Dielectric coated conductive rod resonantly coupled with a cut transmission line as a tunable microwave bandstop filter and sensor.

The resonant interaction of a dielectric-coated conductive rod with the X-band microwave field is investigated. The magnetic field distribution of the Goubau standing radial surface waves was experimentally visualized by using a thermo-elastic optical indicator microscope, and the corresponding electric field distribution was determined via numerical simulations. These field distributions are characterized by a certain pattern of antinodes distinctive for standing waves. An analysis of these field distributions allows one to couple a coated rod with a cut Goubau line. A rod placed in the gap region perpendicular to the Goubau line results in a sharp rejection band in the transmission spectrum which is extremely sensitive to the changes in the surrounding media. The shifting rate of the resonance as a function of the dielectric shell thickness is approximately 1.4 GHz/mm. The Q-factor of copper rods depends on their size and dielectric shell thickness. Longer rods with more energy localization areas have higher Q-factors, typically 1.7 times higher (12.7 vs. 7.5). Moreover, incorporating a dielectric shell enhances energy confinement and can elevate the Q-factor by as much as 22 %. When a 25 mm Cu rod is situated inside a cut Goubau line system, the Q-factor values are significantly higher, with a ratio of 275 to 13. With the addition of a dielectric shell, the Q-factor can be elevated by 58 %. The versatility of the proposed controllable system makes it possible to tune the operating spectrum towards higher GHz and THz frequencies.

Tunable ultra-broadband terahertz metamaterial absorber based on vanadium dioxide strips.

A simple design of an ultra-broadband metamaterial absorber (MMA) of terahertz (THz) radiation based on vanadium dioxide (VO2) configurations is proposed. The system is composed of a top pattern representing orderly distributed VO2 strips, a dielectric spacer and an Au reflector. Theoretical analysis based on the electric dipole approximation is performed to characterize the absorption and scattering properties of an individual VO2 strip. The results then are used to design an MMA composed of such configurations. It is shown that the efficient absorption characteristics of the Au-insulator-VO2 metamaterial structure can be ensured in a broad spectrum of 0.66-1.84 THz with an absorption band relative to the center frequency reaching as high as 94.4%. The spectrum of the efficient absorption can be easily tuned via the corresponding choice of strip dimensions. Wide polarization and incidence angle tolerance for both transverse electric (TE) and transverse magnetic (TM) polarizations are ensured by adding an identical parallel layer rotated by 90 degrees in respect to the first one. Interference theory is applied to elucidate the absorption mechanism of the structure. The possibility of modulation of the electromagnetic response of MMA relying on the tunable THz optical properties of VO2 is demonstrated.