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We demonstrate a polarization-insensitive perfect absorber with multiband and broadband absorption based on a tunable and thin metamaterial, which consists of a double split-ring microstructure (DSRM) on double-layer and a coating substrate. The multiband absorption at different frequencies and broadband absorption with the relative bandwidth of 90.63% from 5.69GHz to 15.12GHz, of which the absorptivity is larger than 90%, can be achieved by changing the rotary angle of the proposed DSRM perfect metamaterial absorber (DSRM-PMA). The advantages of polarized-insensitivity, wide bandwidth and multiband absorption are illuminated by the angular absorptions and the surface current distributions. The DSRM-PMA device with similar geometry in simulation is fabricated and tested to clearly validate the functionality of our design. The simulated and experimental results indicate that the DSRM-PMA performs multiband and broadband absorptions with the rotary angle of 0° and 90° respectively.
© 2015 Optical Society of America
1. Introduction
Perfect metamaterial absorbers (PMAs), which are firstly proposed by Landy et. al in 2008 [1], have attracted increasing attentions in the research of metamaterials due to their remarkable capability of absorbing electromagnetic (EM) waves and their ultrathin structures. Later, researchers made several efforts on the PMAs to achieve wide incident absorption [2–5], polarization-insensitive absorption [6–9], broadband and multi-band absorption [10–19] in the frequency range including microwave, terahertz, and optics. However, the common PMAs exhibit the narrow band due to their single resonant structures at a fixed frequency, which limits their potential applications in many areas [20–24]. It is necessary to design the tunable PMA devices, which can achieve broadband and multiband absorption, with polarization -insensitive for satisfying the practical application.
In order to design the tunable PMA in microwave and terahertz frequencies, several techniques have been proposed and demonstrated in recent years. The diodes and the microelectomechanical system, which control the resonant frequency, have been applied to design tunable PMA [25–27]. Moreover, the stacked grapheme-dielectric sheet, phase-change material (Ge2Sb2Te5), and the special substrate are controlled by the voltage or temperature to design the tunable PMA [28–31]. Another approach is to change the structure or the height of substrate to obtain the tunable absorption with different characters [32–37]. The tunable PMA with narrow band absorption have been designed using these methods design.
Different from the proposed method, we proposed a multiband, broadband and polarization -insensitive perfect metamaterial absorber (PMA) with the three layers based on a tunable and thin double split-ring microstructure (DSRM) in this paper. In our design, the tunable DSRM was the tuning metamaterial by physically modifying the device geometry. The absorption bandwidth was changed by shifting the rotary angle between the split-ring structure I (SRS-I) on the bottom layer and the split-ring structure II (SRS-II) on the middle layer. For the rotary angle of 0°, the DSRM-PMA obtained a multi-band absorption. As the rotary angle increased to 90°, a broadband absorption with relative bandwidth of 90.63% from 5.69GHz to 15.12GHz with the absorptivity larger than 90% was achieved at normal incidence. The advantages were illuminated by angular absorptions and the surface current distributions. The DSRM-PMA devices with similar geometry in simulation were fabricated and measured to clearly validate the functionality of our design. The multi-band, broadband and polarization -insensitive absorption for the tunable metamaterial are illustrated by the simulated and experimental results.
2. Design and simulation analysis
As shown in Fig. 1, the proposed tunable DSRM-PMA consisted of a double-layer double split-ring microstructure (DSRM) and a coating substrate. The coating substrate was Arlon AD430 (εr = 4.3 and tanδ = 0.003) with the thickness of 1.5mm. The middle and bottom substrates were FR4 (εr = 4.4 and tanδ = 0.02) and Modified_epoxy (εr = 4.2 and tanδ = 0.02) with the thickness of 1.5mm, and 1mm respectively. The underside of the bottom substrate was a copper without pattern, so that the transmitted wave could be suppressed. The metallic DSRM was a copper with conductivity 5.8 × 107 S/m. The thickness of the copper is 0.036mm. The β represented the rotary angle between the splits in SRS-I and SRS-II. The DSRM-PMA was simulated and optimized by the High Frequency Structure Simulator (HFSS 14) to enhance the absorption and increase the absorbed bandwidth. The optimized parameters in simulation were as follows: r1 = 2.92, r2 = 3.42, r3 = 4.1, r4 = 4.6, w1 = w2 = 0.5 (units: mm).
Fig. 1 Schematic geometry of the DSRM-PMA unit cell. (a) The DSRM-PMA with three layers. (b) The middle layer with SRS-II. (c) The bottom layer with SRS-I. (d) The perspective of the DSRM-PMA. β represented the rotary angle between the splits of the SRS-II on the middle layer and the SRS-I on the bottom layer.
In order to investigate the tunable absorption for the DSRM-PMA, the Floquet port and periodic boundary conditions (PBCs) were utilized to design the PMA. Both the electric and magnetic resonances could be aroused independently, which would confine the electromagnetic (EM) wave into the DSRM-PMA cell. The EM wave would gradually be absorbed by dielectric and conductive loss. It could achieve that the effective permeability equaled to the permittivity (μ(ω) = ε(ω)) and the imaginary part of the effective impedance was close to zero (Im(Zeff(ω) = 0)), resulting in perfect absorptivity for incident EM wave. In more direct perspective, the absorptivity was defined as
To maximize absorptivity (A), we could minimize the reflection (R, R = |S11|2) and the transmission (T, T = |S21|2) simultaneously at the same frequency range. Because transmission was blocked by the metallic plate on the bottom layer (T = 0), the absorptivity could be calculated by A = 1-R. Hence, the absorptivity could be written asAccording to the Eq. (2), the simulated absorptivity results were shown in Fig. 2. The multiband and broadband absorptions from 2GHz to 18GHz could be achieved as the rotary angle β shifted from 0° to 150°. The DSRM-PMA achieved an absorption peaks at 4.7GHz with the absorptivity larger than 0.8 and β of 150°. The dual and triple absorption peaks were obtained with A>0.8 & β = 120° at 4.76, 16.85GHz and A>0.8 & β = 30° at 16.82, 4.74, and 15.61GHz respectively. The DSRM-PMA exhibited the multiband absorption with β of 0° and 60° and the broadband absorption with β of 90° from 5.69GHz to 15.12GHz for the absorptivity larger than 90% at normal incidence. As shown in Fig. 2, the different absorption bands from 4GHz to 18GHz were obtained for the proposed DSRM-PMA due to the different resonance modes. The tunable absorptions of the narrowband, dual-band, triple band, multiband and broadband for the DSRM-PMA were illustrated in Fig. 2 and Table 1. The relationship between the absorption band and the rotary angle is nonlinear. The relationship can be given by the Eq. (3), which is given as the follows.
Where y is the number of absorption band. The calculated results were showed in Fig. 2(b). We could see that the Eq. (3) could show the nonlinear relationship. The effective impedance was given in Fig. 2(c). From Fig. 2(c), we could see that the imaginary parts were close to zero when rotary angle was 0° at 4.72, 7.13, 10.86, 12.32, 14.8, and 17.42GHz and when rotary angle was 90° at 4.7, 6.1, 8.7, 12.5, 14.9, 16.5, and 17.5GHz. From the Fig. 2, it could be concluded that the DSRM-PMA performed tunable absorption with the different β values from 4GHz to 18GHz. For the limit on the period array, the cases of β = 0° and β = 90° have been fabricated and measured in a microwave anechoic chamber.Fig. 2 (a) Simulated absorptivity results of the DSRM-PMA with the rotary angle β of 0°, 30°, 60°, 90°, 120°, and 150° from 2GHz to 18GHz for the TEM incident wave (θ = 0°). (b) The relationship between the absorption band and the rotary angle. (c) The effective impedance of the DSRM-PMA.
Table 1. The DSRM-PMA exhibited the tunable absorption as rotary angle β of different values.
To interpret the multiband and broadband absorption of the DSRM-PMA, Fig. 3 showed the absorption with the incident angle shifted from 0° to 80° in the TE and TM polarized incidences. From Fig. 3(a), the absorption would decrease as the incident angle shifted from 0° to 80°. But the absorption peaks at 6.9, 10.86, 15.0 and 16.45GHz were remained due to the strong resonance intensity. As displayed in Fig. 3(b), the proposed DSRM-PMA obtained the broadband absorption with incident angle from 0° to 40°. The absorptivity was obviously weakened as the incident angle increased from 60° to 80°. The main reason was that the components along -z direction of the incidence EM wave decreased as the incident angle shifted from 0° to 80°. From Fig. 2, the absorption with the TE polarized incidence equaled to that of TM polarized incidence in the same conditions. Hence the proposed DSRM-PMA performed the polarization-insensitive absorption.
Fig. 3 Simulated absorption results of the DSRM-PMA from 2GHz to 18GHz at incident angles (θ) of 0°, 20°, 40°, 60°, and 80° for the TE and TM polarized incidences. (a) The absorption results of the DSRM-PMA with β = 0°. (b) The absorption results of the DSRM-PMA with β = 90°.
The above analysis could be further identified with the surface current distributions (SCDs) of the multiband DSRM-PMA with the rotary angle β of 0° at 8.0GHz and 10.86GHz when the incident angles were 0°, 40° and 80° for TE and TM polarized incidences in Fig. 4. The phenomena that the SCDs for the TE incidence were similar to that for TM incidence wave could be achieved not only for the incident angle of 0° but also for the incident angle of 40° and 80°. At 10.86GHz, the SCDs for θ of 0° were approximately equal to that for θ of 40° which caused that the absorptivity remained the same. Compared to the SCDs at 8.0GHz, the strong coupling effects between SRS-I and SRS-II produced as θ shifted from 0° to 80° at 10.86GHz all the while, this behavior leaded to the high absorption peaks for the DSRM-PMA in TE and TM polarized incidences. From the Figs. 3(a) and 4, the more coupling effect on the SRSs performed, the more absorptivity of the DSRM-PMA exhibited. It can be concluded that the DSRM-PMA with the multiband absorption obtained the advantage of polarized -insensitivity illuminated by the surface current distributions.
Fig. 4 Surface current distributions of the SRS-I, SRS-II and copper ground for DSRM-PMA with the rotary angle β of 0° at 8.0GHz and 10.86GHz when the incident angles were 0°, 40° and 80° for the TE and TM polarized incidences.
To interpret the polarized-insensitivity of the broadband DSRM-PMA for TE and TM polarized incidences, we presented the SCDs at 8.2GHz and 10.52GHz with β of 90° and θ of 0°, 40° and 80° in Fig. 5. The absorption peaks with θ of 40° and 80° were produced at 10.86GHz, but the absorptivity of θ of 40° and 80° were below 0.8 at 8.2GHz because the coupling effects were too weak, whereas the resonances of SRS-I seemed not to be affected much by the existence of SRS-II. Indeed, the antiparallel surface currents on SRS-II and SRS-I with θ of 40° at 10.52GHz enhanced each other by induced electromotive force. On the other hand, the induced current on SRS-I was more intensive than that on SRS-II one owing to difference in the number of free electrons, and SRS-II got influenced more by the incident wave when the coupling happened. Interestingly, when the incident angle was increased from 0° to 80°, the absorption was enhanced together at 10.86GHz in Fig. 3(b). This behavior could be explained by the fact that the increased number of free electrons on SRS-II affected not only the magnitude of induced surface current but also the full-charging time in the edge area, which was related to the capacitance of structure. It was reported that, compared with the anti-parallel surface currents in neighboring microstructure, the coupling effect between two metallic layer was stronger [38]. It is true that the SCDs for the broadband DSRM-PMA with the TM incidence were similar to that with the TE incidence at the same θ and frequency, so we could conclude that the broadband DSRM-PMA performed the advantage of polarized -insensitivity demonstrated by the surface current distributions in Fig. 5.
Fig. 5 Surface current distributions of the SRS-I, SRS-II and copper ground for DSRM-PMA with the rotary angle β of 90° at 8.2GHz and 10.52GHz when the incident angles were 0°, 40° and 80° for the TE and TM polarized incidences.
The nonlinear relationships between the incident angle and angular absorption for the multiband and broadband DSRM-PMA were shown in Fig. 6. From Fig. 6(a), it could be observed that the angular absorption with β of 0° at 4.72, 7.13, 8.0, 10.86, 12.32 and 14.8GHz decreased sluggishly and drastically for the incident angle increased from 0° to 60° and from 60° to 90° respectively. This phenomenon could be concluded from Fig. 6(b). When the incident wave impinged the DSRM-PMA, there would be direct reflection component due to the impedance-mismatch. The wave which entered the slab was multi-reflected by SRS-I, SRS-II, and copper ground plane and repeatedly absorbed, and a part of it was multi-refracted out of surface. The reflected and multi-refracted components increased as the incident angle increased. Hence the absorption would decrease as the incident angle increased. The absorptivity at 12.32GHz with β of 0° was enhanced due to the electric response as shown in Fig. 6(c). The enhanced electric response was caused by the coupling effect between SRS-I and SRS-II on the bottom and middle layers. From Fig. 6(b), the angular absorption for the broadband DSRM-PMA would increase owing to the strong electric response shown in Fig. 6(d) when the incident angle was 60° at 12.5GHz. From Figs. 6(a) and 6(b), we found that the proposed DSRM-PMA exhibited the wide incident angle at 10.86GHz with β of 0° and at 10.52GHz with β of 90° due to the strong coupling effects between the SRSs.
Fig. 6 Simulated angular absorption results of the DSRM-PMA when the incident angle (θ) shifted from 0° to 90° for the TEM incidence and the electric field and magnetic field with the incident angle (θ) of 60°. (a) Angular absorptivity results with β of 0° at the frequencies of 4.72, 7.13, 8.0, 10.86, 12.32, and 14.8GHz. (b) Angular absorptivity results with β of 90° at the frequencies of 4.7, 6.1, 8.2, 8.7, 10.52, 12.5 and 14.9GHz. (c) The electric and magnetic fields at 12.32GHz with β of 0° and θ of 60°. (d) The electric and magnetic fields at 12.5GHz with β of 90° and θ of 60°.
4. Fabrication and measurement
In order to verify the characters, two 576-cell (24 × 24) devices of the proposed DSRM-PMA with the rotary angle of 0° and 90° were fabricated and illustrated in Fig. 7. The fabricated tolerance is 0.02mm which is important for the fabrication and construction of polarization -insensitive DSRM-PMA nanostructures. The devices had been measured by employing the free-space test method in a microwave anechoic chamber. The multiband and broadband DSRM-PMA samples were fabricated using optical lithographic processes on substrates of Arlon AD430 (εr = 4.3 and tanδ = 0.003) with thickness of 1.5mm, of FR4 (εr = 4.4 and tanδ = 0.02) with the thickness of 1.5mm, and of Modified epoxy (εr = 4.2 and tanδ = 0.02) with the thickness of 1mm, respectively. A vector network analyzer (Agilent N5230C) and two linearly polarized standard-gain horn antennas, which covered 2-18GHz, were used to transmit and receive the EM waves. The devices were placed vertically in the center of a turntable to ensure that the EM wave could be similar to a plane wave on the front of device. The distance between the antennas and the devices under test satisfied the far-field condition.
Fig. 7 Prototypes of the proposed DSRM-PMA. (a) Photograph of the top layer. (b) Photograph of the middle layer. (c) Photograph of the bottom layer. (d) The proposed DSRM -PMA was measured in a microwave anechoic chamber.
Experimental results of the absorption for the multiband and broadband DSRM-PMA sample were given in Figs. 8(a) and (b). When the rotary angle of the multiband DSRM-PMA sample was 0°, the four band absorption with θ of 0° could be obtained for TE and TM polarized incidences from Fig. 8(a). Two wideband absorptions from 6.29GHz to 7.43GHz and from 10.25GHz to 13.06GHz could be achieved for the multiband DSRM-PMA sample with θ of 0°. The number of frequency band was increased as the incident angle increased as shown in Fig. 8 (a), because the more response modes would be produced by the coupling effect for the oblique incidence. Moreover, the broadband absorption could be exhibited from 5.63GHz to 15.46GHz with the absorptivity larger than 80% when the rotary angle of the DSRM-PMA sample was 90° in the TE and TM polarized incidences. It is noted that the absorptivity from 8.85GHz to 10.89GHz was more than 90% as the incident angle increased from 0° to 40° in the TE and TM polarized incidences due to the stable electric and magnetic responses. From Fig. 8, it was obvious that the DSRM-PMA device achieved the characters of multiband absorption, broadband absorption and polarized-insensitivity with the rotary angle β of 0° and 90°. The good agreement was obtained by the experimental and simulated results.
Fig. 8 The experimental absorptivity for the DSRM-PMA samples with β of 0° and 90° in the TE and TM polarized incident waves from 2GHz to 18GHz. (a) The experimental absorptivity of the multiband DSRM-PMA sample with β of 0° and θ of 0°, 40° and 80° in the TE and TM polarized incident waves. (b) The experimental absorptivity of the broadband DSRM-PMA sample with β of 90° and θ of 0°, 40° and 80° in the TE and TM polarized incident waves.
The experimental results of angular absorption for the DSRM-PMA sample with β of 0° and 90° were given in Fig. 9. The measured results illustrated that the angular absorption decreased sluggishly and drastically as the incident angle increased from 0° to 60° and from 60° to 90° respectively in the TE and TM polarized incidences not only for the multiband DSRM-PMA at 4.72, 7.13, 8.0, 10.86, 12.32, and 14.8GHz but also for the broadband DSRM-PMA at 4.7, 6.1, 8.2, 8.7, 10.52, 12.5, and 14.9GHz. The ultra-wide angle absorptions for the multiband DSRM-PMA at 10.86GHz and the broadband DSRM-PMA at 10.52GHz were performed for the TE and TM polarized incidences. The experimental results agreed well with the simulations for the TE and TM polarized incidences. It was noted that the difference between simulated and measured absorption results was addressed by the gaps between the different layers, the fabrication and measurement tolerances and the cable loss.
Fig. 9 Experimental angular absorption results of the DSRM-PMA with β of 0° and 90° when the incident angle (θ) shifted from 0° to 90° in the TE and TM polarized incidences. (a) Experimental angular absorption results of the multiband DSRM -PMA with β of 0° at 4.72, 7.13, 8.0, 10.86, 12.32, and 14.8GHz. (b) Experimental angular absorption results of the broadband DSRM-PMA with β of 90° at 4.7, 6.1, 8.2, 8.7, 10.52, 12.5, and 14.9GHz.
5. Conclusion
In conclusion, we have designed and fabricated a tunable DSRM-PMA which performed multiband and broadband absorption at different frequency controlled by the rotary angle between the SRS-I on the bottom layer and SRS-II on the middle layer. The tunable absorption of the narrowband, dual-band, triple-band, multiband and broadband for the DSRM-PMA could be achieved for the rotary angle β of 150°, 120°, 30°, 0°(60°) and 90°. The DSRM-PMA respectively exhibited the broadband absorption from 5.69GHz to 15.12GHz for the absorptivity larger than 90% and the multi-band absorption with the rotary angle of 90° and 0°. The performance of polarized-insensitivity is illustrated by the simulated surface current distribution and angular absorptions in TE and TM polarized incidences. The DSRM-PMA devices were measured in different polarizations and the agreement between experiment and simulation was good. In addition, the wide angle absorptions for the multiband DSRM-PMA with rotary angle β of 0° at 10.86GHz and the broadband DSRM -PMA with rotary angle β of 90° at 10.52GHz were obtained in the TE and TM polarized incidences. This DSRM-PMA device with the innovation of using rotary angle is promising for many practical applications such as radar cross scatter reduction.
Acknowledgments
This work is supported by the National Natural Science Foundation of China under Grant (No.61271100, No.61471389), the Natural Science Foundational Research Fund of Shaanxi Province (No.2010JZ6010, No.2012JM8003), and Doctoral Foundation under Grant of (No.KGD080914002) at Air Force Engineering University. They also thank the reviewers for their valuable comments.
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