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Abstract
Combining multiple physical fields with programmable metasurfaces in realistic scenarios is a hot topic. There are numerous studies on controlling metasurfaces using light-field, thermal fields, and so on. Due to its excellent penetration and invisibility, ultraviolet (UV) has benefits that conventional light does not possess. However, previous works that apply UV-light to metasurfaces and modulate electromagnetic (EM) waves using UV-light sensing can only sense very few points. This paper proposes a UV-sensing metasurface integrated with an 8*8 sensor array and can achieve a complicated UV-information input and more complicated EM-filed manipulation, including dual-beam, chess-board patterns, and RCS-reduction. By assembling a UV-sensor and an embedded PIN diode on each metasurface supercell, each supercell (2*2 elements) not only can independently sense and feed back the change of UV-light intensity, but also be programmed for diverse EM functions. After elaborate simulation and experiment, the experimental outcomes are in good agreement with the simulative outcomes, which verifies the feasibility of the scheme. Such matrix UV-light field input builds a new interactive channel with electromagnetic information, which is suitable for application scenarios with flexible requirements for communications and imaging.
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1. Introduction
Metamaterial [1,2] is a kind of artificial electromagnetic material, which gets rid of the limitation of the inherent electromagnetic parameters of natural materials and have excellent electromagnetic wave control ability [3–5]. Through experiments, it is found that it can realize many strange physical phenomena, such as negative refraction [6–8], perfect lens [9,10], perfect absorber [11], and invisibility cloak [12]. Metasurface is a two-dimensional(2D) equivalent form of metamaterials. It not only inherits the prominent electromagnetic wave control performance of metamaterials, but also is widely used in scenes with high integration and complex functional requirements [13–17]. To dynamically control electromagnetic waves in real time, Professor Cui Tie Jun team proposed the concept of “digital coding and programmable metasurface “ in 2014 [2]. The discrete coding of phase values can greatly simplify the design and optimization process, and facilitate the combination of metasurface and information processing technology [18,19]. The “0” and “1” digital states are electrically modulated by controlling the on and off state of the loaded switching diodes, and the incident electromagnetic waves can be modulated in real time by using a field programmable gate array (FPGA) [20–22] hardware system to input the codes into the digital metasurface phase distribution in real time. This has promising applications in wireless communication and radar cross section (RCS) [23–25].
In recent years, a variety of metasurfaces with modulation mechanisms have been proposed, which contain metasurface operating in a wide range of wavelengths, such as microwave [26,27], infrared [28–30] and visible light [14,31]. Compared to conventional programmable metasurface, light-controlled metasurface have the advantage of not requiring complex physical control circuits. This manner of regulation does not require manual control, but by controlling the optical field distribution, the metasurface can dynamically modulate the reflection phase of the unit as required, achieving high tunability. In addition, the integration of Ultraviolet-regulated metasurface and FPGA contains a strategy to directly regulate electromagnetic waves by pre-programmed coding sequences on FPGA. It is worth noting that it also includes an additional approach that does not preset the FPGA. This allows real-time programming of ultraviolet light by modifying the control program of FPGA, achieving the function of regulating electromagnetic waves. Moreover, light-controlled metasurface combine light-filed and microwaves to modulate the optical fields in a non-contact manner, enabling remote real-time modulation of electromagnetic waves. Based on the above features, the academic community has conducted in-depth research on light-controlled metasurface. For instance, light-controlled metasurfaces without control circuit, with variable frequency and controllable modulation range of reflection phase have been proposed successively [32–34]. However, there are drawbacks of using ordinary visible light, which is visible and susceptible to environmental effects. Ultraviolet (UV) light is invisible, more penetrating than visible light, and can expand the frequency band of sensors and controllers. Therefore, applying UV-light to metasurfaces is a good proposition. It can also be combined with algorithms to obtain a stronger real-time adjustment ability [35–39], resulting in an adaptive intelligent metasurface [40–43].
Based on the previous research on UV light controlled metasurface [44], an ultraviolet metasurface is proposed, in which each supercell can individually sense the changes of external ultraviolet light. An 8*8 UV-sensor array enables complex light-field information input, which can be programmed into richer EM functions like chess-board patterns and RCS-reduction. The programmable metasurface embedded with PIN diodes is designed to modulate the reflection-phase. The state of the PIN diode is controlled by the UV sensors and the voltage driving module of FPGA. Specifically, UV sensors can sense a two-dimensional distribution of UV light and encode them into distinct coding sequences. Combined with the voltage control module, the PIN diodes of metasurface can works independently in diverse states. The reflection-phase of the metasurface will change, resulting in various beam deflections. We designed four patterns for simulation the experimental verifications. The results show that our simulative outcomes are in good agreement with the experimental results.
2. Principle and results
Figure 1 depicts the conceptual diagram of the proposed ultraviolet light-controlled coding metasurface. The UV-sensor array and FPGA were applied to this design in order to implement the adaptive UV light sensing function on the metasurface. The metasurface composed of 16*16 elements is designed, integrated with 8*8 UV-sensor array on the back-surface of the metasurface. Reflection phase is modulated using a 1-bit coding metasurface embedded with a PIN diode in each element front-surface. PIN diodes have the exactly same phase response and bias voltage. When there is a 180° difference in the phase response element, it is encoded as binary “0” or “1“, accordingly. For a particular UV wave band, the UV sensor is highly sensitive. With this feature, it can efficiently detect if UV-light is present or absent, establishing the groundwork for UV-light control of the metasurface. FPGA can receive sensing signal and output digital signal, which is indispensable in this design. Specifically, after ultraviolet light irradiation, the ultraviolet light signal (photo current) signal is converted into analog signal (DC voltage) through the ultraviolet sensor, and then converted into digital signal through the ADC converter in the MCU, and finally output to the FPGA. At the same time, the pre-designed coding sequences are stored on the FPGA chip. According to the collected digital signal form ADC, the FPGA outputs the corresponding control signal to metasurface elements, so as to achieve a variety of electromagnetic functions. To show more clearly how our proposed metasurface works, we provide illustrative diagrams, as shown in Fig. 1 (b).
Fig. 1. (a) Schematic diagram of ultraviolet sensing metasurface. The metasurface contains ultraviolet sensor and PIN diode, combined with FPGA. When the ultraviolet sensor senses the stimulus of ultraviolet light, it transmits the sensing signal to the FPGA, and the FPGA controls the diode state to code metasurface, generating obvious abnormal reflection. (b) Illustrative diagram of the working principle.
The element design of the proposed adaptive UV light controlled metasurface is shown in Fig. 2 (a) and (b). Figure 2 (a) also shows the working state of PIN diode. A three-layer structure may be seen in this unit. Metal patch layer, with a thickness of 0.02 mm, is the top layer. It is made up of two metal patches. The middle is a FR-4lossy dielectric substrate with a dielectric constant of 4.3 and a loss tangent of 0.025. Having a t = 0.02 mm thickness, the bottom is made of metal. As a resistor-inductor-capacitor (RLC) paradigm, a PIN diode (Skyworks SMP 1320) is implanted between two planar metal patches and connected to the metal behind through the hole to generate a bias network. The geometric parameters of the two metal patches are b = 1.8 mm and w = 8 mm, and the period of this element is a = 12. The symmetrical patch has a wire width of c = 0.1 mm on either side. The diameter of the through hole is 0.3 mm. The dielectric substrate measures h = 3.32 mm in thickness. These parameters are set to achieve satisfactory electromagnetic performance.
Fig. 2. Illustration of element used for wavefront manipulation and electromagnetic response. (a) A three-dimensional perspective view of an element and the switch status equivalent diagram of PIN diode. (b) Front view of the element. (c) and (d) are the phase and amplitude response diagrams of the element respectively.
We use RLC equivalent model to simulate PIN diode. The OFF state of the diode is determined by the parameters R, C, L. When R = 2.2 Ω, C = 0pf, L = 0.4nH, it is OFF. The ON state of the diode corresponds to the parameters R = 0 Ω, C = 0.4pF, L = 0.4nH. In order to achieve efficient modulation of UV-light controlled metasurface, we use CST Microwave Studio to simulate and observe the element structure, and optimize it. The y-polarized plane incident wave is used as the excitation signal. For the sake of making the reflection phase of the metasurface have a phase-difference of 180 °, and the amplitude can meet the demand correspondingly, we preliminarily optimized some metal patch parameters, such as b and w, which can determine the cell performance. Fixing these two parameters and optimizing other parameters can obtain ideal results.
Figures 2 (c) and (d) show the phase and amplitude response diagram of optimized unit, and the digital states “0” and “1” are shown in blue and pink. The phase difference of 180° occurs when the frequency point is 5.1 GHz, and the corresponding amplitudes are related samples are -1.0 dB and -0.4 dB respectively. In addition, ML8511 is selected as the model of the UV sensor in this design. From the datasheet, we can know that the relationship between the UV light intensity and the output voltage is linear at different temperatures, and the light sensitive band of the UV-sensor is in the range of 280∼390 nm. We use these features to complete this design.
We have carefully designed a metasurface supercell to achieve effective UV light control, and its detailed structure is shown in Fig. 3. Each supercell is integrated with on UV-sensor, while 8*8 UV-sensor array is integrated in the whole metasurface. The structure is composed of five levels. A designed metasurface unit with a PIN diode, metal wire, through-hole, and metal patch make up the top layer. The third layer is metal ground, the bottom layer is a microcircuit, the second and fourth layers are dielectric layers. The through-hole is used as a bridge to connect the metal patch at the top and the metal ground at the third layer. Finally, it is connected with the microcircuit at the bottom layer, which enables the microcircuit to accurately feedback a series of electrical characteristics changes caused by different ultraviolet light stimuli received by the ultraviolet sensor to the metasurface element. The microcircuit includes an ultraviolet sensor module, which can transmit the detected sensor signal to the FPGA. At the same time, the FPGA can also be used to control the status of the PIN diode, which can be coded into different patterns according to needs, so that the ultraviolet light-controlled metasurface can be programmed in reality. For the sake of achieving flexible modulation of electromagnetic waves, each supercell of the metasurface in this design is equipped with a UV-sensor and four PIN diodes. Through the microcircuit, each unit on the metasurface can independently sense the UV-light and regulate electromagnetic waves. In the microcircuit system, the part marked black is the ML8511 ultraviolet sensor.
Fig. 3. Metasurface supercell diagram, integrated one UV-sensor in each supercell. It is a five-layer structure. The first layer is a metal patch layer, the second and fourth layers are dielectric layers, the middle third layer is a metal ground, and the bottom layer is a microcircuit. The layers are connected with each other through holes.
We established four distinct patterns, as shown in Fig. 4, to evaluate the properties of the proposed UV light controlled metasurface that can be independently perceived and controlled. Figures 4(a) and 4(b) are respectively the pattern A with horizontal coding of “00011100011000110001 “and the pattern B with horizontal coding of “00001111100001111 “, and Fig. 4 (c) is the chessboard coded metasurface with horizontal and vertical coding of “000000111110000 “. Figure 4 (d) shows the RCS pattern. Figure 4 also lists their corresponding far-field results at the central frequency point of 5.1 GHz. When the y-polarized plane wave incident on the metasurface, for pattern A, the incident wave is mainly reflected into three beams with a deflection angle of about 45 ° at 5.1 GHz, and most of the energy is concentrated in the side beam. The peak energy of the double beams is -0.2 dB. Pattern B still reflects three beams, but the deflection angle is small, about 35 °. The energy is also concentrated in the side beam. The peak energy of the double beam is 0.62 dB. The chessboard coded metasurface is designed to reflect the incident wave into five beams with almost the same elevation angle at 5.1 GHz. The energy of the middle beam is much lower than that of the other four side beams, and the peak energy of the multi beam is -1.9 dB. The last RCS pattern has a far field result that is significantly different from the coded array and chessboard.
Fig. 4. Diagram of the far-field simulation for four different patterns at 5.1 GHz. (a) Pattern A with horizontal code of “0001110001110001 “. (b) Pattern B with horizontal code of “000011100001111 “. (c) Pattern C uses a chessboard to code the metasurface. Its horizontal and vertical codes are both “00000011111110000 “. (d) RCS Reduction.
We examined the far-field outcomes in a typical microwave chamber to further illustrate the precisely designed metasurface. Figure 5 depicts the schematic diagram of the metasurface sample and the testing setting. Each of the 16*16 units that make up the UV light-controlled metasurface are equipped with UV sensors. With a 10-meter separation between them, the feed-source and receiver are two rectangular horn antennas. The feed-source horn is mounted simultaneously with the metasurface on the rotating workstation, 1.5 m from the metasurface. The far-field data on the two-dimensional plane can be measured when the workbench rotates.
Fig. 5. (a) The real experimental environment for Far-Field measure. (b) The demonstration of the Far-Field measure principle.
In Fig. 6, we show the comparison between the simulation results of the three modes and the far-field experimental results. In order to compare the simulation and measurement results more intuitively, both are marked in the figure at the same time. The blue curve represents the simulation results, and the red curve represents the measurement results. When the angle is within the range of [-90°,90°], the characteristics of double beam and multi beam are obvious, and the simulation results are in good agreement with the measured beam characteristics. In addition, the blue line and the red line have approximately the same trend, indicating a high degree of consistency between the measured and simulated results. The slight differences between the two results can be attributed to the following aspects: (1) There may be errors caused by manual operation during measurement; (2) The error caused by the additional reflection of the photosensitive module and the non-ideal excitation of the horn antenna.
Fig. 6. Far-field results of three patterns. (a), (b) and (c) are the comparison of the far-field results between the simulation and experiment of the three patterns.
3. Conclusion
In this paper, a programmable 1-bit metasurface controlled by ultraviolet is proposed. To complete various coding sequences of the element metasurface under a complex UV-light, an 8*8 UV-sensor array, FPGA, PIN diode, and microcircuit module are used, to achieve richer electromagnetic wave modulation function. The 2D variation in the UV-light can be sensed and fed back to the FPGA by the UV-sensor array, which is extremely sensitive in a particular band, allowing the FPGA to modify the PIN diode switch state. Because of this, the metasurface is sensitive to UV-light and may modify the reflection phase effectively. In addition, the UV-sensor and the PIN diode are simultaneously integrated in each cell of the metasurface, allowing the control mode to modulate electromagnetic waves by individually manipulating each element rather than just one column. Microwave simulation and measurement data demonstrate that the simulated results and the experimental results are quite consistent. The proposed UV-light controlled metasurface is real-time modifiable, and the electromagnetic wave control method is highly efficient and adaptable, with cheap cost, and may be applied to imaging, lithography, and other situations.
Funding
National Natural Science Foundation of China (11404207, 52177185); SHIEP Foundation (K2014-054, Z2015-086); Local Colleges and Universities Capacity Building Program of the Shanghai Science and Technology Committee, China (15110500900).
Acknowledgments
This work was supported by the National Key Research and Development Program of the National Natural Science Foundation of China (11404207), 52177185), SHIEP Foundation (K2014-054, Z2015-086), and the Local Colleges and Universities Capacity Building Program of the Shanghai Science and Technology Committee, China (15110500900).
Disclosures
The authors declare no conflict of interest.
Author Contributions
Conceptualization, Z.S; and HR. T; methodology, Z.S; software, FJ. Y; validation, SS. L; FJ. Y; and HR. T; formal analysis, Z.S; investigation, HY.C; writing—original draft preparation, HR. T; writing—review and editing, HY.C; visualization, SS. L; project administration, Z.S; funding acquisition, L.C. All authors have read and agreed to the published version of the manuscript.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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