplitude code "0" and "1" represent the total reflection and transmission, respectively. The introduction of the R–T amplitude code enables a full‐space communication system and larger information capacity by associating with phase codes. For this purpose, first a novel polarization‐dependent R–T amplitude code is proposed, which is distinguished by the polarization of the incidence. Further positive–intrinsic–negative (PIN) diodes are adopted to realize dynamic R–T amplitude code, resulting in real‐time controls of far‐field radiation characteristics. Good agreements between simulations and experiments demonstrate the powerful ability to manipulate the EM waves by combining the R–T amplitude and phase codes, indicating great potentials in new digital metasurface‐based information, radar, and imaging systems. [ABSTRACT FROM AUTHOR]

In recent years, an intense research effort has focused on the flexible manipulation of electromagnetic (EM) waves using metamaterials. As an important category, gradient metamaterials (metamaterials with gradient phases) can impart subwavelength‐scale space‐variant phase changes during the electromagnetic wave propagations to shape EM radiations in which arrays of elements with certain phase gradients can form EM beam propagations as desired. Here, the concept of space‐frequency‐domain gradient metamaterial is proposed, in which new degrees of freedom are attained by introducing frequency‐domain phase gradients to the metamaterial. The frequency gradients of the metamaterial are obtained by the derivative of phase gradients with respect to frequency, which can be used to shape the uniformly distributed phase gradients to vary as demand in the working frequency band without any active devices. The combinations of space and frequency gradients in metamaterials in 1D/2D open new possibilities for more powerful controls of EM waves, thereby yielding more complicated functions such as 2D flexible beam scanning, continuous transformations of vortex modes, and generation of multiple beams with desired transmission directions and transmission modes. In this work, a new concept of space‐frequency‐domain gradient metamaterials is proposed in which the space gradients of the metamaterial determine the electromagnetic responses at the initial frequency, while the frequency gradients generate frequency‐variant uniform phase patterns in the working band, creating intriguing functions such as flexible beam scanning, continuous mode transformation, and time‐domain multiple beam patterns. [ABSTRACT FROM AUTHOR]

Digital coding metasurface is aimed at building up a bridge between physics and information science. Higher information capacity of the digital coding metasurface means more powerful ability to control electromagnetic waves. Here, a multiband digital coding metasurface to improve the information capacity is proposed. The digital coding structures can provide 2‐bit digital states at three separate frequency bands (C, X, and Ku). It is shown that the proposed metasurface can eliminate the in‐band interference and path degradation by introducing an operator of frequency‐hopping spread spectra, from which flexible beam controls can be designed independently in every operating band. To demonstrate the capability and the compatibility, a multifunctional digital coding metasurface which can perform optical illusion, scattering reduction, and generation of orbital angular momentum in a shared aperture is presented. Numerical simulations and measured results have very good agreements, verifying excellent performance of the multiband digital coding metasurface. The proposed method opens opportunities to improve the information capacity of the digital coding metasurface and paves novel ways to multitasking systems on photonic applications. A multitasking system is engineered by implementing a multiband digital coding metasurface in a shared aperture. To demonstrate the capability and compatibility, a multifunctional digital coding metasurface which performs optical illusion, scattering reduction, and generation of orbital angular momentum is proposed and verified. The proposed method paves the way for multitasking systems on microwave and photonic applications in the future. [ABSTRACT FROM AUTHOR]

The development of chiral metasurfaces with spin-selective reflection or transmission provides a new way to control the circularly polarized (CP) waves. However, it is still a great challenge to independently manipulate the polarization, frequency, and phase of the spin-selective reflected waves in different operating bands, which may have potential applications in improving the data capacity of microwave and optical communication systems. Here, a dual-band chiral metasurface is proposed to generate gigantic intrinsic chirality with strong circular dichroism (CD) in two different frequency bands by piecing two typical mono-chiral units together. The polarization, frequency and phase of the spin-selective reflected waves can also be independently designed in the two operating bands by adjusting the configuration of the chiral unit structures. Based on the proposed chiral structures, a dual-band chiral metasurface with spin-selective anomalous reflections is designed and demonstrated by both simulations and experiments. The results show that the polarization of spin-selective reflected waves can be customized by selecting appreciate chiral structures, while the wavefront of the spin-selective reflected waves can be further controlled by designing their arrangement.