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.
At an extremely low temperature of 20 mK, we measured the loop current in a tunable rf superconducting quantum interference device (SQUID) with a dc-SQUID. By adjusting the magnetic flux applied to the rf-SQUID loop (Φ f ) and the small dc-SQUID (Φ cjj f ), respectively, the potential shape of the system can be fully controlled in situ. Variation in the transition step and overlap size in the switching current with a barrier flux bias are analyzed, from which we can obtain some relevant device parameters and build a model to explain the experimental phenomenon.
