Thermal and ultrafast optical tuning of ultrathin high-temperature superconducting terahertz metamaterials

We demonstrate thermal and ultrafast optical tuning in planar terahertz (THz) superconducting metamaterials. The fundamental resonance of an array of split-ring resonators (SRRs) fabricated from a 50-nm-thick high-temperature superconducting (HTS) YBa ₂Cu ₃O _7-δ (YBCO) film is characterized as a function of temperature and near-infrared photoexcitation fluence. The HTS metamaterial exhibits a very strong resonant response at temperatures much lower than the transition temperature T _c. Increasing the temperature reduces the density of Cooper pairs, which results in a dramatically decreasing imaginary part of the complex conductivity, and thereby tunes the metamaterial resonance. We observe switched resonance strength and large red shift of resonance frequency when the temperature increases from 20 K to T _c. Similar resonance switching and frequency tuning is also demonstrated in an ultrafast time scale through near-infrared femtosecond laser excitation. We further compare the thermal tuning behaviour of the 50-nm-thick HTS metamaterial with a metamaterial sample comprised of gold SRRs with identical geometry and dimensions, which has negligible tunability.