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Broadband perfect absorption enabled by using terahertz metamaterial resonator
Superlattices and Microstructures 135 (2019) 106240
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Broadband perfect absorption enabled by using terahertz metamaterial resonator Ben-Xin Wang *, Chao Tang, Qingshan Niu, Yuanhao He, Huaxin Zhu, Wei-Qing Huang School of Science, Jiangnan University, Wuxi, 214122, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Metamaterials Broadband absorption Terahertz
Broadband metamaterial perfect absorbers are of paramount importance in many applications. However, these broadband absorbers are typically based on complex structure designs, the manufacturing process of them is highly challenging. This paper shows that a fairly simple structure formed by a rectangular metallic patch with two rectangular holes backed by two layers of dielectric sheet and metallic mirror is utilized to obtain the broadband absorption. The structure possesses three different but similar resonance peaks, and the superposition of them can realize a bandwidth of 1.00 THz with absorption >80%. The absorber physical origin is eluci dated by means of the corresponding field distributions of the three absorption modes. Further more, the device bandwidth can be tuned via changing the sizes of the resonator. These absorption properties using the simple structure design can benefit a wide range of applications.
1. Introduction Achieving near 100% (or 100%) absorption of incident waves in various frequency ranges is of particular importance in many aspects of bolometric detector, selective thermal emitters, thermal imaging, and energy collection. However, early studies of the light absorbers at microwave frequency range encountered some issues of thick and bulky [1,2]. Recently, it is found that the design concept of light absorption devices based on metamaterials can be used to build the resonance features of ultra-thin and light-weight. The first ultra-thin light absorber with measured absorption rate of 88% at microwave region was demonstrated by Landy et al. [3]. After that, many types of metamaterial absorbers in different spectral ranges have been designed to realize near 100% absorption for various applications [4–10]. Nevertheless, most of the designed light absorbers have single-band, and in particular of narrow-band absorption response. This is not advisable for some applications. Several ways have been presented to overcome the narrow-band characteristic and obtain the broadband absorption devices. Generally, these suggested approaches are mainly divided into two categories: The first one is to design large size of coplanar superunit structure consisted of several different dimensions of sub-units [11–18]. The second is the integration of multiple metallic ele ments (or components) into the stacked resonance structure [19–30]. However, these two frequently used methods suffer from some shortcomings. Firstly, the coplanar super-units not only have large dimension of structures which are opposite to design trend of miniaturization, but also have coupling between the sub-units which can deteriorate the high absorption performance of the broad spectral. Secondly, the manufacturing processes of the stacked structures are typically complex and challenging because of a large * Corresponding author. E-mail address: [email protected] (B.-X. Wang). https://doi.org/10.1016/j.spmi.2019.106240 Received 15 May 2019; Received in revised form 30 July 2019; Accepted 28 August 2019 Available online 29 August 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 135 (2019) 106240
B.-X. Wang et al.
number of the layers. Thirdly, it is extremely time-consuming to optimize the resonance structures of the BMAs using the methods of coplanar super-unit and stacked. Therefore, there is a pressing need to demonstrate a new type of broadband absorber which can overcome the issues faced using previous methods. Herein, we show that only one (or single-sized) metallic resonator can be used to obtain broadband absorption response. This kind of design strategy over previous works cannot only simplify the complexity of the structure significantly, but also has excellent broadband absorption performance. For example, a broad spectra of 1.00 THz with absorption >80% is realized. The FWHM of the device with respect to the central resonance frequency can reach 45.02%. The main physical origin of the broadband device is dis cussed with the aid of the field distributions. Results further reveal that the broadband spectral range of the device can be adjusted by reshaping the dimensions of the single-sized metallic resonator. These excellent properties show that it is a promising device and can have applications in energy harvesting and stealth technology. 2. Structure design The simple single-sized resonator consisted of a rectangular Au patch with two symmetrical rectangular holes is illustrated in Fig. 1 (a). In fact, it is difficult to achieve broadband absorption by employing the resonator on top of a dielectric layer (DL). Here we introduce an Au board on bottom of the DL (polyimide) to block the transmission of the incident waves. By carefully optimizing the DL thickness, a wide bandwidth of absorber can be obtained. Therefore, the basic cell of the designed absorber consists of three layers, the inset of Fig. 1(b) gives its side-view. The top layer of the absorber is shown in Fig. 1(a), the rectangular patch has length of l ¼ 70 μm, width of w ¼ 55 μm, thickness of 0.4 μm, conductivity of 4.09 � 107 S/m. The center of the rectangular patch coincides with the center of the unit cell. In other words, the rectangular patch is placed at the center of the unit cell. Their common center is represented by point O in red, see Fig. 1(a). Two rectangular holes with same sizes are introduced into the rectangular patch, and the two holes are symmetrically distributed in the unit cell (or the red point O). The distance between the center position of the rectangular hole and the center position of the unit cell is expressed by the deviation δ. The length and width of the two rectangular holes are marked with g and s, respectively. The detailed dimensions of two rectangular holes are as follows: length of g ¼ 12 μm, width of s ¼ 40 μm, deviation of δ ¼ 12 μm. The basic cell sizes are respectively Px ¼ 88 μm, and Py ¼ 68 μm. The DL has thickness of t ¼ 12 μm, and refractive index of n ¼ 1.73 þ i0.1. Finite dif ference time domain method is utilized to simulate the absorption performance of the designed device. In the simulations, the basic cell is vertically illuminated by a plane electromagnetic wave where its E-field is parallel to the x-axis. Periodic boundary conditions and perfectly matched layers are respectively chosen in plane of x-y and propagation direction (z-axis) of the incident waves.
Fig. 1. (a) and (b) are respectively the front view and absorption spectra of the broadband absorption device; (c) absorption spectra of the designed devices with/without rectangular holes; (d) absorption spectra of the designed device in polarization angles of 00 and 900; Inset of (b) is the side view of the broadband absorption device. 2
Superlattices and Microstructures 135 (2019) 106240
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3. Results and discussion For the simulation results of Fig. 1(b), three resonance peaks marked A1, A2 and A3 are observed, the absorbing rates of them can respectively reach 99.90%, 99.83%, and 99.85%. A wide absorption bandwidth can be obtained because of the three closely adjacent peaks. For example, a broad spectra of 1.00 THz with absorption >80% is achieved. Moreover, > 50% absorption can be obtained in a wider bandwidth of 1.28 THz. Considering the device central frequency to be 2.87 THz, its relative bandwidth is 44.60%, which is superior to previous works having polarization-dependent designs without experimental validation, for TE and TM modes respectively. The value of relative bandwidth can be further increased by reshaping the length l, see below Fig. 4(d). Compared with previous reports, this work not only can simplify the structure complexity significantly, but also has excellent absorption performance, including >99% absorption and wide resonance bandwidth. These features are essential in the design of a new type of broadband device. Note that the device absorption performance is not insensitive to polarization angles because of the asymmetric of the resonance structure. Blue curve of Fig. 1(d) shows that only one resonance peak is formed when the E-field is parallel to the y-axis (i.e., 900 polarization). To explore the main mechanism of the wide-band absorption, an additional absorber based on a rectangular patch and a DL deposited on a metallic mirror is introduced. It is remarkable that the rectangular patch of the newly introduced absorber has no holes, while other parameters are the same as those of broadband absorber. The black curve of Fig. 1(c) presents the absorption spectra of the newly introduced absorber. As shown, single resonance band located at 2.50 THz with absorption of 94.28% is realized. We partic ularly found that this mode is very close to the A1 of the broadband absorption device. Therefore, we can tentatively judge that the two modes should have the similar physical picture. However, the A2 and A3 should originate from the introduction of the holes in the rectangular patch because the existence of the two holes can rearrange their electromagnetic fields, see below Fig. 3. In fact, the physical picture of the single-band absorption of the newly introduced absorber is caused by the three-order response of the rectan gular patch because its magnetic field (perpendicular to the rectangular patch along the dotted line of Fig. 2(a)) distributions have three strong aggregation regions in the DL, see Fig. 2(b). Correspondingly, the single-band absorption of the designed device in 900 polarization (see blue curve of Fig. 1(d)) should be due to the three-order resonance of the patterned resonator because three strong magnetic field (perpendicular to the metallic resonator along the dotted line of Fig. 2(c)) distribution areas are observed in the DL, as shown in Fig. 2(d). We next give the corresponding field distributions of the A1, A2, and A3 to better explore the main mechanism of the broadband absorption. As observed in Fig. 3(a) of the mode A1, its electric fields are localized mainly on both flanks of the rectangular resonator, while the fields in both flanks of the two holes are negligible, which are very similar to that of the field distributions of the single-band absorption of newly introduced absorber in Fig. 2(a). Moreover, three magnetic field domains are observed in the DL of the broadband device, see Fig. 3(d) and (g). As a result, mode A1 can be explained by the three-order response of the rectangular resonator. On the basis of the above analysis, we can know that the introduced rectangular holes have negligible (o slight) effect on the original field distributions in Fig. 2(a) and (b). However, different from the case of the A1, we observed that the electric fields of the A2 in Fig. 3(b) (or A3 in Fig. 3(c)) are clustered chiefly in both flanks of the two introduced holes, which have nothing in common with the electric fields of the A1 in Fig. 3(a). Meanwhile, the magnetic fields of the A2 in Fig. 3(e) and (h) (or A3 in Fig. 3(f) and (i)) are also different from the case of the A1 in Fig. 3(d) and (g). A2 and A3 field distributions prove that the existence of the two holes in the rectangular patch can significantly rearrange the original field distributions in Fig. 2(a) and (b). That is to say, the main mechanisms of the A2 and A3 are caused by the introduction of the two holes in the rectangular patch.
Fig. 2. (a) and (b) are respectively the electric and magnetic field distributions of the resonance mode of the newly introduced absorber; (c) and (d) are respectively the electric and magnetic field distributions of the designed device in 900 polarization. 3
Superlattices and Microstructures 135 (2019) 106240
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Fig. 3. (a), (b), and (c) are respectively the A1, A2, and A3 electric field distributions; (d), (e), and (f) are respectively the A1, A2, and A3 magnetic field distributions in the plane that is perpendicular to the rectangular patch along the red dotted line; (g), (h), and (i) are respectively the A1, A2, and A3 magnetic field distributions in the plane that is perpendicular to the rectangular patch along the white short line. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. Dependence of the resonance performance of the broadband device on the parameter variations of (a) hole length g, (b) hole width s, (c) hole deviation δ, and (d) rectangular patch length l.
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Superlattices and Microstructures 135 (2019) 106240
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Although the A2 and A3 are due to the introduction of the two holes, the mechanisms (or the corresponding field distributions) of the two modes are different. Concretely speaking, for A2, besides strong electric fields at both flanks of the holes, four corners of the rectangular resonator have significant field aggregations, see Fig. 3(b). This is why we found that its magnetic fields show strong aggregations in both planes that are respectively perpendicular to the rectangular patch along the red dotted line and white short line in 3(b). However, for A3, its electric fields in four corners of the rectangular patch are negligible, see Fig. 3(c). As a result, we do not observe obvious magnetic distributions (see Fig. 3(i)) in the plane that is perpendicular to the rectangular patch along the white short line of Fig. 3(a). Therefore, its magnetic fields are only focused on the plane that is perpendicular to the patch along the red dotted line (see Fig. 3(f)) because its electric fields are confined mostly in both flanks of the holes and the rectangular patch. We can conclude that the main physical pictures of the A2 and A3 are different. However, the emergence of the A2 and A3 originates from the introduction of the two holes in the rectangular patch, which can make the original electromagnetic fields in Fig. 2(a) and (b) to be redistributed. Because the main mechanisms of the A2 and A3 are due to the two introduced holes in the rectangular patch, the resonance performance of the A2 and A3 should be strongly affected by the hole parameter changes. As shown in Fig. 4(a)–(c), the variations of the length g, width s, and deviation δ of the two holes indeed can obviously influence the resonance features of the A2 and A3, including the frequencies and absorptivities. The dependence of the absorption spectra on the parameter changes of the rectangular holes can provide further evidence to support the physical origin of the A2 and A3. Although the A2 and A3 performance have large dependence of the hole parameters, the resonance bandwidth of the designed device has not changed significantly. We found that, however, the length l change of the rectangular patch provides the ability to broaden the resonance bandwidth of the absorption device. As illus trated in Fig. 4(d), the A1 frequency can be intensely influenced by the length l, while the A2 and A3 have merely slight frequency shifts. Based on this resonance feature, the device bandwidth with absorption >50% can be increased with the decrease of the l. As a special example, a wide spectra of 1.35 THz with absorption >50% at central frequency of 2.72 THz is realized. Its relative absorption bandwidth is 49.63%, which is superior to previous broadband devices. This also reflects that the designed broadband absorber has the adjustable absorption performance [31–38]. 4. Conclusion In conclusion, a new type of terahertz metamaterial absorber formed by a rectangular metallic patch with two symmetric distributed rectangular holes and a dielectric sheet placed on a metallic ground plane is demonstrated to obtain broadband absorption performance. The device possesses three different but narrowing separated absorption peaks, the combination of them provides the ability to realize a wide bandwidth of 1.28 THz with absorption higher than 50%. The relative absorption bandwidth of the absorber can reach 45.02%, which is superior to prior broadband devices. The broadband device main mechanisms are discussed through analyzing the electric and magnetic fields of the three closely adjacent peaks. We also study the influence of structure parameters on the performance of the broadband absorption, and the results confirm that the relative absorption bandwidth (or resonance band width) can be further broadened via decreasing the rectangular patch length. These findings provide the opportunity to develop broadband terahertz absorbers with simple structure design. Funding This work was supported by National Natural Science Foundation of China (11647143), Natural Science Foundation of Jiangsu (BK20160189), Postdoctoral Science Foundation of China (2019M651692), Postdoctoral Science Foundation of Jiangsu (2018K113C), and the Fundamental Research Funds for the Central Universities (JUSRP51721B). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2019.106240.
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Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Broadband perfect absorption enabled by using terahertz metamaterial resonator Ben-Xin Wang *, Chao Tang, Qingshan Niu, Yuanhao He, Huaxin Zhu, Wei-Qing Huang School of Science, Jiangnan University, Wuxi, 214122, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Metamaterials Broadband absorption Terahertz
Broadband metamaterial perfect absorbers are of paramount importance in many applications. However, these broadband absorbers are typically based on complex structure designs, the manufacturing process of them is highly challenging. This paper shows that a fairly simple structure formed by a rectangular metallic patch with two rectangular holes backed by two layers of dielectric sheet and metallic mirror is utilized to obtain the broadband absorption. The structure possesses three different but similar resonance peaks, and the superposition of them can realize a bandwidth of 1.00 THz with absorption >80%. The absorber physical origin is eluci dated by means of the corresponding field distributions of the three absorption modes. Further more, the device bandwidth can be tuned via changing the sizes of the resonator. These absorption properties using the simple structure design can benefit a wide range of applications.
1. Introduction Achieving near 100% (or 100%) absorption of incident waves in various frequency ranges is of particular importance in many aspects of bolometric detector, selective thermal emitters, thermal imaging, and energy collection. However, early studies of the light absorbers at microwave frequency range encountered some issues of thick and bulky [1,2]. Recently, it is found that the design concept of light absorption devices based on metamaterials can be used to build the resonance features of ultra-thin and light-weight. The first ultra-thin light absorber with measured absorption rate of 88% at microwave region was demonstrated by Landy et al. [3]. After that, many types of metamaterial absorbers in different spectral ranges have been designed to realize near 100% absorption for various applications [4–10]. Nevertheless, most of the designed light absorbers have single-band, and in particular of narrow-band absorption response. This is not advisable for some applications. Several ways have been presented to overcome the narrow-band characteristic and obtain the broadband absorption devices. Generally, these suggested approaches are mainly divided into two categories: The first one is to design large size of coplanar superunit structure consisted of several different dimensions of sub-units [11–18]. The second is the integration of multiple metallic ele ments (or components) into the stacked resonance structure [19–30]. However, these two frequently used methods suffer from some shortcomings. Firstly, the coplanar super-units not only have large dimension of structures which are opposite to design trend of miniaturization, but also have coupling between the sub-units which can deteriorate the high absorption performance of the broad spectral. Secondly, the manufacturing processes of the stacked structures are typically complex and challenging because of a large * Corresponding author. E-mail address: [email protected] (B.-X. Wang). https://doi.org/10.1016/j.spmi.2019.106240 Received 15 May 2019; Received in revised form 30 July 2019; Accepted 28 August 2019 Available online 29 August 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 135 (2019) 106240
B.-X. Wang et al.
number of the layers. Thirdly, it is extremely time-consuming to optimize the resonance structures of the BMAs using the methods of coplanar super-unit and stacked. Therefore, there is a pressing need to demonstrate a new type of broadband absorber which can overcome the issues faced using previous methods. Herein, we show that only one (or single-sized) metallic resonator can be used to obtain broadband absorption response. This kind of design strategy over previous works cannot only simplify the complexity of the structure significantly, but also has excellent broadband absorption performance. For example, a broad spectra of 1.00 THz with absorption >80% is realized. The FWHM of the device with respect to the central resonance frequency can reach 45.02%. The main physical origin of the broadband device is dis cussed with the aid of the field distributions. Results further reveal that the broadband spectral range of the device can be adjusted by reshaping the dimensions of the single-sized metallic resonator. These excellent properties show that it is a promising device and can have applications in energy harvesting and stealth technology. 2. Structure design The simple single-sized resonator consisted of a rectangular Au patch with two symmetrical rectangular holes is illustrated in Fig. 1 (a). In fact, it is difficult to achieve broadband absorption by employing the resonator on top of a dielectric layer (DL). Here we introduce an Au board on bottom of the DL (polyimide) to block the transmission of the incident waves. By carefully optimizing the DL thickness, a wide bandwidth of absorber can be obtained. Therefore, the basic cell of the designed absorber consists of three layers, the inset of Fig. 1(b) gives its side-view. The top layer of the absorber is shown in Fig. 1(a), the rectangular patch has length of l ¼ 70 μm, width of w ¼ 55 μm, thickness of 0.4 μm, conductivity of 4.09 � 107 S/m. The center of the rectangular patch coincides with the center of the unit cell. In other words, the rectangular patch is placed at the center of the unit cell. Their common center is represented by point O in red, see Fig. 1(a). Two rectangular holes with same sizes are introduced into the rectangular patch, and the two holes are symmetrically distributed in the unit cell (or the red point O). The distance between the center position of the rectangular hole and the center position of the unit cell is expressed by the deviation δ. The length and width of the two rectangular holes are marked with g and s, respectively. The detailed dimensions of two rectangular holes are as follows: length of g ¼ 12 μm, width of s ¼ 40 μm, deviation of δ ¼ 12 μm. The basic cell sizes are respectively Px ¼ 88 μm, and Py ¼ 68 μm. The DL has thickness of t ¼ 12 μm, and refractive index of n ¼ 1.73 þ i0.1. Finite dif ference time domain method is utilized to simulate the absorption performance of the designed device. In the simulations, the basic cell is vertically illuminated by a plane electromagnetic wave where its E-field is parallel to the x-axis. Periodic boundary conditions and perfectly matched layers are respectively chosen in plane of x-y and propagation direction (z-axis) of the incident waves.
Fig. 1. (a) and (b) are respectively the front view and absorption spectra of the broadband absorption device; (c) absorption spectra of the designed devices with/without rectangular holes; (d) absorption spectra of the designed device in polarization angles of 00 and 900; Inset of (b) is the side view of the broadband absorption device. 2
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3. Results and discussion For the simulation results of Fig. 1(b), three resonance peaks marked A1, A2 and A3 are observed, the absorbing rates of them can respectively reach 99.90%, 99.83%, and 99.85%. A wide absorption bandwidth can be obtained because of the three closely adjacent peaks. For example, a broad spectra of 1.00 THz with absorption >80% is achieved. Moreover, > 50% absorption can be obtained in a wider bandwidth of 1.28 THz. Considering the device central frequency to be 2.87 THz, its relative bandwidth is 44.60%, which is superior to previous works having polarization-dependent designs without experimental validation, for TE and TM modes respectively. The value of relative bandwidth can be further increased by reshaping the length l, see below Fig. 4(d). Compared with previous reports, this work not only can simplify the structure complexity significantly, but also has excellent absorption performance, including >99% absorption and wide resonance bandwidth. These features are essential in the design of a new type of broadband device. Note that the device absorption performance is not insensitive to polarization angles because of the asymmetric of the resonance structure. Blue curve of Fig. 1(d) shows that only one resonance peak is formed when the E-field is parallel to the y-axis (i.e., 900 polarization). To explore the main mechanism of the wide-band absorption, an additional absorber based on a rectangular patch and a DL deposited on a metallic mirror is introduced. It is remarkable that the rectangular patch of the newly introduced absorber has no holes, while other parameters are the same as those of broadband absorber. The black curve of Fig. 1(c) presents the absorption spectra of the newly introduced absorber. As shown, single resonance band located at 2.50 THz with absorption of 94.28% is realized. We partic ularly found that this mode is very close to the A1 of the broadband absorption device. Therefore, we can tentatively judge that the two modes should have the similar physical picture. However, the A2 and A3 should originate from the introduction of the holes in the rectangular patch because the existence of the two holes can rearrange their electromagnetic fields, see below Fig. 3. In fact, the physical picture of the single-band absorption of the newly introduced absorber is caused by the three-order response of the rectan gular patch because its magnetic field (perpendicular to the rectangular patch along the dotted line of Fig. 2(a)) distributions have three strong aggregation regions in the DL, see Fig. 2(b). Correspondingly, the single-band absorption of the designed device in 900 polarization (see blue curve of Fig. 1(d)) should be due to the three-order resonance of the patterned resonator because three strong magnetic field (perpendicular to the metallic resonator along the dotted line of Fig. 2(c)) distribution areas are observed in the DL, as shown in Fig. 2(d). We next give the corresponding field distributions of the A1, A2, and A3 to better explore the main mechanism of the broadband absorption. As observed in Fig. 3(a) of the mode A1, its electric fields are localized mainly on both flanks of the rectangular resonator, while the fields in both flanks of the two holes are negligible, which are very similar to that of the field distributions of the single-band absorption of newly introduced absorber in Fig. 2(a). Moreover, three magnetic field domains are observed in the DL of the broadband device, see Fig. 3(d) and (g). As a result, mode A1 can be explained by the three-order response of the rectangular resonator. On the basis of the above analysis, we can know that the introduced rectangular holes have negligible (o slight) effect on the original field distributions in Fig. 2(a) and (b). However, different from the case of the A1, we observed that the electric fields of the A2 in Fig. 3(b) (or A3 in Fig. 3(c)) are clustered chiefly in both flanks of the two introduced holes, which have nothing in common with the electric fields of the A1 in Fig. 3(a). Meanwhile, the magnetic fields of the A2 in Fig. 3(e) and (h) (or A3 in Fig. 3(f) and (i)) are also different from the case of the A1 in Fig. 3(d) and (g). A2 and A3 field distributions prove that the existence of the two holes in the rectangular patch can significantly rearrange the original field distributions in Fig. 2(a) and (b). That is to say, the main mechanisms of the A2 and A3 are caused by the introduction of the two holes in the rectangular patch.
Fig. 2. (a) and (b) are respectively the electric and magnetic field distributions of the resonance mode of the newly introduced absorber; (c) and (d) are respectively the electric and magnetic field distributions of the designed device in 900 polarization. 3
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Fig. 3. (a), (b), and (c) are respectively the A1, A2, and A3 electric field distributions; (d), (e), and (f) are respectively the A1, A2, and A3 magnetic field distributions in the plane that is perpendicular to the rectangular patch along the red dotted line; (g), (h), and (i) are respectively the A1, A2, and A3 magnetic field distributions in the plane that is perpendicular to the rectangular patch along the white short line. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. Dependence of the resonance performance of the broadband device on the parameter variations of (a) hole length g, (b) hole width s, (c) hole deviation δ, and (d) rectangular patch length l.
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Although the A2 and A3 are due to the introduction of the two holes, the mechanisms (or the corresponding field distributions) of the two modes are different. Concretely speaking, for A2, besides strong electric fields at both flanks of the holes, four corners of the rectangular resonator have significant field aggregations, see Fig. 3(b). This is why we found that its magnetic fields show strong aggregations in both planes that are respectively perpendicular to the rectangular patch along the red dotted line and white short line in 3(b). However, for A3, its electric fields in four corners of the rectangular patch are negligible, see Fig. 3(c). As a result, we do not observe obvious magnetic distributions (see Fig. 3(i)) in the plane that is perpendicular to the rectangular patch along the white short line of Fig. 3(a). Therefore, its magnetic fields are only focused on the plane that is perpendicular to the patch along the red dotted line (see Fig. 3(f)) because its electric fields are confined mostly in both flanks of the holes and the rectangular patch. We can conclude that the main physical pictures of the A2 and A3 are different. However, the emergence of the A2 and A3 originates from the introduction of the two holes in the rectangular patch, which can make the original electromagnetic fields in Fig. 2(a) and (b) to be redistributed. Because the main mechanisms of the A2 and A3 are due to the two introduced holes in the rectangular patch, the resonance performance of the A2 and A3 should be strongly affected by the hole parameter changes. As shown in Fig. 4(a)–(c), the variations of the length g, width s, and deviation δ of the two holes indeed can obviously influence the resonance features of the A2 and A3, including the frequencies and absorptivities. The dependence of the absorption spectra on the parameter changes of the rectangular holes can provide further evidence to support the physical origin of the A2 and A3. Although the A2 and A3 performance have large dependence of the hole parameters, the resonance bandwidth of the designed device has not changed significantly. We found that, however, the length l change of the rectangular patch provides the ability to broaden the resonance bandwidth of the absorption device. As illus trated in Fig. 4(d), the A1 frequency can be intensely influenced by the length l, while the A2 and A3 have merely slight frequency shifts. Based on this resonance feature, the device bandwidth with absorption >50% can be increased with the decrease of the l. As a special example, a wide spectra of 1.35 THz with absorption >50% at central frequency of 2.72 THz is realized. Its relative absorption bandwidth is 49.63%, which is superior to previous broadband devices. This also reflects that the designed broadband absorber has the adjustable absorption performance [31–38]. 4. Conclusion In conclusion, a new type of terahertz metamaterial absorber formed by a rectangular metallic patch with two symmetric distributed rectangular holes and a dielectric sheet placed on a metallic ground plane is demonstrated to obtain broadband absorption performance. The device possesses three different but narrowing separated absorption peaks, the combination of them provides the ability to realize a wide bandwidth of 1.28 THz with absorption higher than 50%. The relative absorption bandwidth of the absorber can reach 45.02%, which is superior to prior broadband devices. The broadband device main mechanisms are discussed through analyzing the electric and magnetic fields of the three closely adjacent peaks. We also study the influence of structure parameters on the performance of the broadband absorption, and the results confirm that the relative absorption bandwidth (or resonance band width) can be further broadened via decreasing the rectangular patch length. These findings provide the opportunity to develop broadband terahertz absorbers with simple structure design. Funding This work was supported by National Natural Science Foundation of China (11647143), Natural Science Foundation of Jiangsu (BK20160189), Postdoctoral Science Foundation of China (2019M651692), Postdoctoral Science Foundation of Jiangsu (2018K113C), and the Fundamental Research Funds for the Central Universities (JUSRP51721B). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2019.106240.
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