The electromagnetic response of 3D-printed periodic carbon structures was investigated numerically and experimentally in the microwave (26-37 GHz) and terahertz (0.2-1.2 THz) frequency ranges. The reflection, transmission and absorption spectra, as well as the effects of the concentration of electromagnetic waves, were analysed and discussed. High broadband absorption was observed for the 3D-printed cellular structures based on a moderately conductive (1 -30 S•m -1 ) skeleton, whereas perfect tuneable resonant absorption could be achieved by 3D meshes made of highly conductive (1 200 -2 000 S•m -1 ) glassy carbon. We show that laser stereolithography (SLA) or fused deposition modelling (FDM) 3D-printing technique should be preferred for getting pre-defined required electromagnetic performances depending on the intended application.
We propose an original technique for the grating metasurfaces fabrication by low-power ultraviolet (UV) laser treatment of fluorinated graphene (FG) films with the focus on terahertz applications. The laser treatment reduces dielectric FG to its conductive counterparts, increasing DC conductivity to 170 S·m-1 for treated areas. The electromagnetic (EM) response of the grating metasurfaces studied by THz time-domain spectroscopy in the 100 GHz – 1 THz frequency range demonstrates enhanced resonant transmittance through metasurfaces. The intensity and position of transmittance peak could be tuned by changing the metasurface geometry, i.e. the period of the structure and width of the reduced and unreduced areas. In particular, the decrease of the reduced FG area width from 400 µm to 170 µm leads to the shift of the resonance peak from 0.45 THz to the higher frequencies, 0.85 THz. Theoretical description based on the multipole theory supported by finite element numerical calculations confirms the excitation of the dark state in the metasurface unit cells comprising reduced and unreduced FG areas at resonance frequency determined by the structure geometrical features. Fabricated metasurfaces have been proved to be efficient narrowband polarizers being rotated by 50◦ about the incident THz field vector.
We introduce diffraction-theory-inspired analytic description of the metasurface comprising an array of graphene subwavelength hemispheres. Our theory describes light interaction with the random metasurface, in which the periodicity is broken by accidentally damaged meta-atoms in the nodes of a two-dimensional periodic lattice. Both numerical modeling and experiment show that such a nm-thin metasurface possesses giant broadband absorption in the THz spectral range that remains intact even when a substantial portion of meta-atoms, i.e. graphene hemispheres, is damaged. Moreover, defective fabrication of graphene free-standing metasurface may enhance the absorptive properties.
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