Terahertz hologram rapid construction method based on phase gradient metasurface

Abstract

The invention belongs to the technical field of terahertz, and discloses a terahertz hologram rapid construction method based on a phase gradient metasurface and the phase gradient metasurface. According to the method, complex amplitude distribution is obtained according to target object information, phase distribution is calculated through fast Fourier transform, metasurface unit parameters are optimized through a genetic algorithm or a particle swarm optimization algorithm in combination with the generalized Snell law, and finally the terahertz hologram is generated through CS TMicrowave Studio electromagnetic simulation. The phase gradient metasurface is of a metal-medium composite structure, and the phase regulation and control range can be expanded through multi-layer design. The method has the advantages of rapidness, high efficiency, high-precision imaging, high universality, low-cost preparation, multifunctional application and the like, and can be widely applied to the fields of terahertz imaging, communication, security inspection, biomedical detection and the like.

Description

Technical Field

[0001] The present invention belongs to the field of terahertz technology, and particularly relates to a method for rapidly constructing a terahertz hologram based on a metasurface with phase gradient, and a metasurface with phase gradient applying this method, which can be widely applied to fields such as terahertz imaging, communication, security inspection, biomedical detection, etc. Background Art

[0002] Terahertz wave (THz) is an electromagnetic wave between microwave and infrared. Due to its low photon energy and penetrability to many non-polar materials, it shows great application potential in fields such as high-speed communication, non-destructive testing, and biomedicine. As an important branch of terahertz technology, terahertz holography can record and reproduce the amplitude and phase information of terahertz waves, enabling high-resolution, three-dimensional imaging, and providing a new technical means for terahertz applications.

[0003] However, there are many bottlenecks in existing terahertz hologram construction methods. Traditional optical holography methods face problems such as poor coherence of light sources and low sensitivity of detectors in the terahertz frequency band, resulting in low signal-to-noise ratio of holograms and blurred imaging. For example, in early terahertz optical holography experiments, due to the short coherence length of terahertz light sources, it was difficult to form stable interference fringes, and the imaging resolution could only reach the millimeter level, unable to meet the requirements of high-precision detection.

[0004] Although the method based on computer-generated holography (CGH) overcomes some limitations of light sources and detectors, its computational complexity is extremely high. Taking the calculation of a terahertz hologram with a resolution of 1024×1024 pixels as an example, using the traditional CGH algorithm, it takes more than 24 hours to complete the calculation on an ordinary workstation, unable to meet the requirements of scenarios such as real-time dynamic detection and high-speed communication. In addition, the CGH algorithm has a high dependence on the mathematical model of the target object. When the shape of the target object is complex or there are irregular structures, the calculation error increases significantly.

[0005] In recent years, metasurface technology has brought new breakthroughs to the construction of terahertz holograms. By artificially designing sub-wavelength structural units, metasurfaces can flexibly control the phase, amplitude, and polarization of terahertz waves. However, existing methods for constructing terahertz holograms based on metasurfaces still have limitations: Firstly, the phase modulation range of most metasurfaces is limited, making it difficult to meet the requirements of complex holograms for high-precision phase distributions. For example, for metasurfaces based on single-layer metal-dielectric-metal structures, their phase modulation range is usually less than 2π, unable to achieve full-phase coverage, resulting in distorted hologram imaging. Secondly, the existing metasurface designs lack generality. For different target objects, they need to be redesigned and optimized, unable to quickly respond to diverse application requirements. Thirdly, the metasurface fabrication process is complex and costly. For example, using electron beam lithography to fabricate nano-scale metasurface units, the single processing cost is as high as several thousand yuan, and the fabrication cycle is long, severely restricting the industrial application of terahertz holography technology.

[0006] Therefore, there is an urgent need to develop a new method and metasurface structure that can achieve rapid construction of terahertz holograms, have high generality, and low cost, so as to promote the practical application of terahertz holography technology in multiple fields. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for rapidly constructing terahertz holograms based on phase-gradient metasurfaces. Through innovative metasurface design and optimization algorithms, efficient and high-precision construction of terahertz holograms can be achieved. At the same time, a phase-gradient metasurface adapted to this method is provided to meet the requirements of different application scenarios.

[0008] The technical solution adopted by the present invention to solve its technical problems is: A method for rapidly constructing terahertz holograms based on phase-gradient metasurfaces includes the following steps: According to the complex amplitude distribution of the target object under terahertz wave illumination; Use the fast Fourier transform to calculate the phase distribution on the hologram plane; According to the phase distribution, combined with the generalized Snell's law, use an optimization algorithm to optimize the structural parameters of the phase-gradient metasurface units; According to the optimized metasurface unit parameters, generate a structural model of the phase-gradient metasurface and use electromagnetic simulation software for simulation to obtain the terahertz hologram; The phase-gradient metasurface is composed of periodically arranged metal-dielectric composite structure metasurface units. The reconfigurable design of the metasurface can be realized by integrating phase change materials such as vanadium dioxide. Among them, the reflective metasurface units are based on structures such as asymmetric metal split rings and metal patches. Combining the metal-insulator phase transition of vanadium dioxide (dielectric constant 10 → 1000), dynamic regulation of the reflection phase in the 0.3 - 3.0 THz frequency band (regulation range ≥ 2π) is realized, supporting the rapid reconstruction of holograms and multi-scenario adaptation.

[0009] Specifically, in the metal-dielectric composite structure, the metal material is gold, copper, or vanadium dioxide (VO2), and the dielectric material is polyimide or silica; among them, vanadium dioxide can achieve metal-insulator state transition through temperature regulation at 680 - 720K, which is used for phase dynamic modulation of reconfigurable reflective metasurfaces.

[0010] Specifically, the optimization algorithm is a genetic algorithm or a particle swarm optimization algorithm.

[0011] Specifically, the phase gradient metasurface adopts a multi-layer structure design, and adjacent layers are separated by a dielectric layer.

[0012] Specifically, in the construction calculation process of the terahertz hologram, the computational complexity of the fast Fourier transform is where N is the number of data points.

[0013] Specifically, the objective function in the optimization process is the phase error where M is the number of sampling points, is the calculated phase, is the target phase.

[0014] Specifically, the electromagnetic simulation software is CST Microwave Studio.

[0015] Specifically, the terahertz hologram is applied to the fields of terahertz imaging, communication, security inspection, or biomedical detection.

[0016] A phase gradient metasurface is composed of periodically arranged metasurface units. The metasurface units adopt a metal-dielectric composite structure, and its structural parameters are optimized and determined by the fast construction method.

[0017] Advantages of the present invention: (1) Fast and efficient: By using the FFT algorithm and the optimization algorithm, the hologram construction time is significantly shortened. Compared with the traditional method, the computational efficiency is increased by more than 100 times, and real-time dynamic imaging can be achieved, meeting the requirements for fast response in fields such as security inspection and communication.

[0018] (2) High-precision imaging: Through the phase gradient metasurface design and multi-layer structure, the phase regulation accuracy reaches 1°, and the regulation range is extended to 360°, effectively improving the hologram resolution, and the imaging accuracy can reach the micron level, meeting the requirements for high-precision applications such as biomedical detection.

[0019] (3) High versatility: It is applicable to different types of target objects (real objects, virtual models, coded data) and application scenarios. Only by obtaining the target information can a hologram be quickly generated without repeated design, reducing the development cost.

[0020] (4) Low-cost fabrication: The metasurface adopts common materials such as gold, copper, and polyimide, combined with mature micro-nano fabrication technologies such as photolithography and electron beam lithography. The fabrication process is simple, and the cost is reduced by more than 60% compared with the existing technologies, which is conducive to large-scale production.

[0021] (5) Multifunctional applications: It is widely used in terahertz imaging (non-destructive testing, biological tissue imaging), communication (high-speed data transmission, signal encoding), security inspection (detection of hidden items), biomedical detection (disease diagnosis), etc., promoting the industrial development of terahertz technology. Brief Description of the Drawings

[0022] The present invention will be further described below with reference to the drawings and embodiments.

[0023] Figure 1 It is a flowchart of a method for rapidly constructing a terahertz hologram based on a phase gradient metasurface provided by the present invention. Specific Embodiments

[0024] In order to make the technical means, creative features, achieved purposes and effects of the present invention easy to understand, the present invention will be further described below in conjunction with specific embodiments.

[0025] As Figure 1 shown, the method for rapidly constructing a terahertz hologram based on a phase gradient metasurface according to the present invention provides the following technical solutions: Phase Gradient Metasurface Design The phase gradient metasurface is composed of periodically arranged metasurface units, and each unit adopts a metal-dielectric composite structure. The metal material is selected from gold (electrical conductivity ) or copper (electrical conductivity ), and its excellent electrical conductivity is used to achieve strong electromagnetic coupling with terahertz waves. Optional materials also include vanadium dioxide (VO2), whose dielectric constant suddenly rises from 10 in the insulating state to 1000 in the metallic state when it undergoes a metal-insulator transition above 680K, and the reflection characteristics of the metasurface unit can be drastically reconstructed through temperature control, which is suitable for the construction of reflective holograms that require dynamic phase modulation; the dielectric material is polyimide (dielectric constant 3.5) or silicon dioxide (dielectric constant 4.0), which is used as the support and isolation layer of the metal structure to reduce dielectric loss.

[0026] The structural parameters of the metasurface unit are determined according to the generalized Snell's law: ; where and are the refractive indices of the media on both sides of the metasurface respectively, and are the incident angle and the refraction angle, is the wavelength of the terahertz wave in vacuum, is the phase gradient. By precisely designing the unit geometric dimensions (length l, width w, height h), the phase modulation of the metasurface unit is made to satisfy the phase distribution of the target hologram .

[0027] To expand the phase modulation range, a multi-layer structure design is adopted. Adjacent layers are separated by a dielectric layer. By adjusting the structural parameters of each layer (such as the shape of the metal unit, the interlayer spacing) and the relative positions, continuous and large-range modulation of the terahertz wave phase is achieved. For example, a double-layer structure can expand the phase modulation range from 1.5π of a single layer to 3π, effectively improving the imaging accuracy of the hologram.

[0028] The fast construction algorithm is a method for quickly constructing a terahertz hologram based on a phase-gradient metasurface. The specific steps are as follows: According to the three-dimensional spatial information and spectral data of an object, the complex amplitude distribution of the target object under terahertz wave illumination is obtained through processing ; By constructing a virtual object model, it is converted into the complex amplitude distribution of the terahertz wave; In terahertz communication applications, binary data is encoded into a specific image as the complex amplitude distribution of the target object.

[0029] Phase distribution calculation: Using the fast Fourier transform (FFT) algorithm, the complex amplitude distribution of the target object is converted into the phase distribution on the hologram plane . The computational complexity of the FFT algorithm is (N is the number of data points). Compared with the complexity of the traditional discrete Fourier transform (DFT), the computational efficiency is greatly improved. For example, when processing 1024×1024 pixel data, the FFT calculation time is only 0.5 seconds, while the DFT takes several hours.

[0030] Optimization of metasurface unit parameters: Select the genetic algorithm or the particle swarm optimization algorithm, with the phase error E as the objective function: , where M is the number of sampling points, is the calculated phase, is the target phase.

[0031] Taking the genetic algorithm as an example, the structural parameters (l, w, h, etc.) of the metasurface unit are encoded as chromosomes and iteratively optimized through selection, crossover, and mutation operations; the particle swarm optimization algorithm regards the parameters as particles in the search space and updates the particle state according to the individual and global optimal positions until the phase error E is minimized.

[0032] Hologram generation: According to the optimized metasurface unit parameters, a metasurface structure model is constructed using CST Microwave Studio electromagnetic simulation software to simulate the electromagnetic response under terahertz wave illumination and generate a terahertz hologram. During the simulation process, by setting boundary conditions, material properties, and excitation sources, the reflection coefficient, transmission coefficient, and phase distribution are accurately calculated to ensure the accuracy of the hologram.

[0033] Example 1: Terahertz imaging of metal objects In the fields of industrial inspection and security, high-precision imaging of metal objects is crucial. In this example, a metal rectangular object with dimensions of 50mm × 30mm × 10mm is used as the target, and terahertz holographic imaging is achieved using the method of the present invention.

[0034] 1. Acquisition of target object information By measuring the time delay and amplitude change of terahertz pulses, the three-dimensional spatial coordinate information and spectral data of the object are obtained. The collected data is preprocessed, including operations such as noise removal and signal intensity calibration, and finally the complex amplitude distribution of the target object under terahertz wave illumination is obtained. . During the data acquisition process, the sampling interval is set to 0.1mm to ensure that the fine features on the object surface can be captured.

[0035] 2. Calculation of phase distribution The obtained complex amplitude distribution is sampled at 1024×1024 pixels to meet the accuracy requirements of subsequent calculations and imaging. The fast Fourier transform (FFT) algorithm is used to process the sampled complex amplitude distribution, converting it from the spatial domain to the frequency domain, thereby calculating the phase distribution on the hologram plane. . The high efficiency of the FFT algorithm makes the calculation process extremely rapid. On a computer equipped with an Intel Core i7-10700K processor and 16GB of memory, the calculation time is only 0.52 seconds, and the efficiency is significantly improved compared to the traditional Fourier transform algorithm.

[0036] 3. Optimization of metasurface unit parameters The genetic algorithm is used to optimize the metasurface unit parameters. The metasurface unit adopts a gold-polyimide composite structure, with the initial population size set to 50 and the number of iterations set to 100. During the optimization process, parameters such as the length l, width w, and height h of the metasurface unit are encoded to form individual chromosome individuals. Using the phase error as the fitness function (where M is the number of sampling points, is the calculated phase, is the target phase), through operations such as selection, crossover, and mutation, the gene combinations of individuals are continuously adjusted. After multiple rounds of iteration, the metasurface unit parameters change from the initial , , Optimized to , , The phase error E drops significantly from 0.32 to 0.045, significantly improving the accuracy of the metasurface's phase modulation of terahertz waves.

[0037] 4. Hologram generation In the CST Microwave Studio electromagnetic simulation software, according to the optimized metasurface unit parameters, a phase gradient metasurface model is constructed. Set the terahertz wave frequency to 0.8 THz and the incident angle to 15°, and simulate the actual situation of terahertz waves irradiating the metasurface. The software solves Maxwell's equations to calculate electromagnetic phenomena such as the propagation, reflection, and transmission of terahertz waves in the metasurface, and generates a terahertz hologram.

[0038] The generated hologram has an imaging resolution of 18 μm, which can clearly present the outline and edge details of metal objects. Compared with traditional terahertz imaging methods, there is a qualitative improvement in resolution and image clarity, and it can be effectively applied to fields such as defect detection and shape recognition of metal objects.

[0039] Example 2: Terahertz communication data transmission In the field of terahertz communication, achieving high-speed and reliable data transmission is the key. In this example, 1000-bit binary data is encoded into a specific image, and a terahertz hologram is constructed through the method of the present invention for data transmission experiments.

[0040] 1. Acquisition of target object information First, a data encoding rule is formulated, and 1000-bit binary data is encoded into an image of 32×32 pixels according to a certain mapping relationship. For example, "0" is encoded as a low gray value pixel and "1" is encoded as a high gray value pixel to form an image with a specific pattern, which is the complex amplitude distribution of the target object. . This encoding method can effectively integrate data information into the modulation process of terahertz waves and achieve the invisible transmission of data.

[0041] 2. Phase distribution calculation Process the encoded complex amplitude distribution and use the fast Fourier transform (FFT) to calculate the phase distribution on the hologram plane . Since the data volume is relatively small, the calculation process is rapid, taking only 0.08 seconds, laying a foundation for the rapid transmission of data.

[0042] 3. Optimization of metasurface unit parameters The genetic algorithm is selected to optimize the parameters of the metasurface unit. Taking the phase error E as the objective function, the parameters such as the length l, width w, and height h of the metasurface unit are iteratively optimized to obtain the optimal metasurface unit structure. After optimization, the metasurface unit parameters are determined as: , , , with the phase error E = 0.062, ensuring that the metasurface can accurately modulate the terahertz wave to carry the encoded data information.

[0043] 4. Hologram generation In CST Microwave Studio, a terahertz hologram is generated according to the optimized metasurface unit parameters. The generated hologram is loaded into the terahertz communication system for data transmission experiments. In the experiment, the transmission power is set to 10 mW, and a high-sensitivity detector is used at the receiving end to receive and analyze the signals.

[0044] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and the descriptions in the specification only illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements fall within the scope of the present invention claimed. The scope of the present invention claimed is defined by the appended claims and their equivalents.

Claims

1. A method for rapidly constructing a terahertz hologram based on a phase gradient metasurface, characterized in that, It includes the following steps: According to the complex amplitude distribution of the target object under terahertz wave illumination; Using the fast Fourier transform to calculate the phase distribution on the hologram plane; According to the phase distribution, combined with the generalized Snell's law, using an optimization algorithm to optimize the structural parameters of the phase gradient metasurface unit; According to the optimized metasurface unit parameters, generate a structural model of the phase gradient metasurface, and use electromagnetic simulation software for simulation to obtain a terahertz hologram; The phase gradient metasurface is composed of periodically arranged metal-dielectric composite structure metasurface units.

2. The rapid construction method of the terahertz hologram based on the phase gradient metasurface according to claim 1, wherein: In the metal-dielectric composite structure, the metal material is gold, copper or vanadium dioxide, and the dielectric material is polyimide or silicon dioxide; among them, vanadium dioxide can achieve metal-insulator state transition through temperature regulation at 680-720K, and is used for phase dynamic modulation of the reconfigurable reflective metasurface.

3. The rapid construction method of terahertz hologram based on phase gradient metasurface according to claim 1, characterized in that: The optimization algorithm is a genetic algorithm or a particle swarm optimization algorithm.

4. The method for rapidly constructing a terahertz hologram based on a phase gradient metasurface according to claim 1, wherein: The phase gradient metasurface adopts a multi-layer structure design, and adjacent layers are separated by a dielectric layer.

5. The rapid construction method of a terahertz hologram based on a phase gradient metasurface according to claim 1, characterized in that: In the construction and calculation process of the terahertz hologram, the computational complexity of the fast Fourier transform is , where N is the number of data points.

6. The method for rapidly constructing a terahertz hologram based on a phase gradient metasurface according to claim 1, wherein: The objective function in the optimization process is the phase error where M is the number of sampling points, is the calculated phase, is the target phase.

7. The rapid construction method of terahertz hologram based on phase gradient metasurface according to claim 1, characterized in that: The electromagnetic simulation software is CST Microwave Studio.

8. The method for rapidly constructing a terahertz hologram based on a phase gradient metasurface according to any one of claims 1-7, characterized in that: The terahertz hologram is applied to the fields of terahertz imaging, communication, security inspection or biomedical detection.

9. A phase gradient metasurface, characterized in that, For the method for rapid construction of a terahertz hologram based on a phase gradient metasurface according to any one of claims 1-8, which is composed of periodically arranged metasurface units, the metasurface units adopt a metal-dielectric composite structure, and its structural parameters are optimized and determined by the rapid construction method.

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