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Abstract
An optically transparent water-based radar-infrared compatible stealth structure is proposed in this paper, which acts as a wideband microwave switchable absorber/reflector and features low infrared radiation at the same time. The enhanced wideband switching function between the microwave absorbing state (10.52–20.04 GHz for TE mode and 10.52–20.21 GHz for TM mode) and the reflecting state in the entire band (5–26.5 GHz) is realized through the injection and discharge of pure water. The low infrared radiation characteristics can be obtained by properly designing the infrared shielding layer (IR-SL) on the top and the temperature management through water circulation. The unique properties of the proposed structure in microwave, optical and infrared bands are validated by separate experiments. Owing to its multifunctional performance, the proposed design is expected to be applied in various potential applications, especially the multispectral stealth area.
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1. Introduction
Over the past few decades, electromagnetic absorbing materials have been extensively investigated due to their manifold applications in various fields such as target stealth, electromagnetic compatibility (EMC) and electromagnetic interference (EMI) suppression [1]. The Salisbury screen, which exhibits narrow band absorbing properties, is realized by placing a homogeneous resistive sheet λ/4 above a conductive backplane [2]. As an extension of the Salisbury screen, the Jaumann absorber is developed to broaden the absorption bandwidth by simply cascading multiple resistive layers and substrates in between at the expense of an increasing thickness [3]. The frequency selective surface (FSS) absorber, as a kind of circuit analog (CA) absorber, performs as a potential candidate to simultaneously enhance the absorbing performance and reduce the structure profile [4]. Due to the practical requirements in stealth systems, considerable attention has been paid to wideband absorbers [5–8]. Wideband switchable absorber/reflector structures, which enhance the survivability at absorbing state and work as a reflector for easy detection and discovery at reflecting state, are highly demanded to adapt to the complex electromagnetic environment [9–13]. Meanwhile, transparent wideband absorbers are highly demanded in the fields where optical transparency or aesthetic effect is required (e.g. windows of stealth targets) [14–17]. Water is a cheap, abundant and environmentally friendly resource in nature. Due to its optical transparency, microwave frequency dispersion and high dielectric loss, water is highly attractive in designing transparent all-dielectric metamaterials with wideband absorbing characteristics [18,19]. However, most of the studies previously reported mainly concentrate on the radar absorbing properties and less attention has been paid to the infrared radiation performance of the structure.
With the rapid development of the multispectral composite detection technology, the realization of stealth in a single band can no longer satisfy the camouflage requirement of weapons and equipments. Nowadays, the radar detection and the infrared detection are the two main detection methods. It is crucial to achieve the radar-infrared compatible stealth camouflage considering that the composite detection mode in both radar and infrared bands are usually simultaneously applied in practice. Nonetheless, a paradoxical issue lies in the fact that low reflection and high absorption are required in radar stealth; on the contrary, high reflection and low absorption are demanded to achieve infrared stealth. In addition, an increase in the temperature of the object will be resulted from the heat produced by the absorption and dissipation of incident radar waves, which further hampers the infrared stealth. Fortunately, the large interval between the radar and infrared bands offers a possibility of realizing radar absorbing materials with infrared camouflage.
Under the law of Stefan-Boltzmann, the infrared radiation (M) can be described by the equation below:
where σ is the Stefan-Boltzmann constant, T is the absolute temperature of an object and ɛ(T) is the infrared emissivity determined by the surface properties of an object. It can be seen from Eq. (1) that both the surface infrared emissivity and the body temperature of an object have an influence on the infrared radiation. Although traditional materials, including nano materials, conductive polymer materials, doped oxide semiconductors and photonic crystals, have been explored in achieving radar-infrared compatible stealth [20–23], it is usually difficult to realize wideband and highly efficient radar absorption as well as low infrared radiation at the same time, let alone to independently manipulate the characteristics in different frequency bands. Compared to other composite stealth materials, the electromagnetic metamaterials characterized with promising capabilities of manipulating electromagnetic waves can offer more freedoms and possibilities for radar-infrared compatible stealth structures. In [24], a thin radar-infrared compatible stealth structure, consisting of a metallic FSS layer, a resistive FSS layer and a metal plate from top to bottom, has been successfully designed. A hybrid metasurface that features reduction of microwave reflection and infrared radiation has been proposed [25]. The structures aforementioned are opaque to the visible light, which cannot meet the requirement of optical transparency in practical applications. To address the challenging but intriguing issue, some radar-infrared bi-stealth structures with optical transparence have been presented recently [26–30]. In [29] and [30], flexible materials are employed in designing flexible and transparent radar-infrared compatible stealth structures, which can be applied in nonplanar scenarios. However, the radar-infrared compatible stealth structures reported aforementioned only achieve radar absorbing function without the reconfigurable capability presented in [9–13] so that the microwave performance is fixed once built.In this paper, a flexible and optically transparent radar-infrared compatible stealth structure is developed, which deals with the challenging issues mentioned above. To the best of our knowledge, it is the first multifunctional and multispectral design that simultaneously integrates the advantages of wideband switchable radar absorbing/reflecting states, optical transparency and low infrared radiation. Indium tin oxide (ITO) films deposited on ultrathin polyethylene terephthalate (PET) substrates are employed to design the top-layer lowpass filter and the bottom-layer reflective backplane with optical transparency. By properly adjusting the filling ratio of the ITO structure in the top layer, a low emissivity can be obtained in the infrared band with the microwave transmittance larger than 95%. The wideband absorption performance is mainly implemented by the clear structured water encapsulated in the transparent polydimethylsiloxane (PDMS) microfluidic structure. Due to the frequency dispersion and high dielectric loss of pure water, highly efficient absorption properties can be guaranteed in a wide frequency band. One unique feature of the proposed design lies in that a switching function between the absorbing and reflecting states with a large isolation can be obtained by injecting and discharging pure water. Meanwhile, the infrared radiation of the proposed structure can be also managed by controlling the temperature of pure water through dynamic circulation. The feasibility of the proposed design is validated by simulations and measurements, demonstrating its potential application in the multifunctional and multispectral stealth field.
The remainder of the paper is organized as follows. In Section 2, a detailed design description of the transparent radar-infrared compatible structure is presented. Then, design prototypes are fabricated and the properties in microwave, optical and infrared bands are validated by separate experiments in Section 3. Finally, a conclusion is drawn in Section 4.
2. Design of the transparent radar-infrared compatible structure
The radar-infrared compatible stealth composite structure integrated with both wideband microwave absorbing/reflecting characteristics and low infrared radiation features can be developed by placing an infrared shielding layer (IR-SL) on the top of a water-based switchable radar absorber/reflector (R-A/R).
According to the Kirchhoff’s law, the emissivity equals to the absorbing rate of an object at the thermal equilibrium. Therefore, the metals with high reflection, such as copper, aluminum, silver, gold and platinum, are conventional materials to implement infrared stealth. Although most metal materials can be employed for infrared camouflage, their infrared emissivities increase dramatically as metal oxides form on the surface. The transparent ITO material, which fills in the gap of metal materials, is a potential candidate to realize radar-infrared compatible stealth. The real part of the ITO permittivity is negative as described by the Drude model, and thus it can behave like a metal in the infrared band. The electromagnetic features of the ITO film in the microwave band are similar to those in the infrared band and most incident waves will be reflected with a low transmittance [30]. In this case, microwave waves are usually not allowed to transmit through continuous ITO films with no gaps. The IR-SL can be achieved by employing capacitive frequency selective surface (FSS) structures, which reflect infrared lights and transmit microwave waves. The top view of the IR-SL is shown in Fig. 1(a). The IR-SL comprises ITO square patch elements deposited on an ultrathin PET substrate whose dielectric constant, loss tangent and thickness are ɛpet1 = 3, tanδ1 = 0.06 and Hpet1 = 50 µm, respectively. The period and the side length of the ITO square patch are denoted by m and l, respectively. The gap between adjacent elements is denoted by s, which satisfies the relation of m = l + s. The parameters l and s are the critical ones that simultaneously decide the microwave transmittance and the filling ratio of the ITO structure.
Fig. 1. The IR-SL. (a) Top view of the IR-SL. Performance of the IR-SL (b) when m = 1 mm and l varies; (c) when s/l = 3/7 and m varies.
To investigate the microwave properties, the transmission performance of the IR-SL is acquired by using the electromagnetic simulation software HFSS. Two pairs of Master-Slave boundaries are assigned to mimic an infinite periodic array and the Floquet port is employed as the excitation. The Impedance boundary condition is adopted to model the ITO structure, and the surface resistance of the ITO film (RITO1) is set as RITO1 = 6 Ω/sq to obtain metal-like properties. A high microwave transmittance over 95% can be obtained within the frequency range (5–26.5 GHz) by choosing appropriate parameters. The transmission coefficients of the IR-SL under different geometric parameters are illustrated in Fig. 1(b) and Fig. 1(c). As shown in Fig. 1(b), the microwave transmittance decreases as the side length l increases at a given period size (m = 1.0 mm). It is depicted in Fig. 1(c) that the transmission reduces with an increasing period m for the same ratio of s/l (s/l = 3/7).
The infrared emissivity (ɛ) of the IR-SL is a combination of the emissivities of both the ITO film and the PET substrate due to the discontinuity of the IR-SL, which can be calculated by the following equation:
where ɛi and ɛs are the infrared emissivities of the ITO film and the PET substrate, and the measured values of them are about 0.09 [30] and 0.9 [31], respectively. The filling ratio fi of the ITO structure can be obtained by the following equation:According to Eq. (3), the filling ratio of the ITO structure under various sets of geometric parameters (m, l, s) are listed in the legends of Fig. 1(b) and Fig. 1(c). It can be concluded that the filling ratio of the ITO structure becomes larger as its side length increases at a given period size. Besides, the filling ratio of the ITO structure remains unchanged under different period sizes as long as the ratio of s/l is the same. The infrared emissivities corresponding to different filling ratios are also calculated by using Eq. (2) after obtaining the values of filling ratios, indicating that a low infrared emissivity can be realized by choosing proper geometric sizes of the ITO structure.
Obviously, it can be inferred that there is a compromise between the infrared emissivity and the microwave transmittance. Based on the comprehensive consideration of low infrared emissivity, high microwave transmittance and easy fabrication accuracy, one specific case (i.e. m = 1 mm, l = 0.7 mm, s = 0.3 mm) is chosen as the geometric parameter set of the IR-SL in the following section of this manuscript. In this case, the calculated infrared emissivity is 0.5031 and the microwave transmittivity is higher than 95% in the entire microwave band.
The schematic diagrams of the super-element and the array of the proposed radar-infrared compatible absorber/reflector are illustrated in Fig. 2. The structure mainly consists of two parts including an IR-SL and an R-A/R right below. The upper IR-SL comprises a 6 × 6 array of ITO square patches in a single super-element. The R-A/R is composed of a PDMS cover plate, a structured water layer, a PDMS container and an ITO-PET film as the backplane. The composite structure possesses flexibility and optical transparency since the materials (i.e. ITO, PET, PDMS and pure water) used in the design are all flexible and optically transparent in the visible band. The permittivity and loss tangent of the PDMS material are 2.72 and 0.0027, respectively. The surface resistance of the ITO film (RITO2), the permittivity (ɛpet2), the loss tangent (tanδ2) and the thickness (Hpet2) of the PET substrate in bottom layer are the same as those of the IR-SL (i.e. RITO1 = RITO2, ɛpet1 = ɛpet2, tanδ1 = tanδ2 and Hpet1 = Hpet2). The detailed geometric parameters of the R-A/R are listed below: p = 6 mm, a = 3 mm, b = 1 mm, c = 2.6 mm, d = 5 mm, hw = 0.4 mm, Hpdms1 = 1.2 mm, Hpdms2 = 4.6 mm, Hpdms3 = Hpdms2 – c = 2 mm.
Fig. 2. The schematic diagrams of the proposed design. (a) Perspective view of the unit cell. (b) Perspective view of the array.
The structured water in this paper is designed as a combination of four standing-up water blocks around a PDMS cylinder and a cross water plate beneath the cylinder structure, which guarantees the connection and circulation of water between adjacent unit cells. The complex permittivity of the pure water can be predicted by the Debye model [32], and the general equation is presented in Eq. (4):
By using the Debye model, the variations of the complex permittivity of pure water as a function of frequency and temperature are shown in Fig. 3. It can be observed that the complex permittivity exhibits obvious frequency dispersion characteristics, which is beneficial to realize wideband absorbing properties. The real part of the complex permittivity strictly decreases while the imaginary part climbs up and then declines as the frequency increases at a fixed temperature. The permittivity and the loss tangent of pure water at the room temperature of 25 ℃ under a standard atmospheric pressure are imported into the simulation software to introduce the frequency dispersion and high dielectric loss characteristics of the pure water.
Fig. 3. The variations of the complex permittivity of pure water as a function of frequency and temperature. (a) Real part. (b) Imaginary part.
The reflection coefficients of the proposed design for different polarizations under oblique incidence are shown in Fig. 4. When the interior structure is filled with pure water, wideband absorbing properties can be attained, and the −10 dB absorbing bandwidths under normal incidence are 9.52 GHz (10.52–20.04 GHz) and 9.69 GHz (10.52–20.21 GHz) for TE and TM polarizations, respectively. On the contrary, when the water is extracted out of the structure, the proposed design switches to an opposite function and operates as a wideband reflector almost in the whole frequency band. Therefore, an evident wideband switching function between the absorbing and reflecting states can be realized by simply injecting and expelling the pure water. Besides, the situation of the oblique incidence (0–30°) is also considered in Fig. 4. In addition, the polarization independence is guaranteed by the symmetry of the proposed structure. Compared to the switchable absorber/reflector designs that stimulated by external dc bias, the water-based wideband switchable radar absorber/reflector requires no extra lumped devices and bias lines, and the strong and wideband reflection performance can be guaranteed since the structured water with high dielectric loss is discharged at reflecting state.
Fig. 4. The simulated reflection coefficients of the proposed design with and without water for different polarizations under oblique incidence. (a) TE polarization. (b) TM polarization.
In order to investigate the working mechanism of the proposed design, the distributions of the electric field and the magnetic field at different resonant frequencies are plotted in Fig. 5(a) and Fig. 5(b). At the first resonant point of 12.62 GHz, a strong electric field distribution can be observed in the IR-SL, which demonstrates its apparent capacitive feature. The magnetic field mainly occurs in the standing-up water blocks and the region of the upper PDMS layer right over the water blocks, which reflects the simultaneous presence of both electric and magnetic resonances at this frequency point. At the second resonant point of 19.98 GHz, the electric field still concentrates in the IR-SL even though it is weaker than that at the first resonance. The magnetic field strongly forms in the upper part of the standing-up water blocks where they contact with the upper PDMS layer. Therefore, multiple electromagnetic resonances are produced at different resonant frequencies and the interaction of all theses resonant modes facilitate the realization of the wideband absorption. In addition, it can be found that the electric field distributions are very weak in the region below the cross water plate since most incident waves are absorbed and dissipated into heat in the structured water. The significant role of the pure water in absorption is further illustrated by the volume power loss densities in Fig. 5(c). It is evident that the power losses simultaneously occur in the IR-SL and the structured water whilst the power losses inside the structured water are much stronger, indicating that the high dielectric loss of the structured water is critical in acquiring the strong absorption in a wide frequency band.
Fig. 5. The distributions of the proposed design at different resonant frequencies of 12.62 GHz (left two) and 19.98 GHz (right two). (a) The vector electric field. (b) The vector magnetic field. (c) The volume power loss density.
3. Experimental results
To validate the microwave, optical and infrared performance of the proposed design, a prototype with an overall dimension of 146 mm × 146 mm is fabricated as shown in Fig. 6(a) and Fig. 6(b). As part of the sample, the ITO-PET films of the IR-SL and the reflective backplane, which possess the advantages of flexible, ultrathin, stable conductivity, high optical transmittance and temperature resistance, are processed by the magnetron sputtering technology. In our design, the transparent conductive thin ITO coating (SnO2: In2O3 = 1: 9) with a thickness of 185 nm and a surface resistance of 6 Ω/sq is deposited on a 50 µm thick transparent organic film PET substrate, and the ITO square patches are etched by the laser lithography technique. The PDMS microfluidic structure encapsulating the pure water is shown Fig. 6(c). Two moulds are firstly manufactured and processed, and prepolymers and curing agents are mixed into the moulds to cast for the PDMS microfluidic structure, which are reverse to the moulds. Then, the PDMS cover layer and container can be obtained after demoulding, separately. Finally, the two demoulded parts are bonded to complete the PDMS microfluidic structure, inside which the liquid can be sealed efficiently. The PDMS microfluidic structure is brought into contact with the two ITO-PET films by using colorless transparent ultrathin optical clear adhesives (OCAs). For injecting pure water, two holes with a diameter of 1 mm are drilled at the corners in a diagonal of both the ITO-PET backplane and the bottom of the PDMS container, one as the water filling nozzle and the other as the pumping port. Hollow steel needles are inserted into the holes for better connection with the PTFE ducts. To reduce the occurrence of air bubbles during the injection procedure, the fabricated sample is placed upside down to ensure that the standing-up water blocks are full of pure water before the pure water is filled in the cross water plate. The pure water can be smoothly discharged from inside when being normally placed. The injection and discharge of the pure water are implemented by using a syringe in the experiments, although a micropump can be further employed to provide a faster and more stable control at a constant speed.
Fig. 6. The fabricated prototype. (a) 3D sketch diagram and (b) Side view of the proposed structure. (c) PDMS microfluidic structure (left) and the two moulds (right).
Microwave Absorbing and Reflecting Characteristics: In order to obtain the microwave properties of the proposed design, the free-space method is employed in an anechoic chamber and the measurement setup is shown in Fig. 7. Two pairs of horn antennas (1–18 GHz and 18–26.5 GHz) are used to cover the entire working frequency band (5–26.5 GHz). The transmitting and receiving antennas are connected by a vector network analyzer (VNA) Agilent N5245A to measure the reflection coefficients of the sample. The time domain gating technique in the VNA is adopted to smooth the measured curves by filtering unwanted reflected waves during the procedure. A two-step process is carried out to conduct the measurement: (1) the reflection curves with the sample are tested at first; (2) then, the reflective backplane with the same dimension is measured. Finally, the measured reflection data in the previous two steps are given normalized treatment and illustrated in curves.
The experimental reflection results of the proposed design in the frequency band ranging from 5 GHz to 26.5 GHz are presented in Fig. 8. The measured results are compared with the simulated ones. Some frequency shift and resonant depth variation exist, which can be attributed to the fabrication error, the tolerances of different materials properties (e.g. the surface resistance of ITO films, the permittivity and thickness of PET and PDMS substrates) and the existence of air bubbles in the structured water. In spite of these deviations, a good consistency between the test and simulation results can be observed, and an obvious wideband switching function between the absorbing and reflecting states can be realized through the injection and discharge of the pure water. In addition, the performance in multiple situations is also explored, and the proposed design proves to be stable under different polarization states and incident angles up to 30°, demonstrating its evident function as a wideband switchable absorber/reflector.
Fig. 8. The measured reflection coefficients of the proposed design with and without water under oblique incidence. (a) 5–18 GHz at the TE polarization. (b) 18–26.5 GHz at the TE polarization. (c) 5–18 GHz at the TM polarization. (d) 18–26.5 GHz at the TM polarization.
Optical Transmittance: The optical transparency of the proposed design is guaranteed by utilizing transparent constituents including ITO films, PET substrates, PDMS microfluidic structures and pure water. The photographs of the fabricated prototype with and without pure water are shown in Fig. 9(a) and Fig. 9(b), and the images below the samples are clear to naked eyes although the situation with pure water is even better. In order to accurately characterize the optical transparency, the light transmittance meter LH221 in Fig. 9(c) is used to measure the optical transmittances in visible wavelengths (380–760 nm). Firstly, the average optical transmittances of each part are measured separately. Since the data from single measurement may be inaccurate, a more robust solution is implemented by using the weighted average approach and 40 sets of data are recorded for each part. The average optical transmittances with corresponding standard deviations (σ) of the IR-SL, PDMS microfluidic structure and reflective backplane are 74.13% (σIR-SL = 0.37%), 73.06% (σPDMS = 1.86%) and 72.64% (σbackplane = 0.65%), respectively. The total optical transparency of the proposed design can be calculated by multiplying the transmittance of each part, and it reduces to 39.34%. The proposed design without pure water is also measured as a whole with an average optical transmittance of 40.92% (σtotal_w/o = 1.73%), which approximates the calculated one. When the structured water is changed from “empty state” to “full state”, the average optical transmittance of the proposed structure with pure water increases to 47.74% (σtotal_w = 3.18%), which is higher than that of the one without pure water. This can be explained by the fact that the internal rough surface of the PDMS microfluidic structure becomes smoother when it is filled with pure water, which reduces the diffuse reflection and improves the optical transmittance. Besides, it should be noted that the standard deviation of the proposed design with pure water is higher than the others, which can be attributed to the existence of air bubbles that results into an uneven liquid distribution during the transmittance test.
Fig. 9. The photographs of the fabricated prototype (a) with pure water and (b) without pure water. (c) The optical transmittance meter LH221.
Infrared Radiation Performance: Besides the wideband switchable microwave properties and the optical transparency in the visible band discussed above, the stealth performance of the proposed design in the infrared band is also discussed. As indicated in Eq. (1), the infrared radiation of an object is more affected by the infrared emissivity of the object surface and the dynamic temperature of the body. In this paper, the infrared stealth capability is enhanced by employing the IR-SL with low infrared emissivity as well as the structured water with dynamic circulation capacity. Compared to most common substances, the water has a much larger specific heat capacity so that the body temperature is primarily controlled by the temperature of the structured water and rarely affected by the ambient temperature. Considering the structure size of the proposed design is much larger than the operating wavelength in the infrared band, it is difficult to use the available computing resources to simulate the emissivity of an electrically large object. Thus, the infrared radiation performance is directly validated by experiments in this paper. The IR-SL is put on the heating table with a temperature of 50 ℃, and meanwhile, a PDMS sheet, a metal block and a piece of paper are added as the control group. Then, a hand-held thermal imaging device (Hikon Vision H11) with the response wave band ranging from 8 µm to 14 µm is used to record the thermal infrared images after the temperature is stabilized over an adequate period of time. As shown in Fig. 10(a), the infrared radiation of the paper is the strongest and the PDMS sheet also shows very high thermal radiation. In contrast, the metal block has the lowest amount of heat radiation and the thermal radiation of the IR-SL is also very small compared to the other two materials (i.e. paper and PDMS), which means that it is less likely to be detected in the infrared band. To validate the capability of pure water in managing the infrared radiation, two prototypes of the proposed design with and without pure water are put on the heating table with a temperature of 50 ℃ for comparison, and the thermal radiations are recorded after 3 minutes. As shown in Fig. 10(b), although the infrared radiation of the sample with pure water is enhanced, its radiation is still weaker than that of the one without pure water. Furthermore, it can be observed in Fig. 10(c) that the infrared radiation is obviously weakened after the pure water is replaced by the water with a lower temperature. The heat can be carried away through the dynamic circulation capability of the pure water, and then the infrared radiation can be easily managed by the temperature of the injected water.
Fig. 10. The thermal infrared images. (a) The IR-SL and the contrast group (i.e. PDMS, metal and paper). The prototypes of the proposed design with and without pure water placed on the heating table with a temperature of 50 ℃ (b) after 3 minutes and (c) after water circulation.
4. Conclusion
In this paper, a multispectral structure that integrates stealth features in both microwave (5–26. 5 GHz) and infrared (8–14 µm) bands and exhibits optical transparency in the visible light band is designed, fabricated and experimentally verified. The proposed structure consists of a top-layer IR-SL with low infrared emissivity and a water-based R-A/R right below. The optical transparency is ensured by utilizing transparent materials (i.e. ITO, PET, PDMS and pure water) in the design procedure. The transparent pure water with frequency dispersion and high dielectric loss are structured to attain enhanced wideband absorbing performance. Furthermore, through the dynamic circulation of pure water, the reconfigurable function of switching between absorbing and reflecting states can be realized, and meanwhile the thermal infrared radiation can be managed by regulating the water temperature. A comprehensive comparison has been presented in Table 1 to illustrate the advantages of the proposed design over the wideband absorber/reflector designs and the radar-infrared compatible stealth structures reported previously. Some critical parameters, including the radar absorption/reflection bandwidth, the polarization sensitivity, the angle stability, the infrared emissivity, the infrared radiation management, the optical transparency and the flexibility, are highlighted in the table. Compared to the studies aforementioned, the proposed multifunctional and multispectral design in this paper simultaneously integrates the advantages of wideband microwave switching function, optical transparency and low infrared radiation, which make the proposed design possess extensive application prospects in the multispectral stealth field. In future, as mentioned in the previous study [33], 5% or 10% propylene glycol (PG), as a kind of antifreeze, can be added in pure water to reduce the freezing point below 0 ℃, which will further push the application boundaries of the proposed design.
Table 1. Comparison between the Proposed Structure and Other Designsa
Funding
Key Laboratory of Radar Imaging and Microwave Photonics (Nanjing University of Aeronautics and Astronautics), Ministry of Education (NJ20210002); National Natural Science Foundation of China (61871219).
Acknowledgments
The authors acknowledge Dr. Lei Xing for the useful discussions and valuable suggestions. Dr. Huangyan Li thanks the Ministry of Education for its Grant of Key Laboratory of Radar Imaging and Microwave Photonics for young scientists.
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.
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