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Abstract
An excellently transparent metamaterial-based electromagnetic interference (EMI) shielding window with broadband absorption is presented theoretically and demonstrated experimentally. The window is composed of double split circular ring (SCR) elements whose absorption spectra feature two mild resonant peaks. Indium–tin–oxide (ITO) with resonant patterns is utilized as the material to induce high ohmic loss and broaden the absorption bandwidth. The window achieves strong absorptivity, > 90%, covering an ultrawide frequency range of 7.8–18.0 GHz. Moreover, the measured shielding effectiveness (SE) of the window is > 18.25 dB, at 7.0–18.0 GHz, while the average optical transmittance is fixed at ∼73.10% in the visible–near-infrared (Vis–NIR) region of 400–1,500 nm. Further, the absorption mechanism is revealed by designing an equivalent circuit model and studying the distributions of the electric field and surface currents of the structure. Furthermore, a specific design feature also makes our device insensitive to the incident angle and the polarization state of the impinging microwave. The 90% absorption and shielding performance of the proposed optical window avail it for a wide range of great potential applications, such as the displays of military and medical precision devices.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The prosperous developments of radiofrequency techniques and electronic equipment have resulted in serious electromagnetic (EM) pollution, interference with precision detection devices, and even threats to human health [1,2]. To eliminate these negative impacts, electromagnetic interference (EMI) shielding materials are widely employed to isolate sophisticated instruments or human settlements from EMI sources [3–6]. Practically, EMI shielding materials generally require good optical transmittance while maintaining their strong EMI shielding performance, especially the optical windows of detectors or sensors in military aircraft, aeronautic and research facilities, medical apparatus, and instruments [7–11]. In fact, the simultaneous achievement of high optical transmittance and strong shielding efficiency is a persisting challenge. Further, there are tremendous efforts globally, to study new structures and materials for the development of the transparent EMI shielding technology. Currently, high-performance EMI shielding windows predominantly adopt transparent conductive materials, including various shapes of microscale metallic grids or networks [12–15], metallic nanowires [16], and transparent conductive oxides [17], for the reflection of incident microwave and radio wave. Nevertheless, despite the strong shielding performance and high optical transparency that can be obtained, the strong background scattering signals caused by pollution from secondary EM radiation in space cannot be eliminated in these reflection-dominated shielding materials [18,19]. Theoretically, a perfect EMI shielding window should completely absorb the incident microwave interference.
Recently, metamaterial absorbers (MMAs), composed of artificial assemblies of sub-wavelength periodic structured unit cells, have received increased interests from many researchers [20–22]. By properly designing the geometrical parameters of the structures, the effective permittivity and permeability of MMAs could be manipulated separately in a wide range beyond the ranges of conventional materials, resulting in impedance-matching of the free space [23–26]. The transmitted power is thereafter absorbed and dissipated by the structure because of the lossy components of effective permittivity and permeability. Compared with traditional absorption materials, MMAs have demonstrated better advantages, such as near-perfect absorptions, thinner thicknesses, lighter weights, and wider bandwidths, simultaneously [27–30]. With extensive investigations in recent years, various types of MMAs have been developed for a wide range of applications. However, most of conventional MMAs are mostly opaque because of the dark colors of the formed materials, which prevent their further developments as transparent EMI shielding windows [31–35]. To overcome this challenge, enormous efforts have been committed toward designing optically transparent MMAs [36–45]. For example, Shen et al. developed an optically transparent MMA with water-substrate incorporation units, which achieved broadband absorption, with absorptivity of > 90% in the range of 6.4–23.7 GHz [36]. An optically transparent MMA with windmill-shaped elements, which obtained a microwave absorption of > 90%, in the range of 8.3–17.4 GHz was also reported by Zhang et al. [37]. Chen et al. also developed a coding metasurface for broadband microwave scattering reduction with optical transparency, which achieved >10 dB scattering reduction at 8–15 GHz [45]. Although these optically transparent absorbers exhibited good broadband absorption and optical transmission performance, they generally utilized indium–tin–oxide (ITO) films as their back layers, and ITO patterns with a large duty cycle (the ratio of the area covered with conductive materials to the whole area of the structure) as the resonant layer. ITO thin films exhibit poor infrared (IR) transmissions while they are transparent at visible (Vis) wavelengths, thus limiting the applications of the EMI shielding windows, especially in the near-IR (NIR) region. Generally, wave, in the NIR region, is crucial in military and commercial detections, imaging, investigations, and monitoring. Additionally, for the ITO films, a better EMI shielding performance is associated with a worse optical transmittance. Compared to ITO films, micro-metal grids can afford enhanced EM shielding efficiency at the same optical transmittance. Therefore, the utilization of micro-metal mesh instead of ITO ground planes and the resonant patterns with low duty cycle in the design of broadband MMAs with high optical transparency, i.e., from Vis to IR, and strong EMI shielding performances are of great significance for both physics and other application research on absorption-dominated EMI shielding windows.
In this paper, we proposed an excellent broadband EMI shielding optical window with high Vis and NIR transparency, based on MMA. The unit cell consisted of ITO double split circular ring (SCR) resonant patterns with a low duty cycle, which were placed on polyethylene terephthalate (PET), and the aluminum mesh with a micro-period, etched on the quartz glass, was separated by a polymethyl methacrylate (PMMA) substrate. The metamaterial-based optical window simultaneously achieved strong microwave absorption and shielding performance over an ultrawide microwave region, as well as high optical transmittance from Vis–NIR. Moreover, the functionality of our device was its polarization-independence and insensitivity to incident angles of the impinging microwave, based on its symmetrical resonant patterns. We obtained the measured absorptivities for both the transverse electric (TE) and transverse magnetic (TM) mode waves through tests in the microwave anechoic chamber. The experimental measurements are in good agreement with the simulated results. Such a high-performance metamaterial-based optical window exhibited good application prospects in the field of high-performance transparent EMI shielding materials, which are urgently desirable in modern optoelectronic devices.
2. Numerical simulation and discussion
The schematic of the MMA-based broadband EMI shielding optical window in which a multilayered structure was employed to achieve the desired functionality is presented in Fig. 1(a). The single unit cell consisted of the double SCRs, which were placed at the top of the PET films; the PMMA substrate; and the aluminum mesh with a micro-period, etched on the quartz glass, in sequence. An aluminum grid was employed to replace the ITO film, as the back layer of the structure, which enabled visible light to pass through the window and also ensured the transmission of IR light. In the numerical simulations, the ITO films with a sheet resistance of 6 Ω/sq were employed as the material resonant patterns, which could induce high ohmic loss and broaden the absorption bandwidth. The aluminum grid exhibited a conductivity of 3.5 × 107 S/m, and the thickness of the aluminum mesh was 200 nm. Further, PET, PMMA, and quartz glass possessed dielectric constants of 3.0 (1-j0.03), 2.25 (1-j0.01), and 3.80 (1-j0.0004), respectively. The geometric details of the ITO resonant patterns are shown in Fig. 1(b); the period of a single unit cell is a = 10 mm. Additionally, the radius of the inner SCR was R1 = 4.3 mm and that of the outer SCR was R2 = 4.6 mm. The width of the split was d = 100 μm, the linewidth of the whole SCRs was w = 100 μm, the diagonal splits angle was β = 90°, and the rotation angle of the inner ring related to the outer one was 45°. The bottom square aluminum grid, as shown in Fig. 1(c), exhibited a period of p = 350 μm, and the linewidth was d0 = 10 μm. The thicknesses of the PET film, PMMA, and quartz glass substrates were h1 = 0.175 mm, h2 = 3.5 mm, and h3 = 1.2 mm, respectively. Therefore, the proposed EMI shielding optical window exhibited a slightly low duty cycle, which contributed to its optical transmissions, like Vis and IR.
Fig. 1. (a) Schematic diagram of the MMA-based optical window. The geometries of (b) the ITO resonant patterns and (c) bottom square aluminum mesh. The dimensions of the single unit cell are a = 10 mm, R1 = 4.3 mm, R2 = 4.6 mm, d = 100 μm, w = 100 μm, β = 90°, p = 350 μm, d0 = 10 μm, h1 = 0.175 mm, h2 = 3.5 mm, and h3 = 1.2 mm. The resonant patterns of SCRs were produced from ITO with a sheet resistance of 6 Ω/sq. (d) Simulated absorptivity, transmittance, and reflectivity curves of the metamaterial-based optical window. (e) Retrieved effective EM parameters; complex permittivity, $\varepsilon_r = \varepsilon_r^{\prime} + \varepsilon_r^{\prime\prime}$ and permeability, $\mu_r = \mu_r^{\prime} + \mu_r^{\prime\prime}$.
Next, we performed the numerical simulations of the metamaterial-based optical window, employing the Ansoft high frequency structure simulator (HFSS 15.0) software. A transverse EM wave whose electric and magnetic field polarizations were set to be excited along the x and y directions, respectively, was incident normally from the front of the window. From the simulations, we obtained the complex frequency-dependent S-parameters, S11 and S21, where the reflectivity, transmittance, and absorptivity are given by $R(\omega ) = \textrm{|S}_11{\textrm{|}^2}$, $T(\omega ) = |S_21{|^2}$, and $A(\omega ) = 1 - R(\omega ) - T(\omega )$, respectively. As shown in Fig. 1(d), the absorptivity value of > 90% covered a wide frequency range of 7.8–18.3 GHz. Moreover, the two absorption peaks, located at 9.6 and 16.0 GHz, respectively, and the highest absorptivity could attain ∼99.00%, thereby satisfying the critical requirement of an optical window of an aircraft. Moreover, the transmittance was generally < 0.9%, thus demonstrating the excellent performances of microwave interception and shielding. Through the S-parameters retrieval method [46], we successfully retrieved εr and μr of the absorber. As shown in Fig. 1(e), the real part of εr was very close to that of μr at the broadband absorption band of 7.8–18.3 GHz, implying that impedance-matching of the free space was achieved to minimize the reflected wave. Since the microwave transmittance of the metamaterial optical window was relatively low, reducing the reflection of the incident wave would enhance its absorption performance. Simultaneously, the imaginary parts of the effective EM parameters were positive in the absorption band, indicating the effect of dissipation on the incident EM wave. Additionally, it could be observed from the effective EM parameters that the electric and magnetic resonance occurred simultaneously, at two resonant absorption frequencies (9.6 and 16.0 GHz), implying that the absorption was contributed by the electric and magnetic resonance, which improved the absorption strength and broadened the absorption peaks.
Further, to gain detailed insights on the absorption mechanism of the optical window, we studied the distributions of the electric fields and surface current vectors on the top and bottom of the unit cell, at the two nearly perfect absorption peak frequencies, 9.6 and 16.0 GHz, as shown in Fig. 2. At the low-resonant frequency, most of the electric fields were mainly localized at the splits of the double SCRs [Fig. 2(a)]. The electric field was strongly limited in the gaps between the outer and inner SCRs, at the high-resonant frequency [Fig. 2(b)]. Figures 2(c) and 2(e) show the distributions of the surface current vectors on the ITO resonant layer and the cross-sectional view of the induced surface current vectors at 9.6 GHz, respectively. The induced current, excited by external electric fields on both sides of SCRs, were along the negative direction of the x-axis and parallel to the direction of the electric field, thereby generating an electric resonance. Because the ITO resonant layer was close to the aluminum grid, near-field coupling was induced to generate anti-parallel currents between the resonant region and the mesh layer [Fig. 2(e)], and a closed-loop was formed on the z-axis to generate a magnetic resonance. Moreover, by adjusting the electric and magnetic resonance to the same frequency, EM coupling was induced. Thereafter, EM coupling regulated and tailored the equivalent EM parameters of the metamaterial, resulting in the impedance-matching of the free space and structure and the resultant suppression of the reflection of incidence. This finding verified that the aforementioned electrical and magnetic resonance occurred simultaneously at the resonant absorption frequency. At 16.0 GHz, the distributions of the surface current vectors on the resonant layer and the cross-sectional view of the induced surface current vectors are shown in Figs. 2(d) and 2(f), which is like the distribution, at 9.6 GHz, except for the directions of the currents. Generally, the induced surface currents on the resonant layer combining the effective resistance from ITO SCRs will result in a significant ohmic loss to enhance the absorption performance according to $Ploss = {I^2}RITO$, where I is the induced current, and RITO is the surface resistance of the ITO film.
Fig. 2. (a, b) Electric field distributions of the ITO resonant pattern layers, as well as the directions of the induced surface current vectors on (c and d) the resonant layers and (e and f) the cross-sectional view of the induced surface current vectors of the metamaterial-based optical window at two absorption peaks.
To further understand the working mechanism, a general equivalent circuit model was constructed to describe the optical window, as shown in Fig. 3(a). To simplify the operation, we assumed here that the transparent dielectric substrate was lossless and that the incident wave was incident normally into the absorber. The transparent dielectric substrate was regarded as a transmission line with characteristic impedance, $Z_T = Z_0{{\sqrt {\varepsilon_r} } / {\sqrt {\mu_r} }}$, where Z0 = 377 Ω is the impedance of the free space, εr and μr are the relative permittivity and permeability of the transparent dielectric substrate, respectively. Further, the transmittance was generally < 0.9%, thus the transmitted wave was not considered in the equivalent circuit model. In this circuit, the cascaded resistance, R; inductance, L; and capacitance, C, were associated with the ITO resonant layer, and LAl was described as the equivalent inductance of the aluminum grid. According to the transmission line theory, the reflection coefficient, r, could be defined as follows [47,48]:
where Zin is the input impedance of the metamaterial-based optical window.Fig. 3. (a) Equivalent circuit model of the metamaterial-based optical window. The calculated (b) R–f relationship curves of the structure and (c) L–C relationship curves, at the two resonant absorption frequencies (9.6 and 16.0 GHz), as well as (d) the Smith chart of Zin, at 7–19 GHz.
In the resistance, inductance, and capacitance (RLC) series equivalent circuit theory, the equivalent surface impedance (ZR) of the ITO resonant layer is given by Eq. (2):
where j is the imaginary unit and ω = 2πf is the angular frequency in the resonant point.The grounded transparent dielectric slab behaved as an inductor, and its equivalent impedance (ZA) could be computed analytically as follows:
where β = $\mathrm{\omega }\sqrt {{\varepsilon _r}{\mu _r}} /{c_0}$ (c0 is the velocity of the wave in a vacuum) and h is the thickness of the transparent dielectric. Since the thickness of PET is relatively low, quartz glass was placed under the grid, and PMMA was crucial in the overall structure, εr and μr of PMMA were employed to describe the transparent dielectric.Hence, based on the transmission line theory [46,49], the overall input impedance (Zin) can be expressed as follows:
By combined Eqs. (1), (2), (3), and (4), the real and imaginary parts of Zin could be derived as follows:The thickness of the transparent dielectric substrates and the size of resonant patterns were optimized to obtain an excellent absorption performance. In addition to these parameters, another important factor to consider in MMA is the surface resistance of the ITO resonant shapes. Figure 4 shows the simulated absorption spectra of the metamaterial-based optical window containing the double SCRs with different sheet resistance. As the sheet resistance changed from 6 to 10 Ω/sq, the two notable resonant peaks became a single one. When the sheet resistance further increased to 22 Ω/sq, the peak absorption reduced from 0.991 to 0.862, and the peak frequency was fixed at 13.8 GHz where one-quarter of the wavelength was the thickness of the absorber. Precisely, in this case, the absorber behaved like a Salisbury screen [50]. Further, the increase in the surface resistance of the ITO film layer could weaken the induced current, which had been excited by the applied electric field; reduce the EM coupling strength, and change the impedance-matching condition, thereby resulting in a decrease in the absorption performance. When the sheet resistance of the ITO patterns was 6 Ω/sq, both the absorption bandwidth and performance were optimal.
Fig. 4. Simulated absorption curves of the metamaterial-based optical window with different ITO sheet resistance from 6 to 22 Ω/sq, at the normal incidence of the TE waves.
The proposed metamaterial-based optical window exhibited robust functionalities that were highly desired in practical applications. Figures 5(a) and 5(b) depict the simulated absorption efficiencies versus the frequencies (x-axis) and incident angles (y-axis) in the TE and TM mode polarizations, respectively. Noteworthily, in terms of both the TE and TM polarized waves, the fabricated structure could maintain a rather broad absorption band while the incident angle varied from 0° to 45°, and the absorption was fixed at > 85%. However, the absorptivity decreased clearly when the incident angle was > 45°. Additionally, there were some minor differences between the two modes. In the TE polarization mode, as the incident angle increased, a slight blue shift occurred in the resonant absorption peaks, at high frequency. Further, in the TM polarization mode, as the incident angle increased, there was largely no shift in the two resonant peaks, although their absorption bandwidths shrunk slightly. The reduction of the absorptivity with the TE wave was mainly due to the decrease in the magnetic flux of the structure with the increase in the incident angle. Further, the intensity of the circulating currents, induced by the incident magnetic field in the structure, became gradually weak to influence the impedance-matching and reduce the absorption property [51]. Similarly, with the TM wave, the interactions between the incident electric field and ITO resonant patterns were weakened as the incident angle increased, thus resulting in a decrease in absorption.
Fig. 5. Simulated absorption map of the proposed metamaterial-based optical window with different incident angles from 0° to 75° for (a) TE and (b) TM polarized waves. (c) Simulated absorption characteristics for normal incident waves with different φs.
Next, we investigated the polarization angle dependence of the proposed metamaterial-based optical window. Figure 5(c) depicts its absorption spectra with the polarization angle, φ, of the incident waves varied from 0° to 45° with a step of 15° for the normal incident case. Clearly, the absorptivity of our device was strictly invariant to the polarization angle. It benefited from the fact that the design was a four-fold center symmetric, which equipped it with identical features in both the TM and TE plane mode waves [52–54]. As discussed thus far, the proposed window exhibited excellent polarization-independence and possessed wide incident angle absorption properties for both the TE and TM polarizations.
3. Experimental verification
To experimentally validate the optical and microwave performances of the proposed design, the metamaterial-based optical window was fabricated, as described in Fig. S1 in Supplement 1. The sample (150 × 150 mm) consisting of 225 unit cells is shown in the left inset of Fig. 6(a) where letters, printed on a paper under the sample were clearly seen, proving that the optical window exhibited high optical transmission. The right insets of Fig. 6(a) also exhibit the ITO patterns and aluminum networks, which were separately recorded by a 10× lens, respectively. The average linewidth of the aluminum is ∼10.25 μm, and the geometric parameters of the ITO resonant patterns agree well with those of the simulation. The optical transmittance of the sample was measured with a UV/Vis/NIR spectrometer (Lambda 750), at a wavelength range of 400–1,500 nm, as shown in Fig. 6(a). Moreover, the optical transmittance is > 62.77%, and the average value attained ∼73.10%, indicating its excellent Vis–NIR transparency. Actually, the choices of PMMA and quartz glass substrates also affected the optical transmittance. Therefore, the degraded optical transparency could be further enhanced by maintaining the purity of the ITO films during laser processing and selecting PMMA and quartz glass substrates with higher optical penetrability.
Fig. 6. (a) Photograph of the fabricated sample and the optical transmittance of the sample, at a wavelength of 400–1,500 nm. The measured absorptivity spectra of the metamaterial-based optical window under (b) TE and (c) TM waves at different incident angles, θ1, as well as (d) measured EMI SE and SEA and the simulated EMI SE and SEA under TE polarization.
The microwave transmittance and reflectance were measured in a microwave anechoic chamber, and the measurement details are shown in Fig. S2 in Supplement 1. The measured absorption spectra of the sample are illustrated in Fig. 6(b) with the TE polarization waves, which are largely consistent with the simulated results, shown in Fig. 5(a). The absorption rate at normal incidence was slightly < the simulated value. Moreover, as the incident angle increased from 0° to 45°, the broadband absorption was still achieved with an absorptivity of > 90%, i.e., from 7.8 to 18.0 GHz. The measured microwave absorption curves, at the illumination of TM polarization waves [Fig. 6(c)], retained a large absorptivity (> 88.7%) from 7.7 to 18.0 GHz, at all the incident angles, as predicted by the simulations in Fig. 5(b). Similarly, the absorptivity, at normal incidence, was slightly < that of the simulation. Moreover, it was observed that the two resonant absorption peaks, at normal incidence, in the simulation results, could be observed near the corresponding points of the measured absorption curves. The absorption spectra in the experiment were slightly different from those in the simulation because of the difference between the materials, employed in the manufacturing process, and the simulated values; a slight deviation in the surface resistance and the size of the resonant units; negligence of the thickness of the optically clear adhesive; and the error of the experimental equipment. Moreover, as the incident angle increased, the absorption curves generated a slight shift toward high frequency, as revealed by the results from previous simulation analysis. Generally, the wave absorption performance of the designed window is in good accordance with the simulation results, at 7–18 GHz, and even at 18 GHz, and more than 90% absorption could be maintained. Thus, it was predictable that its absorption performance, at 18 GHz, gradually reduced, and this result will also be consistent with that of the simulation. Therefore, the designed optical window exhibited excellent stabilities, namely a wide-angle of incidence and strong broadband absorption performance.
Regarding the EMI shielding window, one of the main parameters was SE, which could evaluate its ability to intercept and shield EM waves. The total SE comprised the reflection (SER) and absorption (SEA). We calculated SE and SEA from the microwave reflectance, transmittance, and absorption, as follows: $SE(dB) ={-} 10\log 10|T|$, $SER(dB) ={-} 10\log 10|1 - R|$, $SEA(dB) ={-} 10\log 10|T/(1 - R)|$, respectively. As shown in Fig. 6(d), the simulated SE and SEA clearly overlapped, from 7.8 to 18.0 GHz with the TE polarization, indicating that the mechanism of shielding was dominant in absorption while contributing minimally to the reflection construction. Furthermore, the measured SE and SEA were > 18.25 dB and > 17.56 dB, respectively, from 7.0 to 18.0 GHz, and they are very identical in the broadband absorption region, thus verifying that SE mainly contributed to the absorption in this band. However, the measured SE and SEA are slightly < those of the simulated results. Such a phenomenon was caused by the irregularities in the metal thicknesses, linewidths, and the surface oxidations of the aluminum mesh, which would degrade the reflectivity of the incident waves of the sample and reduce the shielding performance. Additionally, the test system also exhibited a slight measurement error. Only the periods, linewidths, and thicknesses of the bottom grids were related to SE. On the premise of maintaining high optical transmittance, the choice of the metal mesh structure did not affect the broadband absorption distribution of the window.
Comparisons between this metamaterial optical window and its counterparts are outlined in Table 1. It exhibits that the designed metamaterial window in this paper has excellent broadband microwave absorption, strong EMI shielding effectiveness and polarization-independent performance, especially high optical transmittance from Vis to NIR region at 400-1500 nm.
Table 1. Comparison between the metamaterial-based optical window in this work and its counterparts.
4. Conclusion
Conclusively, we demonstrated a metamaterial-based EMI shielding optical window with Vis–NIR transparency to achieve broadband, polarization-independent, and incident angle-insensitive perfect microwave absorption and shielding. The experimental results indicated that the proposed window achieved an absorptivity of > 90%, covering an ultrawide frequency band of 7.8–18.0 GHz. Moreover, the measured SE was > 18.25 dB, from 7.0 to 18.0 GHz, which agrees well with the values obtained from the numerical simulations. Furthermore, the average normalized transmittance of the window was fixed at ∼73.10% in the Vis–NIR region of 400–1,500 nm. These excellent properties availed great potentials for the applications of microwave absorption, such as EMI shielding optical windows and electronic displays for aeronautic, medical, civilian, and research facilities.
Funding
Strategic Priority Research Program of the Chinese Academy of Sciences (XDB16030700); High-level talents Program (2019CT001); Shanghai Rising-Star Program; Youth Top-notch Talent Support Program in Shanghai; National Natural Science Foundation of China (61675219, 61875256).
Disclosures
The authors declare no conflicts of interest.
See Supplement 1 for supporting content.
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