Corresponding author: Andrey V. Sabluk (sablukandrey@gmail.com)

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, non-destructive defectoscopy and security systems. The design of high-efficiency terahertz absorbers and converters is currently the main task in the development of terahertz technologies. In this work a frequency selective high-Q metamaterial is used for the fabrication of a terahertz-to-infrared converter. The converter consists of a metamaterial-based terahertz absorber coated with a micrometer-thick 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.

The terahertz

The use of existing well-proven 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 high-Q 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 non-resonance ones. The absorbers of the former category are distinguished by perfect absorption in a narrow transmittance band, whereas the non-resonance absorbers are used for non-perfect wide-band 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: (_{1} = 0.535 mm, _{2} = _{d} = 0.585 mm, and thicknesses _{1} = _{2} = 0.024 mm and _{d} = 0.067 mm. The overall thickness of the metamaterial is 0.115 mm; (

Another category of the resonance absorbers is the frequency selective surface [_{p}

where ^{*} 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 ^{*} 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. ^{2} sized array consisting of 1600 metaatoms (Fig.

Results of numerical simulation of radiation absorption: (^{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 (

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: (

The radiation converter efficiency was calculated as the ratio of the heat radiation energy _{out} to the incident terahertz radiation energy _{in}. The IR radiation energy _{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 ^{2} and the thermal energy accumulation time by the metamaterial was chosen to be _{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:

where ε is the radiation coefficient, σ is the Stefan–Boltzmann constant, _{mm} is the thermal energy accumulation time by the metamaterial cell and _{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 metamaterial-based 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 0718-2020-0025).