Metal-free super-wideband THz absorber for electromagnetic shielding

A technique is implemented for obtaining the high absorption over super-wideband (SWB) in a metal-free THz absorber. The multiple resonant modes with wide spectra are generated in a graphite-based resonator placed on a dielectric cavity merging of which provides the SWB response. The low permittivity dielectric slab sandwiched between the graphite sheet at its bottom and graphite resonator at its top acts as the Fabry–Perot cavity where absorption takes place. The high absorption rate of graphite in the THz regime can make it a suitable candidate for its utilization in implementing the broadband absorber. Thus, the molecular transition due to interaction of energy in graphite also provides the high absorption. The absorption bandwidth can further be enhanced by stacking of multiple layers in two different configurations of the proposed unit cell. The absorber maintains the polarization insensitivity due to symmetry and allows the high absorption for the electromagnetic wave incident up to the angle of more than 75 ° . The proposed absorber can be utilized in the THz electromagnetic shielding applications due to its SWB response.

Narrowband-to-broadband switchable and polarization-insensitive terahertz metasurface absorber enabled by phase-change material

A terahertz absorber with controllable and switchable bandwidth and insensitive to polarization is of great interest. Here, we propose and demonstrate a metasurface absorber with switchable bandwidth based on a phase-change material of vanadium dioxide (VO2) and verify its performance by the finite element method simulations. The metasurface absorber is composed of a hybrid cross fractal as a resonator separated from a gold ground-plane by a polyimide spacer. Switching from narrowband to broadband absorber is achieved via connecting VO2 patches to the gold first-order cross fractal converting the resonator to a third-order cross fractal. In the insulator phase of VO2, the main narrowband absorption occurs at the frequency of 6.05 THz with a 0.99 absorption and a full-width half-maximum (FWHM) of 0.35 THz. Upon insulator-to-metal transition of VO2, the metasurface achieves a broadband absorption with the FWHM of 6.17 THz. The simulations indicate that by controlling the partial phase-transition of VO2, we can tune the bandwidth and absorption level of the absorber. Moreover, the designed absorber is insensitive to polarization due to symmetry and works well for a very wide range of incident angles. In the metallic state of VO2, the absorber has an absorption exceeding 0.5 in the 3.57-8.45 THz frequency range with incident angles up to 65°.

Electrochemical growth mechanism of nanoporous platinum layers

Porous platinum is a frequently used catalyst material in electrosynthesis and a robust broadband absorber in thermoelectrics. Pore size distribution and localization determine its properties by a large extent. However, the pore formation mechanism during the growth of the material remains unclear. In this work we elucidate the mechanism underlying electrochemical growth of nanoporous platinum layers and its control by ionic concentration and current density during electrolysis. The electrode kinetics and reduction steps of PtCl4 on platinum electrodes are investigated by cyclic voltammetry and impedance measurements. Cyclic voltammograms show three reduction steps: two steps relate to the platinum cation reduction, and one step relates to the hydrogen reduction. Hydrogen is not involved in the reduction of PtCl4, however it enables the formation of nanopores in the layers. These findings contribute to the understanding of electrochemical growth of nanoporous platinum layers in isopropanol with thickness of 100 nm to 500 nm.