Here, Keiser and co-authors have made a saturable terahertz absorber by creating a metamaterial from a high-temperature superconductor. The metamaterial elements in the form of split ring resonators are etched in the 100 nm thick YBa2Cu3O7 film and cooled down to 10 K. Then the authors experimentally studied the performance of this absorber with changing temperature and with changing power of the terahertz radiation. By increasing the temperature by 60 K they observed the decrease of absorption from 80% to 20%, while by changing the intensity in the available range, they reduced the absorption from 70% to 40%. The authors explained the observed behavior using a two-fluid model. The dependence of the electromagnetic properties of materials on the intensity of waves is at the essence of nonlinear optics, and this work makes an important contribution to nonlinear physics at terahertz frequencies.
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