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) YBa2Cu3O7-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 Tc. 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 Tc. 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.
We fabricated and studied a planar composite material consisting of sub-wavelength double split ring resonator structures made of Gold on a Silicon substrate. Our measurements reveal a strong transmission dip at 0.6 THz. Experimental and numerical results indicate that there is an Inductor-Capacitor resonance at 0.6 THz, characterized by enhanced electric field strength across the ring gap. Our results also indicate a shift in the resonance to higher frequencies as thickness is increased. Spectral properties of the composite material were measured using THz Time Domain Spectroscopy in the range from 0.1 THz to 3.5 THz. Simulations were carried out using the commercially available electromagnetic solver, Microwave StudioTM. Fabrication of the structures was done with Proton Beam Writing, a nanolithography technique based on focused MeV protons. The direct-write technique allowed us to fabricate structures much thicker than otherwise possible. For this work, the ring resonator structures had overall dimensions of 38 μm and a thickness of 8 μm with highly vertical and smooth sidewalls with minimum critical dimensions of 2 μm.
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