Metasurfaces, two-dimensional (2D) counterparts of metamaterials composed of subwavelength periodic or quasi-periodic atoms [1] according to the design requirement, exhibit flexible and powerful modulation in broadband electromagnetic (EM) waves from the visible band to the microwave band [2,3]. The abundant mechanisms behind metasurfaces provide superior performance in controlling EM wave characteristics [4,5], such as amplitude, phase, and polarization mode [6–11]. Through the extraordinary ability to control EM waves by metasurfaces, the functions of EM devices have been effectively enriched. With versatile modulation of EM waves, metasurfaces present some meaningful schemes in different frequency regimes [12]. Some fascinating applications in theory and practice are expanded, such as perfect absorbers, polarization-control devices, cloaking devices, and planar lenses [13–16].

In the camouflage field, the appearance of a metasurface realizes a multispectrum compatible design from the perspective of structural design [17,18]. Camouflage to the frequency bands can be divided into microwave camouflage, infrared camouflage, and optical camouflage. In the microwave and optical bands, the main camouflage goal is to reduce the radar cross section (RCS), while in the infrared band, the goal is to reduce the distinguishability between the target and the background. At present, there have been some mature mechanisms to realize single-spectrum camouflage. In microwave bands, the low RCS can be realized by reducing the amplitude and phase interference of microwaves. Absorption is the main approach to reducing amplitude, and the mechanisms behind it include cavity resonance and resistance film loss. Through realizing the impedance matching between the stealth materials and air, the incident EM wave can maximumly enter the interior of the materials. For naturally absorbing materials such as water, the EM wave can be absorbed in the process of mutual loss between EM waves and microscopic particles. For the resistance film, the energy of EM waves can be converted into thermal energy, which realizes the absorption effect. In addition, the EM wave can be lost through the interference superposition inside the cavity if the depth of the cavity can be adjusted to one-quarter of the wavelength in a specific frequency band. For polarization modulation, the main approaches include changing the polarization state of the reflected wave and random diffusion scattering [19–23]. The reflected energy can be divided into different directions, and the RCS is reduced, especially in the vertical direction. In the infrared band, the low emissivity materials such as metal are coated on the metasurfaces to camouflage in a low-infrared emissivity environment. To better adapt to the changing environment, phase-change materials such as Ge2Sb2Te5 (GST) and VO2 are fabricated for films coating on metasurfaces, which can realize broadband-switchable infrared absorbers. For the high-temperature environment, efficient thermal management coatings such as hyperbolic metamaterials (HMMs), near-zero gradient (G-ENZ) materials, and polymeride are used for radiation cooling [23,24]. In addition, metals and indium tin oxide (ITO) with metallic properties have low infrared emissivity; by designing the occupation ratios of the surface, the overall infrared emissivity can be custom-designed [25–27]. Infrared waves and microwaves all belong to the EM wave. Therefore, the camouflage mechanisms in amplitude and phase are similar to the stealth in microwaves. However, according to Kirchhoff’s law, the condition of thermal equilibrium and transmittance is zero, and the emissivity is equal to 1−R, in which R is the reflectivity. The realization of low emissivity needs high reflectivity, which is different from microwave stealth. For the visible band, materials with high transparency and electrical conductivity such as ITO can be compatible with other functional devices to realize low visibility [28–30].

In actual application, with the development of the detected technologies, reconnaissance means are becoming more and more varied. Therefore, the demand for camouflage has also expanded from a single-frequency band to multiple-frequency bands. To meet the requirements of multiband camouflage, metasurfaces need to harmonize the compatibility among different frequency spectra. The flexible regulations on microwave performance such as amplitude, phase, and wavefront provide more camouflage methods in microwave bands [6–11,31]. These methods can reduce the requirements of materials but improve the level of structural design. The infrared modulation films are coated on the metasurfaces with microwave absorption, which can independently realize the microwave and infrared camouflage. For example, a wavelength-selective emitter such as multilayer Ge/ZnS can realize radiative cooling, and absorptive metasurfaces such as Cu-ITO-Cu can realize microwave camouflage [31–34]. This method takes full advantage of the wavelength difference of the spectral bands, which solves the contradiction among visible, middle infrared, and laser camouflage [35,36]. However, the complexity of the structural design is great, and the optimization of parameters involves time and cost. Meanwhile, the multilayer structure enhances the difficulty of fabrication. To simplify the design, one layer of periodic conducting patches with a high occupation ratio put on the top of the metasurface can reduce the infrared emissivity effectively [25–27]; the patches can also be used as a frequency-selective surface to couple with the microwave layer. However, the infrared and microwave layers are still separated. Structures still have a lot of potential for integration.

To concentrate the multispectral functional layers into a single layer, this paper proposes a multimechanism-empowered ITO-polymethyl methacrylate (PMMA)-ITO metasurface comprising four meta-atoms. In the microwave band, the backward scattering is reduced through the simultaneous modulation of amplitude and phase. Under the effect of the longitudinal cavity bound, the energy of EM waves is maximally concentrated on the surface. Therefore, the reflected waves can be absorbed through the loss of resistance in the film. The amplitude modulation is completed. For phase modulation, the meta-atoms are capable of polarization conversion. The remaining copolarized reflected waves are converted into cross-polarized reflected waves. By rotating the patterns on the meta-atoms, the phase can be modulated according to the Pancharatnam–Berry (PB) phase theory. The meta-atoms with different reflective phases are randomly distributed, which can form diffuse reflection. The reflected waves can interfere with each other. The reflected energy is scattered in other directions, and the microwave scattering in the incidence direction is reduced. The above mechanisms enable the metasurface to achieve camouflage through the reduction of backward scattering in the 8–14 GHz frequency range. In the infrared range, four meta-atoms with varying occupation ratios can effectively cover infrared radiation from 0.90 to 0.44. The overall infrared radiation characteristic can be designed by the spatial arrangement of the four metapatterns according to the environmental infrared radiation. For the visible range, ITO films with high conductivity and transparency are used to ensure compatibility with low visibility and microwave absorption. The camouflage in three bands has been successfully integrated into the structural design of the top layer of the metasurface. To illustrate the design method, a metasurface sample was devised and constructed. The simulation and experimental results demonstrate that the metasurface exhibits effective low backward scattering, a digital infrared effect, and high optical transparency. Additionally, microwave performance exhibits angle stability. The metasurface fully exploits the modulation ability of EM waves and effectively balances camouflage in different bands. This paves the way for a new approach to designing customized infrared camouflage adapted to complex environments. Furthermore, the integration design offers significant advantages in cost control and large-scale fabrication, which has great potential in equipment survival and military communications.

Figure 1 gives the schematic diagram of a multimechanism-empowered functional metasurface. To fully drag the modulation potential of the metasurface, three typical mechanisms in amplitude and phase are introduced to the meta-atoms. For the amplitude modulation, the half-wave effect is considered in the cavity thickness design to form a longitudinal cavity bound, in which the energy of reflective waves in the central wavelength can be maximally concentrated into the meta-atom surface. Then through the resistance film, the energy can be maximumly absorbed by the resistance film. The polarization state can be changed through polarization conversion, which transfers the copolarized wave to a cross-polarized wave. The phase of reflected waves depends on the patterns of the meta-atoms. For the overall metasurface, the reflected waves will cause destructive interference, and the scattering energy will be reduced in the incidence direction. Due to the different occupation ratios of every meta-atom, infrared radiation presents digital characteristics in thermal imagery.

In the longitudinal cavity, the interference superposition of the incident wave and reflective wave will produce the cavity resonance according to Eq. (1) [37–39], Es=E0ejωt−γz(1+r0e2γz).(1)

The direction of the EM wave is along the z axis. The meta-atom is in the x–o–y plane. The incident wave is in a positive direction, and the reflective wave is in a negative direction. Take the reflection coefficient at z=0 as r0. In the Eq. (1), E0 is the electric field intensity of the incident wave. γ is the transmission coefficient, which can be expressed as γ=j2π/λ+δ,(2)where δ is the attenuation coefficient of the dielectric material. For the reflective meta-atom, r0=−1. Transfer Eq. (1) to complex number form: Es=E0ej(θ+ωt).(3)

The real part can be obtained from Eq. (4), Re(Es)=E0(e2δz+e−2δz+2cos4πzλ)12.(4)

When the cavity depth is z=nλ/4, n∈Z, the electric field intensity reaches its maximum value. In that case, the energy of EM waves can be maximally worn out through resonance or absorption in the longitudinal dimension. The polarization phase can be fully modulated in the horizontal dimension through the design of meta-atoms.

For the anisotropic symmetric patterns in the meta-atoms, the polarization vectors of incident EM waves can be customized and divided into orthogonal basis vectors. Through the electric and magnetic resonances [40], Fano resonance [41], and multiorder plasmon resonances [42], a 180°-phase difference is generated between the two orthogonal components of reflected waves [40–42], in which the polarization direction of the reflected wave is rotated 90°. The polarization conversion ratio (PCR) is defined as [42] PCR=rxy2rxx2+rxy2=ryx2ryy2+ryx2,(5)where rxx=ryy represents copolarization and rxy=ryx represents cross-polarization, respectively.

Under the guidance of the above mechanisms, three types of patterns are designed on the surface of meta-atoms to realize the ultrawideband low reflection with absorption and polarization conversion. Figure 2(a) shows the structural hierarchy of the meta-atoms. Particularly, all the meta-atoms have the same structure, with three layers. The top layer is a functional layer, which is ITO thin film with 10Ω/sq. The ITO is attached to the polyethylene terephthalate (PET) with a dielectric constant of 3.0 (1-j0.06). The middle dielectric layer is PMMA with a dielectric constant of 2.65 (1-j0.001). The bottom is ITO with 10Ω/sq as well. The bottom and the dielectric layer are attached by the PET. The thicknesses of the PET and ITO are fixed due to the processing technology with 0.05 mm and 200 nm. Figures 2(b)–2(d) show the structural details of three types of meta-atoms, respectively. The detailed values are shown in Table 1 in Section 5.

To realize the ideal effect of longitudinal cavity modulation, the depth of the dielectric layer h2 is 3 mm, which is one-quarter of the wavelength of a 10 GHz EM wave, the central frequency of the functional bandwidth. After the optimization, three meta-atoms can all realize the regulation effect in the same frequency band of 8–14 GHz.

To verify the performance, the S11 and S12 of three meta-atoms are simulated by the frequency domain solver of CST Microwave Studio with unit cell boundary conditions to achieve the reflection coefficients, with incident angles from 0° to 60°; and the results are shown in Figs. 3(a)–3(c), respectively. The copolarization reflectivity of the meta-atoms is reduced below 10 dB at 8–14 GHz and has great stability of angle. When the incident angle increases to 60°, the reflective amplitude is still less than −3dB. However, with the increase of the incident angle, the reflection coefficients will reduce for three reasons. First, with the increase of the incident angle, the equivalent wavelength will decrease, and the original resonance will be weakened, which results in the fluctuation of reflection coefficients. Second, part of the waves are specularly reflected in other directions, which cannot be received by the experimental antenna. With the increasing incident angle, the EM waves undergo specular reflection, which is increasing as well. Finally, when the incident angle increases, a leakage wave phenomenon will appear, which influences the stability of reflection coefficients. To further analyze the contribution of different mechanisms to low scattering, the efficiency proportion of different mechanisms at 10 GHz with vertical incidence waves is shown in Fig. 3(d). From the results, the absorption caused by cavity resonance and resistance film loss is the main mechanism for ultralow scattering. The polarization conversion is the second factor for low microwave scattering. A detailed analysis of the above mechanisms is given in Section 6. The introduction of multimechanisms is to realize the same stealth frequency band in different meta-atoms with various occupation ratios, which can form gradient infrared emissivity. The various meta-atoms are the foundation for designing a metasurface with digital camouflage characteristics. Moreover, the multimechanisms can enhance the angular stability in stealth performance.

To expand the range of the infrared emissivity, code 4 is introduced, which has no ITO patterns on the surface. For the cross-polarization reflected waves, the phases are shown in Fig. 4(a). The phase difference from meta-atoms covers 0°–180°, which can be called code 0, code 1, code 2, and code 3, respectively.

The microwave scattering reduction for the metasurface depends on the reflection phase φi, amplitude A, and ratio ki of the types of meta-atoms according to Ref. [43], Scatteringreduction=10log[k1·A1exp(jφ1)+k2·A2exp(jφ2)+k3·A3exp(jφ3)+(1−k1−k2−k3)·A4exp(jφ4)]2,(6)where ki denotes the proportion of the meta-atom “‘i” in the metasurface, φi indicates the reflected phase, and Ai indicates the amplitude of meta-atoms. Considering the loss of microwave energy, the scattering reduction cannot be realized just through the phase difference [44,45]. To further analyze the scattering field of the metasurface, the concept of coding the metasurface is introduced [21]. Given that the low-scattering coding metasurface is assumed to be composed of M×N atoms with four different types, the reflected amplitude of each atom should be considered in the calculation. According to the theoretical equation, AF(θ,φ)=∑m=1M∑n=1N|A(m,n)|·e−jφ(m,n)·e−jkPsinθ[(m−1/2)cosφ+(n−1/2)sinφ],(7)where θ and φ denote the angles of elevation and azimuth of the incident wave, respectively. k=2π/λ indicates the wavenumber vector, and P represents the period of subarrays. A(m,n) and φ(m,n) are the reflected amplitude and reflected phase of the subarrays at the point of (m,n), respectively. From the equation, the far-field pattern of the metasurface interferes in a random way when φ(m,n) is randomly arranged at 0° and 180°, leading to the reduction of the backscattering. However, the phase difference among the meta-atoms in the metasurface is more complex, which results in a more diffuse scattering field distribution. Compared to the four-beam or two-beam scattering field distribution produced by a chessboard or keyboard metasurface, the metasurface with a randomized atom arrangement can disperse the reflected energy more evenly in all directions, which realizes the scattering reduction in a wide angular domain. Moreover, the randomized arrangement will also favor customized digital infrared camouflage. More details will be described in the next section.

To verify the design effects, the scattering far fields of the metasurface are designed and simulated at 8, 10, and 12 GHz. The incident angle covers 0°–60°. The metasurface is composed of 40meta‐atoms×40meta‐atoms, and the overall size is 400 mm. To make the phase distribution of the metasurface more intuitive, Fig. 4(a) gives the 2D visualization of the phase. According to the reflected phase curves, the phase difference among meta-atoms I to III maintains 180° at 6–18 GHz, which covers the stealth frequency band. The phase difference between meta-atom IV and meta-atom III covers from 0° to 180° at 9–14 GHz. To intuitively show the phase relationship among the meta-atoms I to IV, the distribution is given in Fig. 4(a). The distribution represents the typical phase relative relationship at 9–14 GHz. The black parts represent the phase of meta-atoms II and III. The white parts represent the phase of meta-atom I. The gray parts represent the phase of meta-atom IV. According to Eq. (7), the energy can be dispersed in random directions. The backward scattering will be reduced. The RCS values of the metasurface and ITO plate in the incident direction between 6 and 18 GHz are collected and shown in Fig. 4(b). From the results, the RCS of the metasurface is far less than that of the ITO plate. To more intuitively show the effect of microwave diffuse scattering, the far-field scattering results of the metasurface are simulated and shown in Fig. 4(c), and the ITO plate with the same size is shown in Fig. 4(d). Compared to the ITO plate, the reflected energy is randomly dispersed into other directions, and the energy in the incident direction is greatly reduced.

Compared with traditional low-infrared emissivity camouflage methods, it will be more obviously observed in environments characterized by high infrared radiation intensity or hybrid infrared radiation backgrounds where low- and high-intensity regions coexist, such as forests, shrublands, and densely built urban areas. Consequently, a digital camouflage pattern incorporating variable infrared radiation signatures is essential. For the design of the infrared camouflage layer, the key is to design and regulate the surface emissivity of the metasurface. Generally, metals have high reflectivity and low emissivity in the infrared regime. ITO also shows high infrared reflectivity and low infrared emissivity like metals due to their excess free electrons [34]. Furthermore, the zero transmission and low absorption in the whole infrared band of the ITO manifest a superior infrared camouflage property. The surface emissivity of the metasurface can be calculated by averaging all the emissivity pixels of the area according to an empirical formula [46], ε=εm·fm+εd·(1−fm),(8)where ε is the total emissivity of the metasurface; εm is the emissivity of ITO, which is about 0.05; εd is the emissivity of PET, which is about 0.9; and fm is the area percentage of the ITO part [46].

The surface patterns of the meta-atoms, classified as Types I through IV, exhibit distinct characteristics. Following the specifications outlined in Eq. (8), the occupation ratios of the four types of meta-atoms are 0.35, 0.38, 0.57, and 0. The infrared emissivity, in theory, is 0.62, 0.59, 0.44, and 0.90, respectively. The occupation ratios span a range from low to high, while the infrared emissivity of each atom presents a graduation from high to low. Under the random phase encoding distribution of the metasurface, each coding unit represents a digital unit. The localized emissivity designability of metasurface can be customized camouflage through the design of the coding sequence, which exhibits characteristics of digital camouflage from the overall infrared imagery.

The infrared and microwave performance of the metasurface can be simultaneously controlled and designed. In the microwave band, according to the cross-polarization reflective phase, the meta-atoms can be divided into code 0, code 1, code 2, and code 3. Four types of meta-atoms are arranged randomly to constitute the metasurface with diffuse phase distribution. When the EM wave is incident on the metasurface, the reflected energy will be canceled in the normal direction and randomly redirected towards the full space. Combined with the EM absorption mechanism, broadband ultralow scattering can be realized in the metasurface. In the infrared band, due to the infrared emissivity gradient of code 0 to code 3, the overall space infrared radiation characteristics can be custom-designed through the spatial arrangement of different types of meta-atoms. According to the infrared background environment, the high-infrared emissivity areas and low-infrared emissivity areas can appear simultaneously to form a digital camouflage effect, which can be effectively camouflaged in a more complex environment.

According to the above design ideas, the ultralow metasurface, simultaneously with visible transparency and digital infrared camouflage, is designed and fabricated. The process is as follows. First, the ITO patterns on the top surface are realized by etching a conductive ITO film on an optically transparent PET substrate using laser etching techniques. Another ITO-coated PET sheet serves as the back plane. Then adhering these components to the sides of a PMMA board, the metasurface is constructed. Notably, the details of the functional layer on the top surface are fabricated with precise laser etching on conductive ITO-coated PET thin films. The fabricated sample was measured as 300mm×300mm.

In the microwave experiment, as shown in Fig. 5(a), the sample is put on the platform of the arch frame. The receive and transmit antennas can be moved on the arch frame to detect the RCS of the sample at different angles. Then, the RCS values in the incident direction are monitored as the angle increases from 0° to 30°. The results are shown in Fig. 5(b). The experimental results are consistent with the simulated results. The errors are mainly produced by coating precision, noise interference, and antenna loss. In addition, the coating precision can affect the square resistance of ITO. The noise and antenna loss can interfere with the signal collection.

In the optical experiment, the sample is put in front of the plants, which can show the visual transparency of the metasurface. Then, the optical transmittance is quantified and illustrated in Fig. 5(d) with the aid of the test instrument. The results demonstrate that the sample exhibits high visible transparency, with a value of approximately 60% within the 400–800 nm range.

In the infrared experiment, the localized emissivity designability is proved first. Infrared emissivity results with varying occupation ratios were obtained using a TSS-5X IR emissivity meter in the course of the infrared experiment. In this configuration, the infrared probe serves as the detector, while the metasurface is mounted horizontally on a platform. An infrared detector is positioned on the metasurface to obtain the infrared emissivity in different meta-atoms. The results are presented in Fig. 6(a). To gain further insight into the radiation characteristics of the infrared spectrum, the local infrared emissivity of the metasurface in different regions was measured in the 3–14 μm range using an infrared spectroscopy instrument. The instrument and results are presented in Fig. 6(b). Due to the different patterns of the meta-atoms, the local regions on the metasurface show the gradient infrared emissivity. The designability of localized infrared emissivity of the metasurface is proved. Notably, the fluctuation around 6–9 μm is due to the material properties of PMMA. The larger the proportion of PMMA in the local region, the greater the fluctuation of infrared emissivity. However, the fluctuation will not influence the designability of localized infrared emissivity.

Based on the localized emissivity designability, a scenario was selected to assess the practical viability of the sample for camouflage through the infrared imager (Thermao GEAR), as illustrated in Fig. 6(c). Four distinct temperature levels were achieved by simultaneously heating a selection of leaves on a hot plate for varying periods. These temperatures were used to represent four different categories of protected targets, each exhibiting unique infrared radiation characteristics. In thermal imaging, the leaves exhibit a range of colors, allowing for clear differentiation from the background. Subsequently, the leaves were covered with the sample. In visible-light imagery, the sample exhibits effective transparency, allowing for clear observation of the leaves. Conversely, in infrared imagery, the leaves covered with the sample are effectively concealed, akin to the appearance of camouflage.

Finally, the temperature stability of the metasurface is proved. The sample was put on the heating platform, and the temperature was increased from 40° to 120°C. From the infrared imaging of the sample in Fig. 6(d), the sample always maintained a discernible digital infrared camouflage feature. Due to the uneven heating of the heating table, the temperature in the middle part is higher than at its edges. The above experimental phenomenon convincingly proves the infrared camouflage ability is compatible with microwave scattering reduction and visible transparency in complex infrared backgrounds.

To demonstrate the advantages of this work, Table 2 presents a comprehensive comparison of various characteristics between our proposed metasurface and previously reported advanced camouflage devices. From the table, the metasurface proposed in this work exhibits a simpler structure with a single functional layer compatible with microwave and infrared camouflage. Moreover, a broader infrared emissivity gradient is realized, which can camouflage in various environments.Table 1.

Comparison of the Proposed Metasurface with Previously Reported Studies

In this study, we propose a multimechanism-empowered metasurface comprising four types of ITO-based meta-atoms that can realize ultralow microwave scattering, customized infrared digital camouflage, and visible transparency. In the microwave band, the scattering reduction is realized by simultaneous modulation of amplitude and phase. The meta-atoms of the metasurface can realize radar stealth at 8–14 GHz. The reflective amplitude is less than −10dB. When the incident angle increases to 60°, the reflective amplitude is still less than −3dB. In the infrared band, four meta-atoms with varying occupation ratios can effectively cover infrared radiation from 0.8 to 0.3. The overall infrared radiation characteristic can be designed by the spatial arrangement of the four metapatterns according to the environmental infrared radiation. For the visible range, ITO films with high conductivity and transparency are used to ensure compatibility with low visibility and microwave absorption. The camouflage in three bands has been successfully integrated into the structure design of the top layer of the metasurface. This work presents what we believe is a novel approach to achieving multispectrum functions with integrated multimechanisms in a single functional layer, especially with the characteristics of digital camouflage that can be customized and designed according to the environmental background. Furthermore, the integration design offers significant advantages in cost control and large-scale fabrication.

The parameters of meta-atoms can be divided into two categories: common parameters and structural parameters. The common parameters are applicable to every type of meta-atoms, including depth of the dielectric layer, period size of every meta-atom. The structural parameters depend on the pattern structures from different types of meta-atoms, which are shown in Figs. 2(b)–2(d).

For the meta-atoms from Type I, Type II, and Type III, the common absorption frequency band is 7.5–13 GHz from the reflection curve. Here, three typical frequency points (8, 10, and 12 GHz) are chosen to observe the electric field of different meta-atoms through numerical simulation. In the longitudinal dimension, as shown in Figs. 7(a)–7(c), cross-sectional diagrams of the Type I, Type II, and Type III, meta-atoms are illustrated. It can be observed that all the electric vectors on the surface are opposite to the electric vectors on the bottom, which are interconnected to generate one big magnetic dipole, resulting in a powerful magnetic resonance. Additionally, the height of the cavity is approximately equal to one-quarter of the wavelength, which can concentrate the energy of the wave crest on the surface.

In the lateral dimension, the energy of waves is lost through the current vortex and electric resonance. For meta-atom I, the electric field intensity distribution at typical frequencies is shown in Fig. 8(a), and the electric field is focused on the edges of arrows, especially around the bottom corner. The resonance is generated through the mutual coupling between the edges of adjacent triangular arrows, which can accelerate the loss of energy and polarization transformation of the wave. For meta-atom II, as Fig. 8(b) shows, the electric field distribution is like meta-atom I, with a similar arrow structure, in which the arrow ends play a key role in resonance. Meta-atom III is different, as Fig. 8(c) shows: the electric energy is focused on the split square ring and the edges of the patches. The electric resonance is caused by the interaction between the ring and the patches. And the patches provide the full space to generate the vortex field of EM waves, which can accelerate the loss of the EM waves.

Polarization conversion is realized through the phase gradient in different directions of the reflected waves.

We take y polarization as the incident wave, and the u and v coordinate system is introduced. The u axis is along a 45° direction concerning the y axis and is perpendicular to the v axis, as shown in Fig. 9(a). Then we consider that the incident EM wave is polarized along the y axis. The electric field can be decomposed into two orthogonal components.

When the EM wave is incident onto the surface, the electrons motivated by the waves will move along the copper pattern, and the equivalent direction is toward the u axis, which means in the other direction, the z axis will not affect the performance of the EM waves (be equivalent to the perfect electric conductor), so Δφ=0 and ru=rv. However, in the u axis direction, because of the resonance oscillation between the surface current and bottom current, the phase of the reflected waves will change, and when Δφ=π, the field synthesized by Eru and Erv will be changed to the x direction. Therefore, the direction of reflective polarization is rotated by 90°.

Here we discuss the reasons why Δφ=π. The production of the phase gradient mainly comes from the multiple plasmon resonances. Here, the surface current distributions of the meta-atoms are simulated in CST and shown in Figs. 9(b)–9(d), respectively. For meta-atom I, there are two basic modes. As Fig. 9(b) shows, in mode (i), the surface current is mainly along the v direction and the bottom current is mainly along the u direction. It means the u component of the incident wave successfully enters the bottom of the metasurface and specular reflection occurs, while the v component of the incident wave interacts with the ITO on the surface and resonance occurs. For the reflective wave, the phase difference of the u component is 2π, which is composed of the reflected phase π and transmitted phase π. The phase difference of the v component is π, which is the resonant phase. So, Δφ=π. In mode (ii), the situation is reversed. The v component of the incident wave successfully enters the bottom of the metasurface and specular reflection occurs, while the u component of the incident wave interacts with the ITO on the surface and resonance occurs. Mode (i) and mode (ii) form the broadband polarization conversion.

For meta-atom II, the modes are like those of meta-atom I. However, due to the difference in the patterns, the current paths are different as well. In mode (i), the equivalent current direction is along the negative v axis, while the bottom current is along the positive v axis. So, the phase difference of the v component is 3π, which is composed of the reflected phase π, transmitted phase π, and resonant phase π. There is no phase difference in the u component. Mode (ii) is the opposite of mode (i), in which the equivalent current direction is along the negative u axis, while the bottom current is along the positive u axis. The phase difference of the u component is 2π, which is composed of the reflected phase π, transmitted phase π, and resonant phase π. There is no phase difference in the v component. Mode (i) and mode (ii) form the broadband polarization conversion.

For meta-atom III, the situation is more complex. Due to the patches in the center of the surface, most of the incident waves are restricted on the surface. The coupling between the patches and the split square ring is the main factor in polarization conversion. There are two basic modes. In mode (i), the current is concentrated on the split square ring, and the equivalent direction is along the u axis. The resonant phase difference π of the u component exists. In mode (ii), the current is concentrated on the patches, and the equivalent direction is along the v axis. The resonant phase difference π of the v component exists. Mode (i) and mode (ii) form the broadband polarization conversion.