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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 Tc, 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.