Introduction

Metamaterials are composite and artificial materials that have been engineered [1], [2], [3], [4], [5]. They can also be described as materials that exhibit abnormal, unique, and special properties. Therefore, study and research in the field of metamaterials are interdisciplinary and are used in the fields of electromagnetics, optics, optoelectronics, semiconductor engineering, and solid-state physics. One of the most important researchers in this field dates back to 1968 when Veselago was the first person to introduce a negative refractive index (NRI) material with ε, μ < 0 [6]. At that time, Veslago's work did not receive much consideration until in 1996, Pendry succeeded in using an array of metal wires to artificially fabricate a material with a negative permeability coefficient [7]. Among the applications that can be mentioned from metamaterials are: negative refractive index, reverse doppler effects, invisibility cloak, perfect lens, negative refraction, artificial magnetism, transparency, perfect imaging, hyperbolic dispersion, sensing [8], [9], [10], [11], [12], perfect absorption [13], [14], [15], [16], [17], [18], filters [19], [20], antennas [21], super lensing, switches [22], modulators, polarization rotation, energy harvesting, multiplexers and so on [23]. Most of these applications can be used over a wide range of frequencies, such as microwaves, sometimes up to terahertz, or a little more energy such as infrared or visible regime.

Today, the terahertz frequency range (0.3–10 THz) has attracted the attention of many scientists and researchers, and various platforms have been introduced for designing devices in this frequency range. The term terahertz refers to electromagnetic radiation in the frequency range between the microwave frequency spectrum of 300 GHz and the 3000 GHz infrared frequency spectrum [24]. In terms of wavelength, the range of these waves is between 0.1 mm and 1 mm. It is important to note that terahertz is a frequency range of electromagnetic waves that have neither the properties of a microwave nor the properties of high-energy waves. Practically terahertz waves are a range in electromagnetic-wave that is far from direct access and must be measured by periodic properties of wavelength or energy. The penetration depth of terahertz radiation is less than microwave and greater than infrared, these waves pass through a wide range of materials such as plastic, wood, cardboard, plaster, and building materials, but these waves are not useful for telecommunications and are absorbed by water vapor in the Earth's atmosphere. Of course, one of the advantages of terahertz waves, which have attracted the attention of many researchers and scientists in recent years, is the phenomenon that THz waves are non-ionizing.

Another advantage that can be mentioned today, is due to the use of the majority of applications at frequencies below terahertz, they are associated with the great challenge of dimensions, bulky equipment, heavy, portable, and expensive. Jumping to terahertz solves some of these problems. Given that the advent of terahertz communications is going to revolutionize the transportation industry, allowing cars to be controlled automatically, it is also welcomed in medical imaging, such as imaging of cancerous tissue or images of the internal structure of a tooth, in agriculture and industry.

The THz technology has applications in detecting the different kinds of biomolecules, biomaterials, nucleic acids, and proteins. Moreover, the THz frequency regime has applications in biomedical imaging for various types of tissues according to the paper [25].

One of the applications of metamaterials in terahertz frequencies is absorber structures. A lot of metamaterials absorber papers have been published, including single-band [26], [27], double-band [28], [29], [30], broadband [31], [32], [33], and multi-bands [34], [35], [36], [37].

In the paper [38], comprising pixelated metasurface based on graphene-strontium titanate has been proposed. The advantages of this structure also are sharp transmission, red and blue shifts under changing the ambient temperature, insensitive characteristics to the and polarization on transmission. This device can be utilized in tunable temperature and strain-sensing devices as well. Furthermore, this device has clear on/off states for the transmission and a high Q-factor was reported for this device.

In another paper [39], the pixelated metasurface based on dielectric was proposed, in which it has a single fano resonance that can be tuned by scaling the meta-atom. This presented configuration also has a considerable Q-factor and precise nondestructive identification for molecular fingerprints. But, as we have known, these configurations need high-skilled fabrication methods and they will be costly for the producers. [40] has proposed a novel method to exploit chirality of highly sensitive graphene plasmonic metasurfaces to characterize complex refractive indexes (RI) of viruses by detecting the polarization state of the reflected electric fields in the THz spectrum. Also [41] has proposed a new strategy for active chiral metasurfaces (by tunning the conductivity and Fermi level of graphene by electric doping, and analyzed the circular polarization reflectivity and circular dichroism of the chiral units with different Fermi levels) which allows the metasurfaces to switch between “on” and “off” states.

In the first part of this paper, a super perfect absorber with 99.99 % absorption at the frequency of 7.536 THz is presented. This structure is also insensitive to the polarization angle and can support a wide incident angle. In the next part of the paper, this idea is used to design a terahertz sensor to detect malaria. Simulation results show that a sensor with an average sensitivity of 2.538 THz/RIU and an average Q-factor of 23.4 can be obtained, which is more competitive and superior to similar models in terms of simplicity.

Section snippets

Theory and designing perfect metamaterials absorber

As shown in Fig. 1, the unit cell of the proposed absorber structure is located in four different facades consisting of four layers of graphene-dielectric- polysilicon-gold. The middle layer is made of SiO2, which has a dielectric constant of 2.25 and a height of T1 = 3 

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. The proposed structure also has a unit-cell periodicity of Px = Py = 5 

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. The bottom layer is gold with a conductivity of 4.56 

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 and height of TAu = 0.2 

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)which is much greater than the penetration depth for gold(.

Simulation results and sensing performance

To better understand how this structure is absorbed, the electric field and magnetic field distribution under the TE mode at the resonance frequency of the proposed structure are shown in Fig. 6. The proposed absorption mechanism of the structure is such that after the electromagnetic wave hits the structure, the top layer, which is graphene, acts as a resonator and excites the surface plasmon polaritons.

The most electric field is also distributed in four disks that are symmetrically located in 

Conclusion

In summary, a super-perfect single-band absorber structure with absorption of 99.99 % at a frequency of 7.57 THz is introduced. The proposed structure has the best absorption rate among other adsorbents and in addition to perfect absorption, it is polarization insensitive and has a wide incident angle. In the next step, the transmission line theory was presented with the help of a transmission matrix, impedance matrix, and scattering matrix to analyze this structure and the relations between

CRediT authorship contribution statement

Pouria Zamzam: Conceptualization, Methodology, Formal analysis, Software, Writing – original draft, Writing – review & editing. Pejman Rezaei: Validation, Formal analysis, Writing – review & editing, Supervision. Yadgar I. Abdulkarim: Validation, Formal analysis, Software, Writing – review & editing. Omid Mohsen Daraei: Writing – review & editing, Formal analysis, Validation.

Declaration of Competing Interest

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.

Acknowledgment

The authors acknowledge the Semnan University staff for their beneficial and professional help. Also, the authors would like to thank the Editor, and reviewers for their constructive comments.