Design, Fabrication and Characterization of a Wideband Metamaterial Absorber for THz Imaging ^†

Abstract

In this paper, the design and optimization of a wideband THz metamaterial absorber (MMA) are proposed. By simulation, we reached four structures with absorptions higher than 50%, 70%, 80%, and 90%, with relative absorption bandwidths (RABWs) of 1.43, 1.29, 0.93, and 0.72, respectively. Terahertz absorbers can be used in many potential applications, such as in imaging, energy harvesting, scattering reduction, and thermal sensing. Our intended application was to use the optimal absorber on a thermal detector for detectivity over a wide THz range. Since broadband absorption in the range of 0.3 to 2 terahertz is considered for use in medical imaging, the MMA with more than 50% absorption in the range of 0.35-2.1 THz was selected. The designs were also intended to have the capability of being implemented on different devices, such as bolometers. The cost of the fabrication of the proposed absorber was also low because of the implementation of a single-layer MMA design and the utilization of affordable and more accessible materials and techniques. Our proposed structure had a minimum feature size of 3 μm, making the fabrication process convenient using the standard photolithography method as well. We used thin layers of nickel as the metal for both the single-layer pattern and ground layer, which were placed on the front and back sides of the structure, respectively. The nickel thin film layers were deposited using the sputtering technique and separated by a dielectric layer. The material chosen for the dielectric layer was SU8, which has proper electromagnetic properties and also good adhesion to nickel. Characterization of the fabricated absorber was performed using a terahertz spectroscopy system, and the experimental results verified the high absorption of the sample.

1. Introduction

The extraordinary electromagnetic properties of materials, many of which are not found in nature, have been widely studied in recent years from the visible to the microwave [1,2]. They have received research attention for characteristics like negative refraction [3], invisible cloaking [4], energy harvesting [5], imaging lenses [6], and wave absorption [7,8]. The latter has been widely used in applications like THz radiation detection, imaging, filters, and spectroscopy.

To absorb the incident wave in a detector, we need an absorber of electromagnetic (EM) waves and we know that the detection rate is proportional to the amount of absorption [9]. Therefore, we need to design an absorber with significant absorption over a wide frequency range in the terahertz region, in other words, a broadband terahertz absorber. Most materials in nature absorb at separate frequencies, which results in small absorption bandwidths. This is where metamaterials come into play. Electromagnetic metamaterials are artificially engineered materials arranged in sub-wavelength dimensions and can be modeled as materials with negative effective electrical permittivity (ε(ω)) and magnetic permeability (μ(ω)) values. Therefore, they have a refractive index of less than zero [10,11]. Veselago was the first person to publish a theoretical analysis of materials with negative electric permittivity and magnetic permeability [12]. Metamaterials are an excellent choice for use in electromagnetic wave absorbers in order to increase the absorption bandwidth [13]. Landy and Tao designed and fabricated the first metamaterial absorber [14].

Early-design metamaterial absorbers were not broadband, did not absorb (e.g., absorption above 50%) over a significant frequency range, and only had absorption peaks at some isolated frequencies. The need to increase the absorption bandwidth (in order to access a larger frequency range depending on their application) encouraged scientists to look at the design of broadband structures [15,16,17,18,19,20,21,22].

Here, we have designed four different THz broadband absorbers for different purposes. In these designs, we have used nickel as an absorbent layer material, which causes high losses and increased absorption due to its high permeability coefficient. Also, for the dielectric layer, we used a low-cost and available photoresist material so that the fabrication process can be achieved easily and at a low cost, and these absorbers can be implemented on different devices. In addition to the appropriate selection of materials, we have developed suitable designs for different structures. Thus, by using a combination of substructures with different dimensions and optimizing these dimensions, the absorption peaks caused by each substructure were brought closer to one another, and we finally achieved broadband structures. The proposed absorbers were designed and simulated using the numerical electromagnetic solver, Computer Simulation Technology (CST). The absorption results of these structures were compared with other similar works, and we realized excellent performance by the presented structures. The fabrication process was easily achieved using the standard processes of sputtering and photolithography. The structures and results are described in the following sections.

2. Materials and Methods

Generally, metamaterial absorbers are designed in three layers, with the metamaterial, being an array of periodic structures, separated from the ground plane (uniform metal plate) by a dielectric layer [23]. The existence of a metallic layer with high conductivity as the ground plane at the back of the structure is the reason that all of the incident wave that reaches the ground plane will be reflected, and all we have is absorption and reflection. In other words, because the transmission is zero (T = 0), we have:

A = 1 − R,

(1)

where $R$ and $A$ are the reflectivity and absorptivity of the structure, respectively. For the reflectivity $\left(R\right)$ of TE and TM polarization of the incident wave with the angle of incidence ($\theta$), by modeling the absorber by a material with $\epsilon \left(\omega \right)={\epsilon }_0{\epsilon }_r\left(\omega \right)$ and $\mu \left(\omega \right)={\mu }_0{\mu }_r\left(\omega \right),$ where ${\epsilon }_0=\frac{1}{36\pi }\times {10}^{-9}F{m}^{-1}$ and ${\mu }_0=4\pi \times {10}^{-7}H{m}^{-1},$ and ${\epsilon }_r\left(\omega \right)$ and ${\mu }_r\left(\omega \right)$ are the relative permittivity and relative permeability, respectively, we have:

$$R_{TE}={\left|r_{TE}\right|}^2={\left|\frac{\cos \theta –{\mu }_r^{-1}\sqrt{n^2–{\sin }^2\theta }}{\cos \theta +{\mu }_r^{-1}\sqrt{n^2–{\sin }^2\theta }}\right|}^2,R_{TM}={\left|r_{TM}\right|}^2={\left|\frac{{\epsilon }_r\mathit{cos}\theta –\sqrt{n^2–{\mathit{sin}}^2\theta }}{{\epsilon }_r\mathit{cos}\theta +\sqrt{n^2–{\mathit{sin}}^2\theta }}\right|}^2$$

(2)

where $r$ is the reflection coefficient and $n$ is the refractive index of the absorber ($n=\sqrt{{\epsilon }_r\left(\omega \right){\mu }_r\left(\omega \right)})$. So, the absorptivity $\left(A\right)$ of the absorber can be easily calculated from the reflectivity $\left(R\right)$ of the incident wave.

To design our structure and choose the feature sizes to achieve broadband absorption, we took advantage of the fact that each part of the pattern in the MMA induces resonance in the structure, and this resonance results in a peak in the absorption spectra, $\frac{{\lambda }_0}{4}\propto L$, where ${\lambda }_0$ is the wavelength corresponding to the resonance frequency and L is the length of the resonator in the structure. The absorption was maximized through independent engineering of the structural parameters. Due to the different effective permittivity and permeability of each material, optimizations were carried out for each material to enhance the overall absorption. The size of the metallic structure has the highest contribution to frequency tuning. The periodicity, fill factor, and dielectric thickness can modify the frequency as well, although they mainly affect the peak absorption and quality factor. The permeability coefficient of metallic components is also a key parameter to extend the bandwidth, since higher permeability shows a wider bandwidth. Therefore, we utilized nickel as the metallic layer as it is a ferromagnetic material and has a high permeability coefficient.

The designs reported in this paper combine different sizes of rings in a single layer to achieve broadband absorption behavior. Another phenomenon in these structures is that when the size values become a bit closer, coupling between neighbors of different sizes takes place as well. This adds an extra component in the frequency response of the absorber. We utilized this behavior to extend the absorption bandwidth.

We designed and simulated four different structures in the CST environment, as shown in Figure 1. Each structure’s unit cell consisted of four rings that caused a high absorption bandwidth. Nickel was utilized as the ground and metamaterial metallic layer material, with a relative permeability of 600.

In

Table 1. Comparison of different structures based on number of layers, absorption BW, and absorption rate.

Ref.	MM Layer Material	Number of Layers	Absorption Frequency Range (THz)	Center Frequency (THz)	RABW	% Absorption
[24]	Gold	3	1.24–2.86	2.05	0.79	>50%
[25]	Gold and Graphene	4	22.02–36.61	29.3	0.50	>68%
[26]	Gold	19	5–7.75	6.37	0.43	>80%
[27]	Black phosphorus	10	4.77–6.49	5.63	0.30	>90%
This paper (#1)	Nickel	3	0.35–2.1	1.22	1.43	>50%
This paper (#2)	Nickel	3	0.45–2.07	1.25	1.29	>70%
This paper (#3)	Nickel	3	0.55–1.5	1.02	0.93	>80%
This paper (#4)	Nickel	3	0.7–1.5	1.1	0.72	>90%

, we compare some basic properties of our MMAs with some other recent MMAs designed for THz frequencies, which indicates that the bandwidths of our proposed metamaterial absorbers were higher than those of similar works.

Due to proper design and suitable selection of materials and parameters, fabrication of the absorber was easy, rapid, and low cost. We used standard photolithography for fabrication and the steps are given in Figure 2. Glass was chosen as the substrate, which can be replaced by any other device surface. After the RPA cleaning process, nickel was sputtered on the glass as the ground layer. Then, SU8 negative photoresist was spin-coated as the dielectric layer. For the metamaterial layer, nickel was sputtered again. For patterning, first S1813 positive photoresist was spin-coated. After chromium mask alignment and UV exposure, the excess photoresist was removed using NaOH as the developer. FeCl₃ was chosen as the Ni etchant. Finally, the photoresist was removed using acetone.

To evaluate the fabrication process and performance of the absorbers, we used structure #2, with the slight difference that instead of 37 µm, we coated a dielectric layer of 30 µm. This difference was implemented in order to investigate the effect of dielectric thickness on structure absorption. The final fabricated absorber is shown in Figure 3. The simulation and measurement results of this absorber are presented in the next section.

3. Results and Discussion

We used a time-domain THz spectroscopy system to evaluate the absorption of our design in the fabricated structure. As shown in Figure 4, the simulation and measurement results were consistent. The slight difference was because we used a 100 nm thick gold layer to calibrate and normalize the graph. But, as we know, for this purpose we need a perfect reflective reference and gold material itself has a fingerprint in this frequency range. Also, due to its symmetrical design, this structure was polarization-independent, which is an advantage in many applications.

4. Conclusions

We introduced four ultra-wideband metamaterial THz absorbers that can be implemented on various devices, such as bolometer detectors, to increase detectivity. The absorptions of the structures were more than 50%, 70%, 80%, and 90%, with relative absorption bandwidths (RABWs) of 1.43, 1.29, 0.93, and 0.72, respectively. Simulations in the CST Studio Suite environment confirmed the correct operation of the designs. We used nickel as the ground and metamaterial layer material due to its ferromagnetic nature and high permeability coefficient, which caused more absorption by the structures. Also, we utilized SU8 negative photoresist as the dielectric material, which can be easily spin-coated on various surfaces. Also, the minimum feature size of the designs was 3 μm. Using these available and low-cost materials and suitable designs, we successfully fabricated an absorber using a rapid, standard, and low-cost method. On the other hand, we designed the structures with only one patterned layer, which made our fabrication process more cost-effective, and this is one of the outstanding advantages of our MMAs over other broadband MMAs that employ more than one layer. Finally, we measured the absorption of the fabricated structure using a time-domain THz spectroscopy system. This special design with broadband absorption will allow us to use the structure in various applications, such as bolometric imaging, scattering reduction, and thermal sensing.