Polarization Tunable Terahertz Metamaterial Absorber - IEEE Xplore

Polarization Tunable Terahertz Metamaterial Absorber Volume 7, Number 4, August 2015 Ben-Xin Wang Gui-Zhen Wang Xiang Zhai Ling-Ling Wang

DOI: 10.1109/JPHOT.2015.2448718 1943-0655 Ó 2015 IEEE

IEEE Photonics Journal

Polarization Tunable Metamaterial Absorber

Polarization Tunable Terahertz Metamaterial Absorber Ben-Xin Wang,1,2 Gui-Zhen Wang,3 Xiang Zhai,1 and Ling-Ling Wang1 1

School of Physics and Electronics, Hunan University, Changsha 410082, China 2 School of Science, Jiangnan University, Wuxi 214122, China Modern Educational Technology Center, Hunan Traditional Chinese Medical College, Zhuzhou 412012, China

3

DOI: 10.1109/JPHOT.2015.2448718 1943-0655 Ó 2015 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received May 31, 2015; revised June 19, 2015; accepted June 19, 2015. Date of publication June 23, 2015; date of current version August 3, 2015. This work was supported in part by the National Natural Science Foundation of China under Grant 61176116 and Grant 11074069 and in part by the Specialized Research Fund for the Doctoral Program of Higher Education of China under Grant 20120161130003. Corresponding authors: X. Zhai and L. L. Wang (e-mail: [email protected]; [email protected]).

Abstract: Metamaterial-based perfect absorbers have attracted considerable attention due to their potential for practical applications. The existing absorbers, however, are mostly polarization insensitive or only sensitive to one direction, which is inapplicable in some areas. Polarization tunable or high absorption in two orthogonal directions is very useful and necessary. Herein, we present a polarization tunable absorber formed by an asymmetric patch and a dielectric layer on top of a metallic board. With this structure, the frequency of the absorber can be tuned by merely changing the polarization of the incident. The tunable mechanism originates from the different length of the patch along the two orthogonal directions. The concept is rather general and applicable to various absorbers, as long as the asymmetric design is valid. The absorber can find practical applications in manipulation of the polarization of the light and detecting waves with specific polarization. Index Terms: Metamaterial, perfect absorber, polarization tunable, terahertz.

1. Introduction Metamaterials have attracted significant attention due to their exotic properties that are unavailable in nature, such as invisibility cloaking [1], perfect lensing [2], and negative refraction. Since most proposed metamaterials are metallic resonant structures and rely on strong resonances, the absorption losses are inevitable; the loss of the metamaterials often degrades their performance (such as sensing and filtering performances). However, the loss becomes useful and could be significantly enhanced in creating perfect absorber materials. Since its first presentation [3], metamaterial-based absorbers have received considerable attention and many absorbers have been proposed [4]–[7]. However, these efforts have common shortcomings of single-band absorption, which is inapplicable in some areas. An effective method to broaden the absorption bandwidth is to make the metamaterial units resonate at several neighboring frequencies. Following this design strategy, polarization insensitive or polarization dependent broadband absorbers have been widely demonstrated from microwave to optical [8]–[23]. For example, Huang et al. [11] presented the polarization dependent

Vol. 7, No. 4, August 2015

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Fig. 1. (a) Cross section and (b) top view of the unit cell of the proposed asymmetric metallic patch absorber. (c) Simulated absorption spectra for different polarizations.

broadband absorber based on three I-shaped resonators. Grant et al. [12] obtained polarization insensitive wideband absorbers by stacking multiple patterns. However, for certain applications, such as controlling thermal emissivity [6], [10], polarization tunable absorbers, as opposed to absorbers that are polarization insensitive or only sensitive to one direction, could be used for wavelength conversion. During this process, the incident light with a specific polarization can be first absorbed by the structure; then the structure can absorb another frequency along the perpendicular orientation. This could be certainly achieved by constructing two different absorber structures or stacking at least five layers. However, these approaches result in an increased thickness and fabrication complexity. Alternative ways of obtaining polarization tunable absorption would thus be of great importance. In this paper, we present a polarization tunable absorber formed by only an asymmetric metallic patch and a metallic board separated by a dielectric layer. With this structure, we are able to tune the absorber frequency by merely changing the polarization of the light. The tunable mechanism originates from the variation of the effective length of the absorber. This feature makes the proposed structure very useful in manipulating polarization of light and detecting waves with specific polarization. The concept also applies to other types of asymmetric structure and could be readily extended to other regimes. Furthermore, we demonstrate the polarization tunable dual-band and broadband absorbers by combing several such asymmetric patterns.

2. Structure and Design The unit cell of the structure is illustrated in Fig. 1; it consists of an asymmetric patch and a dielectric layer on top of a ground plane. The repeat period is P ¼ Px ¼ Py ¼ 85 m, and the lengths of the patch along the x - and y -axis are lx ¼ 44 m and ly ¼ 40 m, respectively. The thickness of the patterned gold is 0.4 m with a conductivity of ¼ 4:09 107 Sm1 . The dielectric constant of dielectric (polyimide) is 3(1 þ i0:06) [14], and its thickness is t ¼ 4:5 m. The computational model and suitable boundary conditions that we assumed in our simulations are available in [17].


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Fig. 2. (a)–(c) Distributions of the electric ðjE jÞ, real ðEzÞ, and magnetic fields at the frequency of 1. 79 THz, respectively. (d)–(f) Distributions of the electric ðjE jÞ, real ðEzÞ, and magnetic fields at the frequency of 1.94 THz, respectively.

Fig. 1(c) shows the absorption spectra of the absorber for various polarization angles. As shown, the frequency switches from 1.79 THz to 1.94 THz gradually when the angle increases from 0 to 90 . The bandwidths, which are defined as the full width at half maximum, are 0.19 THz and 0.21 THz for the x - and y -polarized waves, respectively, and the off-resonance absorption is very small. These results indicate a frequency selectivity of the absorber due to the narrow resonance bandwidth. To better understand the physical origins and the tune of the frequency, we give the calculated electric (jE j and real ðEzÞ) and magnetic field ðjHy jÞ distributions corresponding to the absorption in 1.79 THz (x -polarization) and 1.94 THz (y -polarization) in Fig. 2. For resonance at 1.74 THz, it is obvious that the electric field ðjE jÞ is mainly focused on both sides of the patch [see Fig. 2(a)], indicating larger charge accumulates at the edges of the array [24], [25]. As shown in Fig. 2(b), opposite charges accumulate at sides of the patch, indicating the excitation of the electric dipole resonance in the metallic array. This electric dipole is strongly coupled with the bottom board, and an anti-parallel surface current on the top and bottom layers can be obtained. As a result, a magnetic polariton is formed, and this induces a strong magnetic resonance [see Fig. 2(c)]. The coupling of the electric dipoles and the magnetic response is mainly determined by the dielectric layer thickness [13]. In addition, it is clear that the field distribution for resonance at 1.94 THz is similar to that of 1.79 THz. Thus, the effectiveness of the absorber is attributed to the coupling of the electric and magnetic resonances. Furthermore, according to the LC circuit model [15], the absorber frequency is inversely proportional to the patch length. Thus, the frequency at x -polarization is less than that of the y -polarization because the length lx is larger than that of ly . The proposed concept can be realized at lower frequencies by changing the dimensions of the absorber. For a typical example, we can give a design in the microwave range. Similar to the structure of Fig. 1(b), the dielectric is simulated with electric permittivity " ¼ 4 and loss tangent tan ðÞ ¼ 0:02 [15], and the metallic are modeled as lossy copper with a conductivity of 5:8 107 Sm1 and the thickness of 0.2 mm. The unit cell of the structure is shown in Fig. 3(a), and the optimal parameters are followed in millimeters: P ¼ Px ¼ Py ¼ 10, lx ¼ 7, ly ¼ 6,


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Fig. 3. Unit cell of the asymmetric microwave patch absorber (a) and the asymmetric terahertz cross absorber (c). (b) and (d) Dependence of the absorption spectra for different polarizations in microwave and terahertz ranges, respectively.

t ¼ 0:15. Fig. 3(b) shows the absorption spectra of the proposed absorber. It can be seen from Fig. 3(b) that, for x -polarized (0 ) GHz wave, there is a distinct absorption peak at 8.49 GHz with the absorptivity of 99.98Q. When the GHz wave turns into y -polarized (90 ), the absorption shifts to 11.1 GHz. Obviously, the asymmetric metallic patch structure is appropriate for designing the polarization tunable microwave absorber. Although our approach is presented using the asymmetric patch structure, it is a genetic method and can also apply to other type structure designs. Here we employ the asymmetric cross as an example. The unit cell of the structure is shown in Fig. 3(c), the parameters are followed in micrometers: P ¼ Px ¼ Py ¼ 60, lx ¼ 34, ly ¼ 38, w ¼ 4, t ¼ 5:5. The conductivity of the gold and the dielectric constant of the dielectric are the same in the terahertz absorber. Fig. 3(d) shows the absorption spectra of the structure for various polarization angles. As shown, the frequency switches from 2.65 THz to 2.38 THz gradually when the angle increases from 0 to 90 . Therefore, the asymmetric cross structure is suitable for designing the polarization tunable absorber.

3. Polarization Tunable Dual-Band and Broadband Absorbers Below we try to increase the number of the absorption peaks to achieve the polarization tunable dual-band and broadband absorbers. As we know, the absorber frequency is primarily determined by the pattern length. Thus, dual-band or broadband absorbers can be obtained by combining two different sized patches on a stacked structure. (Fig. 4(a)–(c) shows the cross section and the top views of the dual-band and broadband tunable absorbers, respectively.) The optimal parameters for the tunable dual-band and broadband absorbers are listed in Table 1.


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Fig. 4. (a) Cross section of the unit cell of the dual-band and broadband tunable absorbers, (b) and (c) Top views of top and bottom layers of the dual-band and broadband absorbers, respectively. (d) and (g) Absorption spectra in 0 and 90 for dual-band and broadband absorbers, respectively. (e) and (f) Magnetic field distributions of the dual-band absorber at the frequencies of 1.70 THz and 2.14 THz, respectively. (h) and (i) Field distributions of the broadband absorber at the frequencies of 1.72 THz and 1.90 THz, respectively.

TABLE 1 Parameters for the proposed dual-band and broadband absorbers

For the sake of clarity, we first study the dual-band absorber. The absorption for x -polarized wave, with two frequencies centered at 1.70 THz ðf1 Þ and 2.14 THz ðf2 Þ, is plotted in Fig. 4(a). As the wave turns into y -polarized, those two peaks shift to 2.12 THz and 2.77 THz, respectively. The shift of the peaks toward higher frequencies is explained by the decrease in the length of the patterns. To understand the physical insight, in Fig. 4(e) and (f), the distributions of the magnetic field for modes f1 and f2 are investigated. As shown in Fig. 4(e), the resonance at 1.70 THz is associated with the excitation of the bottom layer (i.e., dielectric layer t2 ), while the resonance at 2.14 THz is mainly a consequence of excitation of the top layer (i.e., dielectric layer t1 ) [see Fig. 4(f)]. Therefore, the combination of those two peaks determines the dual-band absorption.


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Fig. 5. Dependence of the absorption spectra on the length (a) l1 and (b) l3 of the dual-band absorber.

The mechanism of the dual-band absorber can be better explained by changing the patch sizes. As discussed above, the absorber frequency is inversely proportional to the patterned length. Thus, with the other parameters fixed, the length l1 (or l3 ) change can nearly determine the frequency of the mode f2 (or f1 ). As shown in Fig. 5(a), it is obvious that as the length l1 is increased, mode f2 becomes small while mode f1 is nearly fixed. Similarly, for the change of l3 , mode f1 gradually increases with the decrease of l3 , while mode f2 change is neglected [see Fig. 5(b)]. Taking a further step, we also investigate the tunable broadband absorber. It can be seen from Fig. 4(g) that the absorption spectra for x -polarized THz wave consists of two resonances, located at the frequencies of 1.72 and 1.90 THz. Owing to the two closely positioned peaks, we obtain a wide frequency band, from 1.59 to 2.04 THz, where the absorption is over 50Q. When the THz wave turns into y -polarized, those two peaks are both shift to higher frequencies, and thus the polarization tunable broadband absorber is obtained. Particularly, the field distributions for the broadband absorber are similar to that of the dual-band absorber. Consequently, the mechanism of the broadband absorber is attributed to the decrease in the length of the absorber.

4. Conclusion In conclusion, we demonstrate a polarization tunable absorber formed by an asymmetric metallic patch and a dielectric layer on top of a metallic board. The frequency of the absorber can be controlled by changing the polarization of the incident light wave. The tuning mechanism of the absorber is attributed to the variation of the effective length of the absorber. Moreover, the concept is applicable to other types of asymmetric absorber structure and can readily be scaled up to the structures that are working in the microwave frequency range. Finally, we demonstrate the polarization tunable dual-band and broadband absorbers by combing several such asymmetric metallic patterns.

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