In the terahertz regime, due to perfect conductivity of most metals, it is hard to realize a strong confinement of Surface plasmon polaritons (SPPs) although a propagation loss could be sufficiently low. We experimentally demonstrated a structure with periodic pillars arranged on a thin metal surface that supports bound modes of spoof SPPs at terahertz (THz) frequencies. By using scanning near-field THz microscopy, the electric field distribution above the metal surface within a distance of 130 μm was mapped. The results proved that this structure could guide spoof SPPs propagating along subwavelength waveguides, and at the same time reduce field expansion into free space. Further, for the development of integrated optical circuits, several components including straight waveguide, S-bend, Y-splitter and directional couplers were designed and characterized by the same method. We believe that the waveguide components proposed here will pave a new way for the development of flexible, wideband and compact photonic circuits operating at THz frequencies.
Ultrathin metasurfaces with local phase compensation deliver new schemes to cloaking devices. We demonstrate a remarkable large size carpet cloak realized by an ultrathin metasurface at terahertz frequencies. The metasurface cloak is constructed by periodically arranging 12 different elements. The reflected wave front is perfectly reconstructed by an ultrathin metasurface cloak, which perform well under both intensity-sensitive and phase-sensitive detectors. The invisibility is verified when the cloak is placed on a reflecting triangular surface (bump). The multi-step discrete phase design method would greatly simplify the design process and is probable to achieve large-dimension cloaks, for applications in radar and antenna systems as a thin and easy-to-fabricate solution for radio and terahertz frequencies.
A tunable terahertz (THz) filter is reported, which was based on surface plasmonic effects from a subwavelength copper hole array, and the tunability was made possible by a substrate of phase-transition vanadium dioxide (VO<sub>2</sub>) film. The phase transition of the VO<sub>2</sub> film was induced by femtosecond laser pulses and the modulation depth of the THz pulse peak by the VO<sub>2</sub> film was measured by THz time-domain spectroscopy to be 80% under optimal fabrication conditions. The change of the conductivity of the substrate film could lead to a shift of the center resonance frequency from 1.33 to 0.92 THz, a relative shift of 30.8%. This tunable THz filter holds great promise for various applications.