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Design and measurement of a narrow band metamaterial absorber in terahertz range
Optical Materials 100 (2020) 109712
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Design and measurement of a narrow band metamaterial absorber in terahertz range Min Zhong Hezhou University, Hezhou, 542899, China
A R T I C L E I N F O
A B S T R A C T
OCIS codes: (160.3918) Metamaterials (300.1030) Absorption (260.5740) Resonance (230.0230) Optical devices
In this paper, we proposed and measured a metamaterial absorber with cross metal array in 12–28 THz range. A narrow absorption band (the absorption amplitude is 96%) is obtained at resonance frequency 19.24 THz, which is excited by the bright-bright modes coupling effect. Impedance matching is achieved at this resonance fre quency between samples and free space. This absorption peak is enhanced and its resonance frequency is shifted with lattice constant reducing or structural parameter w1 increasing. The absorption peak is highly sensitive to the variation of structural parameters due to the bright-bright modes coupling effect on the cross strips. An equivalent LC circuit mode is proposed to understand the physical mechanism of the resonance frequency shifting. The effect of three gases on this absorption peak is measured. Moreover, high absorption performance is revealed when the incident angle reaches 480.
1. Introduction In the last decades, metamaterial absorbers have become a scientific hotspot [1–3]. A variety of metamaterial absorbers with excellent per formances have been confirmed in a broad frequency band [4–7]. Niu et al. proposed a 2D photonic crystals absorber for solar thermophoto voltaics, which shows high performance and wide-angle absorption (average absorptivity is 84.5%) under 45� oblique incidence [8]. Wang et al. proposed and simulated a single-band metamaterial absorber in THz band, which demonstrated polarization-insensitive and wide-angle properties [9]. Ding et al. show a dual-band metamaterial absorber based on the interaction effect between LSP and SPP modes [10]. Shen et al. designed and measured a triple-band metamaterial absorber in 2–12 GHz, which revealed the polarization-independent and wide-angle properties [11]. Moreover, many schemes for modulating the oper ating frequency of the metamaterial absorber have been proposed, such as illumination, fluid filling, electronic charge, heat radiation, and so on [12–14]. Zhang et al. designed and measured a graphe ne–electrolyte–grapheme structure perfect absorber [15]. The absorp tive properties can be controlled by changing Fermi level. Moreover, the absorption band is also can be modulated by stacking GSS layer. Another important modulation method is to modulate the absorption properties by changing the amount of stacked materials [16–19]. For example, Wang, et al. proposed a one-dimensional photonic crystals, the band width and center frequency can be controlled through changing the
amount of stacking of Ge and ZnS layers [20]. Metamaterial absorbers can be used in a variety of fields [21–24]. To date, different plasmonic metamaterial are applied in sensors [25–27]. Two obvious features are both revealed by these metamaterial absorbers: The first feature is that the working frequency segment can be moved largely. The second feature is that the absorption peak is a narrow band, which is sensitive to the changes of signals. Moreover, the electromagnetic wave loss is inevitable in plasmonic metallic metamaterials. Therefore, it makes sense to apply metamaterial absorber to the sensor [28–30]. The feasi bility of metamaterial absorbers in the field of sensing has been proven by many works based on optimizing the electric-magnetic responses in the target band. In this paper, a metamaterial absorber with cross metal array is designed and measured. A narrow absorption band is obtained at reso nance frequency 19.24 THz. The effect of lattice constant and structural parameter w1 on the absorption peak is measured. Moreover, three kinds of gases are measured and obtained three absorption reactions. Finally, the effect of incident angle on the absorption peak is also revealed. 2. Unit cell, simulation, and experiments The designed unit cell can be found in Fig. 1. This unit cell contains three layers: the top layer is a cross metal array, an intermediate medium layer, and a bottom metal layer. Metal layers can be given as follows:
E-mail address: [email protected]. https://doi.org/10.1016/j.optmat.2020.109712 Received 22 December 2019; Received in revised form 12 January 2020; Accepted 20 January 2020 Available online 24 January 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved.
M. Zhong
εðωÞ ¼ 1
Optical Materials 100 (2020) 109712
ω2P ω iωγD 2
Table 1 Geometric parameters.
(1)
Here, γ D ¼ 9 � 1013 s 1 stands for the collision frequency, ωp ¼ 1:37 � 1016 s 1 stands for the plasma frequency [31]. The thicknesses of metal layers and SU-8 layer are t1 ¼ 0:05μm, t2 ¼ 1:5μm, and t3 ¼ 0:1μm, respectively, which results in a total thickness of 1:65μm.metamaterial. Lattice constant of the proposed unit cell is P ¼ 4μm. More detailed structural parameters can be found in Table 1. Simulations of the pro posed unit cell are performed by full-wave HFSS Ansoft. Two floquet ports can be applied at bottom and top of the structure, while four pe riodic boundary conditions can be applied for sidewalls [32]. The designed samples are manufactured as follows: First, a 2 mm thickness wafer is cleaned through ethanol and ultrapure water, then is dried by (using device: C-MAG HP10). This wafer is as the substrate of our samples. Second, a t3 ¼ 0:1μm thick metal layer can be deposited on the substrate (using device: ZZL-U400C, the vacuuming time set to 5.5 h, the warm-up time is set to be 40 min, at a rate 1.9 Å s 1, the working pressure is set to be 55e-10(atm)) as the bottom layer of samples. Third, the intermediate dielectric layer SU-8 is spun on (using device: MSC-400Bz-6N spinner, the spin speed set to 2500 rpm, spin time is 5 min) the bottom metal layer. Then this SU-8 layer is dried (using device: C-MAG HP10, dried for 55 s at 90 � C). Fourth, the top metal layer is also deposited on (using device: ZZL-U400C, the vacuuming time set to 5.5 h, the warm-up time is set to be 25 min, at a rate 1.9 Å s 1, the working pressure is set to be 55e-10(atm)) the SU-8 layer. Finally, the designed unit cell array can be defined by electron beam lithography (using de vice: CABL-9000C). The achieved samples will be cleaned through the ultrasonic (using device: VGT-QTD). The SEM of samples is revealed by JSM-7610F.
Parameter
P
W1
W2
t1
t2
t3
Value(μm)
4
3
1
0.05
1.5
0.1
Fig. 2. The measured and simulated absorption spectrum in 12–28 THz.
obviously that these bright modes are coupled with each other at reso nant frequency 19.24 THz. A strong field interference effect is achieved between bright modes in a unit cell, as shown in Fig. 3(b). This brightbright modes coupling and interference effect enhances the absorption of electromagnetic waves propagating through the samples and results in the absorption peak at resonant frequency 19.24 THz. When the simulated frequency is deviated from the resonance frequency 19.24 THz, the bright-bright modes coupling effect can’t be excited, as shown in Fig. 3(a, c). It should be indicated that four small peaks are found in Fig. 2. These absorption peaks are originated from the dielectric loss of electromagnetic waves inside the dielectric layer. Another way to understand the basic physical nature of the meta material absorber is using effective optical parameters: the effective permittivity and permeability. Both of effective optical parameters are extracted and shown in Fig. 4 [33,34]. It is found that the real part of the effective permeability reaches 6.08 at resonant frequency (19.24 THz) of the absorption peak. A strong magnetic resonance is achieved at this resonant frequency, as shown in Fig. 4(a). Moreover, the real part of the effective permittivity is revealed as 6.02, which results in a strong electrical resonance, as shown in Fig. 4(b). Therefore, the permittivity and permeability are achieved as μeff ¼ 6:08 þ 3:26i and εeff ¼ 6:02 þ
3. Measurement results and discussion The measured and simulated absorption spectrum is shown in Fig. 2. A highly absorption peak is achieved at resonance frequency 19.24 THz, the measured maximum value is 96%. The simulated result is revealed by HFSS. The simulated result is matched well with the measured result, as shown in Fig. 2. The difference between the measured and simualted results is mainly due to the experimental errors and idealization of simulation conditions. Since these errors are not obvious, the effec tiveness of the measurements in Fig. 2 is not affected. The physical mechanism of this absorption peak can be discussed in depth in two ways. One is the electric field intensity distribution at the resonance frequency. The simulated electric field intensity distribution results are shown in Fig. 3. When the electromagnetic wave reaches the metal surface, the induced current and the induced electric field are excited. At the resonant frequency 19.24 THz, two bright modes are excited on the surface of cross strips due to the interacting between the collective excitation of induced current and the incident light field. It is
Fig. 1. (a) The top view of unit cell. (b) The side view of unit cell. (c) The SEM of sample. The yellow parts are metal layers. The gray parts are SU-8 layers. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 2
M. Zhong
Optical Materials 100 (2020) 109712
Fig. 3. Electric field intensity distribution simulated results. (a) At 18.75 THz. (b) At 19.24 THz. (c) At 19.83 THz.
Fig. 5. The measured absorption spectrum for CO2, CO, and NO. Fig. 4. (a) The real parts and imaginary parts of the extracted effective permeability. (b) The real parts and imaginary parts of the extracted effective permittivity.
of the NO gas, absorption behaviors with higher resonance intensity are obtained. The absorption peak is reduced from 96% to 76.4%, and the resonance frequency is shifted from 19.24 THz to 15.38 THz. The gas sensitivity of this absorber is confirmed by the experimental results in Fig. 5. In order to fully understand the influence of structural parameters on absorption resonance properties, two sets of experiments are revealed. In one set of experiments, the lattice constant is set to be4μm3:9μm3:8μm, and 3:7μm, while other structural parameters and measuring conditions are unchanged. The measured absorption spec trum is found in Fig. 6. The resonance frequency of the absorption peak is reduced from 19.24 THz to 17.88 THz with the lattice constant reducing, as shown in Fig. 6. There is a slight increase in the peak. A 1.36 THz deviation in resonance frequency corresponds to lattice con stant of 0:3μm decreases. The weak correlation between the resonance frequency of the absorber and the lattice constant is confirmed by the experimental results in Fig. 6. In another set of experiments, the struc tural parameter w1 is adopted3μm, 3:1μm, 3:2μm, and 3:3μm, while other structural parameters and measuring conditions remain un changed. The measured absorption spectrum is found in Fig. 7. Simi larly, the absorption peak is also shifted to lower resonance frequencies, from 19.24 THz to 15.36 THz. The absorption peak is also enhanced, as shown in Fig. 7. However, a significantly different feature needs to be pointed out: a 3.88 THz deviation in resonance frequency corresponds to w1 of 0:3μm increases. The strong correlation between the resonance properties and the structural parameter w1 is confirmed by the experi mental results in Fig. 7. To further understand the relationship between structural parame ters and absorption peaks, the experimental results of Figs. 6 and 7 are
8:72i, respectively. Based on these extracted effective parameters, the effective impedance can be given as follows [35,36]: � X2 Fi c20 f 2e � � � Þ μeff ðf Þ ¼ μ0 ð1 (2) i¼1 c2 f 2 c20 f 2mi jc0 γmi fmi 0
εeff ðf Þ ¼ ε0 ðε∞
� c20 f 2e � Þ c0 f 2 jc0 γ e f � 2
Then, the effective impedance is revealed as follows: qffiffiffiffiffiffiffiffiffiffiffiffiffi�ffiffiffiffiffiffiffiffiffiffiffiffiffiffi Zeff ðf Þ ¼ μeff ðf Þ εeff ðf Þ
(3)
(4)
Therefore, the effective impedance is revealed as z ¼ 1:0 þ 0:03i � zo . The impedance matching condition is achieved at the resonant fre quency 19.24 THz, which leads to the reflection negligible and the ab sorption rate maximize. In order to verify the feasibility of the absorber in gas sensing ap plications, the samples and measured gases are both placed in a closed vessel for absorption spectrum measurements. These measured gases are CO2, CO, and NO, respectively [37–39]. The measured absorption spectrum for these gases is revealed in Fig. 5. It is found that the ab sorption peak is enhanced slightly and deviated from the resonance frequency. When the CO2 is replaced by the CO gas, the absorption peak is reduced from 96% to 91.3%, and the resonance frequency is shifted from 19.24 THz to 17.46 THz, as shown in Fig. 5. Similarly, for the case 3
M. Zhong
Optical Materials 100 (2020) 109712
Fig. 6. Measured absorption spectrum with different lattice constants.
Fig. 9. Statistics of different structural parameter W1. (a) Absorption peak. (b) Resonance frequency.
Figs. 8 and 9, as follows: yFrequency ¼ 2:27xLattice constant þ 10:17
(5)
yPeak ¼
(6)
0:05xLattice constant þ 1:16
yFrequency ¼ 6:47xw1 yPeak ¼
6:63
0:063xw1 þ 1:212
(7) (8)
Based on these Eqs (5)–(8), it is found that the absorption peak reveals approximate linear modulation for lattice constant or structural parameter w1. Obviously, the slope of Equation (7) is significantly higher than the slope of Equation (5), which indicates that the absorp tion peak is more sensitive to changes in structural parameter w1. This is mainly due to the fact that the absorption peak is derived from the coupling between two bright modes of the cross-shaped metal strips, as shown in Fig. 3. The absorption peak is shifted to lower resonance frequencies with lattice constant reducing or structural parameter w1 increasing in Figs. 6 and 7. This is because the coupling effect between adjacent unit cells of samples can be modulated through changing structural parameters. In order to better reveal the effect of the electrical coupling between such unit cells at the resonant frequency, an LC equivalent resonant circuit is proposed, as shown in Fig. 10. From the view of the macroscopic elec tromagnetic, electrical coupling between the proposed unit cells can be achieved as Fig. 10(a). When the electromagnetic waves are incident on the surface of samples, induced surface current and induced electric field are excited on the surface of the cross metal strips simultaneously. The electrical coupling effect is achieved based on these induced electric fields, as shown in Fig. 10(a). At the same time, the equivalent capaci tance Cc and resistance Rcof the cross metal strips are also obtained synchronously based on electric-coupling effect [40]. And the Co is capacitance between adjacent cross metal strips. The Ro is the equiva lent resistance which is derived from intermediate areas of adjacent cross metal strips. The Lo is the equivalent inductance. The total capacitance C of the proposed cross metal strips can be obtained as follows:
Fig. 7. Measured absorption spectrum with different w1.
statistically. The relationships between the absorptive properties and structural parameters are revealed in Fig. 8 and Fig. 9. The shift in the resonant frequency exhibits a regular change. In order to facilitate the description of changes in resonance frequency and peak value, four fitting equations are proposed based on these statistically results in
1 1 1 ¼ þ C Cc Co C¼
Cc ⋅Co ðCc þ Co Þ
(9) (10)
Therefore, the resonance frequency of absorption peak is achieved as follows:
Fig. 8. Statistics of different lattice constants. (a) Absorption peak. (b) Reso nance frequency. 4
M. Zhong
Optical Materials 100 (2020) 109712
Fig. 11. Measured absorption spectrum with different incident angles.
on this absorption peak is measured. This absorber can achieve an ab sorption rate of over 90% when the incident angle reaches 480.
Fig. 10. (a) Two adjacent cells in the sample. (b) The LC equivalent resonant circuit of the unit.
fResonance ¼
1 pffiffiffiffiffiffiffiffi 2π Lo C
Declaration of competing interest
(11)
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
An equivalent LC circuit mode is achieved based on Equations (9-11). The measured resonance frequency is 19.24 THz, and the simulated result is 19.16 THz, as shown in Fig. 2. Based on this equivalent LC circuit mode, the calculated result is 19.35 THz. The error between the calculated and experimental results is 0.15 THz. These resonance be haviors in Figs. 6 and 7 can be understood based on this LC circuit mode. As the lattice constant reduces (other structural parameters are un changed), the horizontal distance between two cross metal strips is reduced and the capacitance Co is enhanced. According to equation (10), the total capacitance C of the proposed cross metal strips is increased and the electric-coupling effect is enhanced synchronously. According to equation (11), the resonance frequency is reduced and the absorption peak is shifted to lower frequencies, as shown in Fig. 6. Similarly, as the structural parameter w1 increases, the electric-coupling effect is also enhanced and the total capacitance C of the proposed cross metal strips is increased. The absorption peak is also shifted to lower frequencies to equation (11), as shown in Fig. 7. Moreover, it should be indicated that the absorption peak is enhanced with lattice constant reducing or structural parameter w1 increasing. This is due to the equivalent resistance is enhanced with the lattice constant reducing or structural parameter w1 increasing [41,42]. Finally, the angle insensitivity is also experimentally validated. The measured absorption spectrum for different incident angles is shown in Fig. 11. When the incident angle is lower than 480, the absorption peak is reduced slightly and the resonance frequency is shifted for a small deviation. However, when the incident angle higher than 480, the maximum absorption rate is rapidly reduced, as shown in Fig. 11. This is due to the coupling between the incident electromagnetic wave and the induced electric field of metal strips is failure, which leads to the reso nance intensity of bright modes suppress and the absorption peak decrease. These measured results reveal that the absorber is insensitive to the incident angle between 00 to 480.
CRediT authorship contribution statement Min Zhong: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administra tion, Resources, Software, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing. Acknowledgments This research was financially supported by the Doctor’s Scientific Research Foundation (No. HZUBS201503), the Young and Middle Teachers’ Basic Ability Improvement Project of Guangxi (No. KY2016YB453), the Mathematical Support Autonomous Discipline Project of Hezhou University (No. 2016HZXYSX01), and the Innovation and Entrepreneurship Students Project of Hezhou University (Nos. 201611838018, 201911838062, 201911838071, 201911838179). References [1] Xiang Yin, Long Chang, Junhao Li, Hua Zhu, Lin Chen, Jianguo Guan, Xun Li, Ultra-wideband microwave absorber by connecting multiple absorption bands of two different sized hyperbolic metamaterial waveguide arrays, Sci. Rep. 5 (2015) 15367. [2] Tsung-Yu Huang, Ching-Wei Tseng, Ting-Tso Yeh, Tien-Tien Yeh, Chih-Wei Luo, Tahsin Akalin, Ta-Jen Yen, Experimental realization of ultrathin, double-sided metamaterial perfect absorber at terahertz gap through stochastic design process, Sci. Rep. 5 (2015) 18605. [3] Fei Ding, Dai Jin, Yiting Chen, Jianfei Zhu, Yi Jin, I. Sergey, Bozhevolnyi, Broadband near-infrared metamaterial absorbers utilizing highly lossy metals, Sci. Rep. 6 (2016) 39445. [4] Yunsong Xie, Xin Fan, Yunpeng Chen, Jeffrey D. Wilson, Rainee N. Simons, John Q. Xiao, A subwavelength resolution microwave/6.3 GHz camera based on a metamaterial absorber, Sci. Rep. 7 (2017) 40490–40497. [5] Y.Q. Xu, P.H. Zhou, H.B. Zhang, L. Chen, L.J. Deng, A wide-angle planar metamaterial absorber based on split ring resonator coupling, J. Appl. Phys. 110 (2011), 044102. [6] Thomas Roger, Stefano Vezzoli, Eliot Bolduc, Joao Valente, J. Julius, F. Heitz, John Jeffers, Cesare Soci, Jonathan Leach, Christophe Couteau, Nikolay I. Zheludev, Daniele Faccio, Coherent perfect absorption in deeply subwavelength films in the single-photon regime, Nat. Commun. 6 (2015) 7031–7035. [7] Bui Xuan Khuyen, Bui Son Tung, Young Joon Yoo, Young Ju Kim, Ki Won Kim, Liang-Yao Chen, Vu Dinh Lam, YoungPak Lee, Miniaturization for ultrathin metamaterial perfect absorber in the VHF band, Sci. Rep. 7 (2017) 45151–45157.
4. Conclusion We proposed and measured a narrow band metamaterial absorber based on the bright-bright modes coupling effect. The absorption peak is enhanced through reducing the lattice constant or increasing the structural parameter w1. Moreover, the resonance frequency can be controlled by changing structural parameters. The effect of three gases 5
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6
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Design and measurement of a narrow band metamaterial absorber in terahertz range Min Zhong Hezhou University, Hezhou, 542899, China
A R T I C L E I N F O
A B S T R A C T
OCIS codes: (160.3918) Metamaterials (300.1030) Absorption (260.5740) Resonance (230.0230) Optical devices
In this paper, we proposed and measured a metamaterial absorber with cross metal array in 12–28 THz range. A narrow absorption band (the absorption amplitude is 96%) is obtained at resonance frequency 19.24 THz, which is excited by the bright-bright modes coupling effect. Impedance matching is achieved at this resonance fre quency between samples and free space. This absorption peak is enhanced and its resonance frequency is shifted with lattice constant reducing or structural parameter w1 increasing. The absorption peak is highly sensitive to the variation of structural parameters due to the bright-bright modes coupling effect on the cross strips. An equivalent LC circuit mode is proposed to understand the physical mechanism of the resonance frequency shifting. The effect of three gases on this absorption peak is measured. Moreover, high absorption performance is revealed when the incident angle reaches 480.
1. Introduction In the last decades, metamaterial absorbers have become a scientific hotspot [1–3]. A variety of metamaterial absorbers with excellent per formances have been confirmed in a broad frequency band [4–7]. Niu et al. proposed a 2D photonic crystals absorber for solar thermophoto voltaics, which shows high performance and wide-angle absorption (average absorptivity is 84.5%) under 45� oblique incidence [8]. Wang et al. proposed and simulated a single-band metamaterial absorber in THz band, which demonstrated polarization-insensitive and wide-angle properties [9]. Ding et al. show a dual-band metamaterial absorber based on the interaction effect between LSP and SPP modes [10]. Shen et al. designed and measured a triple-band metamaterial absorber in 2–12 GHz, which revealed the polarization-independent and wide-angle properties [11]. Moreover, many schemes for modulating the oper ating frequency of the metamaterial absorber have been proposed, such as illumination, fluid filling, electronic charge, heat radiation, and so on [12–14]. Zhang et al. designed and measured a graphe ne–electrolyte–grapheme structure perfect absorber [15]. The absorp tive properties can be controlled by changing Fermi level. Moreover, the absorption band is also can be modulated by stacking GSS layer. Another important modulation method is to modulate the absorption properties by changing the amount of stacked materials [16–19]. For example, Wang, et al. proposed a one-dimensional photonic crystals, the band width and center frequency can be controlled through changing the
amount of stacking of Ge and ZnS layers [20]. Metamaterial absorbers can be used in a variety of fields [21–24]. To date, different plasmonic metamaterial are applied in sensors [25–27]. Two obvious features are both revealed by these metamaterial absorbers: The first feature is that the working frequency segment can be moved largely. The second feature is that the absorption peak is a narrow band, which is sensitive to the changes of signals. Moreover, the electromagnetic wave loss is inevitable in plasmonic metallic metamaterials. Therefore, it makes sense to apply metamaterial absorber to the sensor [28–30]. The feasi bility of metamaterial absorbers in the field of sensing has been proven by many works based on optimizing the electric-magnetic responses in the target band. In this paper, a metamaterial absorber with cross metal array is designed and measured. A narrow absorption band is obtained at reso nance frequency 19.24 THz. The effect of lattice constant and structural parameter w1 on the absorption peak is measured. Moreover, three kinds of gases are measured and obtained three absorption reactions. Finally, the effect of incident angle on the absorption peak is also revealed. 2. Unit cell, simulation, and experiments The designed unit cell can be found in Fig. 1. This unit cell contains three layers: the top layer is a cross metal array, an intermediate medium layer, and a bottom metal layer. Metal layers can be given as follows:
E-mail address: [email protected]. https://doi.org/10.1016/j.optmat.2020.109712 Received 22 December 2019; Received in revised form 12 January 2020; Accepted 20 January 2020 Available online 24 January 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved.
M. Zhong
εðωÞ ¼ 1
Optical Materials 100 (2020) 109712
ω2P ω iωγD 2
Table 1 Geometric parameters.
(1)
Here, γ D ¼ 9 � 1013 s 1 stands for the collision frequency, ωp ¼ 1:37 � 1016 s 1 stands for the plasma frequency [31]. The thicknesses of metal layers and SU-8 layer are t1 ¼ 0:05μm, t2 ¼ 1:5μm, and t3 ¼ 0:1μm, respectively, which results in a total thickness of 1:65μm.metamaterial. Lattice constant of the proposed unit cell is P ¼ 4μm. More detailed structural parameters can be found in Table 1. Simulations of the pro posed unit cell are performed by full-wave HFSS Ansoft. Two floquet ports can be applied at bottom and top of the structure, while four pe riodic boundary conditions can be applied for sidewalls [32]. The designed samples are manufactured as follows: First, a 2 mm thickness wafer is cleaned through ethanol and ultrapure water, then is dried by (using device: C-MAG HP10). This wafer is as the substrate of our samples. Second, a t3 ¼ 0:1μm thick metal layer can be deposited on the substrate (using device: ZZL-U400C, the vacuuming time set to 5.5 h, the warm-up time is set to be 40 min, at a rate 1.9 Å s 1, the working pressure is set to be 55e-10(atm)) as the bottom layer of samples. Third, the intermediate dielectric layer SU-8 is spun on (using device: MSC-400Bz-6N spinner, the spin speed set to 2500 rpm, spin time is 5 min) the bottom metal layer. Then this SU-8 layer is dried (using device: C-MAG HP10, dried for 55 s at 90 � C). Fourth, the top metal layer is also deposited on (using device: ZZL-U400C, the vacuuming time set to 5.5 h, the warm-up time is set to be 25 min, at a rate 1.9 Å s 1, the working pressure is set to be 55e-10(atm)) the SU-8 layer. Finally, the designed unit cell array can be defined by electron beam lithography (using de vice: CABL-9000C). The achieved samples will be cleaned through the ultrasonic (using device: VGT-QTD). The SEM of samples is revealed by JSM-7610F.
Parameter
P
W1
W2
t1
t2
t3
Value(μm)
4
3
1
0.05
1.5
0.1
Fig. 2. The measured and simulated absorption spectrum in 12–28 THz.
obviously that these bright modes are coupled with each other at reso nant frequency 19.24 THz. A strong field interference effect is achieved between bright modes in a unit cell, as shown in Fig. 3(b). This brightbright modes coupling and interference effect enhances the absorption of electromagnetic waves propagating through the samples and results in the absorption peak at resonant frequency 19.24 THz. When the simulated frequency is deviated from the resonance frequency 19.24 THz, the bright-bright modes coupling effect can’t be excited, as shown in Fig. 3(a, c). It should be indicated that four small peaks are found in Fig. 2. These absorption peaks are originated from the dielectric loss of electromagnetic waves inside the dielectric layer. Another way to understand the basic physical nature of the meta material absorber is using effective optical parameters: the effective permittivity and permeability. Both of effective optical parameters are extracted and shown in Fig. 4 [33,34]. It is found that the real part of the effective permeability reaches 6.08 at resonant frequency (19.24 THz) of the absorption peak. A strong magnetic resonance is achieved at this resonant frequency, as shown in Fig. 4(a). Moreover, the real part of the effective permittivity is revealed as 6.02, which results in a strong electrical resonance, as shown in Fig. 4(b). Therefore, the permittivity and permeability are achieved as μeff ¼ 6:08 þ 3:26i and εeff ¼ 6:02 þ
3. Measurement results and discussion The measured and simulated absorption spectrum is shown in Fig. 2. A highly absorption peak is achieved at resonance frequency 19.24 THz, the measured maximum value is 96%. The simulated result is revealed by HFSS. The simulated result is matched well with the measured result, as shown in Fig. 2. The difference between the measured and simualted results is mainly due to the experimental errors and idealization of simulation conditions. Since these errors are not obvious, the effec tiveness of the measurements in Fig. 2 is not affected. The physical mechanism of this absorption peak can be discussed in depth in two ways. One is the electric field intensity distribution at the resonance frequency. The simulated electric field intensity distribution results are shown in Fig. 3. When the electromagnetic wave reaches the metal surface, the induced current and the induced electric field are excited. At the resonant frequency 19.24 THz, two bright modes are excited on the surface of cross strips due to the interacting between the collective excitation of induced current and the incident light field. It is
Fig. 1. (a) The top view of unit cell. (b) The side view of unit cell. (c) The SEM of sample. The yellow parts are metal layers. The gray parts are SU-8 layers. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 2
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Optical Materials 100 (2020) 109712
Fig. 3. Electric field intensity distribution simulated results. (a) At 18.75 THz. (b) At 19.24 THz. (c) At 19.83 THz.
Fig. 5. The measured absorption spectrum for CO2, CO, and NO. Fig. 4. (a) The real parts and imaginary parts of the extracted effective permeability. (b) The real parts and imaginary parts of the extracted effective permittivity.
of the NO gas, absorption behaviors with higher resonance intensity are obtained. The absorption peak is reduced from 96% to 76.4%, and the resonance frequency is shifted from 19.24 THz to 15.38 THz. The gas sensitivity of this absorber is confirmed by the experimental results in Fig. 5. In order to fully understand the influence of structural parameters on absorption resonance properties, two sets of experiments are revealed. In one set of experiments, the lattice constant is set to be4μm3:9μm3:8μm, and 3:7μm, while other structural parameters and measuring conditions are unchanged. The measured absorption spec trum is found in Fig. 6. The resonance frequency of the absorption peak is reduced from 19.24 THz to 17.88 THz with the lattice constant reducing, as shown in Fig. 6. There is a slight increase in the peak. A 1.36 THz deviation in resonance frequency corresponds to lattice con stant of 0:3μm decreases. The weak correlation between the resonance frequency of the absorber and the lattice constant is confirmed by the experimental results in Fig. 6. In another set of experiments, the struc tural parameter w1 is adopted3μm, 3:1μm, 3:2μm, and 3:3μm, while other structural parameters and measuring conditions remain un changed. The measured absorption spectrum is found in Fig. 7. Simi larly, the absorption peak is also shifted to lower resonance frequencies, from 19.24 THz to 15.36 THz. The absorption peak is also enhanced, as shown in Fig. 7. However, a significantly different feature needs to be pointed out: a 3.88 THz deviation in resonance frequency corresponds to w1 of 0:3μm increases. The strong correlation between the resonance properties and the structural parameter w1 is confirmed by the experi mental results in Fig. 7. To further understand the relationship between structural parame ters and absorption peaks, the experimental results of Figs. 6 and 7 are
8:72i, respectively. Based on these extracted effective parameters, the effective impedance can be given as follows [35,36]: � X2 Fi c20 f 2e � � � Þ μeff ðf Þ ¼ μ0 ð1 (2) i¼1 c2 f 2 c20 f 2mi jc0 γmi fmi 0
εeff ðf Þ ¼ ε0 ðε∞
� c20 f 2e � Þ c0 f 2 jc0 γ e f � 2
Then, the effective impedance is revealed as follows: qffiffiffiffiffiffiffiffiffiffiffiffiffi�ffiffiffiffiffiffiffiffiffiffiffiffiffiffi Zeff ðf Þ ¼ μeff ðf Þ εeff ðf Þ
(3)
(4)
Therefore, the effective impedance is revealed as z ¼ 1:0 þ 0:03i � zo . The impedance matching condition is achieved at the resonant fre quency 19.24 THz, which leads to the reflection negligible and the ab sorption rate maximize. In order to verify the feasibility of the absorber in gas sensing ap plications, the samples and measured gases are both placed in a closed vessel for absorption spectrum measurements. These measured gases are CO2, CO, and NO, respectively [37–39]. The measured absorption spectrum for these gases is revealed in Fig. 5. It is found that the ab sorption peak is enhanced slightly and deviated from the resonance frequency. When the CO2 is replaced by the CO gas, the absorption peak is reduced from 96% to 91.3%, and the resonance frequency is shifted from 19.24 THz to 17.46 THz, as shown in Fig. 5. Similarly, for the case 3
M. Zhong
Optical Materials 100 (2020) 109712
Fig. 6. Measured absorption spectrum with different lattice constants.
Fig. 9. Statistics of different structural parameter W1. (a) Absorption peak. (b) Resonance frequency.
Figs. 8 and 9, as follows: yFrequency ¼ 2:27xLattice constant þ 10:17
(5)
yPeak ¼
(6)
0:05xLattice constant þ 1:16
yFrequency ¼ 6:47xw1 yPeak ¼
6:63
0:063xw1 þ 1:212
(7) (8)
Based on these Eqs (5)–(8), it is found that the absorption peak reveals approximate linear modulation for lattice constant or structural parameter w1. Obviously, the slope of Equation (7) is significantly higher than the slope of Equation (5), which indicates that the absorp tion peak is more sensitive to changes in structural parameter w1. This is mainly due to the fact that the absorption peak is derived from the coupling between two bright modes of the cross-shaped metal strips, as shown in Fig. 3. The absorption peak is shifted to lower resonance frequencies with lattice constant reducing or structural parameter w1 increasing in Figs. 6 and 7. This is because the coupling effect between adjacent unit cells of samples can be modulated through changing structural parameters. In order to better reveal the effect of the electrical coupling between such unit cells at the resonant frequency, an LC equivalent resonant circuit is proposed, as shown in Fig. 10. From the view of the macroscopic elec tromagnetic, electrical coupling between the proposed unit cells can be achieved as Fig. 10(a). When the electromagnetic waves are incident on the surface of samples, induced surface current and induced electric field are excited on the surface of the cross metal strips simultaneously. The electrical coupling effect is achieved based on these induced electric fields, as shown in Fig. 10(a). At the same time, the equivalent capaci tance Cc and resistance Rcof the cross metal strips are also obtained synchronously based on electric-coupling effect [40]. And the Co is capacitance between adjacent cross metal strips. The Ro is the equiva lent resistance which is derived from intermediate areas of adjacent cross metal strips. The Lo is the equivalent inductance. The total capacitance C of the proposed cross metal strips can be obtained as follows:
Fig. 7. Measured absorption spectrum with different w1.
statistically. The relationships between the absorptive properties and structural parameters are revealed in Fig. 8 and Fig. 9. The shift in the resonant frequency exhibits a regular change. In order to facilitate the description of changes in resonance frequency and peak value, four fitting equations are proposed based on these statistically results in
1 1 1 ¼ þ C Cc Co C¼
Cc ⋅Co ðCc þ Co Þ
(9) (10)
Therefore, the resonance frequency of absorption peak is achieved as follows:
Fig. 8. Statistics of different lattice constants. (a) Absorption peak. (b) Reso nance frequency. 4
M. Zhong
Optical Materials 100 (2020) 109712
Fig. 11. Measured absorption spectrum with different incident angles.
on this absorption peak is measured. This absorber can achieve an ab sorption rate of over 90% when the incident angle reaches 480.
Fig. 10. (a) Two adjacent cells in the sample. (b) The LC equivalent resonant circuit of the unit.
fResonance ¼
1 pffiffiffiffiffiffiffiffi 2π Lo C
Declaration of competing interest
(11)
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
An equivalent LC circuit mode is achieved based on Equations (9-11). The measured resonance frequency is 19.24 THz, and the simulated result is 19.16 THz, as shown in Fig. 2. Based on this equivalent LC circuit mode, the calculated result is 19.35 THz. The error between the calculated and experimental results is 0.15 THz. These resonance be haviors in Figs. 6 and 7 can be understood based on this LC circuit mode. As the lattice constant reduces (other structural parameters are un changed), the horizontal distance between two cross metal strips is reduced and the capacitance Co is enhanced. According to equation (10), the total capacitance C of the proposed cross metal strips is increased and the electric-coupling effect is enhanced synchronously. According to equation (11), the resonance frequency is reduced and the absorption peak is shifted to lower frequencies, as shown in Fig. 6. Similarly, as the structural parameter w1 increases, the electric-coupling effect is also enhanced and the total capacitance C of the proposed cross metal strips is increased. The absorption peak is also shifted to lower frequencies to equation (11), as shown in Fig. 7. Moreover, it should be indicated that the absorption peak is enhanced with lattice constant reducing or structural parameter w1 increasing. This is due to the equivalent resistance is enhanced with the lattice constant reducing or structural parameter w1 increasing [41,42]. Finally, the angle insensitivity is also experimentally validated. The measured absorption spectrum for different incident angles is shown in Fig. 11. When the incident angle is lower than 480, the absorption peak is reduced slightly and the resonance frequency is shifted for a small deviation. However, when the incident angle higher than 480, the maximum absorption rate is rapidly reduced, as shown in Fig. 11. This is due to the coupling between the incident electromagnetic wave and the induced electric field of metal strips is failure, which leads to the reso nance intensity of bright modes suppress and the absorption peak decrease. These measured results reveal that the absorber is insensitive to the incident angle between 00 to 480.
CRediT authorship contribution statement Min Zhong: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administra tion, Resources, Software, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing. Acknowledgments This research was financially supported by the Doctor’s Scientific Research Foundation (No. HZUBS201503), the Young and Middle Teachers’ Basic Ability Improvement Project of Guangxi (No. KY2016YB453), the Mathematical Support Autonomous Discipline Project of Hezhou University (No. 2016HZXYSX01), and the Innovation and Entrepreneurship Students Project of Hezhou University (Nos. 201611838018, 201911838062, 201911838071, 201911838179). References [1] Xiang Yin, Long Chang, Junhao Li, Hua Zhu, Lin Chen, Jianguo Guan, Xun Li, Ultra-wideband microwave absorber by connecting multiple absorption bands of two different sized hyperbolic metamaterial waveguide arrays, Sci. Rep. 5 (2015) 15367. [2] Tsung-Yu Huang, Ching-Wei Tseng, Ting-Tso Yeh, Tien-Tien Yeh, Chih-Wei Luo, Tahsin Akalin, Ta-Jen Yen, Experimental realization of ultrathin, double-sided metamaterial perfect absorber at terahertz gap through stochastic design process, Sci. Rep. 5 (2015) 18605. [3] Fei Ding, Dai Jin, Yiting Chen, Jianfei Zhu, Yi Jin, I. Sergey, Bozhevolnyi, Broadband near-infrared metamaterial absorbers utilizing highly lossy metals, Sci. Rep. 6 (2016) 39445. [4] Yunsong Xie, Xin Fan, Yunpeng Chen, Jeffrey D. Wilson, Rainee N. Simons, John Q. Xiao, A subwavelength resolution microwave/6.3 GHz camera based on a metamaterial absorber, Sci. Rep. 7 (2017) 40490–40497. [5] Y.Q. Xu, P.H. Zhou, H.B. Zhang, L. Chen, L.J. Deng, A wide-angle planar metamaterial absorber based on split ring resonator coupling, J. Appl. Phys. 110 (2011), 044102. [6] Thomas Roger, Stefano Vezzoli, Eliot Bolduc, Joao Valente, J. Julius, F. Heitz, John Jeffers, Cesare Soci, Jonathan Leach, Christophe Couteau, Nikolay I. Zheludev, Daniele Faccio, Coherent perfect absorption in deeply subwavelength films in the single-photon regime, Nat. Commun. 6 (2015) 7031–7035. [7] Bui Xuan Khuyen, Bui Son Tung, Young Joon Yoo, Young Ju Kim, Ki Won Kim, Liang-Yao Chen, Vu Dinh Lam, YoungPak Lee, Miniaturization for ultrathin metamaterial perfect absorber in the VHF band, Sci. Rep. 7 (2017) 45151–45157.
4. Conclusion We proposed and measured a narrow band metamaterial absorber based on the bright-bright modes coupling effect. The absorption peak is enhanced through reducing the lattice constant or increasing the structural parameter w1. Moreover, the resonance frequency can be controlled by changing structural parameters. The effect of three gases 5
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