Research Article 
Corresponding author: Andrey V. Sabluk ( sablukandrey@gmail.com ) © 2022 Andrey V. Sabluk, Alexey A. Basharin.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Sabluk AV, Basharin AA (2022) Metamaterialbased terahertz converter. Modern Electronic Materials 8(4): 149155. https://doi.org/10.3897/j.moem.8.4.98919

Since the early 1980s the terahertz range (0.1 to 10 THz) attracts permanent attention of fundamental and applied science. Due to its unique properties terahertz radiation is used in a wide range of applications such as spectroscopy, nondestructive defectoscopy and security systems. The design of highefficiency terahertz absorbers and converters is currently the main task in the development of terahertz technologies. In this work a frequency selective highQ metamaterial is used for the fabrication of a terahertztoinfrared converter. The converter consists of a metamaterialbased terahertz absorber coated with a micrometerthick graphite layer that reemits the absorbed energy in the infrared range. We have carried out electrodynamic and the related thermodynamic calculations of the suggested radiation converter. Numerical simulations yield an electromagnetic radiation absorption coefficient of 99.998% and an analytically calculated converter efficiency of 93.8%. Thanks to these advanced parameters suggested terahertz converter can find it’s applications in a wide range of transportation security inspection and defectoscopy tasks.
metamaterial, terahertz radiation, infrared radiation, radiation converter, frequency selective surface
The terahertz (THz) frequency range (0.1 to 10 THz) [
The use of existing wellproven infrared focal plane array matrices for the processing of THz radiation converted to the IR range is a promising approach. Radiation conversion from one frequency range to another can be achieved using specially structured artificial media, i.e., metamaterials [
In recent years special attention has been drawn to metamaterials due to the possibility of the implementation on their basis of highQ resonators operating over a wide terahertz frequency range [
The importance of converting terahertz radiation is dictated not by the necessity of reducing or increasing the frequency but rather by the practical convenience of devices and equipment operating in some technically achievable frequency range which can be adapted to operation in another range. The design of the converter includes two basic components. By analogy with detectors, the main component of the converter is the receiver, the other component being the emitter. A good signal receiver is equivalent to the perfect absorber which can be made from metamaterials [
By their nature, electromagnetic wave absorbers can be divided in two categories: resonance and nonresonance ones. The absorbers of the former category are distinguished by perfect absorption in a narrow transmittance band, whereas the nonresonance absorbers are used for nonperfect wideband absorption. To understand the reason of the choice of metamaterials as the material for the perfect absorber as component of the suggested converter, we will analyze the working principle of the resonance absorbers. The first resonance electromagnetic wave absorber was invented by the American engineer Winfield Salisbury [
Physical working principles and main parameters of converter: (a) Salisbury screen [31] consisting of a thick screening metallic layer coated with a dielectric layer having a specially selected thickness and a thin transparent metallic screen; (b) frequency selective surface in the form of a 2D array of metallic elements on a dielectric substrate which reflects or absorbs electromagnetic waves at the preset frequency depending on the surface topology; (c) the metaatom considered herein is in the form of three superimposed layers with square crosssections having the following dimensions: L_{1} = 0.535 mm, L_{2} = L_{d} = 0.585 mm, and thicknesses s_{1} = s_{2} = 0.024 mm and s_{d} = 0.067 mm. The overall thickness of the metamaterial is 0.115 mm; (d) frequency selective surfaces of two topologies and their equivalent circuits. Tiles are used in the lowfrequency range, and grid is used at high frequencies; (e) the top and bottom conducting layers in the metaatom are nickel, the dielectric being fiberglass plastic. The bottom surface of the metamaterial is coated with graphite for radiation conversion; (f) example of THz converter application in security domain. The person 2 is in the inspection zone between the 96 GHz electromagnetic source 1 and the screen 3 which is the THz converter. The radiation passes through the transparent media and is incident on the terahertz converter for further conversion to IR radiation. The infrared radiation of the converter is read by the IR camera 4
Another category of the resonance absorbers is the frequency selective surface [
${\omega}_{p}=\sqrt{\frac{n{e}^{2}}{{m}^{*}{\epsilon}_{0}}}$, (1)
where e is the electric charge, n is the charge density and m^{*} is the effective mass of electron. As can be seen from Eq. (1), the only method to reduce the plasma frequency is to reduce the charge density n because the other parameters (e, m^{*} and ε_{0}) are constants. The charge density can be reduced by specifically designing the metallic elements of the top layer of the frequency selective surface. The metamaterial developed in this work (Fig.
One application of the terahertz converter suggested herein is shown in Fig.
The reflection and absorption coefficients of the material suggested in this work in the 50 to 150 GHz range were calculated with the Microwave Studio CST electrodynamic simulation software package. At 96 GHz corresponding to the wavelength λ = 3.122 mm, the absorption coefficient of the material is the highest, 99.998% (Fig.
Results of numerical simulation of radiation absorption: (a) absorption coefficient (red curve) and reflection coefficient (green curve) as a function of incident radiation wavelength. Electrodynamic simulation of the newly designed metamaterial with Microwave Studio CST shows an absorption coefficient of 99.998% at 96 GHz corresponding to the wavelength λ = 3.122 mm; (b) example of detection of a triangular steel plate with leg sizes of 3 and 6 mm by a 23.4 × 23.4 mm^{2} sized metasurface consisting of 40 × 40 cells. The steel plate is located at a 4.7 mm vertical distance and a 0.9 mm horizontal distance from the top right corner of the metasurface. The distance between the steel plate and the metasurface is 20 mm. The electromagnetic response of the metamaterial was visualized by calculation in the Microwave Studio CST electromagnetic simulation software using the finite difference time domain method (FDTD) [36]. The simulation output data were (c) electric field magnitude in the dielectric layer, (d) magnetic field magnitude and (e) surface current density in the metallic layer. The top right corner of the metasurface exhibits a typical diffraction pattern. The screen area in front of the object shows a bright spot corresponding to the peak magnitude of the electric and magnetic fields and the highest surface current density. As one moves away from the spot center the excitation is spread over the metasurface in the form of coaxial circles with a declining intensity
For THz radiation conversion to infrared range the metamaterial absorber is coated with a reemitting layer having the highest degree of blackness the thickness of which should be chosen taking into account converter response time requirements. Another important condition is a uniform distribution of the reemitting material over the surface of the bottom absorber layer. Therefore the reemitting material should have the capability of being applied onto the surface in the form of a sufficiently thin layer and have good adhesion to the surface. Graphite was chosen as the reemitting material in view of the above requirements and taking into account its good adhesion to nickel [
Radiation conversion results: (a) Radially symmetrical model of radiation converter cell (1 is the fiberglass plastic, 2 is the top nickel layer of the metamaterial cell, 3 is detected THz radiation, 4 is IR radiation, 5 is the bottom nickel layer of the metamaterial cell and 6 is the graphite emitting layer). The temperature at the converter perimeter is 293.15 K. Electromagnetic and thermal calculation results obtained in the Microwave Studio CST environment. (b) Heat flow density of converted THz radiation for three graphite layer thicknesses (0.2, 1 and 2 μm). The best result was obtained for the 0.2 μm thick graphite layer as it delivers the highest heat flow density and hence the greatest image contrast on the focal plane array matrix of the thermal imaging system. Images of (c) surface heat flow density and (d) bulk heat flow density suggest that it is the emitting layer in the radiation converter that provides for heat emission
The radiation converter efficiency was calculated as the ratio of the heat radiation energy E_{out} to the incident terahertz radiation energy E_{in}. The IR radiation energy E_{out} was evaluated with CST using conjugate calculation. The terahertz radiation pulse duration was 3 s. The boundary conditions were as follows: the converter temperature was accepted to be constant and equal to 293.15 K, the maximum energy flow density of incident radiation P was accepted to be 10 W/m^{2} and the thermal energy accumulation time by the metamaterial was chosen to be t_{mm} = 20 seconds due to the relatively low thermal conductivity and significant thickness of the dielectric. The converter efficiency η in percents was calculated using the following equations:
$\eta =\frac{{E}_{\text{out}}}{{E}_{\text{in}}}\times 100\%$;
${E}_{\mathrm{in}}={\int}_{0}^{{t}_{\mathrm{imp}}}\mathrm{d}t{\int}_{0}^{R}P(t,r)2\pi r\mathrm{d}r$;
${E}_{\text{out}}={\int}_{0}^{{t}_{\mathrm{mm}}}\mathrm{d}t{\int}_{0}^{R}\epsilon \sigma \left({T}^{4}{T}_{0}^{4}\right)2\pi r\mathrm{d}r$.
where ε is the radiation coefficient, σ is the Stefan–Boltzmann constant, P (t,r) is the maximum energy flow density of incident radiation, t_{mm} is the thermal energy accumulation time by the metamaterial cell and t_{imp} is the THz radiation pulse duration. The best converter efficiency reaching 93.8% was achieved for the minimum emitting layer thickness of 0.2 mm.
A model of a metamaterialbased terahertz converter was suggested. The working principle of the converter is based on the absorption of THz radiation by a resonance metamaterial absorber, converter heating and heat reemission by a thin graphite layer. The emitted IR radiation can be detected by the focal plane array matrix of a thermal imaging system. The metamaterial absorber is designed for operation at 96 GHz at which the absorption coefficient of the converter is 99.998%. The converter has low heat capacity and therefore high sensitivity. The best converter efficiency as calculated in the course of numerical simulation was 93.8%. This converter can find application in THz radiation visualization for defect diagnostics in industry and in security domain.
The Authors are grateful to N.A. Maleeva for her mentoring in academic writing. The work was carried out with financial support from the Ministry of Science and Higher Education of the Russian Federation within State Assignment for the National University of Science and Technology MISiS (Assignment Code 071820200025).