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Abstract
With the large-scale commercialization of broadband wireless communication technologies, interference between spatial electromagnetic waves (EMWs) is becoming increasingly serious. As devices for solving electromagnetic interference (EMI) problems, microwave absorbers (MAs) have become increasingly mature with the rapid development of artificial electromagnetic structures. However, current MAs still face challenges in terms of wideband absorption, wide-angle absorption, thickness, etc., which limits their application scenarios. Therefore, this paper proposes a transparent ultrawideband polarization-insensitive absorber based on a grid indium tin oxide (ITO) structure. This design adopts a multilayer structure with a total thickness of 3.525 mm (0.114λL), close to the predicted minimum thickness, to expand the bandwidth. The grid-patterned ITO film resonant structure and ITO film ground plane are designed to further improve optical transmittance by increasing duty ratios. Simulation results demonstrate over 90% absorption from 9.7 GHz to 69.2 GHz, with a fractional bandwidth (FBW) of 150.8%. The absorber has good incident angle stability and polarization insensitivity. When the incident angle varies within 0-60°, the proposed absorber maintains an absorption close to 80% over an ultrawideband range, and the absorption remains nearly constant when the polarization angle changes from 0° to 90°. The fabricated sample shows a visible light transmittance of approximately 73.2%, and shows an absorption performance consistent with simulations in the 6-38 GHz frequency range, validating the effectiveness of the proposed design. Due to its transparent, ultrawideband absorption, wide incident angle stability and polarization insensitivity properties, this proposed absorber has important guiding significance for applications in high-frequency transparent electronics, radars, communications systems and other devices working in the microwave frequency band.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the rapid development of communication, navigation, and electronic devices in recent years, EMWs technology has made significant progress [1–3]. In particular, with the deepening research on 5G wireless communication technology, some enterprises have begun testing and studying high-frequency millimeter-wave transmission for 5G technology. From a global perspective, millimeter wave networks are spreading rapidly worldwide, enabling the ever-expanding ecosystem to benefit from economies of scale. As an important medium for information transmission, high-frequency EMWs have had a profound impact on human life [4–7]. In fields such as communication, navigation, and radar, they play a crucial role, significantly improving human productivity and quality of life [8–13]. However, the propagation of EMWs can cause radiation problems, which may interfere with electronic devices and cause harm to human health [14–17]. Long-term exposure to an electromagnetic environment can increase the risk of cancer, and a complex electromagnetic environment can also affect the normal operation of electronic devices. MAs, as an effective means of controlling EMWs, can efficiently absorb electromagnetic energy and have important application value in fields such as communication and radar [18,19].
With emerging design methods such as the use of nanomaterials [20,21], metamaterials [22,23], resistive films [24–27], and metal grid structures [28,29], the design of MAs has gradually evolved from perfect absorption at a single frequency to absorbers with characteristics such as multiband absorption, adjustable bandwidth, ultrawideband absorption, and optical transparency [30–34]. Some of these designs have universality and good absorption performance; for example, Amer et al. [35] designed a multiband microwave absorber (MA) based on a split-ring structure and a multilayered resonant pattern, achieving over 98% electromagnetic absorption at four frequency points. This design has important guiding significance for multifrequency resonant absorbers. Similarly, Huang et al. [36] designed a dual-band absorber for the industrial scientific medical (ISM) band. The unit structure is center-symmetric and features a new cross-shaped pattern etched on a polytetrafluoroethylene (F4B) substrate. This design exhibits excellent stability at 2.45 GHz and 5.8 GHz under different polarizations and incident angles. To extend multifrequency resonant absorption to broadband absorption, resistive materials or direct introduction of chip resistors can be used. For example, Nguyen et al. [37] designed a sandwich structure broadband absorber. The unit structure is composed of split rings and lumped resistors, achieving a broadband absorption response in the frequency range of 7.8-12.6 GHz under normal incidence at all polarization angles, with an absorption of more than 90%. Compared to lump resistors, resistive conductive films have a thinner thickness, better conductivity, and more convenient fabrication methods and are favored by researchers [38,39]. Xiong et al. [40] designed a broadband metamaterial absorber (MMA) using high-optical-transparency polymethyl methacrylate (PMMA) as the substrate placed on a periodically arranged ITO film. The MMA is optically transparent and has an absorption of more than 90% within the 6-17.8 GHz frequency range, with three distinct absorption peaks. Park et al. [41] implemented a wideband and high absorption MMA using resistive materials such as an ITO film. The absorber features highly symmetric ITO film split-ring resonators deposited on a PET substrate with transparency and flexibility, exhibiting insensitivity to the polarization angle. With a total thickness of only 0.171$\lambda _L$, where $\lambda _L$ is the wavelength corresponding to the low-frequency point of the absorption bandwidth, this design exhibits high-efficiency electromagnetic absorption above 90% within the 7.2-27 GHz frequency range. However, combining the various advantages of such absorbers, such as ultrawideband absorption, wide-angle stability, high absorption, high optical transparency, polarization insensitivity, and ultrathin thickness, remains a challenging task [42–44].
This paper proposes a transparent ultrawideband polarization-insensitive absorber based on a grid ITO structure. A multilayer structure using ITO films and optical transparent PET and PMMA substrates is designed to achieve wideband absorption. The total thickness of 3.525 mm, close to the minimum required thickness of 3.34 mm for the target bandwidth, is determined. The ITO films are patterned into grid structures to enhance optical transmittance while maintaining wideband absorption. Simulations are performed to analyze the effects of different ITO duty ratios on balancing transparency and absorption performance. The central symmetry of the resonant pattern leads to polarization insensitivity. Simulation results demonstrate over 90% absorption from 9.7 GHz to 69.2 GHz under normal incidence. Good absorption over 80% from 0° to 60° incident angles is attained for both TE and TM polarizations. To validate the simulations, samples are fabricated and tested from 6 GHz to 38 GHz. The experimental results are consistent with simulations, confirming the effectiveness of the proposed transparent ultrawideband absorber design.
2. Design and simulation
Figure 1(a) illustrates that the proposed absorber can efficiently absorb EMWs within a specified band while maintaining good optical transparency in the visible light frequency range. Figures 1(b-f) depict the specific structures of the absorber periodic unit. The absorber periodic unit is a multilayer structure consisting of, from bottom to top, an ITO ground plane, PET, PMMA, ITO Layer1, PET, PMMA, ITO Layer2, PET, and PMMA. The PET and PMMA substrates provide an optical transparency of more than 90%, while the ITO films provide an optical transparency of more than 80%. The design of the grid structure further improves the overall optical transparency of the absorber. The grid structure is a common structure used for electromagnetic shielding, consisting of a set of parallel metal conductor wires arranged in a grid pattern. When EMWs pass through the grid structure, their electric and magnetic fields are obstructed and reflected by metal conductor wires, thereby weakening or eliminating electromagnetic radiation. The ITO films are direct transition wide-bandgap semiconductor materials with certain optical transparency and electronic conductivity. Their crystal structure is a cubic spinel structure [45,46]. The crystal structure lacks oxygen atoms and has excess free electrons, making it suitable for the production of grid structure absorbers. Compared with circular, triangular, and other irregular patterns, the square pattern is a more reasonable choice to etch the grid structure. The square grid remains effectively connected after etching, which can prevent the ITO from falling off. Meanwhile, the square pattern can also easily control the duty ratio accurately. When meshing in HFSS, the square pattern can obtain a more precise grid and faster simulation. Therefore, the square pattern is chosen to etch the grid structure in the design analysis.
Fig. 1. (a) Array of absorbing materials. Schematic diagram of the unit structure and ITO films: (b) separated structure, (c) cross-sectional diagram, (d) ITO ground plane, (e) ITO Layer1, and (f) ITO Layer2.
To achieve optimal absorption efficiency, Rozanov believes that the maximum thickness of the absorber is related to its bandwidth [47], and the relationship between the absorption bandwidth and thickness of the multilayer absorber can be expressed as:
Based on the preset design targets of $f_1$=5 GHz, $f_2$=9.7 GHz, $f_3$=69.2 GHz, $f_4$=80 GHz, and $S_0$=-15 dB, the minimum thickness of the absorber can be calculated. The calculation yields a minimum thickness of 3.34 mm. However, the thicknesses of the substrates provided by the factory must also be taken into account. After considering these factors, the final thickness of the proposed absorber is determined to be 3.525 mm.
The specific structural parameters of the absorber unit are listed as follows: Each of the three PMMA $(\varepsilon _r=2.25, tan\delta =0.01)$ layers has a thickness of $d1$=1 mm; each of the three PET $(\varepsilon _r=3, tan\delta =0.06)$ layers has a thickness of $d2$=0.175 mm. The thicknesses of the ITO films used for the ground plane and the two resonant pattern layers are $d3$=150 nm, which can be ignored. The ITO film resistance for the ground plane is 7 $\Omega /sq$, and the ITO film resistance for the two resonant pattern layers (Layer1, Layer2) is 15 $\Omega /sq$. The perimeter of the unit is $P$=8 mm, and the side lengths of the resonant patterns are $l1$=3.5 mm and $l2$=7 mm, with corresponding widths of $w1$=1.2 mm and $w2$=1.25 mm. The duty ratios of the ground plane and the two resonant pattern layers are 55% and 75%, respectively.
To investigate the absorption characteristics of the proposed absorber, we conducted simulation and optimization using HFSS2021 three-dimensional electromagnetic simulation software. We selected the x and y directions as the master and slave boundaries and simulated an infinite periodic structure. A Floquet port was set in the z direction to handle EMWs irradiation with arbitrary polarization and incident angle.
The simulation results shown in Fig. 2(a) indicate that S11 and S21 are almost identical for the TE and TM modes. In the frequency range from 9.7 to 69.2 GHz, the reflection coefficient S11 is below −10 dB. Furthermore, five absorption peaks are observed at frequencies of 13.46 GHz, 26.26 GHz, 44.72 GHz, 61.92 GHz, and 66.46 GHz. Overall, the transmission coefficient S21 is less than −21.6 dB. The proposed absorber has a transmittance of less than 1%, which can be neglected. The absorption of the absorber $A(\omega )$ can be computed utilizing Eq. (4):
where $R(\omega )$ and $T(\omega )$ denote the reflectance and transmittance, respectively. Figure 2(b) illustrates the absorption, revealing that the proposed absorber has an absorption exceeding 90% in the frequency range of 9.7 to 69.2 GHz for both the TE and TM modes. The absorber exhibits an absolute absorption bandwidth of 59.5 GHz, with an FBW of 150.8%. To explain how the multilayer absorber structure can improve the absorption bandwidth, we compared the initial single-resonant structure design (Model 1) to the final dual-resonant pattern structure design (Model 2) through simulations. Model 2 adds two layers of PMMA and one ITO-PET resonant pattern, providing more tunable parameters to optimize absorption performance. The results are shown in Fig. 3(a). Model2 further extends the absorption bandwidth at low and high frequencies, in addition to the absorption in the 20-53 GHz frequency range, compared to Model1.The equivalent impedance $Z_{eff}$ can be used to analyze the broadband absorption mechanism of MAs. It can be calculated from the simulated S parameters based on effective medium theory. The calculation formula is as follows:
To achieve ultrawideband absorption, the equivalent impedance $Z_{eff}$ of the proposed absorber needs to match the free-space wave impedance of 377 $\Omega$. Normalization of the equivalent impedance of the absorber with the wave impedance of free space can enable better observation of the matching condition. When the real part of the normalized impedance is equal to 1 and the imaginary part is equal to 0, it is commonly accepted that perfect absorption has been attained by MAs. As shown in Fig. 3(b), in the entire frequency range of 9.7-69.2 GHz, the real part of the normalized impedance fluctuates within the range of 1±0.6, and the imaginary part fluctuates within the range of 0±0.6. At the frequency point of 26.26 GHz and in the two frequency ranges of 42.54-46.72 GHz and 59.64-67.28 GHz, the absorption is greater than 99%, and the real part of the normalized impedance is close to 1, while the imaginary part is close to 0. This indicates that the proposed absorber has good matching with the free-space wave impedance in the operating frequency band.
The thickness of the PMMA substrate layer is one of the tunable parameters of the absorber. To study the effect of PMMA layer thickness on absorption performance, we simulated different thicknesses of PMMA substrates. As shown in Fig. 4, as d1 (PMMA layer thickness) increased from 0.6 mm to 1.4 mm with a step of 0.2mm, the overall absorption bandwidth shifted to lower frequencies. The absorption bandwidth of each thickness is shown in Table 1. It is worth noting that except for 0.6 mm, where the absorption fluctuates greatly in the 84-100 GHz frequency band, leading to a decrease in bandwidth, the relative bandwidth changes little for other thicknesses. In general, the PMMA layer thickness parameter can be used to finely tune the shift of the absorption bandwidth to lower frequencies.
Table 1. The absorption bandwidths of different PMMA layer thicknesses $d1$.
The incident angle stability and the polarization angle stability of the absorber are two important indicators to evaluate the performance of MAs. Figures 5(a) and 5(b) show the relationship between the absorption, frequency and incident angle in the TE and TM modes. The trends are basically the same in the two modes. Taking the TE mode as an example, when the incident angle is below 15°, the absorber maintains an absorption efficiency of more than 90% within the original bandwidth range of 9.7-69.2 GHz. When the incident angle rises from 15° to 30°, the absorption slightly shifts toward higher frequencies, and the bandwidth with absorption higher than 90% changes to 10.4-75 GHz. When the incident angle rises from 30° to 45°, the absorption again shifts toward higher frequencies, and the absorption is greater than 88% in the bandwidth range of 11.41-78.9 GHz. When the incident angle rises from 45° to 60°, the absorption slightly decreases, but the absorption is still close to 80% in the bandwidth range of 12.7-80 GHz. Therefore, the proposed absorber has a stable absorption ability for EMWs with incident angles from 0° to 60°. As shown in Fig. 5(c), during the polarization angle change process from 0° to 90°, the absorption at each frequency point of the proposed absorber is almost the same, indicating that it has a polarization-insensitive characteristic.
Fig. 5. (a) and (b) Absorption under different incident angles in the TE and TM modes respectively. (c) Absorption under different polarization angles.
By analyzing the surface loss densities of Layer1, Layer2, and the ground plane at specific frequencies, the energy loss of the absorber can be understood. According to the analysis in Fig. 2, the proposed absorber has five main resonant frequencies: 13.46 GHz, 26.26 GHz, 44.72 GHz, 61.92 GHz, and 66.46 GHz. Due to the polarization-insensitive characteristic of the absorber, only the case of an EMWs vertically incident in the TE mode is considered. As shown in Figs. 6(a-e), strong surface loss is generated in Layer1 at 26.26 GHz, 44.72 GHz, 61.92 GHz and 66.46 GHz, with the surface loss mainly concentrated in the center of Layer1 at 26.26 GHz and at the edge of Layer1 at 44.72 GHz, 61.92 GHz and 66.46 GHz. As shown in Figs. 6(f-j), at 13.46 GHz, the surface loss generated by Layer2 is mainly concentrated on one side of the cross pattern, and at 26.26 GHz, the surface loss generated by Layer2 is relatively small compared to that at the other resonant frequencies, while at 44.72 GHz, 61.92 GHz, and 66.46 GHz, the surface loss is distributed throughout Layer2. As shown in Figs. 6(k-o), the ground plane basically has no surface loss at 13.46 GHz, 26.26 GHz, and 44.72 GHz and only a weak surface loss at 61.92 GHz and 66.46 GHz. By observing Fig. 6 longitudinally, it can be found that at 13.46 GHz, only Layer1 generates strong surface loss, and at 26.26 GHz, only Layer2 generates strong surface loss, achieving nearly 99% absorption at these two frequencies. At the other three frequencies, both Layer1 and Layer2 have good surface loss, achieving almost perfect absorption. In summary, the proposed absorber relies mainly on Layer1 and Layer2 to dissipate incident electromagnetic energy, while the ground plane prevents the transmission of EMWs.
Fig. 6. Surface loss densities of (a)-(e) Layer1, (f)-(j) Layer2, and (k)-(o) the ground plane at 13.46 GHz, 26.26 GHz, 44.72 GHz, 61.92 GHz, and 66.46 GHz.
The use of a grid structure can improve the optical transmittance of the absorber, and a higher duty ratio typically results in higher optical transmittance. The duty ratio of a grid structure refers to the ratio of the open area to the total ITO pattern area, which is used to measure the proportion of the ITO pattern in the original plane. In Fig. 7, taking the ground plane as an example, ground plane A is considered the total pattern area, which is divided into a 10x10 grid, and each grid is proportionally reduced to obtain the open area B that needs to be etched. The final grid structure ground plane is obtained by performing a Boolean subtraction operation on A and B. Here, B/A represents the duty ratio of the ground plane.
To further study the effect of the grid structure on the absorption performance, parameter scanning was performed on the duty ratios of the ground plane, Layer1, and Layer2. A duty ratio of 0% means that the entire ITO is not structured, i.e., B/A=0, while a duty ratio of 100% means that all of the ITO has been etched away, i.e., B/A=1. Therefore, the parameter scanning range for the duty ratios was set to between 5% and 95%, with default duty ratios of 55%, 75%, and 75% for the ground plane, Layer1, and Layer2, respectively.
As shown in Figs. 8(a) and 8(d), the duty ratio of the ground plane has a weak effect on the absorption in the frequency range of 8-72 GHz when it varies from 5% to 55%. As the duty ratio of the ground plane gradually increases from 55% to 95%, the absorption bandwidth in the frequency range of 8-20 GHz shrinks; in particular, when the duty ratio increases from 75% to 95%, the absorption significantly decreases, with the largest decrease occurring at 8 GHz, where it drops to a minimum of 67%. In the frequency range of 20-80 GHz, the absorption only slightly fluctuates with the change in the duty ratio. Considering that the ground plane is the last line of defense against EMWs transmission, the duty ratio should not be too high.
Fig. 8. Absorption of the ground plane, Layer1 and Layer2 at different duty ratios and frequencies: (a)-(c) three-dimensional graphs show, (d)-(f) two-dimensional graphs show.
As shown in Figs. 8(b) and 8(e), the variation in the duty ratio of Layer1 has a significant impact on the absorption in the frequency ranges of 16-24 GHz and 24-36 GHz. For the frequency range of 16-24 GHz, the lower the duty ratio is, the lower the absorption, with the lowest absorption occurring at 20.46 GHz, where it drops from 95% to 76%. For the frequency range of 24-36 GHz, the absorption starts to decrease when the duty ratio is higher than 75%, and when the duty ratio increases to 95%, the absorption is only 70% at 30.8 GHz. A duty ratio of 65-80% for Layer1 can achieve better absorption.
As shown in Figs. 8(c) and 8(f), the absorption shows a slight shrinking trend with increasing duty ratio of Layer2 in the frequency range of 5-80 GHz. In the frequency range of 11-16 GHz, the absorption is lower than 90% when the duty ratio is between 5% and 60%. In the frequency range of 16-70 GHz, the duty ratio has a small effect on the absorption. To obtain a continuous absorption bandwidth, the duty ratio of Layer2 can be selected in the range of 60-75%.
In summary, to achieve broadband absorption of the absorber and improve the optical transmittance as much as possible, the duty ratios of Layer1 and Layer2 can be selected in the range of 65-75%. The duty ratio of the ground plane should be as low as possible, preferably below 60%, to prevent EMWs transmission.
3. Fabrication and experimental confirmation
To verify the actual absorption performance of the proposed absorber, a 200*200 mm sample was made using 25*25 cells and measured. The key technique for sample preparation is the etching of ITO. First, three layers of ITO-PET were made using laser engraving technology. The sheet resistance of the ground plane ITO is 7 $\Omega /sq$, with a duty ratio of 55%, and the sheet resistances of Layer1 and Layer2 are 15 $\Omega /sq$, with duty ratios of 75%. According to Figs. 1(b) and 1(c), ITO-PET was sequentially laminated on the PMMA transparent substrate. As shown in Fig. 9(a), the prepared sample is optically transparent. As a result of the bonding of multiple medium layers, the optical transmittance of the sample is slightly reduced. Fig. 9(b) shows the measured optical transmittance of sample and ITO films (7 $\Omega /sq$) with duty ratios of 55% and 0% on 0.175 mm thick PET substrates in the wavelength range of 380-800 nm, which are 73.2%, 81.6% and 75.65%, respectively. Since the ITO film is ultrathin, it needs to be fixed on the PET substrate for measurement. The comparison of results indicates that increasing the duty ratio of the grid structure can improve the physical transparency of the absorber. Using a Shydwg transmittance measuring instrument, as shown in Fig. 9(c), the optical transmittance in the visible light frequency range is found to be 73.2%.
Fig. 9. (a) Transparent sample. (b) Transmittance spectra of the sample and ITO (with 0.175 mm thick PET) at duty ratios of 55% and 0%. (c) Transmittance Measurement Results of the Sample. (d) Test environment.
As shown in Fig. 9(d), the entire testing apparatus was set up in an environment with absorbing foam to avoid interference from external factors. Two mechanical arms that could swing left and right were used to control the position of the transmitting and receiving horn antennas, which were used to adjust the incident angle of the EMWs. The minimum angle was 5 degrees, which was approximately normal incidence. Placing a metamaterial shield between the two antennae improved their isolation and prevented transmission of energy directly from the transmitting antenna to the receiving antenna. The sample was placed in the plane of rotation of the two mechanical arms. The two horn antennas were connected to the receiving and transmitting ports of a vector network analyzer via low-loss cables. During the measurement process, we used three different sets of transmitting and receiving antennas covering the frequency bands of 6-18 GHz, 18-26 GHz, and 26-38 GHz. Due to the limitation of experimental equipment, antennas and vector network analyzer above 38 GHz were not available in the lab, thus the measurement could not cover the full designed bandwidth up to 69.2 GHz.
The measurement results are shown in Fig. 10. When comparing the actual measured parameters with the simulated parameters, the measured S11 and absorption deviate toward lower frequencies in the frequency range of 6-22 GHz. In the range of 22-38 GHz, the simulated absorption curve matches well with the measured curve. The sample has two resonant peaks at 9.3 GHz and 24.5 GHz, with corresponding S11 values of −24.8 dB and −39.7 dB. In the range of 14-20 GHz, the measured S11 curve is slightly above −10 dB, with the lowest point at 17 GHz and an absorption of 85.6%. The first resonant peak shifts slightly toward lower frequencies. The possible reasons for this discrepancy include: the adhesive used in the multilayer structure bonding process, the relative positional deviation during bonding, the inconsistent angle between the transmitting and receiving antennas used for testing and the simulated environment, and factors like oxidation of the sample during transportation, etc. All these factors can affect the actual experimental measurement results.
As shown in Table 2, compared with single resonant pattern and dual resonant pattern structures of the same type, the absorber proposed in this paper has significant advantages in bandwidth, and is slightly lower than the triple resonant pattern structure Ref. [27,34]. In terms of transparency, due to the introduction of the grid structure, it leads ahead of absorbers with the same thickness and number of resonant layers. In addition, the absorber performs well in thickness, incident angle stability, polarization and other aspects, and is a balanced absorber design in overall performance.
Table 2. Shape Functions for Quadratic Line Elements.
4. Conclusion
In this paper, a transparent ultrawideband polarization-insensitive absorber with optimal thickness based on a grid ITO structure was proposed. A multilayer structure is adopted for the absorber, which includes multiple PET and PMMA substrates and ITO films. The ITO films are etched into a grid structure to improve the optical transmittance of the absorber. Based on the minimum thickness theory of the absorber, preset frequency response curve, and factory processing capabilities, the absorber thickness is finally determined to be 3.525 mm, which can be considered the optimal thickness. The absorber exhibits excellent absorption performance in the frequency range of 9.7-67.2 GHz, with an absolute absorption bandwidth of up to 59.5 GHz and an FBW of 150.8%. At a frequency of 26.26 GHz and in the frequency ranges of 42.54-46.72 GHz and 59.64-67.28 GHz, the absorption is above 99%. Additionally, in the visible light band, the absorber has an optical transmittance of approximately 73.2%. The normalized impedance of the absorber confirms its good matching with the free-space wave impedance. Compared with existing MAs, this absorber has a wider operating bandwidth, good absorption, and superior optical transmittance. In addition, the absorber is insensitive to the polarization angle of the incident EMWs. When the incident angle changes in the range of 0-60°, the absorber can always maintain an absorption close to 80% in an ultrawideband range. Experiments on fabricated samples validate the simulations from 6 GHz to 38 GHz. This high-performance transparent ultrawideband absorber shows great potential for applications in transparent electronics, electromagnetic interference reduction, energy harvesting, radars and communications systems.
Funding
Open funding of Guangdong Provincial Key Laboratory of Millimeter-Wave and Terahertz (2019B030303021002KF2105); Guangzhou Science and Technology Innovation Development Special Fund Project (202102020330107); Open Funding of State Key Laboratory of Millimeter Waves.
Acknowledgments
We would like to thank Prof. Wenhua Gu and Dr. Chen Fu from Nanjing University of Science and Technology for their help with the testing of the experiment.
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 corresponding author upon reasonable request.
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