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Abstract
Applying multiple physical fields to artificial manipulate electromagnetic waves is a highly stirring research. In this paper, we creatively combine light control with microwave scattering, realizing an optically coding metasurface for beam deflection based on anomalous reflection. A photoresistor and a voltage-driven module are connected to control each row of PIN-diode-loaded unit cells, endowing the reflection phase of the elements with a strong dependence on light. Owing to the high sensitivity of photoresistor, the digital element state “0” or “1” can be switched effectively via light variation sensed by the photoresistor. By modulating the light signal, the arrangement of digital elements can be reprogrammed, generating the specific scattering field. Therefore, the electromagnetic field can be determined by the spatial distribution of light, which induces the connect with the optical information and microwave field. The simulated and experimental results demonstrate the feasibility of our design. This light-steering approach provides a dimension for electromagnetic wave modulation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metamaterials possess outstanding electromagnetic (EM) properties which are unavailable in natural materials. Through elaborate design of metamaterial structure, arbitrary equivalent electromagnetism parameters can be obtained to tailor EM wave effectively. Therefore, metamaterials are extensively studied and plentiful applications are achieved including optic lens [1], EM black holes [2], perfect absorption [3,4] and imaging system [5,6]. Metasurfaces are low-dimensional versions of metamaterials with planar configuration, meriting in low profile, less loss and easy processing. Due to exceptional manipulation for EM wave like phase modulation [7,8], magnitude steering [9,10] and polarization control [11,12], metasurfaces inspire a flourish of research interest. Recently, coding metasurfaces [13] have been proposed, bridging the physical metasurfaces and digital codes. One of the major advantages for coding metasurfaces is that EM properties can be described via limited discrete digital states. For a 1-bit coding metasurface, element of phases response 0° is encoded as binary code “0” and 180° is defined as “1”. Furthermore, the concept of coding metasurfaces can also be extended to multi-bit coding, enabling distinctly different functionalities, such as multi-functional metasurfaces [14], radar cross section (RCS) reduction [13], anomalous scattering [15,16].
For various practical application demands, the creation of tunable metasurfaces is a highly nontrivial task. Reconfigurability can be applied to metasurface by utilizing active devices [17,18], micro-electro-mechanical systems [19,20] (MEMS), mechanical tuning [21,22], micro-fluid [23] and tunable substances [24–26]. Thereinto, lumped devices such as PIN diodes [27,28] and varactors [29] are mainly employed in numerous dynamically tunable metasurfaces. For instance, unit cell of a PIN-diode-loaded metasurface can generate 0° or 180° phase response under ON/OFF states of diode. Generally, computer-controlled field programmable gate array (FPGA) is used to control the bias voltage and further realizes polarization conversion and beam deflection [18,30]. Besides, to extend novel versatile applications, multiple physical parameters and environmental factors are combined with reconfigurable metasurfaces, including thermal regulating [31,32] and salinity tuning [33]. In the recent designs, light control is also gradually employed.
To combine optical domain and microwave fields control, we propose a light-controlled reconfigurable metasurface based on photoresistors and PIN diodes. First, we design a metasurface element embedded a PIN diode to modulate reflective phase. Since the change of light can lead to photoresistor value variation, combining the photoresistor with the PIN diode is able to alter voltage drop across the diode switching the OFF/ON state. Subsequently, the reflective phase of the unit cell can be dynamically modulated as desired, by controlling the light field distribution. To prove our idea, the light-sensitive coding metasurface is fabricated and measured. The simulation and experiment results show that our design can achieve beam deflection and serve as an alternative approach to tune the EM waves in real time. More interestingly, such method provides a converting bridge between the light coding signal and microwave scattering signal, offering a new manipulation dimension.
2. Principle and design
The schematic of proposed light steerable coding metasurface is exhibited in Fig. 1. A photoresistor is connected with a row of diode-loaded metasurface elements via a voltage control module. The photoresistor can sense the variation of light and convert optical signal into electrical one, and thus the light information feeds back to the diodes. When the photoresistor is exposed to light, the PIN diodes work in OFF states, otherwise the diodes turn on. Switching of diode states gives rise to the modulation in reflective phase responses of the unit cells. Consequently, by controlling the light distribution, the metasurface is able to transform the coding sequences as required, generating distinct beam deflections. In order to verify the capability of EM waves manipulation, we design three various coding patterns. Under normal incident waves, the light-encoded metasurfaces produce three different scattering fields in microwave respectively.
Fig. 1. The schematic of the liquid metal metasurface for flexible beam-steering. The liquid metal in specific structures generates distinct phase responses. With appropriate heat stimulus, the structure of liquid metal metasurface can be easily changed, to realize various beam-steering.
Figure 2(a) shows the sketch of designed light-controlled unit cell, which is composed of a topside metallic structure, F4B substrate and metal ground. A PIN diode (Skyworks SMP1320) is welded between two planar symmetrical metallic patches printed on the F4B substrate with a dielectric constant of 3 and loss tangent of 0.002. The metal wires on both sides of each patch are employed to provide the biased voltage for the PIN diode. In order to realize highly efficient tunability based on the diode, we use the software CST Microwave Studio to simulate and optimize the engineered structure. The period of the element is a = 10 mm and the thickness of F4B substrate is h = 3 mm. The thickness of metallic patches and ground is 0.01 mm. The detailed geometric sizes of the patch are chosen as follow: b = 2.77 mm, c = 4.1 mm, d = 4.7 mm, e = 1.7 mm, f = 1 mm and g = 0.1 mm. The width of the bias voltage line is w = 0.1 mm and the length is l = 3 mm. In the simulations, the PIN diode is regarded as resistor-inductor-capacitor (RLC) model ensuring the simulated results more accurate as displayed in Fig. 2(b) and the y-polarized incident wave is adopted. We encode the unit cell with the diode in OFF state as “0” element and the diode in ON state as “1” element. In Figs. 2(c) and 2(d), the simulated results of the reflected EM responses are given when the PIN diode is applied with different voltage. The digital states “0” and “1” are marked in magenta and blue. To achieve the desired phase difference and ensure high reflection amplitude responses, we initially carry out parameter optimization for the parameters c and d of the metal patch, which mainly determine the performance of the element. For more preferable electromagnetic responses, we further fix the parameters c and d and vary e, b, h and other parameters to obtain wider phase range and enhance the magnitude. After optimization, the simulated reflective phase difference between the digital element “0” and element “1” is exactly 180° and the related amplitudes are −0.44 dB and −0.97 dB respectively at 5.76 GHz.
Fig. 2. The illustration of the designed element and the electromagnetic responses. (a) The perspective view of the element embedded PIN diode. (b) RLC models of the PIN diode in OFF and ON states. Simulated reflection (c) phases and (d) amplitudes responses of the coding elements.
3. Results
To achieve effective light control, we elaborately design a voltage-driven module to connect the photoresistor and element, presented in Fig. 3(a). A photoresistor is placed at one end of the control circuit to sense the light information manifesting as the change in its electrical property. Further the control module precisely feeds such transformation back to the unit cell. Figure 3(b) illustrates the specific voltage control circuit. At room temperature, the dark resistance of the photoresistor (R2) marked in red is about 1 MΩ and the resistor decreases to 5 KΩ under the light condition. The photoresistor, other constant value resistors and a dynatron (9013) in the control loop are adopted to alter the operation state of the PIN diode. The fixed resistor values are selected as R1 = 50 KΩ, R4 = 4.7 KΩ and current-limiting resistor R3 = 50 Ω. Consequently, we can easily realize light-tunable metasurface, in which the element works in “0” state (diode is turn off) under light illumination and “1” state (diode is turn on) under no-light condition. As shown in Fig. 3(c), the entire metasurface is composed of 25*27 elements (25 elements along x-axis), integrating 675 diodes. Each row of unit cells is controlled by a photoresistor, sharing a same direct-current (DC) voltage via bias lines and working in identical state. The digital “0” elements are marked in light yellow and the “1” elements are marked in dark blue. Based on the above concept, we encode metasurfaces into three patterns with distinct coding consequences of “00001111”, “0000011111” and “00000000001111111111”.
Fig. 3. The diagram for the control circuit and the coding metasurfaces with three patterns. (a) The connection of the photoresistor and element via voltage driven module. (b) The schematic circuit of the voltage driven module. (c) The metasurface with 25*27 elements (25 elements along x-axis). The pattern with the coding sequence of (d) “00001111”, (e) “0000011111” and (f) “00000000001111111111”.
The simulated scattering fields shaped by the 1-bit coding metasurfaces are given in Fig. 4. In the simulations, a y-polarized plane wave propagating along -z direction is normally incident on the metasurfaces with different coding sequences. For pattern A, there are eight rows in a period: four with diodes off and four with diodes on. In Figs. 4(a) and 4(b), we can clearly observe that pattern A (coding sequence of “00001111”) reflects the incident wave into three beams with the deflection angle θ = ± 45°, at 5.6 GHz. According to the generalized Snell’s law, the pattern B with larger period of coding sequence “0000011111” generates a smaller deflection angle of θ = ± 31°. Under the pattern C with the periodic coding sequence of “00000000001111111111”, the normally incident beam is scattered as two main beams with θ = ± 15°, as indicated in Figs. 4(e) and 4(f). In these scenarios, the scattering field varies from tri-beam to dual-beam and most of the energy is concentrated in the two side beams.
Fig. 4. The simulated far field result of the light-sensitive coding metasurface at 5.6 GHz. (a) and (b) The scattering field for pattern A with coding sequence of “00001111”. (c) and (d) The scattering field of coding pattern B with coding sequence of “0000011111”. (e) and (f) The scattering field of coding pattern C with coding sequence of “00000000001111111111”.
In experiment demonstration, the far-field results are measured in a standard microwave chamber room. The measurement configuration is illustrated in Fig. 5(a), where the fabricated metasurface sample and a feed source are fixed on a rotatable table. Two rectangular horn antennas are employed as the feed source and the receiver respectively. As the rotatable table revolves, a far-field data on the 2D plane is measured. The source antenna is set at 1 meters away from the metasurface sample to obtain a quasi-plane wave, while the receiver is placed at 10 meters away from the turning table. The fabricated metasurface sample is presented in Fig. 5(b), from which a metasurface composed of 25*27 units are clearly observed. The 25 elements in each row shares the same bias voltage, controlled by a photoresistor module. To flexibly and independently control the specific rows, the light-tight plastic sheet is inserted between photoresistor modules, to prevent the interfere, as shown in Fig. 5(c). The white light-emitting diodes (LED) are applied here to trigger the voltage transformation.
Fig. 5. The experimental details. (a) The measurement configuration. (b) The fabricated metasurface sample. (c) The detailed photograph of the light-controlling module.
Figure 6 presents the measured results in the far-field of the above-mentioned patterns. Figures 6(a), (b) and (c) respectively present the measured data of the pattern A, B and C. To clearly exhibit the performance, we include the simulated data in the figure for comparison, in which we can observe that the measured data has good agreement with the simulations. The experimental results are marked in the red color, while the simulation results are in red. The deviation between the measured and simulated results may result from the following reason: (1) the angle deviation is due to the manual arrangement of horn antennas and metasurface sample; (2) the slight error may come from the extra reflection of the light-controlling module and the fabrication error.
Fig. 6. The measured results of the presented three patterns. (a), (b) and (c) The compared results in far-field of the patterns A, B and C.
4. Conclusion
To summarize, this paper presents a 1-bit light-controlled coding metasurface, which connect the optics and microwaves. By integrating the photoresistors and PIN diodes, the various distribution of light can endow the metasurface with diverse coding sequences. Since photoresistors are sensitive to changes in light, the metasurface can achieve high tunability. With the aid of elaborately designed coding elements, the proposed coding metasurface realizes beam deflection. We simulate and experiment this design in microwave frequency. The simulation and measured results have good accordance, which indicates that the scattering pattern can be dynamically manipulated by the coding metasurface reconfigured with light distributes. Due to the low cost and high performance, our design is expected to be applied in special communications scenarios.
Funding
National Natural Science Foundation of China (11404207); SHIEP Foundation (K2014-054, Z2015-086); Local Colleges and Universities Capacity Building Program of the Science and Technology Commission of Shanghai Municipality (15110500900).
Disclosures
The authors declare no conflicts of interest.
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