- Email: [email protected]
Ultra-wideband polarization insensitive UT-shaped metamaterial absorber
Accepted Manuscript Title: Ultra-wideband polarization insensitive UT-shaped metamaterial absorber Authors: Nasrollah Karampour, Najmeh Nozhat PII: DOI: Reference:
S1569-4410(16)30073-6 http://dx.doi.org/doi:10.1016/j.photonics.2017.03.002 PNFA 577
To appear in:
Photonics and Nanostructures – Fundamentals and Applications
Received date: Revised date: Accepted date:
11-10-2016 26-2-2017 2-3-2017
Please cite this article as: Nasrollah Karampour, Najmeh Nozhat, Ultrawideband polarization insensitive UT-shaped metamaterial absorber, Photonics and Nanostructures Fundamentals and Applications http://dx.doi.org/10.1016/j.photonics.2017.03.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultra-wideband polarization metamaterial absorber
insensitive
UT-shaped
Nasrollah Karampour and Najmeh Nozhat* Department of Electrical and Electronics Engineering, Shiraz University of Technology, Shiraz 7155713876, Iran *Corresponding author: [email protected]
Highlights:
An ultra-wideband metamaterial absorber with double layers of UT-shaped resonator and the ground plane which is made of titanium and gold is proposed.
The absorption mechanism is relies on the surface impedance tuning caused by UT-shaped resonator and resistive sheet effect of titanium layer in the structure.
The absorber performance is also analyzed by the interference theory and it is shown that the theoretical results are in good agreement with the simulation results.
A polarization insensitive absorber with two rotated UT-shaped resonators is proposed.
Abstract- In this paper, an ultra-wideband metamaterial absorber (MMA) with U and T shaped resonators has been proposed. The resonators and the ground plane consist of gold (Au) and titanium (Ti) layers. The resistive sheet effect of Ti layer and the resonance elements in the structure cause a broad absorption spectrum. The simulations are based on the finite element method (FEM) and the results show that the absorption of the proposed structure is more than 90% between 150 and 300 THz that is much larger than previous works. Moreover, by applying the interference theory, we have demonstrated that the simulation results are in good agreement with the theoretical results. The primary proposed MMA is polarization sensitive. Therefore, a polarization insensitive metamaterial absorber has been suggested. Also, because of the extra resonance elements the full width at 90% absorption increases about 35 THz. This ultra-wideband MMA has various applications in microbalometer, imaging, thermal emitters, photovoltaic, and energy harvesting.
Keywords: Metamaterial absorber, Nanostructure, Polarization insensitive, Resonator, Ultrawideband.
1. Introduction In the past decades electromagnetic metamaterials have attracted researchers’ attention due to their unusual properties such as negative refractive index (NRI), near zero index (NZI) and magnetic response in optical regime. First NRI metamaterial was proposed by Smith in 2000 [1] and after that a wide range of applications such as antenna, energy harvesting, super lens, waveguides and absorbers have been defined for metamaterials [2]. 1
First metamaterial absorber (MMA) was demonstrated by Landy et al. [3]. Since then, many studies have been done in this area and metamaterial absorbers are the basis of various devices such as switches [4, 5], modulators [5], multiplexers [6], filters [7], solar cells [8] and have been used in cloaking [9] and imaging applications [10]. The operation frequency of MMAs spread from microwave to optical frequencies and according to their absorption spectrum, they are classified as narrowband, wideband, single band and multiband absorbers [11]. The absorption mechanism of traditional absorbers are based on the impedance matching caused by the structure. In this case, the electromagnetic energy is completely transferred to the structure and dissipated at lossy medium. However, for metamaterial absorbers, the absorption is often based on a resonance type phenomenon such as electric-magnetic dipole [12-14], surface plasmon [15] and magnetic polariton [16], that the electromagnetic energy concentrates in the metamaterial structure and decays at lossy dielectric. The electric and magnetic responses can be introduced by a metamaterial resonator or by the coupling between a resonator and a ground plane. Consequently, depending on the resonator behavior some peaks appear at specific frequencies. In this type of MMA, the high quality factor (Q) causes a narrow absorption spectrum. Utilizing the multi-resonance elements and multilayer resonance structures with near resonance frequencies are the methods of broadening the absorption spectrum. In the MMA proposed by Wang et al. the full width at half maximum (FWHM) of about 33% is obtained by using a double layered square patches on top of a metallic ground plane [17]. In Ref. [18] by adding an extra layer the FWHM of the structure is enhanced to 42%. The broadband absorption in these structures is related to the longitudinal coupling between the layers, but the number of layers is limited by the effective-homogeneity condition. For MMA of Ref. [19] with two back-to-back split ring resonators, three resonance peaks and the FWHM of 47% are achieved. Combination of different types of absorbers is another strategy for increasing the bandwidth. Alici et al. have used this method and obtained 121 THz full width at 90% absorption [20]. In [21] a MMA with the FWHM of 86% has been proposed and a wide absorption spectrum is related to the excitation of slow light modes in multi-layered metal-dielectric sawtooth shaped structure. In this paper, a metamaterial absorber that consists of a UT-shaped resonator and an SiO layer as substrate has been proposed. The ground plane of this structure is made of titanium and gold layers. The titanium layer behaves as a lossy medium and so the MMA can have a wide bandwidth. Indeed, the low conductivity of titanium layer leads to increasing the structure losses [17]. Therefore, the quality factor is decreased and the absorption spectrum becomes wider. In the proposed ultra-broadband metamaterial absorber, a smooth and wide spectrum is attained by controlling the surface impedance of the structure. The total thickness of the structure is 260 nm that is much thinner than multilayer absorbers. In the first proposed structure, the absorption is more than 90%, between the frequencies of 150 and 300 THz, and in the second proposed MMA that is polarization insensitive, the full with at 90% absorption is improved to 185 THz. The FWHM of the proposed absorber is up to 100% and the full width at 90% absorption is about 70% respect to its central frequency. Also, the near ideal absorption value of 99.67% occurs at 270 THz. The absorption spectrum has been also calculated by the interference theory and it has been shown that the theoretical results are in good agreement with the simulation results. The wide bandwidth, thin and small unit cell size of the proposed MMA makes it suitable for microbolometers and imaging applications. 2
The paper is organized as follows. The theoretical method is expressed in Section 2, and the geometry of the proposed absorber is shown in Section 3. In Section 4, the simulation results are demonstrated and compared with the theoretical results. In Section 5, a polarization insensitive absorber is proposed and the paper is concluded in Section 6. 2. Theoretical Method The performance of MMAs can be studied by various methods such as constitutive parameters and refractive index interpretation, impedance matching and interference theory that all refer to the same concept. Here, our proposed structures are studied by the semitheoretical interference method to show that the magnetic response is created in top resonator elements. The interaction of a dielectric slab with an incident electromagnetic wave is shown in Fig. 1a. For normal incident, the transmission and reflection coefficients are given by [22]: t t e jk2 z d T t1t2e jk2 z d t1t2 r22e j 3k2 z d t1t2 r24e j 5k2 z d ... 1 22 j 2 k2 z d , (1) 1 r2 e
R r1 t1t2 r2e jk2 z d t1t2 r23e j 3k2 z d ... r1
t1t2 r2e j 2 k2 z d , 1 r22e j 2 k2 z d
(2)
where t 1 and r1 are the transmission and reflection coefficients at the first interface, respectively, t 2 and r2 are transmission and reflection coefficients at the second interface of the slab, respectively. k 2z is the slab wave number in the z direction and d is the slab thickness. This procedure has been investigated for two slabs system in Ref. [23]. Here, we have modified it for the case that the metamaterial layer is modeled as an anisotropic slab. The transmission and reflection coefficients of this system that is shown in Fig. 1b can be expressed as: T1T 2 (3) T t T1T 2 T1T 2 R1R 2 T1T 2 R12 R 22 ... , 1 R1R 2 T T R (4) Rt R1 T1T1R 2 T1T1R 1R 22 ... R1 1 1 2 , 1 R1R 2 where T 1 and R 1 are the total transmission and reflection for the incident wave in left side of slab 1, respectively. T 1 and R1 are the total transmission and reflection of slab 1 for the right side illumination, respectively. T 2 and R 2 are the total transmission and reflection from slab 2. According to the interference theory, when high order reflections are out of phase with the first reflected wave from the slab, the total reflected wave will be canceled. Consequently, by applying the mentioned concept to proposed MMA, the origination of the absorption can be explained. 3. Design and Simulation Method Geometry of the proposed metamaterial absorber is shown in Fig. 2. It consists of a UTshaped metamaterial structure, a composite ground plane and a dielectric layer between them. The ground plane is made of gold (Au) and titanium (Ti) layers, and a layer of Ti is between the UT-shaped structure and SiO dielectric layer. The thicknesses of SiO dielectric layer, top and bottom gold and titanium layers are Ts 128 nm , T Au 24 nm , TTi 20 nm , TTi g 38 nm , and T Au g 50 nm , respectively. 3
Also, the geometrical parameters of the UT-shaped resonator are selected to be Lu 168 nm ,
H u 100 nm , W u 34 nm , Lt 216 nm , H t 100 nm , W t 24 nm , and S 30 nm . The proposed ultra-wideband metamaterial absorber is simulated based on the finite element numerical method with the periodic boundary condition (PBC) in the x and y directions. The structure is illuminated with an electromagnetic wave in the z direction and the absorption spectrum is calculated from A 1 Rt T t 2
2
formula, where T and R are the
transmission and reflection coefficients, respectively. Because of the gold ground layer, the transmission is very low and can be neglected. The Refractive indices of gold and titanium are extracted from Palik’s data [24] and the refractive index of SiO is taken from the data obtained by Hass and Salzberg [25]. 4. Theoretical and Simulation Results and Discussions In order to study the interference theory and show the origination of magnetic response in our proposed absorber, the system must be decoupled into two slabs. The first slab consists of the metamaterial structure and the substrate and the second one is the ground plane. The transmission and reflection of these two slabs which are shown in Fig. 1b are equal to the Sparameters of each slab that are measured in the simulations. The transmission and reflection spectra of the decoupled system are depicted in Fig. 3. By putting these values in Eqs. (3) and (4), the total absorption is attained and demonstrated in Fig. 4. Also, the simulated absorption spectrum of the proposed metamaterial absorber is shown in Fig. 4. The structure is excited by a normal incident lightwave that the electric field is in the x direction. It can be seen that the absorption is more than 90% in the frequency range of 150 to 300 THz. Also, there are two absorption peaks at the frequencies of 170 and 260 THz with the values of 99.34% and 99.71%, respectively. It is obvious that the theoretical result has a good agreement with the simulation result. In the decoupled system, the circular current between the top metal structure and the ground plane cannot be formed. Hence, both the electric and magnetic responses in the structure are created by the UT-shaped element in the first slab.
We have used the UT-shaped resonators due to their abilities to create the magnetic and electric responses, simultaneously, in the top metal structure. The magnetic response, provided by the circular current, is induced in the U shaped element, and the electric response is created between the U and T shaped elements and adjacent unit cells elements, as shown in Fig 5. Moreover, the UT-shaped element provides various design parameters as degrees of freedom. In our proposed structure, both resistive sheet and resonance type elements have been used to attain an ultra-wideband absorber. Titanium layer and the gold plate behind it form the ground plane that behaves as an absorber with the resistive sheet effect. Also, the UT-shaped element contributes to the impedance matching in a wide frequency range. The absorption spectrum of the ground plane and the dissipated power density are depicted in Fig. 6. The reflected power from the ground plane into the structure experiences some internal reflections and finally goes out to the air. According to the interference theory, this wave has a destructive interference with the reflected wave from the first interface and so a wideband absorption spectrum can be achieved. 4
The absorption spectra for different polarization angles are depicted in Fig. 7. As can be seen from Fig. 7, the absorption of proposed MMA is dependent on the polarization. However, the absorption peak is more than 90% for all polarization angles. In order to overcome this dependency, a new structure is proposed in Section 5. Furthermore, the absorption spectra for both TE and TM polarizations and various incident angels are shown in Fig. 8. In TE configuration, the electric field is in the x direction and the wave propagates in the y-z plane, while for TM polarization, the magnetic field is in the y direction and wave propagates in the x-z plane. In TE polarization, the absorption is more than 70% at the incident angle of 60°, while for TM polarization, the absorption is more than 80% at the same incident angle. Overall, the absorption of the structure in TM excitation is better than TE excitation for oblique incident wave. 5. Polarization Insensitive Scheme As mentioned before, the optical response of the MMA is polarization sensitive. The proposed polarization insensitive scheme is depicted in Fig. 9. As shown in Fig. 10 the absorption spectrum of this configuration is completely independent of the lightwave polarization angle. The absorption of this MMA in the frequency range of 160 to 345 THz is more than 90% and the maximum absorption value of 99.67% occurs at 270 THz. Moreover, the full width at 90% absorption is enhanced about 35 THz in comparison to the structure of Fig. 1. The x component of the electric field at the frequency of 250 THz is shown in Fig. 11. In 250 THz frequency, the concentrated electric field between the U and T shaped resonators is the reason of broadening of the absorption spectrum. Also, the absorption spectra of the absorber of Fig. 9 for TE and TM polarizations at different incident angles are demonstrated in Fig. 12. For TM polarization, the absorption is up to 50% for all incident angles.
6. Conclusion We have used various resonance elements in combination with Ti resistive sheet effect to propose an ultra-wideband metamaterial absorber. The absorption spectrum of this structure is more than 90% in the frequency range of 150 to 300 THz. Also, the absorption of the structure has been calculated by the interference theory and its good agreement with the simulation results shows that the circular current between the top layer resonators and the ground plane has poor contribution in the absorption of the structure. In the proposed polarization insensitive MMA, the full width at 90% absorption increases 35 THz because of extra resonance elements. Moreover, since the absorption of the polarization insensitive MMA in the frequency range of 160 to 345 THz is more than 90%, it is suitable for wideband applications. References [1] D. R. Smith, W. J. Padilla, D. Vier, S. C. Nemat-Nasser, and S. Schultz, Composite medium with simultaneously negative permeability and permittivity, Phys. Rev. Lett. 84 (2000) 4184-4187.
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[2] C. M. Watts, X. Liu, and W. J. Padilla, Metamaterial electromagnetic wave absorbers, Adv. Mater. 24 (2012) OP98– OP120. [3] N. Landy, S. Sajuyigbe, J. Mock, D. Smith, and W. Padilla, Perfect metamaterial absorber, Phys. Rev. Lett. 100 (2008) 207402. [4] B. Kang, J. Woo, E. Choi, H. H. Lee, E. Kim, J. Kim, T. J. Hwang, Y. S. Park, D. Kim, and J. Wu, Optical switching of near infrared light transmission in metamaterial-liquid crystal cell structure, Opt. Express 18 (2010) 16492-16498. [5] M. Hajizadegan, V. Ahmadi, and M. Sakhdari, Design and analysis of ultrafast and tunable all optical metamaterial switch enhanced by metal nanocomposite, J. Lightwave Technol. 31 (2013) 1877-1883. [6] H. Liu, and K. J. Webb, Optical devices based on cylindrically anisotropic metamaterials., Lasers and Electro-Optics (CLEO) Conference, 2011. [7] B. Vial, G. Demésy, F. Zolla, A. Nicolet, M. Commandré, C. Hecquet, T. Begou, S. Tisserand, S. Gautier, and V. Sauget, Resonant metamaterial absorbers for infrared spectral filtering: quasimodal analysis, design, fabrication, and characterization, J. Opt. Soc. Am. B 31 (2014) 1339-1346. [8] H. Wang and L. Wang, Perfect selective metamaterial solar absorbers, Opt. Express, 21 (2013) A1078-A1093. [9] D. Shin, Y. Urzhumov, Y. Jung, G. Kang, S. Baek, M. Choi, H. Park, K. Kim, and D. R. Smith, Broadband electromagnetic cloaking with smart metamaterials, Nat. Commun. 3 (2012) 1213. [10] N. Fang, and X. Zhang, Imaging properties of a metamaterial superlens, Appl. Phys. Lett. 82 (2003) 161-163. [11] Y. Cui, Y. He, Y. Jin, F. Ding, L. Yang, Y. Ye, S. Zhong, Y. Lin, and S. He, Plasmonic and metamaterial structures as electromagnetic absorbers, Laser Photon. Rev, 8 (2014) 495-520. [12] H. Zhai, C. Zhan, Z. Li, and C. Liang, A triple-band ultrathin metamaterial absorber with wide-angle and polarization stability, IEEE Antennas Wireless Propag. Lett. 14 (2015) 241-244. [13] Q. Ye, Y. Liu, H. Lin, M. Li, and H. Yang, “Multi-band metamaterial absorber made of multi-gap SRRs structure,” Appl. Phys. A 107 (2012) 155-160. [14] H. Li, L. H. Yuan, B. Zhou, X. P. Shen, Q. Cheng, and T. J. Cui, Ultrathin multiband gigahertz metamaterial absorbers, J. Appl. Phys. 110 (2011) 014909. [15] J. Hao, J. Wang, X. Liu, W. J. Padilla, L. Zhou, and M. Qiu, High performance optical absorber based on a plasmonic metamaterial, Appl. Phys. Lett. 96 (2010) 251104. [16] Y. Bai, L. Zhao, D. Ju, Y. Jiang, and L. Liu, Wide-angle, polarization-independent and dual-band infrared perfect absorber based on L-shaped metamaterial, Opt. Express 23 (2015) 8670-8680. [17] B. X. Wang, L. L. Wang, G. Z. Wang, W. Q. Huang, X. F. Li, and X. Zhai, Metamaterial-based low-conductivity alloy perfect absorber, J. Lightwave Technol. 32 (2014) 2293-2298. [18] B. X. Wang, L. L. Wang, G. Z. Wang, W. Q. Huang, and X. Zhai, Broadband, polarization-insensitive and wideangle terahertz metamaterial absorber, Phys. Scripta 89 (2014) 115501-115505. [19] Y. Wen, W. Ma, J. Bailey, G. Matmon, X. Yu, and G. Aeppli, Planar broadband and high absorption metamaterial using single nested resonator at terahertz frequencies, Opt. Lett. 39 (2014) 1589-1592. [20] K. B. Alici, A. B. Turhan, C. M. Soukoulis, and E. Ozbay, Optically thin composite resonant absorber at the nearinfrared band: a polarization independent and spectrally broadband configuration, Opt. Express. 19 (2011) 1426014267. [21] Y. Cui, K. H. Fung, J. Xu, H. Ma, Y. Jin, S. He, and N. X. Fang, Ultrabroadband light absorption by a sawtooth anisotropic metamaterial slab, Nano Lett. 12 (2012) 1443-1447. [22] C. Caloz, and T. Itoh, Electromagnetic metamaterials: transmission line theory and microwave applications ,John Wiley & Sons, Inc., 2006. [23] A. Yahalom, Y. Pinhasi, E. Shifman, and S. Petnev, Transmission through single and multiple layers in the 3-10 GHz band and the implications for communications of frequency varying material dielectric constants, WSEAS Transactions on Communications 9 (2010) 759-772. [24] E. D. Palik, Handbook of optical constants of solids, Academic press, 1998. [25] G. Hass, and C. D. Salzberg, Optical properties of silicon monoxide in the wavelength region from 0.24 to 14.0
microns, J. Opt. Soc. Am. A 44 (1954) 181-183. 6
(a)
(b)
Fig. 1. Reflected and transmitted waves from (a) single slab and (b) two slabs system.
Fig. 2. Geometry of proposed metamaterial absorber.
7
10
1 0.9
0
0.8
Phase (rad)
Amplitude
0.7 0.6 0.5 0.4
R1
0.3
R'1
0.2
-20
R1 R'1
-30
T1
0.1
-10
T1
T'1
0
50
100
150 200 250 Frequency (THz)
300
350
T'1 -40
400
50
100
150 200 250 Frequency (THz)
(a)
300
350
400
(b) 3.1
1 R2
R2
3.05
0.95
0.9
Phase (rad)
Amplitude
3
0.85
2.95 2.9 2.85 2.8
0.8
2.75 0.75
50
100
150 200 250 Frequency (THz)
300
350
2.7
400
50
100
150 200 250 Frequency (THz)
(c)
300
350
400
(d)
Fig. 3. Scattering parameters of the decoupled system. (a) Amplitude and (b) phase of the transmission and reflection powers at metamaterial-substrate and air interface, and (c) amplitude and (d) phase of the reflection powers at the ground plane and air interface. 1 0.9 0.8
Absorption
0.7 0.6 0.5 0.4 0.3 0.2 Simulation Theory
0.1 0
50
100
150 200 250 Frequency (THz)
300
350
400
Fig. 4. Absorption spectrum of the proposed absorber of Fig. 2. The theoretical result is in good agreement with the simulation one.
8
Fig. 5. The electric field (Ex) and current distributions of the structure of Fig. 2 at the top of gold layer.
1 0.9 0.8
Absorption
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
50
100
150 200 250 Frequency (THz)
300
350
400
(a)
(b)
Fig. 6. (a) Absorption spectrum of the ground plane that consists of titanium layer and gold plate, and (b) total dissipated power density.
1 0.9 0.8
Absorption
0.7 0.6 0.5 0.4 =0 =30 =60 =90
0.3 0.2 0.1 0
50
100
150 200 250 Frequency (THz)
300
350
400
Fig. 7. Absorption spectra of the metamaterial absorber of Fig. 2 for different polarization angles.
9
1
0.9
0.9
0.8
0.8
0.7
0.7
0.6 0.5 0.4
=0 =20 =40 =60 =80
0.3 0.2 0.1 0
50
100
150 200 250 Frequency (THz)
300
350
Absorption
Absorption
1
0.6 0.5 =0 =20 =40 =60 =80
0.4 0.3 0.2 0.1 0
400
50
100
150 200 250 Frequency (THz)
(a)
300
350
400
(b)
Fig. 8. Absorption spectra of the metamaterial absorber of Fig. 2 for (a) TE and (b) TM polarizations and different incident angles.
Fig. 9. Polarization insensitive absorber that consists of two rotated UT-shaped resonators.
1 0.9 0.8
Absorption
0.7 0.6 0.5 0.4 = 0 = 30 = 60 = 90
0.3 0.2 0.1 0
50
100
150 200 250 Frequency (THz)
300
350
400
Fig. 10. Absorption spectra of the absorber of Fig. 9 for different polarization angles.
10
1
1
0.9
0.9
0.8
0.8
0.7
0.7
Absorption
Absorption
Fig. 11. Electric field distribution E x
0.6 0.5 0.4
=0 =20 =40 =60 =80
0.3 0.2 0.1 0
50
100
150 200 250 Frequency (THz)
300
350
at the frequency of 250 THz.
0.6 0.5 0.4
=0 =20 =40 =60 =80
0.3 0.2 0.1 0
400
50
100
150 200 250 Frequency (THz)
300
350
400
(a) (b) Fig. 12. Absorption spectra of the absorber of Fig. 9 for (a) TE and (b) TM polarizations and different incident angles.
11
S1569-4410(16)30073-6 http://dx.doi.org/doi:10.1016/j.photonics.2017.03.002 PNFA 577
To appear in:
Photonics and Nanostructures – Fundamentals and Applications
Received date: Revised date: Accepted date:
11-10-2016 26-2-2017 2-3-2017
Please cite this article as: Nasrollah Karampour, Najmeh Nozhat, Ultrawideband polarization insensitive UT-shaped metamaterial absorber, Photonics and Nanostructures Fundamentals and Applications http://dx.doi.org/10.1016/j.photonics.2017.03.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultra-wideband polarization metamaterial absorber
insensitive
UT-shaped
Nasrollah Karampour and Najmeh Nozhat* Department of Electrical and Electronics Engineering, Shiraz University of Technology, Shiraz 7155713876, Iran *Corresponding author: [email protected]
Highlights:
An ultra-wideband metamaterial absorber with double layers of UT-shaped resonator and the ground plane which is made of titanium and gold is proposed.
The absorption mechanism is relies on the surface impedance tuning caused by UT-shaped resonator and resistive sheet effect of titanium layer in the structure.
The absorber performance is also analyzed by the interference theory and it is shown that the theoretical results are in good agreement with the simulation results.
A polarization insensitive absorber with two rotated UT-shaped resonators is proposed.
Abstract- In this paper, an ultra-wideband metamaterial absorber (MMA) with U and T shaped resonators has been proposed. The resonators and the ground plane consist of gold (Au) and titanium (Ti) layers. The resistive sheet effect of Ti layer and the resonance elements in the structure cause a broad absorption spectrum. The simulations are based on the finite element method (FEM) and the results show that the absorption of the proposed structure is more than 90% between 150 and 300 THz that is much larger than previous works. Moreover, by applying the interference theory, we have demonstrated that the simulation results are in good agreement with the theoretical results. The primary proposed MMA is polarization sensitive. Therefore, a polarization insensitive metamaterial absorber has been suggested. Also, because of the extra resonance elements the full width at 90% absorption increases about 35 THz. This ultra-wideband MMA has various applications in microbalometer, imaging, thermal emitters, photovoltaic, and energy harvesting.
Keywords: Metamaterial absorber, Nanostructure, Polarization insensitive, Resonator, Ultrawideband.
1. Introduction In the past decades electromagnetic metamaterials have attracted researchers’ attention due to their unusual properties such as negative refractive index (NRI), near zero index (NZI) and magnetic response in optical regime. First NRI metamaterial was proposed by Smith in 2000 [1] and after that a wide range of applications such as antenna, energy harvesting, super lens, waveguides and absorbers have been defined for metamaterials [2]. 1
First metamaterial absorber (MMA) was demonstrated by Landy et al. [3]. Since then, many studies have been done in this area and metamaterial absorbers are the basis of various devices such as switches [4, 5], modulators [5], multiplexers [6], filters [7], solar cells [8] and have been used in cloaking [9] and imaging applications [10]. The operation frequency of MMAs spread from microwave to optical frequencies and according to their absorption spectrum, they are classified as narrowband, wideband, single band and multiband absorbers [11]. The absorption mechanism of traditional absorbers are based on the impedance matching caused by the structure. In this case, the electromagnetic energy is completely transferred to the structure and dissipated at lossy medium. However, for metamaterial absorbers, the absorption is often based on a resonance type phenomenon such as electric-magnetic dipole [12-14], surface plasmon [15] and magnetic polariton [16], that the electromagnetic energy concentrates in the metamaterial structure and decays at lossy dielectric. The electric and magnetic responses can be introduced by a metamaterial resonator or by the coupling between a resonator and a ground plane. Consequently, depending on the resonator behavior some peaks appear at specific frequencies. In this type of MMA, the high quality factor (Q) causes a narrow absorption spectrum. Utilizing the multi-resonance elements and multilayer resonance structures with near resonance frequencies are the methods of broadening the absorption spectrum. In the MMA proposed by Wang et al. the full width at half maximum (FWHM) of about 33% is obtained by using a double layered square patches on top of a metallic ground plane [17]. In Ref. [18] by adding an extra layer the FWHM of the structure is enhanced to 42%. The broadband absorption in these structures is related to the longitudinal coupling between the layers, but the number of layers is limited by the effective-homogeneity condition. For MMA of Ref. [19] with two back-to-back split ring resonators, three resonance peaks and the FWHM of 47% are achieved. Combination of different types of absorbers is another strategy for increasing the bandwidth. Alici et al. have used this method and obtained 121 THz full width at 90% absorption [20]. In [21] a MMA with the FWHM of 86% has been proposed and a wide absorption spectrum is related to the excitation of slow light modes in multi-layered metal-dielectric sawtooth shaped structure. In this paper, a metamaterial absorber that consists of a UT-shaped resonator and an SiO layer as substrate has been proposed. The ground plane of this structure is made of titanium and gold layers. The titanium layer behaves as a lossy medium and so the MMA can have a wide bandwidth. Indeed, the low conductivity of titanium layer leads to increasing the structure losses [17]. Therefore, the quality factor is decreased and the absorption spectrum becomes wider. In the proposed ultra-broadband metamaterial absorber, a smooth and wide spectrum is attained by controlling the surface impedance of the structure. The total thickness of the structure is 260 nm that is much thinner than multilayer absorbers. In the first proposed structure, the absorption is more than 90%, between the frequencies of 150 and 300 THz, and in the second proposed MMA that is polarization insensitive, the full with at 90% absorption is improved to 185 THz. The FWHM of the proposed absorber is up to 100% and the full width at 90% absorption is about 70% respect to its central frequency. Also, the near ideal absorption value of 99.67% occurs at 270 THz. The absorption spectrum has been also calculated by the interference theory and it has been shown that the theoretical results are in good agreement with the simulation results. The wide bandwidth, thin and small unit cell size of the proposed MMA makes it suitable for microbolometers and imaging applications. 2
The paper is organized as follows. The theoretical method is expressed in Section 2, and the geometry of the proposed absorber is shown in Section 3. In Section 4, the simulation results are demonstrated and compared with the theoretical results. In Section 5, a polarization insensitive absorber is proposed and the paper is concluded in Section 6. 2. Theoretical Method The performance of MMAs can be studied by various methods such as constitutive parameters and refractive index interpretation, impedance matching and interference theory that all refer to the same concept. Here, our proposed structures are studied by the semitheoretical interference method to show that the magnetic response is created in top resonator elements. The interaction of a dielectric slab with an incident electromagnetic wave is shown in Fig. 1a. For normal incident, the transmission and reflection coefficients are given by [22]: t t e jk2 z d T t1t2e jk2 z d t1t2 r22e j 3k2 z d t1t2 r24e j 5k2 z d ... 1 22 j 2 k2 z d , (1) 1 r2 e
R r1 t1t2 r2e jk2 z d t1t2 r23e j 3k2 z d ... r1
t1t2 r2e j 2 k2 z d , 1 r22e j 2 k2 z d
(2)
where t 1 and r1 are the transmission and reflection coefficients at the first interface, respectively, t 2 and r2 are transmission and reflection coefficients at the second interface of the slab, respectively. k 2z is the slab wave number in the z direction and d is the slab thickness. This procedure has been investigated for two slabs system in Ref. [23]. Here, we have modified it for the case that the metamaterial layer is modeled as an anisotropic slab. The transmission and reflection coefficients of this system that is shown in Fig. 1b can be expressed as: T1T 2 (3) T t T1T 2 T1T 2 R1R 2 T1T 2 R12 R 22 ... , 1 R1R 2 T T R (4) Rt R1 T1T1R 2 T1T1R 1R 22 ... R1 1 1 2 , 1 R1R 2 where T 1 and R 1 are the total transmission and reflection for the incident wave in left side of slab 1, respectively. T 1 and R1 are the total transmission and reflection of slab 1 for the right side illumination, respectively. T 2 and R 2 are the total transmission and reflection from slab 2. According to the interference theory, when high order reflections are out of phase with the first reflected wave from the slab, the total reflected wave will be canceled. Consequently, by applying the mentioned concept to proposed MMA, the origination of the absorption can be explained. 3. Design and Simulation Method Geometry of the proposed metamaterial absorber is shown in Fig. 2. It consists of a UTshaped metamaterial structure, a composite ground plane and a dielectric layer between them. The ground plane is made of gold (Au) and titanium (Ti) layers, and a layer of Ti is between the UT-shaped structure and SiO dielectric layer. The thicknesses of SiO dielectric layer, top and bottom gold and titanium layers are Ts 128 nm , T Au 24 nm , TTi 20 nm , TTi g 38 nm , and T Au g 50 nm , respectively. 3
Also, the geometrical parameters of the UT-shaped resonator are selected to be Lu 168 nm ,
H u 100 nm , W u 34 nm , Lt 216 nm , H t 100 nm , W t 24 nm , and S 30 nm . The proposed ultra-wideband metamaterial absorber is simulated based on the finite element numerical method with the periodic boundary condition (PBC) in the x and y directions. The structure is illuminated with an electromagnetic wave in the z direction and the absorption spectrum is calculated from A 1 Rt T t 2
2
formula, where T and R are the
transmission and reflection coefficients, respectively. Because of the gold ground layer, the transmission is very low and can be neglected. The Refractive indices of gold and titanium are extracted from Palik’s data [24] and the refractive index of SiO is taken from the data obtained by Hass and Salzberg [25]. 4. Theoretical and Simulation Results and Discussions In order to study the interference theory and show the origination of magnetic response in our proposed absorber, the system must be decoupled into two slabs. The first slab consists of the metamaterial structure and the substrate and the second one is the ground plane. The transmission and reflection of these two slabs which are shown in Fig. 1b are equal to the Sparameters of each slab that are measured in the simulations. The transmission and reflection spectra of the decoupled system are depicted in Fig. 3. By putting these values in Eqs. (3) and (4), the total absorption is attained and demonstrated in Fig. 4. Also, the simulated absorption spectrum of the proposed metamaterial absorber is shown in Fig. 4. The structure is excited by a normal incident lightwave that the electric field is in the x direction. It can be seen that the absorption is more than 90% in the frequency range of 150 to 300 THz. Also, there are two absorption peaks at the frequencies of 170 and 260 THz with the values of 99.34% and 99.71%, respectively. It is obvious that the theoretical result has a good agreement with the simulation result. In the decoupled system, the circular current between the top metal structure and the ground plane cannot be formed. Hence, both the electric and magnetic responses in the structure are created by the UT-shaped element in the first slab.
We have used the UT-shaped resonators due to their abilities to create the magnetic and electric responses, simultaneously, in the top metal structure. The magnetic response, provided by the circular current, is induced in the U shaped element, and the electric response is created between the U and T shaped elements and adjacent unit cells elements, as shown in Fig 5. Moreover, the UT-shaped element provides various design parameters as degrees of freedom. In our proposed structure, both resistive sheet and resonance type elements have been used to attain an ultra-wideband absorber. Titanium layer and the gold plate behind it form the ground plane that behaves as an absorber with the resistive sheet effect. Also, the UT-shaped element contributes to the impedance matching in a wide frequency range. The absorption spectrum of the ground plane and the dissipated power density are depicted in Fig. 6. The reflected power from the ground plane into the structure experiences some internal reflections and finally goes out to the air. According to the interference theory, this wave has a destructive interference with the reflected wave from the first interface and so a wideband absorption spectrum can be achieved. 4
The absorption spectra for different polarization angles are depicted in Fig. 7. As can be seen from Fig. 7, the absorption of proposed MMA is dependent on the polarization. However, the absorption peak is more than 90% for all polarization angles. In order to overcome this dependency, a new structure is proposed in Section 5. Furthermore, the absorption spectra for both TE and TM polarizations and various incident angels are shown in Fig. 8. In TE configuration, the electric field is in the x direction and the wave propagates in the y-z plane, while for TM polarization, the magnetic field is in the y direction and wave propagates in the x-z plane. In TE polarization, the absorption is more than 70% at the incident angle of 60°, while for TM polarization, the absorption is more than 80% at the same incident angle. Overall, the absorption of the structure in TM excitation is better than TE excitation for oblique incident wave. 5. Polarization Insensitive Scheme As mentioned before, the optical response of the MMA is polarization sensitive. The proposed polarization insensitive scheme is depicted in Fig. 9. As shown in Fig. 10 the absorption spectrum of this configuration is completely independent of the lightwave polarization angle. The absorption of this MMA in the frequency range of 160 to 345 THz is more than 90% and the maximum absorption value of 99.67% occurs at 270 THz. Moreover, the full width at 90% absorption is enhanced about 35 THz in comparison to the structure of Fig. 1. The x component of the electric field at the frequency of 250 THz is shown in Fig. 11. In 250 THz frequency, the concentrated electric field between the U and T shaped resonators is the reason of broadening of the absorption spectrum. Also, the absorption spectra of the absorber of Fig. 9 for TE and TM polarizations at different incident angles are demonstrated in Fig. 12. For TM polarization, the absorption is up to 50% for all incident angles.
6. Conclusion We have used various resonance elements in combination with Ti resistive sheet effect to propose an ultra-wideband metamaterial absorber. The absorption spectrum of this structure is more than 90% in the frequency range of 150 to 300 THz. Also, the absorption of the structure has been calculated by the interference theory and its good agreement with the simulation results shows that the circular current between the top layer resonators and the ground plane has poor contribution in the absorption of the structure. In the proposed polarization insensitive MMA, the full width at 90% absorption increases 35 THz because of extra resonance elements. Moreover, since the absorption of the polarization insensitive MMA in the frequency range of 160 to 345 THz is more than 90%, it is suitable for wideband applications. References [1] D. R. Smith, W. J. Padilla, D. Vier, S. C. Nemat-Nasser, and S. Schultz, Composite medium with simultaneously negative permeability and permittivity, Phys. Rev. Lett. 84 (2000) 4184-4187.
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[2] C. M. Watts, X. Liu, and W. J. Padilla, Metamaterial electromagnetic wave absorbers, Adv. Mater. 24 (2012) OP98– OP120. [3] N. Landy, S. Sajuyigbe, J. Mock, D. Smith, and W. Padilla, Perfect metamaterial absorber, Phys. Rev. Lett. 100 (2008) 207402. [4] B. Kang, J. Woo, E. Choi, H. H. Lee, E. Kim, J. Kim, T. J. Hwang, Y. S. Park, D. Kim, and J. Wu, Optical switching of near infrared light transmission in metamaterial-liquid crystal cell structure, Opt. Express 18 (2010) 16492-16498. [5] M. Hajizadegan, V. Ahmadi, and M. Sakhdari, Design and analysis of ultrafast and tunable all optical metamaterial switch enhanced by metal nanocomposite, J. Lightwave Technol. 31 (2013) 1877-1883. [6] H. Liu, and K. J. Webb, Optical devices based on cylindrically anisotropic metamaterials., Lasers and Electro-Optics (CLEO) Conference, 2011. [7] B. Vial, G. Demésy, F. Zolla, A. Nicolet, M. Commandré, C. Hecquet, T. Begou, S. Tisserand, S. Gautier, and V. Sauget, Resonant metamaterial absorbers for infrared spectral filtering: quasimodal analysis, design, fabrication, and characterization, J. Opt. Soc. Am. B 31 (2014) 1339-1346. [8] H. Wang and L. Wang, Perfect selective metamaterial solar absorbers, Opt. Express, 21 (2013) A1078-A1093. [9] D. Shin, Y. Urzhumov, Y. Jung, G. Kang, S. Baek, M. Choi, H. Park, K. Kim, and D. R. Smith, Broadband electromagnetic cloaking with smart metamaterials, Nat. Commun. 3 (2012) 1213. [10] N. Fang, and X. Zhang, Imaging properties of a metamaterial superlens, Appl. Phys. Lett. 82 (2003) 161-163. [11] Y. Cui, Y. He, Y. Jin, F. Ding, L. Yang, Y. Ye, S. Zhong, Y. Lin, and S. He, Plasmonic and metamaterial structures as electromagnetic absorbers, Laser Photon. Rev, 8 (2014) 495-520. [12] H. Zhai, C. Zhan, Z. Li, and C. Liang, A triple-band ultrathin metamaterial absorber with wide-angle and polarization stability, IEEE Antennas Wireless Propag. Lett. 14 (2015) 241-244. [13] Q. Ye, Y. Liu, H. Lin, M. Li, and H. Yang, “Multi-band metamaterial absorber made of multi-gap SRRs structure,” Appl. Phys. A 107 (2012) 155-160. [14] H. Li, L. H. Yuan, B. Zhou, X. P. Shen, Q. Cheng, and T. J. Cui, Ultrathin multiband gigahertz metamaterial absorbers, J. Appl. Phys. 110 (2011) 014909. [15] J. Hao, J. Wang, X. Liu, W. J. Padilla, L. Zhou, and M. Qiu, High performance optical absorber based on a plasmonic metamaterial, Appl. Phys. Lett. 96 (2010) 251104. [16] Y. Bai, L. Zhao, D. Ju, Y. Jiang, and L. Liu, Wide-angle, polarization-independent and dual-band infrared perfect absorber based on L-shaped metamaterial, Opt. Express 23 (2015) 8670-8680. [17] B. X. Wang, L. L. Wang, G. Z. Wang, W. Q. Huang, X. F. Li, and X. Zhai, Metamaterial-based low-conductivity alloy perfect absorber, J. Lightwave Technol. 32 (2014) 2293-2298. [18] B. X. Wang, L. L. Wang, G. Z. Wang, W. Q. Huang, and X. Zhai, Broadband, polarization-insensitive and wideangle terahertz metamaterial absorber, Phys. Scripta 89 (2014) 115501-115505. [19] Y. Wen, W. Ma, J. Bailey, G. Matmon, X. Yu, and G. Aeppli, Planar broadband and high absorption metamaterial using single nested resonator at terahertz frequencies, Opt. Lett. 39 (2014) 1589-1592. [20] K. B. Alici, A. B. Turhan, C. M. Soukoulis, and E. Ozbay, Optically thin composite resonant absorber at the nearinfrared band: a polarization independent and spectrally broadband configuration, Opt. Express. 19 (2011) 1426014267. [21] Y. Cui, K. H. Fung, J. Xu, H. Ma, Y. Jin, S. He, and N. X. Fang, Ultrabroadband light absorption by a sawtooth anisotropic metamaterial slab, Nano Lett. 12 (2012) 1443-1447. [22] C. Caloz, and T. Itoh, Electromagnetic metamaterials: transmission line theory and microwave applications ,John Wiley & Sons, Inc., 2006. [23] A. Yahalom, Y. Pinhasi, E. Shifman, and S. Petnev, Transmission through single and multiple layers in the 3-10 GHz band and the implications for communications of frequency varying material dielectric constants, WSEAS Transactions on Communications 9 (2010) 759-772. [24] E. D. Palik, Handbook of optical constants of solids, Academic press, 1998. [25] G. Hass, and C. D. Salzberg, Optical properties of silicon monoxide in the wavelength region from 0.24 to 14.0
microns, J. Opt. Soc. Am. A 44 (1954) 181-183. 6
(a)
(b)
Fig. 1. Reflected and transmitted waves from (a) single slab and (b) two slabs system.
Fig. 2. Geometry of proposed metamaterial absorber.
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10
1 0.9
0
0.8
Phase (rad)
Amplitude
0.7 0.6 0.5 0.4
R1
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1 R2
R2
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Amplitude
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2.7
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150 200 250 Frequency (THz)
(c)
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(d)
Fig. 3. Scattering parameters of the decoupled system. (a) Amplitude and (b) phase of the transmission and reflection powers at metamaterial-substrate and air interface, and (c) amplitude and (d) phase of the reflection powers at the ground plane and air interface. 1 0.9 0.8
Absorption
0.7 0.6 0.5 0.4 0.3 0.2 Simulation Theory
0.1 0
50
100
150 200 250 Frequency (THz)
300
350
400
Fig. 4. Absorption spectrum of the proposed absorber of Fig. 2. The theoretical result is in good agreement with the simulation one.
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Fig. 5. The electric field (Ex) and current distributions of the structure of Fig. 2 at the top of gold layer.
1 0.9 0.8
Absorption
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
50
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150 200 250 Frequency (THz)
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400
(a)
(b)
Fig. 6. (a) Absorption spectrum of the ground plane that consists of titanium layer and gold plate, and (b) total dissipated power density.
1 0.9 0.8
Absorption
0.7 0.6 0.5 0.4 =0 =30 =60 =90
0.3 0.2 0.1 0
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Fig. 7. Absorption spectra of the metamaterial absorber of Fig. 2 for different polarization angles.
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1
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=0 =20 =40 =60 =80
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Absorption
1
0.6 0.5 =0 =20 =40 =60 =80
0.4 0.3 0.2 0.1 0
400
50
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150 200 250 Frequency (THz)
(a)
300
350
400
(b)
Fig. 8. Absorption spectra of the metamaterial absorber of Fig. 2 for (a) TE and (b) TM polarizations and different incident angles.
Fig. 9. Polarization insensitive absorber that consists of two rotated UT-shaped resonators.
1 0.9 0.8
Absorption
0.7 0.6 0.5 0.4 = 0 = 30 = 60 = 90
0.3 0.2 0.1 0
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Fig. 10. Absorption spectra of the absorber of Fig. 9 for different polarization angles.
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Absorption
Fig. 11. Electric field distribution E x
0.6 0.5 0.4
=0 =20 =40 =60 =80
0.3 0.2 0.1 0
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150 200 250 Frequency (THz)
300
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at the frequency of 250 THz.
0.6 0.5 0.4
=0 =20 =40 =60 =80
0.3 0.2 0.1 0
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(a) (b) Fig. 12. Absorption spectra of the absorber of Fig. 9 for (a) TE and (b) TM polarizations and different incident angles.
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