Artificially structured metallic meshes have been widely used in optical transparent devices for electromagnetic interference (EMI) shielding. Traditional meshes have drawbacks such as poor imaging quality from the centralized diffraction distribution and the inferior EMI shielding efficiency. Here, we propose a petal-shaped metallic mesh with anomalistic closed-loop pattern that can effectively improve the EMI shielding efficiency by reducing the equivalent period of the mesh and, meanwhile, suppress the stray light and achieve uniform optical diffraction. By experimentally testing the EMI shielding and diffraction performance of such mesh samples, a Ku-band shielding efficiency (SE) larger than 24 dB with the maximum SE of 32.1 dB at 12.2 GHz is obtained, which is 8 dB higher than that of the classical square mesh, and an optical transmittance as high as 73.4% is achieved at the visible wavelength of 632.8 nm. Petal-shaped metallic mesh with such excellent performance may play important roles in various applications of optical transparent windows.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In complicated electromagnetic environment, electromagnetic interference (EMI) is a severe problem for device stability in civil, commercial, and military applications, especially for transparent optical devices such as optical windows utilized in aircrafts, electronic display, etc [1–6]. Therefore, it is a vital issue for the transparent optical devices to be able to shield EMI effectively while maintain their high optical transmittance [7,8]. Some materials or coatings have been proposed to achieve this goal, such as graphene [9,10], carbon nanoparticles [11–13], metasurfaces [14–16], indium tin oxide (ITO) [17,18], and metallic meshes [19–21]. In particular, metallic meshes with micrometer periods have been widely used in optical transparent devices for EMI shielding . For optical transparent windows coated with metallic meshes, the transmittance is an important figure of merit (FOM) and only the zeroth-order diffraction beam is imaged. The high-order diffraction energy is attributed to stray light that causes low imaging quality . In this regard, traditional meshes such as square meshes have a severe drawback that the diffraction energy mainly falls in the (m, 0)th and (0, n)th diffraction orders that are in two directions parallel to the periods of the mesh, which is unexpected for the imaging purpose . There have been some studies on the design of metallic meshes with high shielding efficiency (SE) and uniform diffraction distribution. For instance, Wang et al. proposed a kind of metallic meshes consisting of rings and sub-rings, which had a 3 dB improvement of SE compared with the square mesh and lower stray light with more uniform diffraction . There are also some reports on using, for example, double-layer metallic meshes to further improve the SE [22,26], which incur more processing steps and increase the difficulty of fabrication. The use of irregularly shaped patterns to reduce the diffraction non-uniformity is an effective way. Halman et al. proposed three metallic meshes, in which two with irregular patterns show more uniform diffraction distributions than square mesh and another one . However, the SE of these metallic meshes were not taken into account. Han et al. proposed a metallic mesh with irregular pattern generated by self-cracking template, which could efficiently reduce the concentration of high-order diffraction energy . Nevertheless, with this fabrication method, the mesh pattern is arbitrary and cannot be reproduced, so that the SE performance may be unstable.
Here, we propose a novel single-layer metallic mesh with petal-shaped pattern (as shown in Fig. 1), which can effectively achieve both high EMI SE and uniform diffraction distribution while maintains relatively high optical transmittance and simple geometry for fabrication. The petal-like pattern is a combination of semicircles and arc segments. The rings can homogenize the diffraction distribution due to their circular geometry , and the arcs can further redistribute the diffractive energy uniformly . The unit cell of the mesh consists of four closed loops, which leads to smaller equivalent period to facilitate EMI shielding. According to Ulrich’s equivalent circuit model , under the circumstance of the same linewidth and metal thickness, the smaller period a metallic mesh takes, the higher SE it can achieve. Therefore, the petal-shaped metallic mesh has remarkable improvement of SE over the square mesh with the similar parameters. In the following, after theoretical analysis and design, the proposed mesh is fabricated and experimentally tested. The comparison with square mesh shows its superior performance for EMI shielding in optical transparent windows.
2. Mesh design
Figure 1(a) depicts the proposed petal-shaped mesh with period g and mesh linewidth 2a. The elementary structure of the 1/4 unit cell, as shown by the area surrounded by the yellow dashed lines in Fig. 1(a), consists of segments of arcs with two different curvature radii, as shown in Fig. 1(b). Therefore, the unit cell is composed of semicircles and arcs tangent to the semicircle, which form the petal pattern with four closed loops. The key of the elementary structure design is the choice of proper radius of the semicircle. If it is too large, the four petal-shaped parts would intersect with each other to form additional smaller closed loops, which may cause stronger diffraction according to the scalar diffraction theory. On the contrary, if the radius of the semicircle is too small, the radius of the arcs connecting the semicircles and the central point of the petal would be larger, so that the shape of the area surrounded by the red dashed lines in Fig. 1(a) tends to be square and leads to the similar diffraction behavior of square mesh. Because of the far larger size of the unit cell than the optical incident wavelength, the small change of the radii of arcs has little influence on the diffraction distribution. Therefore, we choose the radius of semicircles as g/8 and the radius of the connecting arcs as 5g/8 as shown in Fig. 1(b). The period g must be much smaller than the incident microwave wavelength so as to effectively shield EMI . Meanwhile, the optical transmittance of the mesh depends on the obscuration ratio  (i.e., the ratio of the area without metal to the total area), which should satisfy actual demand. Based on the above considerations and also taking into account the fabrication feasibility, we choose the period and linewidth of the mesh as 250 µm and 5 µm, respectively. The thickness of metal is 200 nm and the material is chosen as copper for low cost and high conductivity. In the following, we compare the performance of the proposed mesh and a copper square mesh with the same period, linewidth, and metal thickness. All meshes are supported by sapphire substrates.
3. EMI shielding and optical transmittance analysis
EMI shielding performance is a vital property of metallic mesh when used on optical windows. In reality, metallic meshes usually function as high-pass filters for low frequency microwave shielding [7,23]. The mechanism of EMI shielding can be interpreted by waveguide theory . For example, the unit cell of square metallic mesh can be seen as a rectangular waveguide and the cut-off frequency is almost dependent on the size of the square cross section of the waveguide. The smaller aperture a waveguide takes, the higher cut-off frequency it can achieve and, meanwhile, the higher SE for low frequency wave can be obtained. Therefore, the period and linewidth of the mesh are two key parameters for SE. There are some models for SE analysis of metallic meshes [7,23,29,30], among which the commonly used one is the equivalent circuit model firstly proposed by Ulrich . According to the model, the smaller period and larger linewidth one adopts, the higher SE one can get. However, smaller period and larger linewidth reduce the obscuration ratio of the mesh and, furthermore, diminish its optical transmittance, which are unexpected in many applications. Therefore, with the petal-shaped metallic mesh, we can effectively reduce the equivalent period of the mesh while still maintain the relatively high optical transmittance. In the unit cell, each petal-shaped closed loop is a waveguide and the cut-off frequency depends on the area of the cross section. Therefore, we can obtain the equivalent period g’ = g/2 that is the long axis of the closed loop, which can be substituted into the Ulrich’s model
Using the model, we can calculate the SEs of the square mesh and the petal-shaped mesh in Ku-band (12 GHz to 18 GHz). Numerical calculations with CST Microwave Studio are also performed to validate the results, in which normal incidence of plane wave is assumed. Figure 2 shows the calculation results with both the Ulrich’s model and numerical software. We can see that the SE of the petal mesh is nearly 8dB higher than that of the square mesh, i.e. the microwave transmittance of petal mesh is only 15.8% of that of the square mesh. Therefore, by use of the petal-shaped mesh, the EMI shielding performance is significantly improved.
On the other hand, the optical transmittance of metallic mesh, which depends on the obscuration ratio of mesh, is also important in practical applications. The obscuration ratios of the square mesh and the petal mesh are calculated to be 96.0% and 88.8%, respectively, under the circumstances of the same period and linewidth. Taking into account the optical transmittance of the sapphire substrate (85% at 632.8 nm), we can calculate the transmittances of both meshes supported by sapphire substrates as: square mesh 81.6% and petal mesh 75.5%. Moreover, the use of metallic mesh leads to unwanted stray light caused by diffraction of the periodic pattern, which degrades imaging quality. The distribution of diffraction pattern can be analyzed by Fraunhofer diffraction based on Huygens-Fresnel diffraction theory [25,32]. The unit cell of the petal mesh has C4 symmetry. If we define a function EP(x,y) to represent the closed loop curve, the pupil function of the petal mesh can be expressed as23,32], we can obtain the normalized intensity of the diffraction pattern of the petal mesh as
With the model above, we can calculate the normalized diffraction intensity distributions of the meshes, as shown in Figs. 3(a) and 3(b). It is seen that the diffracted energy of the square mesh mainly concentrates in the (m, 0)th and (0, n)th diffraction orders. In contrast, the diffraction pattern of the petal mesh is obviously much more uniform. In order to evaluate the homogeneity of the diffraction distribution quantitatively, we can calculate the normalized diffraction energy (NDE) of the meshes. The maximum NDE of the side lobes of the petal mesh is −3.09dB (0.081%), which is much smaller than that of the square mesh −2.8dB (0.17%), with a 52.3% drop. Therefore, it can be concluded that the petal mesh can effectively overcome the drawback of diffractive energy concentration of the square mesh while maintain relatively high optical transmittance of the optical windows.
4. Mesh fabrication
We fabricated the metallic meshes by ultraviolet lithography and double-layer photoresist method. The manufacturing processes are as follows: a sapphire substrate was coated with America Micro-chem lift-off resist (LOR), with spinning speed 3000 r/min for the resist thickness of about 500 nm, and baked on a hot plate for 3 min at the temperature of 170 °C; a layer of 1.2 µm thick Suzhou Ruihong RZJ-304 photoresist was coated on the LOR with spinning speed of 3000 r/min and baked on a hot plate for 3 min at the temperature of 100 °C; the substrate with double-layer photoresist was exposed by using a Karl SUSS MA/BA 6 lithography machine for 3.5 s and then put in Ruihong RZX-3038 developer solution for 80 s; a 200 nm thick copper layer was evaporated on the sample by using an e-beam evaporator; the sample was immersed in acetone solution for about 1 h and then washed by deionized water and blew dry with nitrogen; finally, the sample was immersed in RZX-3038 for about 1min to dissolve the remaining LOR and then washed by deionized water and blew dry with nitrogen. The microscopic images of the fabricated metallic mesh samples are shown in Fig. 4.
5. EMI shielding and optical transmittance measurements
We measured the EMI SEs of both meshes in Ku-band by using a Keysight Technologies E5071CEP ENA vector network analyzer (VNA) and two waveguide coaxial converters. The mesh sample was clamped tightly by two waveguide coaxial converters with four screws and nuts, as shown in Fig. 5(a), and the whole setup is connected with VNA by cables. The area of metallic mesh can cover the whole end of the waveguide coaxial converter. The data of SE can be obtained with VNA directly. The measurement results verify that there is a nearly 8 dB improvement of SE for petal mesh in the measurement, which means that the microwave transmittance of the petal mesh is only 15.8% of that of the square mesh and the maximum SE of the petal mesh is −32.1dB at 12.2GHz. This is a remarkable improvement of SE in single-layer metallic mesh, compared with the square mesh with the same period and linewidth. Similar performance has only been reported with double-layer metallic meshes before [22,26], which however have more difficulty in fabrication. We note that there is a deviation of SE values of about 5 dB between the measurement and calculation results, as shown in Fig. 5(b). The main reason is that the Ulrich’s model assumes the metal to be perfect conductor. In experiment, the decrease of electrical conductivity of the metallic mesh due to the oxidation of metal may lead to the decrease of SE. Furthermore, the fabrication errors such as the inhomogeneity of metal and the measurement error may also influence the measurement results.
We also measured the diffraction distributions of both meshes under the illumination of a He-Ne laser at wavelength 632.8 nm, as shown in Figs. 3(c) and 3(d). The diameter of the laser beam was expanded to 8 mm and the whole diffraction pattern was recorded by a Nikon D5300 camera at a distance of 0.6 m behind the mesh sample. The black solid circles in Fig. 3(c) indicate the diameter of the incident beam. We measured the diffraction energy distribution at a distance of 3 m behind the sample by using Thorlabs PM100D optical power meter.
It is seen in Fig. 3(c) that the diffraction energy of the square mesh mainly distributes in the (m, 0)th and (0, n)th diffraction orders. In contrast, the diffraction energy of the petal mesh distributes more uniformly in the whole image plane, as shown in Fig. 3(d), meaning lower stray light and better imaging quality. The transmittance of the petal mesh is 73.45%, with only a 5.7% drop compared with that of the square mesh, meaning that an optical window coated with the petal mesh can still maintain high optical transmittance. We also measured the maximum NDE of the side lobes of the square mesh as −2.74 dB (0.18%) and that of the petal mesh as −2.92 dB (0.12%), meaning that there is a 33.3% drop by using the petal mesh. These conclusions are all in accordance with the theoretical results analyzed above. More importantly, compared with the previous meshes with similar performance and multiscale patterns [25,26,28], the petal mesh has simpler pattern with four loops of the same size, which is easier to fabricate with lift off and has better quality after fabrication. Therefore, the optical windows covered with petal meshes may have better transmittance and imaging quality in applications.
In this work, we proposed a novel metallic mesh with petal-shaped pattern to simultaneously achieve higher EMI SE and uniform diffraction distribution. The petal mesh with closed loops could obtain smaller equivalent period leading to higher SE. The maximal SE of 32.1 dB was obtained at 12.2 GHz in Ku-band and an 8 dB improvement of SE (compared with that of square mesh) was achieved, meaning that the microwave transmittance of the petal mesh was only 15.8% of that of the square mesh. It is the first time that such significant improvement of SE was achieved with a single-layer metallic mesh, compared with the square mesh with the same period and linewidth. The similar performance of EMI SE had only been obtained with double-layer metallic mesh before. Moreover, the petal-shaped pattern consisting of arcs with different radii can efficiently reduce the concentration of diffraction energy and smooth the diffraction distribution. In optical measurement, a significant drop of 33.3% for maximum NDE of the side lobes is obtained by using the petal mesh compared with the square mesh, meaning that the petal mesh can efficiently reduce the stray light caused by high-order diffraction and obtain better imaging quality. Therefore, the petal-shaped metallic mesh has great potential for applications of transparent optical devices.
National Natural Science Foundation of China (NSFC) (11474180, 61775113); Shenzhen Fundamental Research Funding (JCYJ20170412171535171)
The authors thank Min Zhang at Division of Advanced Manufacturing, Graduate School at Shenzhen, Tsinghua University, for his help in metallic mesh fabrication, Guixin Li at Department of Material Science and Engineering, and Materials Characterization and Preparation Center, Southern University of Science and Technology of China, for providing sample fabrication facilities, and Qingfeng Zhang at Department of Electrical and Electronic Engineering, Southern University of Science and Technology of China, for providing Vector Network Analyzer for SE measurement.
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