Extraordinary Transmission through Fractal-Featured Metallic and Superconducting Films at Terahertz Frequency

We report on fractal-featured square and ring-shaped apertures with a Sierpinski carpet pattern (SCP) on metallic and superconducting NbN films. Multiple extraordinary terahertz (THz) transmission peaks are studied in the transmission spectra using both THz time-domain spectroscopy and numerical simulation. The characteristic transmission peaks are found to be associated with the interaction of surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) for ring-shaped apertures. The effect of LSPs is less remarkable in the square apertures. For the superconducting NbN film, when the temperature is slightly lower than the critical transition temperature T_c, the peak magnitude of SPP resonances is most prominent due to the non-monotonic temperature dependence of kinetic inductance. These results provide a new way to design compact and efficient THz devices.