Asymmetric transmission wavefront modulation method based on multilayer metasurface

Abstract

The invention relates to an asymmetric transmission wavefront modulation method based on a multilayer metasurface, and belongs to the technical field of polarization optics application. The problems that an existing metasurface unit is complex in structure and has symmetrical transmission limitation are solved. The cascaded metal grating and the metal'bidirectional arrow 'are used as basic structure units, the spatial symmetry in the electromagnetic wave propagation direction is broken, the asymmetric transmission characteristic is obtained, meanwhile, full-range phase modulation is achieved, electromagnetic waves incident in the two directions can be independently regulated and controlled by changing the size parameters of the unit structures, and the structure is simple. Corresponding deflection and convergence effects are achieved respectively. Therefore, the invention provides a microwave band sub-wavelength unit structure, which has the advantages of easiness in design, easiness in processing and the like, has an asymmetric property, and is applied to the fields of holographic display, information encryption and the like.

Description

Technical Field

[0001] The present invention relates to a wavefront modulation method for asymmetric transmission, and particularly to a wavefront modulation method for asymmetric transmission based on a multi-layer metasurface, belonging to the application technical fields of holographic display, polarization optics, and information encryption. Background Art

[0002] In this era, the rapid development of technology has put forward higher requirements for the transmission and processing of information data. Electromagnetic waves are one of the carriers for realizing information transmission. Whether it is in mountains, oceans, the Internet of Things, etc., electromagnetic waves are integrated into all aspects. However, the regulation of electromagnetic waves using traditional optical devices has problems such as large device size, high cost, and single function. For this reason, the concept of metasurface is introduced, that is, a quasi-two-dimensional metamaterial constructed by arranging planar "artificial atoms" in a specific arrangement, which has unique advantages such as ultra-thin and low loss, providing the possibility for realizing complex multi-functional devices. Therefore, the metasurface has a very high degree of freedom and has great potential in the fields of data storage, information processing, beam shaping, polarization control, etc. However, as a two-dimensional planar structure, the metasurface has a relatively low asymmetry in the light wave propagation direction. Therefore, breaking the symmetry of the propagation direction is essential for many optical devices in communication systems and plays a very important role in many fields.

[0003] In the electromagnetic wave regulation of the metasurface, the electromagnetic waves incident in the forward and reverse directions are usually symmetric, which greatly limits the full utilization of electromagnetic space resources by the metasurface. In addition, methods such as "staggered splicing" and "partition design" are used to divide the metasurface into different spatial regions to correspond to different functions. However, this method cannot solve the contradiction between "multi-functional integration - the mutual restriction of device size": that is, the more integrated functions the metasurface has, the larger the required size, resulting in an increase in crosstalk, processing technology, and manufacturing cost. Therefore, the non-staggered splicing arrangement design of the bidirectional metasurface is worthy of exploration. Our design has an important application scenario in the field of low-cost and miniaturized communication. The present invention breaks the symmetry of space and realizes phase modulation of asymmetric transmission. Summary of the Invention

[0004] The purpose of the present invention is to overcome the problem of symmetrically transmitted electromagnetic waves existing in the traditional form of metasurface, and provide an electromagnetic regulation metasurface and metasurface unit for asymmetric transmission. The present invention can regulate the transmission phase of electromagnetic waves incident in both forward and reverse directions, and the tunability of the phase of electromagnetic waves incident in a specific direction can be realized through this method. At the same frequency point, the separate and independent regulation of the cross-polarized wave transmission of the same polarization wave from two directions is realized. For the electromagnetic wave incident in the forward direction, the transmission channel deflects, while for the electromagnetic wave incident in the reverse direction, the transmission channels converge at one point. This two-way function provides a direction-selective asymmetric channel.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] The phenomenon of asymmetric propagation of electromagnetic waves refers to the different transmission functions of electromagnetic waves propagating along a specific direction and those propagating in the opposite direction when passing through the same metasurface. In order to break the spatial symmetry of electromagnetic waves in the propagation direction and make electromagnetic waves exhibit different wavefront modulation characteristics when propagating forward and backward, it is first necessary to select a suitable unit structure to form a multi-layer metasurface. The asymmetric wavefront modulation method based on a multi-layer metasurface disclosed in the present invention establishes the concepts of metasurface deflection and focusing functions on the basis of two metal layer patches, two dielectric layers and a grating layer structure, and effectively multiplexes the electromagnetic wave regulation function by using the inherent characteristic of the electromagnetic wave propagation direction, thereby realizing a multi-functional electromagnetic device based on the propagation direction. By encoding the target phase into each unit structure of the multi-layer metasurface through the phase algorithm of the deflection and focusing functions, due to the changes in the unit size and angle of different metal edges, the phase modulation of two transmission channels is achieved. The direction selectivity is attributed to the fact that the angle transformation of metal patch A has no effect on the backward incident electromagnetic waves, while the angle transformation of metal patch C has no effect on the forward incident electromagnetic waves.

[0007] The multi-layer metasurface can achieve asymmetric transmission, including metal layer A, dielectric layer, grating layer B, dielectric layer and metal patch C; the metal patch C is arranged in an array at the bottom layer, the grating layer B is located between the two dielectric layers, and the top layer is metal patch A. The angles between the centers of metal patches A and C and the positive x-axis are θ1 and θ2. The changes in the angles and the size changes of the metal patch layer can be selected according to the phase distribution required for different functions.

[0008] The asymmetric transmission wavefront modulation method based on a multi-layer metasurface is characterized in that it includes m×n periodically arranged phase mutation units, where both m and n are positive integers; and both m and n are 51.

[0009] The asymmetric transmission wavefront modulation method based on a multi-layer metasurface is characterized in that the metal layer A, grating layer B and metal layer C are all copper layers, and the thickness is 0.018 mm.

[0010] The asymmetric transmission wavefront modulation method based on a multi-layer metasurface is characterized in that the unit structure is square, the side length is p = 6 mm, and the thickness of the dielectric layer is 1 mm.

[0011] The asymmetric transmission wavefront modulation method based on a multi-layer metasurface is characterized in that linearly polarized waves are incident perpendicularly on the metasurface in two directions. When incident forward, the transmitted cross-polarized wave produces a deflection effect, and when incident backward, the transmitted cross-polarized wave produces a focusing effect.

[0012] The described asymmetric transmission wavefront modulation method based on a multi-layer metasurface is characterized in that: the method for preparing the multi-layer metasurface is manufactured by using traditional printed circuit board (PCB) technology. For the multi-layer sub-surface, first, the metal surface of the dielectric with a double-layer coating is treated to ensure that the surface has no oxide layer, oil, dust, and other contaminants, so as to achieve the purpose of dry film adhesion. Under ultraviolet light irradiation, the non-photosensitive dry film needs to be rinsed with a photoresist developer. After exposure, the metal under the photoresist is retained and not affected by etching, and then the surface roughening and anti-oxidation oxidation process are completed. For the multi-layer samples, each treated copper-clad dielectric board is bonded together with an adhesive. Based on this PCB technology, a metasurface sample is fabricated.

[0013] Beneficial effects:

[0014] The asymmetric transmission wavefront modulation method based on a multi-layer metasurface of the present invention uses multi-layer metal patches, dielectric layers, and grating layers as basic structural units, breaking the spatial symmetry in the direction of electromagnetic wave propagation, and enabling the electromagnetic wave to exhibit different wavefront modulation characteristics according to different transmission channels during forward and reverse transmission. By adjusting the size of the metal patches, full-phase modulation from 0 to 2π can be achieved, so it can be used to encode different functional phases. Combining the encoded phase distribution map with the asymmetric transmission characteristics of the cascaded multi-layer metasurface can realize the deflection and convergence functions combined with the propagation direction, and the two functions are independent of each other and can be applied to fields such as holographic display and information encryption. Description of the Drawings

[0015] Figure 1 It is the unit structure diagram of the asymmetric transmission wavefront modulation method based on a multi-layer metasurface of the present invention; the fixed parameter values of the unit structure in the figure are p = 6mm, b = 0.6mm, s = 1.2mm, w = 0.4mm, l = 5mm.

[0016] Figure 2 It is the amplitude and phase modulation effects of 16 different basic constituent units selected for the multi-layer metasurface disclosed by the present invention for each channel. (a) Transmission amplitude spectrum of the transmitted cross-polarization component of the linearly polarized wave under forward and reverse incidence on the metasurface. (b) Transmission phase spectrum of the transmitted cross-polarization component of the linearly polarized wave under forward and reverse incidence on the metasurface.

[0017] Figure 3Effect diagram of deflection and convergence realized by the multi-layer metasurface disclosed in the present invention. (a) Schematic diagram of the proposed surface of the cross-polarized deflection wavefront and the cross-polarized focusing wavefront under the bidirectional incidence of the linearly polarized wave. (b) and (c) show the spatial phase distributions of the deflected beam and the focused beam. (d) and (f) Simulation and measurement results of the phase distribution in the xoz plane. (e) and (g) Simulation and measurement results of the focused energy distribution in the xoy plane. Detailed implementation manners

[0018] Hereinafter, exemplary embodiments of the present invention will be described in conjunction with the accompanying drawings. For the sake of clarity and conciseness, not all features of the actual implementation manners are described in the specification. However, it should be understood that many implementation-specific decisions must be made during the development of any such actual embodiment in order to achieve the specific goals of the developer, for example, to comply with those constraints related to the system and the business, and these constraints may vary with different implementation manners. In addition, it should also be understood that although the development work may be very complex and time-consuming, for those skilled in the art who benefit from the disclosure of the present invention, such development work is only a routine task.

[0019] Here, it should also be noted that, in order to avoid obscuring the present invention due to unnecessary details, only the device structures and / or processing steps closely related to the solution according to the present invention are shown in the drawings, while other details less related to the present invention are omitted.

[0020] The present invention is designed through the following steps

[0021] Detailed implementation manner one: In combination with Figure 1 、 Figure 2 and Figure 3 This embodiment is specifically described. The unit structure adopted in the present invention is as shown in Figure 1 . The metal strips are separated by a dielectric substrate with a thickness of h = 3 mm and a dielectric constant of 2.65. At the top and bottom layers, metal patches with a two-way arrow (labeled A and C respectively) are adopted, and the middle layer is composed of metal gratings with equal intervals (labeled B). It has the following fixed parameters: p = 6 mm, b = 0.6 mm, s = 1.2 mm, w = 0.4 mm, l = 5 mm.

[0022] Detailed implementation manner two: Set the center frequency of the functional structure of the metasurface to 15 GHz.

[0023] Embodiment 3: To achieve the bidirectional function, the calculated phase distribution is discretized into "0", "π / 2", "π", and "3π / 2", and is represented by the code elements "00", "01", "10", and "11" respectively. We further achieve the corresponding change in the unit phase by changing the size and angle of the unit structure, and the change rule is shown in Table 1. Figure 2 (a) and (b) show the amplitude spectra of the transmitted x-polarized waves of the unit for the forward and backward incidence of the y-polarized wave by all 16 coding elements. Figure 2 (c) and (d) show the transmitted phases of the transmitted cross-polarization components (x-polarized waves) for the forward and backward incidence of the y-polarized wave by all 16 coding elements.

[0024] Table 1. Structural parameters of 16 coding units of the metasurface (mm)

[0025]

[0026] Embodiment 4: The asymmetric transmission wavefront modulation method based on a multi-layer metasurface is characterized in that: it includes m×n periodically arranged phase mutation units, where both m and n are positive integers; and both m and n are 51. That is, the metasurface consists of 51×51 sub-atoms, and the total area is 306×306 mm 2 .

[0027] Embodiment 5: This metasurface deflects the transmitted wave for forward incidence and acts as a focusing lens for the transmitted wave for backward incidence. Figure 3 (a) shows the schematic diagrams of different functions. The spatial phase distributions for obtaining these two functions are expressed as:

[0028]

[0029] where f and b represent forward and backward respectively, and λ is the operating wavelength.

[0030] Embodiment 6: Set F to be the focal length of 80 mm.

[0031] Embodiment 7: When the forward y-polarized wave is incident on the metasurface, the transmitted cross-polarized wave vector is accurately deflected and propagated at an angle of 30 degrees along the theoretically predicted target angle (i.e., α = 30°).

[0032] Embodiment 8: When the backward y-polarized wave is incident on the metasurface, the transmitted cross-polarized waves converge into a single focus with a focal length of z = 80 mm.

[0033] Embodiment 9: By calculating the phase distribution of the objective function, Figure 3 (b) and 3(c) describe the target phase distributions of the two functions.

[0034] Furthermore, Figure 3 (d) and Figure 3 (f) show the simulated and measured results of the phase distribution in the xoz plane. That is, for the electromagnetic wave incident on the metasurface in the forward direction, a perfect deflection angle of 30° is achieved. Figures 4(e) and 4(g) show the simulated and measured results of the focused energy distribution on the xoy plane. That is, it represents the electromagnetic wave incident on the metasurface in the reverse direction, achieving the focusing effect in the plane. The good agreement between the numerical and experimental results demonstrates the deflection and focusing effects of the transmission channels for the electromagnetic waves incident in the opposite directions.

Claims

1. An asymmetric transmission wavefront modulation method based on a multi-layer metasurface, characterized in that: By adjusting the metal dimensions of the multi-layer metasurface, a full-phase modulation range of 0 to 2π for the orthogonal polarization transmission channel can be achieved; according to the phase distribution of the corresponding functions, the dimensions of all metal strips are determined one by one, that is, different functional phase information can be encoded in the multi-layer metasurface; a deflection effect can be observed in the channel corresponding to a specific polarization and transmission direction, and a converging effect can be observed in another transmission direction and polarization channel. The multi-layer metasurface can achieve asymmetric transmission, including metal layer A, dielectric layer, grating layer B, dielectric layer and metal patch C; the array of the metal iron patch C is arranged at the bottom layer, the grating layer B is located in the middle of the two dielectric layers, the top layer is metal patch A, and the angles between the centers of metal patch A and C and the positive x-axis are θ1 and θ2, and the change of the angle and the size change of the metal patch layer can be selected according to the phase distribution required by different functions.

2. The asymmetric transmission wavefront modulation method based on a multi-layer metasurface according to claim 1, wherein: It includes m×n periodically arranged phase mutation units, where both m and n are positive integers; and both m and n are 51.

3. The asymmetric transmission wavefront modulation method based on a multi-layer metasurface according to claim 1, wherein: The metal layer A, grating layer B and metal layer C are all copper layers with a thickness of 0.018 mm.

4. The asymmetric transmission wavefront modulation method based on a multi-layer metasurface according to claim 1, characterized in that: The unit structure is square with a side length of p = 6 mm, and the thickness of the dielectric layer is 1 mm.

5. The asymmetric transmission wavefront modulation method based on a multi-layer metasurface according to claim 1, wherein: Linearly polarized waves are incident on the metasurface perpendicularly in two directions. The transmitted cross-polarized wave for forward incidence produces a deflection effect, and the transmitted cross-polarized wave for backward incidence produces a converging effect.

6. The asymmetric transmission wavefront modulation method based on a multi-layer metasurface according to claim 1, characterized in that: Method for fabricating the multi-layer metasurface: It is fabricated using traditional printed circuit board (PCB) technology. For the multi-layer sub-surface, first, the metal surface of the dielectric with double-layer cladding is treated to ensure that the surface has no oxide layer, oil, dust and other contaminants, so as to achieve the purpose of dry film adhesion. Under ultraviolet light irradiation, the non-photosensitive dry film needs to be rinsed with a photoresist developer. After exposure, the metal under the photoresist is retained and not affected by etching. Then, the surface roughening and anti-oxidation processes are completed. For the multi-layer sample, each treated copper-clad dielectric board is bonded together with an adhesive. Based on this PCB technology, a metasurface sample is fabricated.

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