Design of a broadband infrared metamaterial absorber

Accepted Manuscript Title: Design of a broadband infrared metamaterial absorber Authors: Zhixin Che, Changhui Tian, Xiaoli Chen, Binke Wang, Kexin Wang PII: DOI: Reference:

S0030-4026(18)30801-5 https://doi.org/10.1016/j.ijleo.2018.06.002 IJLEO 61005

To appear in: Received date: Revised date: Accepted date:

18-3-2018 30-5-2018 1-6-2018

Please cite this article as: Che Z, Tian C, Chen X, Wang B, Wang K, Design of a broadband infrared metamaterial absorber, Optik (2018), https://doi.org/10.1016/j.ijleo.2018.06.002 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.

Design of a broadband infrared metamaterial absorber

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ZhixinChe, Changhui Tian*, Xiaoli Chen, Binke Wang, Kexin Wang Department of Basic Science, Air Force Engineering University, Xi'an, Shaanxi 710051, PR China Abstract: A broadband infrared metamaterial absorber based on the combination of four different resonators is presented and numerically examined. There are two resonant layers with different dielectrics in the absorber. The simulated absorptivity is beyond 90% for 8 to 12 μm wavelength and the absorber is polarization independent. We studied the absorbent mechanism by the distribution of magnetic fields and energy dissipation in the absorber. The absorptivity stays quasi constant for the incident angle up to 25°. Further simulation shows that we can change the absorption band through the optimization of the geometrical parameters. Key words: Metamaterial absorber, Infrared, Broadband absorption, Resonance 1.Introduction Metamaterial is a kind of artificial material with special electromagnetic properties unavailable in nature[1-3]. Band-pass, band-stop, filter [4], phase modulation and absorption are often achieved through changes of geometric configuration. Since the first metamaterial absorber with the near unity absorption of 99% at the frequency of 11.48 GHz is theoretically and experimentally demonstrated by Landy et al[5], metamaterial absorber has attracted growing interests. While widely applied in microwave applications such as wireless communication, stealth material and electromagnetic compatibility [6-7], metamaterial absorber is less used in infrared and visible region due to the fabrication technology. However, with the development of micro/nano fabrication technologies, it is possible to fabricate infrared, visible band [8-11] and ultraviolet [12] metamaterial absorber. Infrared metamaterial absorber may find practical applications in selective thermal emitters, infrared detectors, infrared stealth and solar energy harvesting [13-15]. Liu et al. [16] designed and fabricated a mid-infrared metamaterial absorber that achieved an experimental absorption of 97% at 6.0 μm and demonstrated a dual-band absorber with two absorption bands [17]. Jeremy John et al. demonstrated a simple design of an ultra-thin, wide-angle plasmonic absorber exhibiting spectrally selective near-unity absorption [18], and the spectral position of the absorption peak as high as 95% can be controlled. Govind [19] designed a polarization independent absorber at mid-infrared consisted of array of circular metallic patches. Wu et al. [20] investigated a TE polarization spectrum selective absorber exhibiting near 100% absorption for infrared frequencies. Bai et al. [21] designed a wide-angle, polarization independent and dual band infrared absorber based on L-shaped metamaterial. A new architecture that used a multifunctional metamaterial absorber to absorb far-infrared incident wave with a narrowband (560 nm FWHM) is demonstrated by Suen et al.[22]. Since the absorption of the metamaterial absorber is based on the electromagnetic resonance in the structure, the absorption bandwidth is often narrow. The metamaterial absorbers in [8-11], can only achieve high absorption at a certain wavelength. M. A. Baqir and P. K. Choudhury investigated a hyperbolic metamaterial (HMM)-based absorber which has a nearly perfect absorption in the entire ultraviolet [12]. In infrared narrow band absorber can be used for infrared

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Corresponding Author. E-mail addresses: [email protected] (Z. Che), [email protected] (C. Tian).

imaging systems and sensors, and has been investigated from single band to multiband. For some applications, such as solar energy harvesting [15], require broadband infrared absorption. Therefore, broadening the bandwidth of metamaterial absorber becomes a new theme, and some efforts have been made to achieve the broadband absorption. Liu et al. [17] demonstrated a metamaterial absorber which achieved a broadband absorption from 0.8 to 1.6 μm by combing 16 different cross resonators in one unit cell. Ma et al. [23] presented metamaterial absorbers and the bandwidth almost covers the full mid-infrared region with absorption higher than 50%.Govind et al. [24] fabricated a metamaterial absorber using ITO film as ground plane which showed the absorption beyond 70% over the mid-infrared region.

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However, considering the two aspects of simple structure and high absorption, there are still some

problems to be solved. In this paper, we present a simple design of a polarization independent, broadband absorption metamaterial in far-infrared spectrum.

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2.Design and simulation The schematic diagram of a single unit cell of the metamaterial absorber is shown in Fig. 1. A unit cell can be regarded as the combination of four equal square portions, four circular patches with different radius located in the center of each square portion. The absorber has two resonant layers, the upper dielectric layer is Al2O3 (Green part in Fig. 1), the lower dielectric layer is SiO2 (Blue part

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in Fig. 1). The dielectric constant and the loss tangent of Al2O3 are 2.28 and 0.04, respectively. The

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dielectric constant and loss tangent of SiO2 are 3.90 and 0.025， respectively. The ground plane and metal patches in the resonators of the absorber are selected as gold. In infrared region, the

 p  2  2176 THz and collision frequency c  2  6.5 THz

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in which plasma frequency

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 p2  m    1   (  ic )

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dielectric constant of gold is given by the Drude model:

[25]. The period of the unit cell is P = 9.0 μm. The thickness of the upper and lower dielectric layers are the same, and its value is H1 = H3 = 0.29 μm. The thickness of the metal patches in the two

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resonant layers are H2 = H4 = 0.1 μm. Four different radiuses of the circular patches are R1 = 1.70

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μm, R2 = 1.60 μm, R3 = 1.50 μm, R4 = 1.40 μm.

Fig.1. Schematic of a unit cell of the absorber.

The

frequency-dependent

absorption through

metamaterial structure

is defined

as

A()  1  R()  T () with A(ω), R(ω), and T(ω) determining the frequency-dependent absorption,

reflection, and transmission coefficients, respectively. Since there is a metal ground plane at the bottom of the structure and the thickness of the ground plane is larger than the penetration depth of light in infrared, the transmission coefficient T(ω) is nearly zero and absorption coefficient can be simplified as A( )  1  R() . R(ω) is calculated by the S-parameters obtained from CST Microwave Studio using the finite difference time domain(FDTD) method. The unit cell boundary conditions are set in the directions of x and y, and the open add space boundary condition is set in the direction of z. The absorption for TE and TM polarization at normal incidence is given Fig.2.

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Simulation results indicate that the absorbent property of the absorber is insensitive to polarization,

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the absorptions are beyond 90% across the range of 8-12 μm for both TE and TM polarizations.

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Fig. 2. Simulated absorption spectra for TE and TM polarization

3. Absorbing mechanisms and discussions In order to explore the broadband absorption mechanism of the absorber, the CST Microwave

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Studio software is used to extract the current, magnetic field and energy loss distribution at the chosen resonance points 11.70 μm, 11.05 μm and 9.41 μm by setting the field monitors, as shown in Fig. 4, Fig. 5, and Fig. 6, respectively. In these figures, (a), (b), and (c) correspond to wavelengths

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11.70 μm, 11.05 μm and 9.41 μm, respectively. At the resonant wavelength of 11.70 μm, it is obvious that antiparallel currents are excited in the gold patches with radius of R1=1.70 μm and the bottom gold layer. Actually, this is often called a magnetic resonance because of the closed circulating

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currents result in a magnetic moment which can strongly interact with the magnetic field of the incident electromagnetic wave. At resonance, a strong enhancement of the localized electromagnetic field is established between the two layers. Consequently, electromagnetic energy can be efficiently confined in the dielectric spacer and therefore almost no wave is reflected back. While at 11.05 μm, the magnetic resonance and the energy dissipation occur in the lower dielectric with the radius of

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R2=1.60 μm. At 9.41 μm, currents along y-direction are excited by the incident electromagnetic wave in the upper metal resonator with the radius of R1=1.70 μm, while in the lower metal resonator oppositely oriented currents are excited. Currents in upper metal resonator and lower metal resonator form a closed loop, the magnetic field excited by the loop couples with the magnetic field of incident wave, thus there is an excitation of a strong magnetic resonance. As a result, the energy is dissipated in the upper dielectric layer. Based on the above analysis, it is known that the two resonant layers have modulation effect on the absorption peak position because of the dielectric constant difference of the dielectric layers. The resonant units of different sizes in the same resonant

layer also have modulation effect on the absorption peak position. The joint action of the two factors produces a number of adjacent absorption peaks and superimposition each other, widening the absorption bandwidth. Hence, the effect of wide band absorption is achieved. Although it is believed that the electromagnetic wave is usually not able to cross the metal, we believe that the infrared wave entry into the absorber is mainly through the gold patches and secondarily from the side. The reasons are as follows. It is believed that the electromagnetic wave cannot pass through the metal for the bulk metal, and its dielectric constant is regarded as infinity. However, the gold patches we used are very thin, only 0.1 μm. Hence, it is a nano structure material, which is different from the bulk material. Its dielectric constant is limited and it has the properties

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of a certain medium material. Therefore, we think that infrared can pass through the gold patches.

Because the dielectric constant is closely related to the frequency of wave, this conclusion cannot

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be extended to other bands.

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Fig. 3. Surface currents in the absorber at the wavelength of (a) 11.70 μm, (b) 11.05 μm and (c) 9.41 μm.

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Fig. 4. Distribution of the magnetic fields at the wavelength of (a) 11.70 μm, (b) 11.05 μm and (c) 9.41 μm.

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Fig. 5. Energy dissipation distributions in the absorber at the wavelength of (a) 11.70 μm, (b) 11.05 μm and (c)

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9.41 μm.

The above simulations are carried out at normal incidence, and it is necessary to consider the effect of oblique incidence on the absorption. The absorptions of different incident angles for TE and TM polarizations are plotted in Fig. 6. The simulation results indicate that the absorption

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would decrease if the incident angle continues to increase.

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efficiency and bandwidth keep constant when the incident angle is less than 25°. The absorption

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Fig. 6. Absorption spectra as a function of incident angle for (a) TE polarization and for (b) TM polarization

By changing the geometrical parameters we tuned the absorption band from 8-12 μm to 5-8 μm. The optimized geometrical parameters are P = 3.0, R1= 1.15 μm, R2 = 1.10 μm, R3 = 1.00 μm, R4 =

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0.95 μm, H1 = H3 = 0.25 μm, H2 = H4 = 0.1 μm. Fig. 7 shows the absorption spectra of the absorber.

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The absorption is beyond 90% from 5.4 to 8.1 μm for both TE and TM polarizations.

Fig. 7. Simulated absorption spectra of for TE and TM polarization

4. Conclusion In summary, a broadband infrared metamaterial absorber is designed and numerically investigated. The absorber is polarization independent with the absorptivity beyond 90% for 8 to 12 μm wavelength. The analysis of the magnetic fields and energy dissipation at resonant wavelengths reveals that the absorptions result from the magnetic resonance, and the broadening of bandwidth is attributed to the mergence of several absorption peaks. The absorption band can be tuned by changing the geometrical parameters. Reference [1] J. B. Pendry, A. J. Holden, W. J. Stewart, and I. Youngs, Extremely Low Frequency Plasmons in

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