Terahertz Dual-Band/Broadband Metamaterial Absorber Enabled by Sio2: Polyimide and PET Dielectric Substrates with Absorption Characteristics

Bull Mater Sci (2020) 43:201 Ó Indian Academy of Sciences

https://doi.org/10.1007/s12034-020-02183-7Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

Terahertz dual-band/broadband metamaterial absorber enabled by SiO2: polyimide and PET dielectric substrates with absorption characteristics

A ELAKKIYA1,*, S RADHA1, B S SREEJA1 and E MANIKANDAN2 1 Department of ECE, SSN College of Engineering, Chennai 603110, India 2 Department of ECE, B. S. Abdur Rahman Crescent Institute of Science and Technology, Chennai 600048, India *Author for correspondence ([email protected])

MS received 27 March 2020; accepted 20 May 2020

Abstract. A compact dual-band/broadband polarization in-sensitive terahertz (THz) metamaterial absorber (MMA) is discussed in this article. It consists of a simple planar structure as a unit cell and an optically transparent indium tin oxide (ITO) ground plane, which are separated by a 50 lm dielectric substrate. We designed three combinations of MMA, which are ITO–SiO2–ITO, ITO–polyimide–ITO and ITO–polyethylene terephthalate (PET)–ITO for the same planar structure. The proposed structure has dual-band absorbance with peak absorptivity of [98% for all three given combinations. By changing the substrate of the structure, the resonant frequency and bandwidth of the absorber structure is varied and by adjusting the design parameters, broadband absorbance is achieved for the same planar structure. The numerical simulation of the absorber shows that the broadband absorptivity is [98% for all three substrates. Polyimide, PET and SiO2 based absorbers are demonstrated with bandwidth of 0.467, 0.527 and 0.6 THz with covered broadband frequency range of 0.390–0.857, 0.407–0.934 and 0.433–1.03 THz, respectively. ITO–PET–ITO absorber structures also possess optical transparency. These bandwidths are convenient and compatible with electronic and magnetic sources in the terahertz region. This study provides applications in THz detection, sensing, bolometric imaging, and stealth and communication systems. All three absorbers have greater absorbance characteristics for transverse electric and transverse magnetic polarizations. The proposed structure is working well for wide angles of incident and polarization angles wave up to 90°.

Keywords. Dual-band/broadband; terahertz metamaterial absorber; SiO2; polyimide; PET.

1. Introduction analysis, design and experiment verification in multiband [19], broadband [20] and ultra-broadband [21] MMAs. The emerging technology and science related to terahertz However, all are not efficient at all time, as they have (THz) frequency (0.3–10 THz) has a great deal of attention limited frequency ranges and do not support some practical due to their potential applications in the areas of medical applications. Authors mainly used three techniques to imaging [1], communication systems [2], biological science enhance the absorption rate and bandwidth range. The first [3], sensing, energy harvesting [4], solar thermals [5], etc. method is to form a multiple subunit structure with different In different regions of electromagnetic (EM) waves, these sizes in a single large unit cell. It will make the size of unit advantages were investigated, e.g., microwave [6], giga- cell too large, which will increase the result in angular hertz [7], visible [8] and infrared [9]. Unavailable physical dependence because of many connections between the sub- properties in nature gained by periodically patterned sub- units [22]. The next one is using many number of vertically wavelength unit-cells or artificial materials are called stacked metal-dielectric layers into a new single unit-cell metamaterials [10]. To realize the terahertz metamaterial [23]. The last one is based on cascaded cavity design and absorber (MMA) system, some components are used such doped graphene or silicon materials or other composite as filters [11], modulators [12], sensors [13], optical devices structures. The broadband and even ultra-wideband stronger [14], absorbers [15], perfect lens [16], cloaking [17] and absorption could be realized by cascaded cavities and antennas [18], etc. In the area of security and imaging multilayer [24]. However, these two approaches are usually systems, an absorber component with low reflection capa- constrained by fabrication complexity and cost. These bilities and high absorption rate is needed, this property is designs also have dependencies on incident angles and difficult to find in natural materials. In recent years, research polarization angles and some on selective frequency and on terahertz technology is getting attracted by many wavelength. Therefore, a simple design with high- researchers around the world and they did theoretical performance MMAs is still an important issue in the 201 Page 2 of 7 Bull Mater Sci (2020) 43:201 metamaterials field. In effect, metamaterials with high-order dual-band/broadband MMA is shown in figure 1. The resonances are important. It is very helpful to design a optimized geometrical parameters of the MMA structure are multi-band or broadband MMA by combining the basic and shown in table 1. The length and width of ground and high-order resonance modes in a single patterned metallic substrate are the same as 200 9 200 lm. (W, L) are the structure [25]. width and length of the patch. Thickness for all three sub- In this article, we designed a simple planar structure strates is same as 50 lm. All the parameter values are same involving MMA, which provides both dual-band and for both dual-band and broadband absorbers, except for L2. broadband operation. When L2 value is 100 lm it gives dual-band absorption This structure supports three different substrates (SiO2, characteristics and when we increase the value to 116 lmit polyimide and PET). Accordingly, we designed and simu- gives broadband absorption characteristics. lated three combinations of MMAs, which are ITO–SiO2– The results of the dual-band/broadband absorber can be ITO, ITO–polyimide–ITO and ITO–PET–ITO for the same achieved by the use of finite integration technique-based planar structure. It consists of a simple planar structure as a CST microwave studio software. Its electric field is made to unit cell and an optically transparent ITO ground plane, point to the direction of x-axis, where a plane EM wave can which are separated by a 50 lm dielectric substrate. The be used as the stimulating source. Unit cell boundary con- proposed structure has dual-band absorbance with peak ditions are used towards the directions of x-axis and y-axis absorptivity of [98% for all three combinations. By and the open space condition is applied along the direction changing the substrate of the structure, the resonant fre- of the z-axis. For simulation, the absorption rate of the 2 2 quency and bandwidth of the absorber structure is varied incident plane wave is calculated by A(x)=1ÀS11-S21, and by adjusting the design parameters, broadband absor- 2 2 S11 ¼ RðÞx and S21 ¼ TðÞx ; here x, R, T and A represent bance is achieved for the same planar structure. the angular frequency, reflectance, transmittance and Comparing with previous terahertz metamaterials, the absorbance, respectively. The bottom layer ITO can be highlights of our proposed structure are, it has a simple unit- treated as a perfect electric conductor (PEC). It blocks the cell structure and high resonance mechanism; the terahertz incident EM waves so the transmittance (T) value is zero. MMA can provide a dual and broadband high-level To achieve 100% absorption rate, we need to match the absorption performance in a single planar structure; the inductance value to the free space so that the reflectance designed terahertz MMA is angle and polarization insensi- value (R) decreases. tive for both transverse magnetic (TM) and transverse electric (TE) waves. Finally, our novelty is that the same structure supports three different substrates (SiO2, poly- 3. Results and discussion imide and PET) and gives a great absorptivity of dual-band/ broadband operation for all the three combinations of The simulation of the proposed structure is carried out using MMAs, such as ITO–SiO2–ITO, ITO–polyimide–ITO and CST microwave studio software. The simulated absorbance ITO–PET–ITO. Thus, our design has some potential of the proposed terahertz dual-band/broadband MMA for all applications in communication systems, sensing and bolo- three combinations is presented in figure 2. ITO–poly- metric imaging at terahertz frequencies. imide–ITO-based absorber structure resonated at two frequencies, initially at 0.4115 and 0.8621 THz with the absorptivity of 99.6 and 99.3%, respectively. When length 2. Structure and design (L2) is increased from 100 to 116 lm, it gives a broadband response at 0.7566 THz with the absorptivity of 99.6%. The The unit cell of the proposed dual-band/broadband terahertz corresponding bandwidth is 0.462 THz (0.391–0.854 THz). MMA is composed of three layers: (1) ITO at the top, When the substrate is SiO2, the terahertz absorber resonated (2) dielectric substrate (any one of polyimide, SiO2 and PET) at two different frequencies, i.e., 0.4915 and 1.0415 THz at the middle and (3) ITO at the bottom. The dielectric sub- with the absorptivity of 98.6 and 98.3%, respectively. For strate of thickness 50 lm is placed in between ITO ground broadband operation, it covers the frequency ranges from plane and the radiating patch structure. Three combinations 0.416 to 1.03 THz and has the corresponding bandwidth of of terahertz MMA are designed and simulated here, which 0.6 THz. If the substrate is PET, the absorber resonated at are ITO–polyimide–ITO, ITO–PET–ITO and ITO–SiO2– 0.4412 and 0.9298 THz frequencies with the absorptivity of ITO for the same planar structure. Here the ITO–PET–ITO 99.7 and 99.4%, respectively. For broadband operation it MMA structure provides optical transparency and electrical covers the frequency ranges from 0.406 to 0.935 THz and and mechanical flexibility. Polyimide substrate is used as has the corresponding bandwidth of 0.52 THz with the adhesive coating, as it has high thermal stability and elec- absorptivity range of 99.7%. The numerical simulation of trical conductivity. Silicon dioxide (SiO2) substrate is nor- the absorber shows that both dual-band and broadband mally used to design unique patterns. Hence, all these three absorbers have absorptivity [98% for all the three sub- substrates (SiO2, polyimide and PET) confirmed their sup- strates (SiO2, polyimide and PET). The results for both port for terahertz applications. The three-layered terahertz dual-band and broadband operations involved three Bull Mater Sci (2020) 43:201 Page 3 of 7 201

Figure 1. A unit cell of the terahertz metamaterial absorber: (a) polyimide in yellow, (b) PET in green and (c) SiO2 in brown, ITO in pink.

Table 1. Designed parameters of MMA structure. The absorption curves for TE and TM polarizations for the terahertz dual-band and broadband absorbers, respectively, Parameters (lm) are shown in figure 4a and b. It is identified that the proposed structure has identical absorption characteristics for Absorber WLWL W L hh 1 1 2 2 1 orthogonal polarizations. So, when a wave is incident on the Dual-band 130 130 200 200 15 100 50 0.185 absorber structure, its field vectors can be resolved in ver- Broadband 130 130 200 200 15 116 50 0.185 tical and horizontal components, which can easily be absorbed by MMA structure at the resonant frequencies. The structure is demonstrated theoretically under different angles of incident and polarization angles. The effect of different combinations and comparisons are shown in varying oblique angle incidence (h) for ITO–polyimide– tables 2 and 3. The return loss characteristics for both dual- ITO, ITO–SiO2–ITO, ITO–PET–ITO based dual-band and band and broadband absorbers are presented in figure 3a broadband absorbers is given in figure 5a–f, respectively. and b. Polyimide, SiO2 and PET based dual-band and a An electric field will be always aligned towards x-direction broadband absorber structures give return loss below –10 so it maintains the TE polarization. Similarly, magnetic dB, hence it is used to achieve the near-unity absorption. field will be always aligned towards y and z directions, so it

Figure 2. Absorption spectrum for three combinations of MMA dual-band/broadband: (a) ITO–polyimide–ITO, (b) ITO–PET–ITO and (c) ITO–SiO2–ITO.

Table 2. Comparisons of the results for dual-band operations.

Absorber Parameters ITO–polyimide–ITO ITO–SiO2–ITO ITO–PET–ITO

Dual-band Frequency (THz) 0.4115, 0.8621 0.4915, 1.0415 0.4412, 0.9298 Absorptivity (%) 99.6, 99.3 98.6, 98.3 99.7, 99.4 201 Page 4 of 7 Bull Mater Sci (2020) 43:201

Table 3. Comparisons of the results for broadband operations.

Absorber Parameters ITO–polyimide–ITO ITO–SiO2–ITO ITO–PET–ITO

Broadband Frequency (THz) 0.7566 0.4901, 0.9156 0.8202 Absorptivity (%) 99.6 98.8, 98.1 99.7 Bandwidth (THz) 0.462 0.6 0.52

Figure 3. Return loss for three combinations of MMA.

Figure 4. Absorption characteristics for TE and TM polarizations: (a) ITO–polyimide–ITO, (b) ITO–PET–ITO and (c) ITO–SiO2–ITO. maintains the TM polarization. The proposed terahertz Figure 7 clearly shows the electric field and magnetic field MMA structure shows that it can behave as a good absorber distributions for dual-band MMA (ITO–polyimide–ITO, over wide-angle up to 90° of oblique incidence (h) under TE ITO–SiO2–ITO and ITO–PET–ITO) at six different fre- polarization. The effect of varying polarization angle (u) quencies. For the frequency of 0.4115 THz, the electric field for ITO–polyimide–ITO, ITO–SiO2–ITO, ITO–PET–ITO is maximum at the top layer of the absorber and side layers based dual-band and broadband absorbers is given in fig- of the substrate. The magnetic field flows at the left and ure 6a–f, respectively. The direction of wave propagation is right side of the patch structure. For the frequency of 0.8621 always towards z-direction while changing the polarization THz, top layer and substrate side layer have the maximum angles and hence, normal incidences occur. The simulated amount of field and the magnetic field is maximum at left results indicate that the proposed terahertz MMA structure and right sides of the patch structure. If the substrate is is polarization independent under normal incidence. SiO2, the structure is resonated at two different frequencies, Bull Mater Sci (2020) 43:201 Page 5 of 7 201

Figure 5. Simulated absorption characteristics for various angles of incidence for dual-band and broadband structures.

Figure 6. Simulated absorption characteristics for various polarization angles for dual-band and broadband structures. 201 Page 6 of 7 Bull Mater Sci (2020) 43:201

Figure 7. Electric field and magnetic field distribution of dual-band absorber for three different combinations at the resonance frequencies of polyimide—(a, g) 0.4115 THz; (b, h) 0.8621 THz; SiO2—(c, i) 0.4915 THz; (d, j) 1.0415 THz; PET—(e, k) 0.4412 THz; (f, l) 0.9298 THz; respectively. Electric field (a–f), magnetic field (g–l). namely, 0.4915 and 1.0415 THz. For the frequency of clearly shows the electric field and magnetic field distri- 0.4915 THz, the electric field distribution is maximum butions for broadband MMA (ITO–polyimide–ITO, ITO– within the patch structure and the magnetic field is maxi- SiO2–ITO and ITO–PET–ITO) at four different frequencies. mum at the left and right sides of the patch structure. Fre- For the frequency of 0.7565 THz, the electric field is quency 1.0415 THz is identified at inside of the patch maximum at the top layer of the absorber and side layers of structure and the magnetic field is maximum at the left and the substrate, and the magnetic field flows at the left and right sides of the patch structure. For the frequency of right sides of the patch structure. For the frequency of 0.4412 THz, the electric field is maximum at the top layer 0.4901 THz, the electric field is maximum at the top layer and substrate side layer and the magnetic field is maximum of the absorber and side layers of the substrate, and the at left and right sides of the patch structure and the top magnetic field flows at left and right sides of the patch portion of the substrate. For the frequency of 0.9298 THz, structure. For the frequency of 0.9156 and 0.8202 THz, the the electric field is maximum at the top layer of the absorber electric field is maximum at the top layer of the absorber and side layers of the substrate, and the magnetic field flows and side layers of the substrate, and the magnetic field flows at the left and right sides of the patch structure. Figure 8 at left and right sides of the patch structure.

Figure 8. Electric field and magnetic field distribution of broadband absorber for three different combinations at the resonance frequencies of polyimide—(a, e) 0.7565 THz; SiO2—(b, f) 0.4901 THz; (c, g) 0.9156 THz; PET—(d, h) 0.8202 THz; respectively. Electric field (a–d), magnetic field (e–h). Bull Mater Sci (2020) 43:201 Page 7 of 7 201

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