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Abstract
Recently, reconfigurable polarization-manipulation metasurfaces controlled with active components have gained widespread interest due to their adaptability, compact configuration, and low cost. However, due to the inherent non-negligible ohmic loss, the output energy of these tunable metasurfaces is typically diminished, particularly in the microwave region. To surmount the loss problem, herein, we propose an active polarization-converting metasurface with non-reciprocal polarization responses that is integrated with amplifying transistors. In addition, we provide a design strategy for a polarizer that is insensitive to polarization and has energy amplification capabilities. Experiments are conducted in the microwave region, and amplification of the polarization-converting behaviors is observed around 3.95 GHz. The proposed metasurface is prospective for applications in future wireless communication systems, such as spatial isolation, signal enhancement, and electromagnetic environment shaping.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Polarization, one of the fundamental properties of electromagnetic (EM) waves that characterizes the change in orientation of the electric field vector at a fixed point, plays a crucial role in a wide range of practical applications across the whole spectrum. For instance, polarization scattering information measured by polarimetric radar systems can provide a comprehensive method for the identification and classification of targets [1]. Conventional techniques for manipulating polarizations employ ferromagnetic compounds and the optical activity of crystals [2], but these techniques are limited by their bulky size, which may not be preferred by integrated systems because polarization manipulations should be performed in a confined space. Using the magneto-optic effect of gyromagnetic materials, the conventional nonreciprocal design method can result in the rotation of polarized waves. As an example, the gyromagnetic photonic crystal is subjected to a direct current (DC) magnetic field in order to break the time-reversal symmetry [3], which is a complicated method requiring cumbersome devices.
Recently, metasurfaces, the two-dimensional version of metamaterials, which are composed of artificially engineered sub-wavelength elements, have received a great deal of attention due to their considerable advantages of compact configuration, light weight, and low loss, as well as their superior abilities to manipulate the magnitude [4–7], phase [8–12], and polarization [13] of EM waves. By designing the metasurface element with symmetry-broken structures, they provide a platform for manipulating the polarization states at will, enabling versatile polarization optics with the desired performances, such as ultra-thin wave-plates [14], broadband polarization conversion [15], holographic images [16] or beam patterns with versatile polarizations [17]. Moreover, by incorporating active components such as diodes [18], liquid crystal [19], or other tunable materials, the metasurface polarization operations can evolve with tunable and reconfigurable properties that are controlled by an external stimulus. For example, a double-layered active metasurface can alternate between chiral and isotropic structures by switching the working states of the PIN diodes to convert the incident linearly polarized wave into either right-handed or left-handed circularly polarized wave [20]. A reconfigurable metasurface embedded with a single PIN diode can control the polarization and propagation states of EM waves [21]. However, these actively tunable elements usually have non-negligible ohmic loss, especially in microwave region, where PIN or varactor diodes are commonly used due to their ultra-fast switching time and stable performance. Consequently, the amplitude of the cross-polarized output is attenuated in reconfigurable meta-polarizers. Moreover, polarization-converting metasurfaces only have reciprocal polarization responses. Nevertheless, the pressing demands from application side also require non-reciprocal functions because they play irreplaceable roles in numerous fields, such as signal isolation, optical imaging, etc.
To tackle the aforementioned difficulties, we propose an ultrathin single-layer active polarization-converting metasurface with an amplification circuit that enables electrically controlled energy enhancement beyond the passive amplitude limit. The metasurface can reflect the linearly polarized EM wave into the orthogonally polarized wave with tunable amplitude enhancement, while the EM wave is absorbed under the incidence of the orthogonally polarized wave. The unit cell of the metasurface consists of three cascaded components: a receiving element, a tunable radio frequency (RF) amplifier circuit, and a reradiating element. The non-reciprocal polarization manipulation is accomplished with the employment of the RF amplifier circuit. By adjusting the bias voltage level in the amplifier circuit, energy enhancement can be tuned at will. Both simulation and measurement results demonstrate the reliability of the proposed design strategy.
2. Concept and design principle
To realize active polarization-converting metasurface in the microwave region, loading transistors to meta-atoms is an effective method to achieve nonreciprocity and amplification, as demonstrated by a variety of fascinating physical phenomena and devices [22–30], such as magnetless nongyrotropic nonreciprocity, spatial nonreciprocal manipulation, and full-duplex reflective beam steering, etc. The functionality of the proposed metasurface is depicted in Fig. 1(a). The x-linearly polarized (LP) wave traveling along the -z direction is reflected and converted to a y-LP wave with gain enhancement by the metasurface. To achieve magnitude-amplification, a RF amplification circuit based on transistors is incorporated into the unit cell of the metasurface. Hence, the metasurface is mainly composed of three parts, namely the energy receiving structure, the unidirectional amplification circuit, and the re-radiating structure, with the receiving and re-radiating structures operating in orthogonal polarization to enable polarization conversion. In contrast to conventional metasurfaces with reciprocal performances (i.e., the x-LP to y-LP conversion is equal to the y-LP to x-LP conversion in reflection mode), the proposed non-reciprocal metasurface responses distinctly to x-LP and y-LP waves, as shown in Fig. 1(b).
Fig. 1. (a) Schematic of the reflective polarization-conversion metasurface with enhanced magnitude controlled by an external DC voltage source. (b) Nonreciprocal functionality of the proposed metasurface. (c) Schematic of the unit cell with parameters of p = 70 mm, w = 28.1 mm, l = 21.8 mm, and w1 = 5.5 mm. The inset illustrates the circuit model of amplifying component.
Figure 1(c) depicts the schematic of the unit cell, which consists of a dielectric layer sandwiched between two metallic layers. Each unit cell is comprised of two rectangular patches that function as two antenna structures for orthogonal polarizations and are connected by a power amplifying circuit. The two patches have a width of l = 28.1 mm and a length of w = 21.8 mm. The top patches and the metallic ground are separated by an F4B dielectric substrate with a thickness h = 2.0 mm, dielectric constant εr = 2.65, and loss tangent tanδ = 0.0015. The periodicity of the unit cell is p = 70 mm. To accomplish amplified polarization conversion, we adopt a methodology that consists primarily of three steps: receiving incidence, amplifying incidence, and radiating output wave. In detail, the left patch structure firstly receives the x-LP incident EM wave. The energy then propagates unidirectionally through the gain-equipped RF amplification circuit. Finally, the amplified wave is guided to the right patch structure through the microstrip line and then radiated back as a y-LP wave that is antiparallel to the polarization direction of the incident wave.
The input and output ports of the transistor are connected to the two rectangular patch structures in the proposed transistor-based RF amplification circuit, as depicted in Fig. 1(c). The RF amplification circuit consists of one amplification transistor, two decoupling capacitors, one bypass capacitor, and one choke inductor. Mini-Circuits Gali-2+ Darlington pair amplifiers are selected as the unidirectional amplifiers, whose power-amplifying properties are controlled by an external DC biased voltage. The DC voltage is directly applied to the circuit via port Vdc. Capacitor C1 and C2 serve as DC-blocking capacitors to prevent DC current leakage. Inductor L1 is used as the RF choke for blocking RF leakage to the voltage source. All values of the DC-blocking capacitor and the RF-choking inductor are determined by considering their S-parameters embedded in a microstrip line with a characteristic impedance of 50 Ω. Capacitor C3 is a bypass capacitor used to improve the stability of the circuit, whose capacitance values are usually chosen by some typical values (100 pF in high-frequency circuits). The amplifier herein is internally matched to 50 Ω, so we just need to design the impedance of the microstrip line as 50 Ω to achieve impedance match. The line width of the microstrip can be calculated and optimized by empirical formulas.
To validate the proposed metasurface, simulations were conducted for both passive and active cases. Incorporating an amplification circuit into the unit cell design poses a challenge for network stability. To maintain stability of the amplification circuit, it is crucial to ensure that the isolation between the input and output ports is greater than the gain of the amplifier [31]. First, we consider the performance of the passive case, in which the amplification circuit between the two patch structures is removed, and the passive metasurface element without transistors is simulated using the commercial software CST Microwave Studio. To make the model more actual, we have included the pads and DC feed lines of the amplification circuit. Periodic boundary conditions are set along both the x- and y-directions, while an open boundary condition is applied along the z-direction. At the operating frequency, a high isolation of 22.5 dB is obtained between the port 1 and port 2 shown in Fig. 1(c), ensuring the stability of the circuit.
The active case is then numerically calculated with the assistance of Transient EM/Circuit co-simulation in CST Microwave Studio. Two discrete ports are utilized to connect the field and circuit, which are then connected to the amplifier at port 1 and port 2, respectively. The complete model is constructed in the circuit simulator of CST, and the active gain of the unit cell can be determined by adding the S-parameter simulation task. As shown in Fig. 2(a), the co-polarized (cross-polarized) reflection coefficients for x- and y-LP incidence are denoted as Rxx and Ryy (Ryx and Rxy), respectively. The first and second subscripts represent the polarization state of the reflected and incident waves, respectively. The reflection coefficient of cross-polarization at 3.95 GHz is 9.4 dB, indicating that the energy of the reflected y-LP wave is significantly greater than that of the incident x-LP wave, indicating that the amplification circuit is operational. Meanwhile, the amplitude of the co-polarized reflection is 15.9 dB lower than the cross-polarized level, preventing the amplifier from the risk of the self-oscillation. In the case of y-LP incidence, the wave energy will be absorbed by the proposed unit cell, revealing a non-reciprocal property of reflective polarization conversion compared to x-LP incidence. The absorption of y-LP wave is primarily attributed to the unidirectional nature of the amplification circuit, which exhibits transmission and reflection coefficients both less than -10 dB when the incidence wave is reversely input to the port 2 of the chip amplifier. Furthermore, we can enhance the operation bandwidth of the active metasurface by extending the bandwidth of the receiving and radiating structures, e.g., employing aperture coupling method.
Fig. 2. (a) Simulated co-polarized and cross-polarized reflection amplitude of the unit cell for active case. (b) Simulated PCR of the proposed metasurface. Simulated surface current distributions at 3.95 GHz for (c) x-LP and (d) y-LP incidences.
The polarization conversion ratio (PCR), one metric for evaluating the performance of a polarization-converting metasurface, is defined as the ratio of reflected cross-polarized power to total reflected power and can be calculated as [32]:
Here, ${R_ \bot }$ and ${R_\parallel }$ represent the cross-polarized and co-polarized reflection coefficients, respectively. The PCR is always less than 100% due to the existence of co-polarized power. When implementing amplification circuits to enhance the reflected cross-polarized power, PCR is calculated as:
3. Experimental demonstration
To experimentally validate the performance of the proposed metasurface, a prototype is fabricated using the standard printed circuit board technology. The prototype consists of 5 × 5 unit cells and has an overall size of 363 × 350 mm2. The top layer and the ground layer are connected using an array of circular metalized via holes. To facilitate DC supply for the amplifiers, an isolation dielectric layer with a thickness of 0.5 mm is placed on the back of the ground, and the DC feeding lines are set at the bottom of the isolation layer. Amplifiers on the same column are all connected by a metal line whose width is 0.2 mm. Figure 3(a) depicts the top view of the fabricated sample, with an enlarged view of a unit cell.
Fig. 3. (a) Top view of the fabricated metasurface. (b) The measured reflection coefficients of the prototype under normal incidence, where the bias voltage is set as 5 V. (c) The measured co-polarized reflection amplitude of the prototype under different bias voltages at 4 GHz. (d) The measured reflection amplitudes of Ryx for different oblique incidences.
The measurement is performed in a standard microwave anechoic chamber [33] using two horn antennas connected to a vector network analyzer (Agilent E8363A). The standard-gain horn antennas are utilized for transmitting and receiving LP EM waves. Both antennas are set at equal distances from the prototype, acting as the transmitting and receiving antennas to measure the reflection coefficient. A metal plate that has the same size of the metasurface sample is employed as the reference to calibrate the measured amplitude. The incident angle is close to 5°, which is approximated as a normal incidence. The metasurface is connected to a DC voltage source, which supplied power to the RF amplification circuit. For x-LP incidence, the receiving horn antenna is rotated by 90° to receive the y-LP EM wave in order to record the cross-polarized reflection coefficient of the sample. The same steps are also applied for the y-LP incidence.
Figure 3(b) shows the measured co-polarized and cross-polarized reflection coefficients. Within the band of interest, the cross-polarized reflection coefficient remained above 0 dB from 3.88 GHz to 4.14 GHz, indicating that the reflected y-LP wave energy was greater than that of the incident x-LP wave. The peak cross-polarized coefficient of 5.18 dB occurred at 4 GHz, where the co-polarized coefficient is −5.19 dB, indicating an isolation lager than 10 dB between two output polarizations. Furthermore, the reflection amplitude can be dynamically controlled by adjusting the level of the applied DC voltage. As shown in Fig. 3(c), we measured the cross-polarized coefficient under different DC voltage at 4 GHz, and the maximum of the bias voltage is 5 V here. The results show that the tunable range of reflection amplitude can reach up to 20 dB, revealing that the magnitude of the reflected wave can be customized at will by adjusting the DC voltage. To evaluate the performance of the metasurface under oblique incidence, we measure the reflection coefficients for various incident angles from 0° to 40° with a step of 10°, as shown in Fig. 3(d). The metasurface exhibits stable performance under oblique incidence, and the functionality of simultaneous amplification and polarization conversion can be attained even at an incident angle of 30°.
4. Dual-polarization active metasurface
The active polarization-converting metasurface described above provides amplitude-gain performance only for x-LP incidence, while it does not support magnitude enhancement for the incidence with orthogonal polarization. Here, to accomplish a polarization-insensitive polarizer with amplification functions, we also present a design strategy for both x-LP and y-LP incidences to realize dual-polarized properties. The schematic of the proposed structure in depicted in Fig. 4(a), where ‘A1’ and ‘A2’ represent two amplifiers operating for x-LP and y-LP waves, respectively. When voltage is only applied to ‘A1’ while ‘A2’ is non-excited, the incident x-LP EM wave is converted to a y-LP wave with enhanced magnitude, and the incident y-LP EM wave is absorbed. The simulated reflection amplitude responses of the unit cell are shown in Fig. 4(b), where the cross-polarized and co-polarized reflection coefficients at 4 GHz are 11.48 dB and −11.72 dB for x-LP incidence, respectively, while they are −15.27 dB and -10.98 dB for y-LP incidence. In contrast, when the port ‘A2’ is excited with a supplied voltage, as shown in the bottom panel of Fig. 4(b), the incident y-polarized EM wave is converted to x-LP output wave with enhanced magnitude, while the incident x-LP wave is absorbed. Except for the opposite footprint, the simulated reflection coefficients are consistent with the previous case shown in the upper panel of Fig. 4(b).
Fig. 4. (a) Schematic of the dual-polarized active polarization-converting metasurface. (b) The simulated co-polarized and cross-polarized reflection amplitude of the unit cell for the cases that only amplifier ‘A1’ is excited (top panel), and only amplifier ‘A2’ is excited (bottom panel).
5. Conclusion
In summary, we have proposed an active polarization-converting metasurface with dynamically tunable reflection enhancement by modulating the externally biased voltage. It receives an x-LP wave, amplifies its magnitude, and then reradiates to the energy with orthogonal polarization. Meanwhile, a y-LP incidence cannot be amplified effectively due to its nonreciprocal properties. The experimental results roughly agree with the simulations, validating energy enhancement even at an obliquely incident angle as large as 30°. In addition, we have also proposed an amplifying polarization-converting metasurface that operates for both x-LP and y-LP incidences based on the similar design principle. The proposed metasurface may be a prospective candidate for boosting the future wireless communication systems in applications of spatial isolation, signal enhancement, and shaping the EM environment.
Funding
The Joint Fund of Ministry of Education for Equipment Pre-research (8091B032112); National Natural Science Foundation of China (62071215, 62271243).
Disclosures
The authors declare no conflicts of interest.
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.
References
1. X. Wang, “Status and prospects of radar polarimetry techniques,” J. Radars 5(2), 119–131 (2016). [CrossRef]
2. S. Noda, M. Yokoyama, M. Imada, A. Chutinan, and M. Mochizuki, “Polarization Mode Control of Two-Dimensional Photonic Crystal Laser by Unit Cell Structure Design,” Science 293(5532), 1123–1125 (2001). [CrossRef]
3. Z.-Y. Li, R.-J. Liu, L. Gan, J.-X. Fu, and J. Lian, “Nonreciprocal electromagnetic devices in gyromagnetic photonic crystals,” Int. J. Mod. Phys. B 28(02), 1441010 (2014). [CrossRef]
4. H. Wakatsuchi, S. Kim, J. J. Rushton, and D. F. Sievenpiper, “Waveform-dependent absorbing metasurfaces,” Phys. Rev. Lett. 111(24), 245501 (2013). [CrossRef]
5. L. Liu, X. Zhang, M. Kenney, X. Su, N. Xu, C. Ouyang, Y. Shi, J. Han, W. Zhang, and S. Zhang, “Broadband metasurfaces with simultaneous control of phase and amplitude,” Adv. Mater. 26(29), 5031–5036 (2014). [CrossRef]
6. S. Guo, Y. Zhao, Q. Cao, Z. Mao, J. Dong, S. Bie, L. Miao, and J. Jiang, “Multistate active control RCS signature for the continuous adjustment absorber/reflector transformation applications,” Opt. Express 29(15), 24151 (2021). [CrossRef]
7. B. Dong, C. Zhang, G. Guo, X. Zhang, Y. Wang, L. Huang, H. Ma, and Q. Cheng, “BST-silicon hybrid terahertz meta-modulator for dual-stimuli-triggered opposite transmission amplitude control,” Nanophotonics 11(9), 2075–2083 (2022). [CrossRef]
8. B. O. Zhu, J. Zhao, and Y. Feng, “Active impedance metasurface with full 360° reflection phase tuning,” Sci. Rep. 3(1), 3059 (2013). [CrossRef]
9. A. Arbabi, Y. Horie, M. Bagheri, and A. Faraon, “Dielectric metasurfaces for complete control of phase and polarization with subwavelength spatial resolution and high transmission,” Nat. Nanotechnol. 10(11), 937–943 (2015). [CrossRef]
10. K. Chen, Y. Feng, F. Monticone, J. Zhao, B. Zhu, T. Jiang, L. Zhang, Y. Kim, X. Ding, S. Zhang, A. Alù, and C. Qiu, “A Reconfigurable Active Huygens’ Metalens,” Adv. Mater. 29(17), 1606422 (2017). [CrossRef]
11. Y. Yuan, S. Sun, Y. Chen, K. Zhang, X. Ding, B. Ratni, Q. Wu, S. N. Burokur, and C. Qiu, “A fully phase-modulated metasurface as an energy-controllable circular polarization router,” Adv. Sci. 7(18), 2001437 (2020). [CrossRef]
12. Q. Hu, J. Zhao, K. Chen, K. Qu, W. Yang, J. Zhao, T. Jiang, and Y. Feng, “An Intelligent Programmable Omni-Metasurface,” Laser Photonics Rev. 16(6), 2100718 (2022). [CrossRef]
13. W. Yang, K. Chen, X. Luo, K. Qu, J. Zhao, T. Jiang, and Y. Feng, “Polarization-Selective Bifunctional Metasurface for High-Efficiency Millimeter-Wave Folded Transmitarray Antenna With Circular Polarization,” IEEE Trans. Antennas Propag. 70(9), 8184–8194 (2022). [CrossRef]
14. N. Yu, F. Aieta, P. Genevet, M. A. Kats, Z. Gaburro, and F. Capasso, “A broadband, background-free quarter-wave plate based on plasmonic metasurfaces,” Nano Lett. 12(12), 6328–6333 (2012). [CrossRef]
15. Y. Yang, W. Wang, P. Moitra, I. I. Kravchenko, D. P. Briggs, and J. Valentine, “Dielectric meta-reflectarray for broadband linear polarization conversion and optical vortex generation,” Nano Lett. 14(3), 1394–1399 (2014). [CrossRef]
16. J. P. Balthasar Mueller, N. A. Rubin, R. C. Devlin, B. Groever, and F. Capasso, “Metasurface polarization optics: independent phase control of arbitrary orthogonal states of polarization,” Phys. Rev. Lett. 118(11), 113901 (2017). [CrossRef]
17. F. Ding, B. Chang, Q. Wei, L. Huang, X. Guan, and S. I. Bozhevolnyi, “Versatile polarization generation and manipulation using dielectric metasurfaces,” Laser Photonics Rev. 14(11), 2000116 (2020). [CrossRef]
18. M. Cerveny, K. L. Ford, and A. Tennant, “Reflective switchable polarization rotator based on metasurface with PIN diodes,” IEEE Trans. Antennas Propag. 69(3), 1483–1492 (2021). [CrossRef]
19. Y. Hu, X. Ou, T. Zeng, J. Lai, J. Zhang, X. Li, X. Luo, L. Li, F. Fan, and H. Duan, “Electrically tunable multifunctional polarization-dependent metasurfaces integrated with liquid crystals in the visible region,” Nano Lett. 21(11), 4554–4562 (2021). [CrossRef]
20. W. Li, S. Xia, B. He, J. Chen, H. Shi, A. Zhang, Z. Li, and Z. Xu, “A reconfigurable polarization converter using active metasurface and its application in horn antenna,” IEEE Trans. Antennas Propag. 64(12), 5281–5290 (2016). [CrossRef]
21. Z. Tao, X. Wan, B. C. Pan, and T. J. Cui, “Reconfigurable conversions of reflection, transmission, and polarization states using active metasurface,” Appl. Phys. Lett. 110(12), 121901 (2017). [CrossRef]
22. S. Taravati, B. A. Khan, S. Gupta, K. Achouri, and C. Caloz, “Nonreciprocal nongyrotropic magnetless metasurface,” IEEE Trans. Antennas Propag. 65(7), 3589–3597 (2017). [CrossRef]
23. Y. B. Li, S. Y. Wang, H. P. Wang, H. Li, J. L. Shen, and T. J. Cui, “Nonreciprocal control of electromagnetic polarizations applying active metasurfaces,” Adv. Opt. Mater. 10(6), 2102154 (2022). [CrossRef]
24. S. Taravati and G. V. Eleftheriades, “Full-duplex reflective beamsteering metasurface featuring magnetless nonreciprocal amplification,” Nat. Commun. 12(1), 4414 (2021). [CrossRef]
25. T. Qiu, Y. Jia, J. Wang, Q. Cheng, and S. Qu, “Controllable Reflection-Enhancement Metasurfaces via Amplification Excitation of Transistor Circuit,” IEEE Trans. Antennas Propag. 69(3), 1477–1482 (2021). [CrossRef]
26. Q. Ma, L. Chen, H. B. Jing, Q. R. Hong, H. Y. Cui, Y. Liu, L. Li, and T. J. Cui, “Controllable and Programmable Nonreciprocity Based on Detachable Digital Coding Metasurface,” Adv. Opt. Mater. 7(24), 1901285 (2019). [CrossRef]
27. W. Pan, C. Huang, X. Ma, and X. Luo, “An Amplifying Tunable Transmitarray Element,” Antennas Wirel. Propag. Lett. 13, 702–705 (2014). [CrossRef]
28. A. Robinson and M. Bialkowski, “Polarization-independent planar amplifier array,” Microw. Opt. Technol. Lett. 20(6), 380–384 (1999). [CrossRef]
29. X. Wang, J. Han, S. Tian, D. Xia, L. Li, and T. J. Cui, “Amplification and Manipulation of Nonlinear Electromagnetic Waves and Enhanced Nonreciprocity using Transmissive Space-Time-Coding Metasurface,” Adv. Sci. 9(11), 2105960 (2022). [CrossRef]
30. S. Taravati and G. V. Eleftheriades, “Programmable nonreciprocal meta-prism,” Sci. Rep. 11(1), 7377 (2021). [CrossRef]
31. J. Loncar and Z. Sipus, “Challenges in Design of Power-amplifying Active Metasurfaces,” in 2020 International Symposium ELMAR (IEEE, 2020), pp. 9–12.
32. J. Ding, B. Arigong, H. Ren, M. Zhou, J. Shao, Y. Lin, and H. Zhang, “Efficient multiband and broadband cross polarization converters based on slotted L-shaped nanoantennas,” Opt. Express 22(23), 29143 (2014). [CrossRef]
33. B. Sima, K. Chen, X. Luo, J. Zhao, and Y. Feng, “Combining Frequency-Selective Scattering and Specular Reflection Through Phase-Dispersion Tailoring of a Metasurface,” Phys. Rev. Appl. 10(6), 064043 (2018). [CrossRef]
