  Through the integration of semiconductors or complex oxides into metal
resonators, tunable metamaterials have been achieved by a change of environment
using an external stimulus. Metals provide high conductivity to realize a
strong resonant response in metamaterials; however, they contribute very little
to the tunability. The complex conductivity in high-temperature superconducting
films is highly sensitive to external perturbations, which provides new
opportunities in achieving tunable metamaterials resulting directly from the
resonant elements. Here we demonstrate ultrafast dynamical tuning of resonance
in the terahertz (THz) frequency range in YBa_2Cu_3O_7-\delta (YBCO) split-ring
resonator arrays excited by near-infrared femtosecond laser pulses. The
photoexcitation breaks the superconducting Cooper pairs to create the
quasiparticle state. This dramatically modifies the imaginary part of the
complex conductivity and consequently the metamaterial resonance in an
ultrafast timescale. We observed resonance switching accompanied with a wide
range frequency tuning as a function of photoexcitation fluence, which also
strongly depend on the nano-scale thickness of the superconducting films. All
of our experimental results are well reproduced through calculations using an
analytical model, which takes into account the SRR resistance and kinetic
inductance contributed from the complex conductivity of YBCO films. The
theoretical calculations reveal that the increasing SRR resistance upon
increasing photoexcitation fluence is responsible for the reduction of
resonance strength, and both the resistance and kinetic inductance contribute
to the tuning of resonance frequency.
