Holographic Surface Research Articles

Advanced directional beam control via holographic acoustic metasurfaces for multibeam scanning

Acoustic multi-beam leaky-wave antennas, designed with holographic metasurfaces, have great potential for beamforming by achieving directional beam control. We propose a planar holographic metasurface for beam steering using multi-beam acoustic radiation serving as high-gain acoustic leaky wave antennas. Based on the principle of holography, we designed a metasurface that adjusts its admittance in a sinusoidal pattern. This surface consists of periodic cylindrical holes with varying depth profiles. A monopole point source is positioned in the middle of the patterned surface, which generates surface waves on the modulated surface and emits acoustic leaky-wave radiation in the desired directions. The holographic admittance surface is designed to radiate an acoustic beam at a frequency of 20 kHz. Additionally, the concept was extended to generate multi-beam scanning for forward leaky-wave radiation in the holographic process. In this research, we employed 3D additive manufacturing to create acoustic holograms, manipulating surface admittance variation at a resolution smaller than the wavelength. The experimental measurements aligned well with both the theoretical and numerical analysis findings, affirming the effectiveness of the proposed technique. The methodology exhibits broad applicability across various domains, including sonar, ultrasound imaging, waveform manipulation, and acoustic communication.

Highly efficient meta-lens integrated holographic surface antenna

We demonstrate the generation of highly directive circularly polarized beams by proposing a meta-lens to calibrate the near fields over the holographic surface. Such a design possesses the merits of ultra-low profile as a lens system on the basis of phase compensation of a specific radiating aperture rather than the conventional consideration of the lens focus, and the ratio between the overall height of the antenna architecture and diameter of radiating aperture is only 0.09. Especially, the proposed meta-lens integrated holographic surface also offers a satisfactory solution to improve the overall efficiency of holographic antennas, and the experimental results verify the proposed design with 27.6 dBic measured gain and 40.4 % aperture efficiency at 15 GHz. We expect the proposed strategy of integrating phase compensation lens closely over the holographic surface, pave the way for highly efficient meta-radiators with ultra-low profile.

Half-Space Sound Field Reconstruction Based on the Combination of the Helmholtz Equation Least-Squares Method and Equivalent Source Method.

In practical conditions, near-field acoustic holography (NAH) requires the measurement environment to be a free sound field. If vibrating objects are located above the reflective ground, the sound field becomes non-free in the presence of a reflecting surface, and conventional NAH may not identify the sound source. In this work, two types of half-space NAH techniques based on the Helmholtz equation least-squares (HELS) method are developed to reconstruct the sound field above a reflecting plane. The techniques are devised by introducing the concept of equivalent source in HELS-method-based NAH. Two equivalent sources are tested. In one technique, spherical waves are used as the equivalent source, and the sound reflected from the reflecting surface is regarded as a linear superposition of orthogonal spherical wave functions of different orders located below the reflecting surface. In the other technique, some monopoles are considered equivalent sources, and the reflected sound is considered a series of sounds generated by simple sources distributed under the reflecting surface. The sound field is reconstructed by matching the pressure measured on the holographic surface with the orthogonal spherical wave source in the vibrating object and replacing the reflected sound with an equivalent source. Therefore, neither technique is related to the surface impedance of the reflected plane. Compared with the HELS method, both methods show higher reconstruction accuracy for a half-space sound field and are expected to broaden the application range of HELS-method-based NAH techniques.

Holographic surface relief diffraction gratings made of hydrogels for direct label-free biosensing of IgGs

This work describes the development of a label-free (LF) biosensing platform for the direct detection of targets based on diffractive structures fabricated with acrylamide-based hydrogels biofunctionalized with proteins and antibodies. Hydrogels containing Bovine Serum Albumin protein (BSA) with different crosslinking degrees were synthesized and characterized to find the optimal conditions for the suitable fabrication of surface relief gratings (SRGs). The bioavailability of BSA-functionalized hydrogels for the specific recognition of anti-BSA antibodies was verified by fluorescence detection. After the hydrogel-based SRG fabrication, diffraction efficiency measures at two different laser wavelengths were used for the direct LF detection of anti-BSA antibodies. The limit of detection in the sub mg L−1 range was read. Additionally, SRGs were prepared with hydrogels biofunctionalized with anti-rabbit antibodies for the direct detection of IgGs from rabbit serum, obtaining similar analytical performance without the necessity of labeling or applying amplification strategies.

Reconfigurable Holographic Surface-Assisted Wireless Secrecy Communication System

This new antenna, called the reconfigurable holographic surface (RHS), is lightweight and compact, and it can precisely steer many beams at once. Because of its reflecting characteristic, it differs from the reconfigurable intelligent surface (RIS), which is frequently employed as a passive relay. To leverage the holographic technology and generate the necessary beam, RHS is most likely to be integrated with the transceiver as an ultra-thin and lightweight planer antenna. This has enormous potential to satisfy the growing demands of the future generation network. This paper is the first to study a wireless secrecy communication system with a base station that has and is helped by an RHS. We suggest a strategy for simultaneously optimizing the holographic beamforming at the RHS and the digital beamforming at the base station with the introduction of artificial noise (AN) to attain the highest secrecy rate. However, because of its non-convexity and changeable coupling, this problem is challenging to solve. A proficient algorithm that utilizes alternating optimization and is capable of solving the problem below the ideal level is suggested. According to simulation studies, RHS outperforms RIS in terms of enhancing the performance of wireless secrecy communication systems, indicating that RHS has a wide range of potential applications in the realm of physical layer security.

A scalar–tensor impedance metasurface for frequency‐multiplexed multifunctional orbital angular momentum beams generation

AbstractA scalar‐tensor composite holographic impedance meta surface (HIMS) for frequency‐multiplexed is proposed. The scalar and tensor parts operate at low frequency and high frequency, respectively, the coupling between them can be over come, and then more multifunction orbital angular momentum (OAM) beams with good performance can be obtained. As an example, a scalar‐tensor HIMS is designed for three OAM beams: (1) Beam‐I at fL = 11.2 GHz, which is generated by scalar part. (2) At fH = 18.2 GHz, Beam‐II and Beam‐III, which are generated by tensor part. The proposed scalar‐tensor HIMS has the following advantages: (i) More flexible multifunction design due to the decoupling of scalar and tensor parts. (ii) High aperture efficiency (14.93% at 11.2 GHz, much higher than conventional scalar HIMS, 23.3% at 18.2 GHz). The HIMS can be used in future high‐capacity secure communication, imaging, and so on.

High-efficiency prediction method for helicopter global/ground noise based on near-field acoustic holography

Noise reduction program design is an effective approach that relies on efficient noise prediction for reducing ground noise during flight. The existing noise prediction methods have the limitations of being computationally expensive or only applicable to far-fields. In this paper, a High-Efficiency Prediction Method (HEPM) for helicopter global/ground noise based on near-field acoustic holography is proposed. The HEPM can predict the global noise based on acoustic modal analysis and has the advantages of high prediction accuracy and low time cost. The process is given as follows: firstly, the rotor noise on the holographic surface in the specified flight is obtained by simulations or experiments. Secondly, the global noise model, which maps time-domain noise to acoustic modes, is established based on near-field acoustic holography and Fourier acoustic analysis methods. Finally, combined with acoustic modal amplitude, the model established enables efficiently predicting the global/ground noise in the corresponding flight state. To verify the accuracy of the prediction method, a simulation study is conducted in hovering and forward flight states using a model helicopter with a 2-meter rotor and Rotor Body Interaction (ROBIN) fuselage. The comparison of HEPM with numerical results shows that the average prediction errors of the global and ground noise are less than 0.3 dB and 0.2 dB, respectively. For a region containing 100000 observers, the computation time of the HEPM is only one-fifth of that of the acoustic hemisphere method, demonstrating the rapidity of the proposed method.

Approach to active control of scattered sound field from underwater targets.

Active control of scattered sound fields is of great significance for the acoustic stealth of underwater targets. In this paper, we propose an approach to control the target scattered field based on the measurement of a single holographic surface. Compared to existing methods, our approach significantly reduces the required number of hydrophones and only relies on the incident direction as prior information. First, we introduce a sound field separation method that uses the measurement of a single holographic surface to extract scattered field near the scatterer. Then two control strategies are presented to reduce redundant sound power outside the selected direction radiated by secondary sources in different situations. Finally, the proposed method is verified by the simulation based on finite element method and the experiment conducted in an anechoic tank. Experimental results in the tank show that the scattered sound pressure level in far-field is reduced by at least 10 dB at 2 kHz after activating the real-time control system.

Method of acoustic separation on the surface of cylindrical baffle

This paper suggests a statistically optimal near-field acoustic holography method based on the combination of plane waves and cylindrical waves, which is used for underwater cylindrical baffles. The method addresses the issue that the extraction of scattered acoustic waves from the surface of underwater cylindrical baffles requires too many measurement points in practical applications: separation of incident and dispersed sound fields and near-field measurements of sound fields at baffle surfaces.The underwater detection sound source is situated in the scatterer's far-field region, so in the actual application scenario for the surface acoustic scattering separation of the cylindrical baffle, the plane wave expansion is used to represent the surface incident acoustic wave and the cylindrical wave expansion is used to describe the scattered acoustic wave on the baffle surface. As a result, fewer expansion orders are required to describe the entire sound field on the baffle's surface. It is verified by simulation that the method is valid and efficient for near-field total sound field measurement and dispersed sound field separation of roughly cylindrical targets under underwater far-field incidence situations because the measurement holographic surface of the surface permits unlimited geometries.

Digital holographic 3D surface topography measurement based on recording-plane rotation

In the present study, a three-dimensional (3D) surface topography measurement method based on digital holography is proposed that changes the observation vector by rotating a charge-coupled device(CCD) . Two digital holograms with different observation vectors were recorded by rotating the CCD at a small angle. In this paper, the 3D surface topographies of an irregular spheres with a radius of 16 mm and an surface relief denture with a dimensional of 10 mm ×10 mm were obtained. The results matches well with the real value of the hemisphere, which verifies the feasibility of the proposed method. At the same time, the surface topography measurement of the denture crown reflects its contour information well and can be used to guide dentists in the process of restorative teeth and denture making.

Reconfigurable Holographic Surface Aided Collaborative Wireless SLAM Using Federated Learning for Autonomous Driving

Simultaneous Localization and Mapping (SLAM) utilizing millimeter-wave (mmWave) radars is widely recognized as an essential component for autonomous driving applications. In this paper, we present a Reconfigurable Holographic Surface (RHS)-aided SLAM system, incorporating federated learning. The hardware cost of autonomous driving systems can be significantly reduced by replacing the expensive phased array antennas, traditionally used in mmWave radars, with the low-cost RHS metasurface antenna. Furthermore, multiple vehicles can collaborate through the federated learning framework, obtaining additional sensed data to enhance SLAM performance. However, the distinctive radiation structure of the RHS and the information exchange within the federated learning framework introduce complexities to the overall SLAM system design. To address these challenges, we propose a multi-vehicle SLAM protocol that regulates RHS-based radar sensing and data processing across multiple vehicles. Additionally, we design algorithms for RHS radiation optimization and federated learning-based localization and mapping. Simulation results demonstrate the efficacy of the proposed approach when compared to existing phased array-based and non-cooperative schemes.

Conical near-field acoustic holography based on cylindrical wave function of variable radius and nonconformal plane measurement

In practical acoustical measurement for large cones, maintaining coaxial and conformal between the holographic surface and reconstructed surface is usually tedious. To overcome this flaw, a conical near-field acoustic holography (NAH) based on cylindrical wave function of variable radius and nonconformal plane measurement is proposed in this paper. First, cylindrical wave function of variable radius (VR)–based modified statistically optimal cylindrical NAH (SOCNAH) method, entitled VR-SOCNAH, is proposed, which is more applicable to conical surfaces. Second, with the measured sound pressure data on the holographic plane, sound pressure on the conical conformal surface is first reconstructed using statistically optimal planar NAH (SOPNAH), and sound pressure on the reconstructed surface is then reconstructed using VR-SOCNAH. Third, fixed and variable radius measurement methods are comparatively studied, and the former is adopted. Furthermore, a measurement parameter optimization method based on orthogonal experiment is proposed to determine appropriate value ranges. Finally, the proposed NAH is applied to a test bed, and the robustness and accuracy of sound field reconstruction for large cones are significantly enhanced, which provides reliable scientific basis and engineering guidance for noise monitoring and control of mechanical equipment.

Reconfigurable Holographic Surface: A New Paradigm to Implement Holographic Radio

Ultramassive multiple-input, multiple-output (MIMO) is one of the key enablers in the forthcoming 6G networks to provide high-speed data services by exploiting spatial diversity. In this article, we consider a new paradigm, termed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">holographic radio</i> for ultramassive MIMO, where numerous tiny and inexpensive antenna elements are integrated to realize high directive gain with low hardware cost. We propose a practical way to enable holographic radio by a novel metasurface-based antenna, i.e., reconfigurable holographic surface (RHS). Specifically, RHSs incorporating densely packed tunable metamaterial elements are capable of holographic beamforming. Based on the working principle and hardware design of RHSs, we conducted full-wave analyses of RHSs and built an RHS-aided point-to-point communication platform supporting real-time data transmission. Both simulated and experimental results show that the RHS has great potential to achieve high directive gain with a limited size, thereby substantiating the feasibility of RHS-enabled holographic radio. Moreover, future research directions for RHS-enabled holographic radio are also discussed.

Programmable Jigsaw Puzzles of Soft Materials Enabled by Pixelated Holographic Surface Reliefs.

Manual intervention in the self-organization of soft matter to obtain a desired superstructure is a complex but significant project. Specifically, optical components made fully or partially from reconfigurable and stimuli-responsive soft materials, referred to as soft photonics, are poised to form versatile platforms in various areas; however, a limited scale, narrow spectral adaptability, and poor stability are still formidable challenges. Herein, a facile way is developed to program the optical jigsaw puzzle of nematic liquid crystals via pixelated holographic surface reliefs, leading to an era of manufacturing for programmable soft materials with tailored functions. Multiscale jigsaw puzzles are established and endowed with unprecedented stability and durability, further sketching a prospective framework toward customized adaptive photonic architectures. This work demonstrates a reliable and efficient approach for directly assembling soft matter, unlocking the long-sought full potential of stimuli-responsive soft systems, and providing opportunities to inspire the next generation of soft photonics and relevant areas.

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

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

Near-Field Localization for Holographic RIS Assisted mmWave Systems

In this letter, we envision the integration of holographic reconfigurable intelligent surface (HRIS) into a millimeter-wave (mmWave) localization system. The large aperture of HRIS in the mmWave band allows the system to operate in the near-field propagation regime, where the wavefront tends to be spherical and contains distance information. To investigate the fundamental limits of localization accuracy of the proposed system, we obtain the Fisher information matrix (FIM) and Cramér-Rao lower bound (CRLB) taking into account the radiation patterns of antennas. The analytical results reveal that the FIM quadratically increases with the size of HRIS. Then, to explore the potential of HRIS passive beamforming for improving localization quality, we formulate the HRIS phases optimization problem to minimize a worst-case CRLB with respect to an uncertainty set of user equipment (UE) positions. To address this formulated problem, an iterative entropy regularization (IER) based method is proposed. Numerical results evaluate the effect of key parameters on the squared position error bound and verify the effectiveness of the proposed IER-based method.

Holographic Radar: Target Detection Enabled by Reconfigurable Holographic Surfaces

Radars are a widely used technology for target detection, commonly enabled by phased arrays. However, complex hardware circuits such as phase shifters in phased arrays lead to relatively high power and cost. To tackle this issue, the reconfigurable holographic surface (RHS), a novel metamaterial antenna with simple hardware and compact element spacing, has been proposed. In this letter, we propose the holographic radar enabled by RHSs for target detection to achieve high accuracy with low power and cost. Due to the amplitude-controlled structure of the RHS which generates beams by tuning radiation amplitudes of its elements, conventional phase-controlled beamforming schemes for phased array radars do not work anymore. Therefore, we design an RHS amplitude-controlled beamforming scheme to optimize the detection accuracy. Theoretical analysis shows the holographic radar consumes less power than phased array radars with the same detection accuracy and cost, which is also verified by simulation results.

Direct evidence of the non-uniform electrodeposition of zinc at the early stage

Aqueous zinc-ion batteries (ZIBs) are regarded as one of the most promising electrical energy-storage systems for their eco-friendliness, low cost and high-safety. Although there have been a lot of studies recently on the Zn metal anode, little attention has been paid to its initial nucleation and growth behavior, which is critical to the understanding of fundamental mechanisms of the Zn plating. Herein, the digital holographic surface imaging (DHSI) method, combined synchronously with electrochemical tests, is employed to in-situ observe the non-uniform concentration changes on the surface of the copper substrate caused by the Zn nucleation and growth at the early stage. It is the first time to observe the early uneven depositions from a perspective of solution concentration changes in a symmetrical battery system. Results show that the later dendritic growth of Zn is closely related to the nucleation and earlier deposition process. Based on the phase maps obtained with in-situ DHSI technique, the nucleation and growth during the Zn electrodeposition can be closely investigated and vividly demonstrated, offering a promising avenue towards the understanding of the interfacial electrochemistry in all aqueous metal-ion batteries.

Holographic MIMO for LEO Satellite Communications Aided by Reconfigurable Holographic Surfaces

Ultra-dense low-Earth-orbit (LEO) satellite communication networks have significant potential for providing high-speed data services. To compensate the severe path loss in satellite communications, a key conceptual enabler is the holographic multiple input multiple output (HMIMO) with a spatially continuous aperture which can achieve a high directive gain with a small antenna size. In this paper, we consider a novel metamaterial antenna called a reconfigurable holographic surface (RHS) integrated with a user terminal (UT) to support LEO satellite communications. Composing of densely packing sub-wavelength metamaterial elements, the RHS can realize continuous or quasi-continuous apertures and provide a practical way towards the implementation of HMIMO. To obtain the desired beam directions towards the satellites, we propose a LEO satellite tracking scheme based on the temporal variation law such that frequent satellite positioning can be avoided. A holographic beamforming algorithm for sum rate maximization is then developed where a closed-form for the optimal holographic beamformer is derived. The robustness of the algorithm against the tracking errors of the satellites’ positions is also proved. Simulation results verify the theoretical analysis and show that the RHS outperforms the traditional phased array of the same physical dimension in terms of the sum rate when the compact element spacing of the RHS leads to much more RHS elements. Moreover, the RHS also provides a more cost-effective solution for pursuing high data rate compared with the phased array.

Reconfigurable Holographic Surface-Enabled Multi-User Wireless Communications: Amplitude-Controlled Holographic Beamforming

The future sixth generation (6G) networks look forward to providing revolutionary mobile connectivity and high-throughput data services through low-cost communication devices. Benefiting from the programmability and tunability of metamaterials, the reconfigurable holographic surface (RHS) inlaid with numerous metamaterial radiation elements shows its great potential to achieve such a bold vision. By leveraging the holographic principle, the RHS serves as an ultra-thin and lightweight antenna integrated with the transceiver to generate desirable directional beams with low hardware cost and power consumption. In this paper, we consider a downlink RHS-aided multi-user communication system where a base station (BS) equipped with an RHS transmits signals to users. We propose an RHS-based hybrid beamforming scheme where the digital beamforming and the holographic beamforming are performed at the BS and RHS, respectively, together with the receive combining at each user. We formulate a sum-rate maximization problem for RHS-based hybrid beamforming and decompose it into three sub-problems. A joint sum-rate maximization algorithm is then developed to solve the sub-problems in an alternating manner. Simulation results show that based on the proposed RHS-aided hybrid beamforming scheme, a moderate-sized RHS is enough to achieve a satisfactory sum-rate, which is also higher than a same-sized phased array can achieve.