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Abstract
A wideband low-scattering metasurface with optical transparency and flexibility is proposed by using the combination of phase cancellation and absorption mechanisms. Electromagnetic (EM) diffusion is achieved through the random phase distribution design of the two coding elements. The enhanced energy absorption can be obtained in a wide spectrum by using indium tin oxide (ITO) with suitable sheet resistance in the supercells. The experimental results show that the radar cross section (RCS) reductions of less than −10 dB under the planar and conformal cases are in 6.65-19.40 GHz and 6.11-17.37 GHz, corresponding relative bandwidth are 97.89% and 95.91%, respectively. Both theoretical analysis and simulated results are good accordance with the experiment. Furthermore, the analyses of the surface current, EM field distribution and power loss density are given to explain the hybrid RCS reduction mechanism. The proposed composite transparent flexible coding metasurface (CTFCM) maintains good angular stability within 0°-60° oblique incidence and has polarization insensitivity. The CTFCM has excellent flexibility and high optical transparency, which provides a way to reduce RCS in a wider band and has important application potential for stealth aircraft cockpit and transparent radome.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Metasurface with periodic or aperiodic sub-wavelength scale exhibits many exotic properties [1–3], which can arbitrarily manipulate the polarization [4,5], phase [6,7] and amplitude [8] characteristics of EM wave. By arranging elements in different shapes, the metasurface can realize different intriguing functionalities, such as anomalous refraction/reflection [9,10], polarization conversion [11], optical focusing [12], and vortex beam [13], etc. Furthermore, the metasurface becomes a hot topic in basic and applied research due to the increasing demand of military platform for stealth [14–16]. RCS is an important parameter to measure the stealth performance of the target [17], which can be used to determine whether there is a target and the specific location of the target.
Some kinds of metasurface-based technologies have been used to achieve RCS reduction, such as metasurface absorber [18], checkerboard metasurface [19] and diffusion metasurface [20]. The RCS reduction mechanism by the above metasurface is different. The metasurface absorber converts incident wave energy into heat to weaken or eliminate scattered wave energy, but the ability of RCS reduction and the thickness of the metasurface absorber are mutually constrained [21]. In addition, due to the introduction of conductive materials into the metasurface element, it is inevitable to bring resistance, which can produce certain insertion loss. The checkerboard and diffusion metasurfaces are mainly based on phase cancellation to control the angular distribution of scattered wave energies. The metasurface with checkerboard configuration reorients the scattered waves into four main lobes along diagonal and non-diagonal directions. However, the problem of checkerboard metasurface is that the scattered wave directions are relatively fixed and has the high bistatic RCS values. It is worth mentioning that the diffusion metasurface consisting of randomly distributing super-atoms with different reflection phases has been designed to overcome these shortcomings, which can disperse the scattered waves into different directions in the upper region. Therefore, monostatic and bistatic RCS can be significantly inhibited. The basic idea is that the phase difference between the reflection coefficients of the metasurface element should meet 180°±37° [22,23]. However, the frequency band range of 10 dB RCS reduction based on phase cancellation is almost consistent with that of the phase difference of 180°±37° [24–28].
By relying on one mechanism, it is difficult to achieve the metasurface with ultra-wide band, wider angle RCS suppression capability, thinner thickness and lighter mass. A composite stealth metasurface is constructed by combination the phase cancellation and absorption, which can absorb the EM wave energy and the unabsorbed energy can be diffused reflection by phase cancellation. That is, by using the two mechanisms to simultaneously suppress RCS, it is expected to obtain better stealth performance than the single mechanism. Therefore, EM stealth metasurface based on hybrid mechanism becomes the focus of developing high-performance stealth surface. In Ref. [29], the RCS reduction in 13-21.5 GHz is attributed to phase cancellation, while the introducing resistive frequency selective surface can absorb most of the incident wave energy in 21.5-31.5 GHz. The proposed metasurface can finally achieve the 10 dB RCS reduction in 21.5-31.5 GHz (84% bandwidth) and the angular response is to 40°. In Ref. [30], an absorption-diffusion integrated metasurface can obtain over −10 dB RCS reduction in 4.8-16.8 GHz. At the same time, the RCS reduction behavior of −10 dB can be maintained to 45° oblique incidence. In Refs. [31,32], low-scattering metasurfaces were reported by using the above hybrid mechanism, which can simultaneously absorb the incoming wave through the introducing lumped resistors and control the backward scattering direction. However, the above-mentioned metasurfaces are composed of rigid dielectric substrates and do not have transparency, which limits their application in some fields. In Ref. [33], an optically transparent metasurface based on hybrid mechanism is proposed, while its angular stability can only be maintained within 45°. And there is no analysis of RCS reduction under the conformal case. Therefore, how to build the metasurface unit conforming to the composite mechanism to achieve RCS reduction in a larger angular domain, and how to realize the effective fusion of phase cancellation and absorption are still need to be further and systematically studied.
In this paper, the EM stealth metasurface based on phase cancellation and absorption mechanism is studied. In order to meet the visual observation of some special scenes, the constructed metasurface also has the transparency and flexibility, which is composed of two coding elements with random phase distribution. Different from the traditional single RCS reduction mechanism, the use of hybrid mechanism can provide more freedom to achieve ultra-broadband RCS reduction. Experimental results indicate that RCS reductions of less than −10 dB under the planar and conformal cases are in 6.65-19.40 GHz and 6.11-17.37 GHz, corresponding relative bandwidth are 97.89% and 95.91%. The CTFCM maintains good performance when incident angle less than 60° under x- and y-polarizations, which also has excellent polarization insensitivity and the transmittance of 76.8%. The RCS reduction mechanism is revealed by the surface current, EM field distribution and power loss density. The proposed CTFCM in this work has broad application prospects in the fields of transparent radome and EM stealth scattering manipulation of more complex objects.
2. Design principle
According to the classical EM theory, the normalized reflection of 1-bit coding metasurface and metallic plate with the same size for the normal incidence is approximately denoted as,
where RL is normalized reflection, A0 and A1 are the reflection amplitudes of “0” and “1” elements, ${\varphi _0}$ and ${\varphi _1}$ are the reflection phases of “0” and “1” elements. The ratio f is defined as f = n0/(n0 + n1), in which n0 and n1 are the total number of “0” and “1” units, respectively. According to Eq. (1), RCS reduction performance of the CTFCM depends on the ratio in the array, reflection amplitude and reflection phase.Figure 1 illustrates the relationships between f, the phase difference ($\Delta {\varphi _{01}} = {\varphi _0} - {\varphi _1}$) and RCS reduction. In Fig. 1(a), for A0 = A1 = 1, the metasurface is regard as the chessboard structure [34]. RCS reduction with normalized reflection less than 0.1 is obtained in the phase difference range of 180°±37°. As A1 decreases from 1 to 0.1, due to the different proportions of “0” and “1” units, the range of phase difference satisfying −10 dB RCS reduction is not limited to 180°±37°. Particularly, in Fig. 1(d), A0 = 1 and A1 = 0.1 meet the requirement that the −10 dB RCS reduction phase difference range is significantly different from that in Figs. 1(a)–1(c). In this case, the phase difference range becomes more and more relaxed in accordance with the low reflection amplitude of the unit cell, and it is easier to achieve −10 dB RCS reduction. The above analysis indicates that the alternate distribution of absorption peaks of “0” and “1” units combined with optimized phase difference can facilitate the combination of phase cancellation and absorption mechanisms to achieve lower scattering performance in a wider frequency band.
Fig. 1. Normalized reflection calculated by different reflection amplitudes and phase differences of “0” and “1” elements under the normal incidence (a) A0 = 1 A1 = 1 (b) A0 = 1 A1 = 0.8 (c) A0 = 1 A1 = 0.4 (d) A0 = 1 A1 = 0.1.
Therefore, the construction of the CTFCM model is divided into the following three steps. The first step is to design “0” and “1” two independent coding elements, so that they have alternately distributed absorption peaks and the required phase difference in a wider frequency band. The second step is to carry out theoretical calculation to determine the required ratio. The third step is to configurate the “0” and “1” units in space through optimization algorithm to obtain the metasurface with optimal arrangement.
3. Design of CTFCM
3.1 Construction of coding unit model
Figure 2 shows the schematic diagram of the proposed coding unit composed of six layers. In Fig. 2(a), the first, third and fifth layers are the polyethylene glycol terephthalate (PET) thin films (εr = 2.86-j0.001) with the thickness of 0.175 mm. The second and sixth layers (i.e. the ground plane) are the ITO thin films, of which the second layer is an ITO pattern layer. The fourth layer is a polyvinyl chloride (PVC) dielectric layer (εr = 3-j0.042) with the thickness of 2.75 mm. In Figs. 2(b) and 2(c), the patterns corresponding to “1” and “0” units are ring and patch, respectively. This structure with simple resonance mechanism can avoid more energy loss, which has symmetry and is easy to process [35]. The sheet resistances of “1” and “0” units are R1 and R2, and their values are independent of each other to mainly meet the requirements of ohmic loss and phase response equalization, so as to accurately control the reflection amplitude and phase. Therefore, the design freedom is increased to further improve the operating bandwidth. The sheet resistance of the ITO ground plane is R3. It is worthy to point out that the ITO ground replaces a copper layer to maintain the visible transparency and suppresses EM waves penetration with a high reflectivity.
Fig. 2. Schematic diagram of the coding unit (a)Multi-layer structure (b)Unit cell “1” (c)Unit cell “0”.
The effect of using ITO as the backplane to prevent EM wave penetration is analyzed through the transmission model, as shown in Fig. 3. ${Z_A} = \sqrt {{{{\mu _A}} / {{\varepsilon _A}}}}$ and ${Z_B} = \sqrt {{{{\mu _B}} / {{\varepsilon _B}}}}$ denote the characteristic impedances of medium A and medium B, which are related to the magnetic permeability and dielectric constant of the media. Z is the sheet resistance of the ITO film. The transmission coefficient (T) from medium A to medium B is expressed as [36],
It can be seen from Eq. (2) that the dielectric constant and magnetic permeability of medium A and medium B are both fixed values, so T is determined only by parameter Z. Therefore, the smaller the sheet resistance of the ITO film used as the ground plane, the smaller T is achieved. According to the above analysis, R3 is equal to 6 Ω/sq. Since its average transmittance is about 0.001 [37], it is approximated to mirror reflection, transmission can be avoided.
All the simulations are performed by CST. The unit cell boundary is employed to simulate the infinite periodic structure in x and y directions, and open (add space) boundary is used in z direction. The EM wave is incident along the -z axis. The optimized model parameters are as follows: p = 8.0 mm, s = 5.1 mm, l = 6.4 mm, w = 1.5 mm, R1 = 8.0 Ω/sq, R2 = 150.0 Ω/sq. Figure 4 illustrates the absorptivity, reflection phase and phase difference of “0” and “1” elements under the y-polarized wave normal incidence. In Fig. 4(a), for “1” unit, there are two resonant frequencies at 6.1 GHz and 19.2 GHz. The unit “0” only has one resonant frequency at 12.2 GHz. Moreover, “0” and “1” units have strong absorption capability at their respective resonant frequencies. Therefore, in the reflection spectrum, “1” unit has obvious dips distributing at both end of the frequency band of 6.1-19.2 GHz, while “1” unit has only one dip just in the middle frequency. In Fig. 4(b), the phase curves have good linearity, which are consistent with the phase characteristics of the diffuse reflective coding metasurface. Furthermore, the phase difference between the two elements fluctuates around 180° in the range of 6.2-19.1 GHz, indicating that there is strong phase cancellation. Therefore, the combination of phase cancellation and absorption makes broadband RCS reduction possible.
Fig. 4. Absorptivity, phases and phase difference of “0” and “1” unit cells (a) Absorptivity (b) Reflection phases and phase difference.
3.2 Determination of coding unit proportion
The ratio f in the array directly determines the RCS reduction capability. The optimal value of f is calculated by Eq. (1). Figure 5 shows the theoretically calculated RCS reduction for different f values. The optimal value of f is determined by less than −10 dB RCS reduction and its corresponding bandwidth. Under this constraint, the optimal value of f is 0.48, which indicates that the proportions of “0” and “1” cells in the coding array are 48.44% and 51.56%, respectively. Less than −10 dB RCS reduction is in 6.19-19.66 GHz, and the relative bandwidth is 104.22%.
3.3 Arrangement of coding unit
The proposed 1-bit CTFCM consists of M × N coding matrix. The RCS reduction can be explained by the array theory, the array factor for the ground plane is denoted as,
To better understand how the proposed CTFCM works, the energy ratio (ER) of generating by absorption, reflection and phase cancellation from the metasurface is calculated by Eq. (5) under the normal incidence, as shown in Fig. 8. In Fig. 8(a), 70.6% of the incident energies are absorbed, whereas the rest (20.1%) is mainly diffused and scattered into different directions in the upper space. Only 9.3% of the incident energies are reflected back along the normal direction. The above results indicate that most of the incident energies are absorbed and transferred into ohmic loss, which indicates that the absorption plays a major role. In Fig. 8(b), the absorption ratio is small and the reflection ratio is high in the low frequency (close to 5 GHz) and high frequency (close to 21 GHz) bands. The cause of this phenomenon is analyzed by combination with Fig. 4(b). The reflection phase difference of units “0” and “1” is close to 0° and 315° at 5 GHz and 21 GHz, resulting to weak interference. This indicates that the incident wave energies are reflected back, which increases the reflection ratio. The phase cancellation consumes a lot of energies from 6.2 GHz to 19.1 GHz, which originates from the fact that the reflection phase difference fluctuates about 180° (as shown in Fig. 4(b)). Therefore, phase cancellation becomes stronger. In addition, the frequency values with large absorptivity are consistent with those of resonance frequencies in Fig. 4(a). In Fig. 4(b), the phase differences at 8.35 GHz and 15.67 GHz are 229.9° and 132.4°, respectively. This indicates that the phase difference corresponding to less than −10 dB RCS reduction achieved by the CTFCM need not be restricted to 143°-217°, and the operating bandwidth is further improved. In conclusion, the CTFCM integrated phase cancellation and absorption has more freedom to stimulate various resonances to achieve the expected RCS reduction.
4. Results and discussions
To analyze RCS reduction characteristics of the CTFCM, full-wave numerical simulation is carried out in the planar and conformal cases. Figures 9(a) and 9(b) show the 3D scattered field distributions at 7 GHz, 14 GHz and 17 GHz under x- and y-polarized wave normal incidences. In Figs. 9(a) and 9(b), the CTFCM effectively diffuses incidence waves into different directions due to phase cancellation, which can achieve RCS reduction in a wide band. In contrast, as shown in Fig. 9(c), the metallic plate with the same size has a sharp 3D scattered field distribution along the incident wave direction at the corresponding frequency, and the reflected energies are mainly concentrated on the main lobe. Most of the incident waves are absorbed, while the scattered energies only account for a small part. Moreover, in Figs. 9(a) and 9(b), 3D scattered field distributions are similar, which indicates that the CTFCM has polarization insensitive characteristics. Figures 9(d) and 9(e) show the 2D scattered field distributions in E- and H-planes of the proposed metasurface and metallic plate. Except for a few directions as shown in Figs. 9(d) and 9(e), the scattering field intensity of the proposed structure is lower than that of the metallic plate in the broadband, which fully demonstrates the advantages of phase cancellation and absorption compatibility. The CTFCM has the certain loss characteristics, which makes most of the incident energies be absorbed and converted into heat energies. Only a small part of the energies participates the interference, making the side lobes of the CTFCM mostly smaller than those of the metallic plate. Therefore, the CTFCM can reduce the scattering field in all detection directions of the upper region, thereby reducing the probability of target detection and achieving better stealth performance.
Fig. 9. 3D and 2D scattered field distributions of the CTFCM and the metallic plate with the same dimension at different frequencies (a) x polarization (b) y polarization (c) Metallic plate (d) E-Plane (e) H-Plane.
Figure 10 shows the RCS reduction of theoretical calculation and numerical simulation in the planar case. Less than −10 dB RCS reductions cover 6.30-19.67 and 6.19-19.66 GHz, corresponding relative bandwidths are 102.96% and 104.22%, respectively. The calculation result fits numerical simulation one well.
Fig. 10. RCS reduction achieved by theoretical calculation and numerical simulation in the planar case.
Flexible metasurface can be conformal to curved surfaces or irregularly shaped objects, so they are more practical than rigid metasurface. To analyze RCS reduction property on curved surface, the CTFCM is conformally bent on convex metallic cylinder with the height of 256 mm and the curvature radius of 75 mm. Figure 11 shows the simulated 3D scattered field distributions of the CTFCM and the bare metal cylinder at 7 GHz, 14 GHz and 17 GHz in the conformal case. In Figs. 11(a)–11(c), the energies in the main scattering direction are redistributed to different directions. Therefore, RCS is effectively suppressed. This is essentially different from the bare metal cylinder. As shown in Fig. 11(d), the scattered waves are only radially distributed in the xoz plane.
Fig. 11. 3D scattered field distributions of metal cylinders before and after covering the CTFCM (a)-(c) Metal cylinder (d) Bare metal cylinder.
Figure 12 shows the monostatic RCS reduction in the conformal case. Less than −10 dB RCS reduction is 6.04-17.40 GHz, and the relative bandwidth is 96.93%. Although this result is inferior to the RCS reduction performance of the planar case, it still indicates that the CTFCM has broadband RCS suppression capability under the conformal condition. The relative bandwidths of less than −10 dB RCS reduction under the planar and conformal cases are 102.96% and 96.93%, which indicates that better RCS reduction performance is obtained with aid of simultaneous phase cancellation and absorption.
RCS reduction mechanism of the CTFCM is investigated from the phase cancellation and absorption. On the one hand, the phase difference between the two coding units should be within 180°±37° in the studied band. In Fig. 4(b), the phase difference between the two units fluctuates around 180° in 6.2-19.1 GHz, indicating strong phase cancellation between units. On the other hand, the surface currents, electric field, magnetic field and power loss density under the normal incidence at 6.1 GHz, 12.2 GHz and 19.2 GHz are investigated to understand the absorption, as shown in Fig. 13. In Figs. 13(a) and 13(d), a large number of currents are induced on the square ring and the ITO ground plane at 6.1 GHz. The surface currents of the square ring are mainly concentrated on the left and right ring arms, and the direction is inversely parallel to the ITO ground plane, indicating that the high absorptivity at 6.1 GHz is due to the magnetic resonance excited by the incident EM field. In Figs. 13(b) and 13(e), the surface current of the square patch is inversely parallel to the ITO ground plane at 12.2 GHz, so magnetic resonance is also induced. On the contrary, in Figs. 13(c) and 13(f), the surface current of the ring is in the same direction and parallel to the ITO ground plane at 19.2 GHz, indicating that electrical resonance is induced. Therefore, the square ring with low square resistance can excite strong electrical and magnetic resonances at the same time, and the square patch with high square resistance can induce magnetic resonance. Magnetic resonance and electric resonance can be observed from Figs. 13(g)–13(i). The pattern layer of the CTFCM is composed of ITO material with a certain square resistance, which is the main carrier of energy consumption. In Figs. 13(j)–13(l), most of the incident energies are converted into ohmic loss, and finally consume in the form of heat energies.
Fig. 13. Distributions of surface current, EM field and power loss density at resonant frequencies (a-c) Surface current of ITO pattern layer (d-f) Surface current of ITO ground layer (g) and (h) Magnetic field distribution (i) Electric field distribution (j-l) Power loss density.
According to the above analysis, the CTFCM combined the phase cancellation and absorption can not only adjust the angular distribution of scattered energies, but also convert the incident energies into heat energies through resistive loss materials, thus greatly reducing the scattered energies. Therefore, the proposed CTFCM can weaken the scattered field in almost all directions of the upper region, which has excellent RCS reduction performance in the broadband.
5. Experimental verification
A CTFCM sample with the size of 256 mm × 256 mm × 3.28 mm is fabricated by laser etching. Flexible PET-ITO materials covered with 8.0 Ω/sq and 150.0 Ω/sq on one side are respectively put into the laser etching machine to etch square rings and square patterns. Paste the patterned surface of PET-ITO material with optically clear adhesive (OCA) glue to form a pattern layer. The pattern layer, PVC dielectric layer and PET-ITO film with a square resistance of 6.0 Ω/sq are bonded into a multi-layer structure with OCA. OCA with the thickness of 0.05 mm is hardly affects the optical transmittance and EM characteristics of the proposed metasurface. The images of the fabricated prototype are shown in Fig. 14.
The RCS reduction measurement is performed in the microwave anechoic chamber. Figure 15(a) shows the schematic of the experimental setup in the planar case. The two same horn antennas connected to a vector network analyzer (Agilent, N5224A) are respectively utilized as the receiver and transmitter. Furthermore, the center of the antennas is consistent with that of the sample. The distance between the prototype and the horn antennas is maintained at 2.5 m. The gate-reflect-line calibration is used to further eliminate undesirable signals in the surrounding environment. In the process of measurement, the same-sized metallic plate is firstly measured as the reference, and then the measured RCS reduction of the planar metasurface under the normal incidence of x-polarized plane wave is gained in Fig. 15(b). RCS reduction of less than −10 dB is achieved in 6.65-19.40 GHz, and the corresponding relative bandwidth is 97.89%. As expected, the measured results are in good agreement with the results of the numerical simulation and those theoretical calculated by Eq. (1), demonstrating the phase cancellation and absorption recombination mechanism can achieve efficient and continuous backscattering suppression in ultra-wide band.
Fig. 15. (a) Schematic of the experimental setup in the planar case (b) RCS reductions in the planar case.
The RCS reduction at different oblique incident angles for both x- and y-polarizations is experimentally investigated in the conformal case. The sample is wrapped around a cylinder with the radius of 75 mm and the height of 256 mm. In the measurement, we first measure the RCS of the same-sized bare metallic cylinder, then measure the RCS of the fabricated sample and normalize it by the RCS of the metallic cylinder. Figure 16 shows the schematic of the test scenario and the experimental results of monostatic RCS reduction. In Figs. 16(b) and 16(c), it is obvious that the −10 dB RCS reduction over a broad frequency band can be kept all the same for x- and y-polarized wave incidences. Angular stability is up to 60° under both x- and y-polarized wave incidences, which is verified through experiment. Compared with the planar case, the RCS reduction performance of conformal operation is decreased, but it can still achieve greater than 10 dB RCS reduction. Because for the metallic cylinder, it possesses an inherent uniform far-field scattering pattern in the radial section. The scattering reduction mainly comes from the scattered energy redistribution in the axial section. Therefore, the overall average reduction is lower. However, if we only focus on the backward RCS reduction, the non-uniform scattered energy distribution of the random metasurface in the racial section can be optimized to obtain a specific design, which has lower scattering in the backward direction. Furthermore, the coding element is rotational symmetric, so the proposed CTFCM exhibits insensitive to the incident polarizations, which further confirms its potential value in the conformal applications.
Fig. 16. Schematic of the measured scenario and RCS reduction corresponding to different incident angles under the conformal case (a) Schematic of the test scenario (b) x polarization (c) y polarization.
To characterize the visible light transmission performance of the sample, the optical transmittance is tested with a spectrometer (Ocean Optics, OFS-2500) in the range of 380-780 nm. In the measurement, the fabricated sample was cut into 10 × 10 mm due to the limitation of the sample pool. The results are shown in Fig. 17. The averaged transmittance is approximately 76.8%, which shows that the sample has good transmittance and can meet the needs of transparent devices.
In general, bandwidth is a tradeoff between thickness and reflectivity. To highlight the RCS reduction performance of the proposed CTFCM, the figure of merit (FoM) is used for evaluation. FoM is the ratio of the relative bandwidth of −10 dB RCS reduction to the thickness of the metasurface:
Table 1. Comparisons Between This Work and Other Designs.
6. Conclusion
A CTFCM with wideband RCS reduction characteristics is proposed. The experimental results indicate that less than −10 dB RCS reductions under the planar and conformal cases are in 6.65-19.40 GHz and 6.11-17.37 GHz, corresponding relative bandwidth are 97.89% and 95.91%, respectively. The experimental results are basically consistent with the simulated ones. The mechanism of wideband RCS reduction originates from the phase cancellation and absorption, which is deeply analyzed by the surface current, EM field distribution and power loss density. Compared with the traditional single RCS reduction mechanism, the use of hybrid mechanisms in this paper can provide more freedom to achieve ultra-broadband RCS reduction. Moreover, the CTFCM has excellent polarization insensitivity and angular stability. And its light transmittance is 76.8%. The proposed CTFCM gives consideration to both microwave stealth and optical flexibility and transparency, which has important application potential for special occasions such as stealth aircraft cockpit and transparent radome.
Funding
Industry Foresight and Key Technology Projects of Suzhou of China (SYC2022149); Young academic leaders of Blue Project of Universities in Jiangsu Province of China (202201022); National Natural Science Foundation of China (52174021); Natural Science Foundation of Heilongjiang Province (LH2020E012).
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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