| Summary: | <p>Femtosecond optical laser pulses have been comprehensively applied in the past to control the material properties of high critical temperature superconductors. However, since the superconducting gap is of the order of several millielectronvolts, optical photons have the potential to deplete the condensate by breaking Cooper-pairs.</p><p>This thesis takes a novel direction as it reports on the non-dissipative control of the Josephson plasma in the layered cuprate superconductor La<sub>1:84</sub>Sr<sub>0:16</sub>CuO<sub>4</sub> using terahertz electromagnetic waves. To achieve sufficiently intense sub-millimetre radiation, two complementary experimental approaches are taken. First, the table-top tilted pulse front technique is employed as a source of microjoule broadband terahertz pulses. Second, a large-scale free electron laser is operated to achieve multi-cycle pulses of less than two percent relative bandwidth.</p><p>Using the tilted pulse front technique, out-of-plane superconducting transport in La<sub>1:84</sub>Sr<sub>0:16</sub>CuO<sub>4</sub> is gated bi-directionally on ultrafast timescales. The applied electric field modulates the interlayer coupling, leading to picosecond oscillations between superconducting and resistive states. Thereby, the modulation frequency is determined by the electric field strength in spirit of the a. c. Josephson effect. Throughout the oscillations, in-plane properties remain unperturbed, revealing an exotic state in which the dimensionality of the superconductivity is time-dependent.</p><p>Using a free electron laser, it is shown that resonant terahertz excitation of nonlinear Josephson plasma waves in La<sub>1:84</sub>Sr<sub>0:16</sub>CuO<sub>4</sub> creates a metastable state that is transparent over a narrow spectral region. This finding is interpreted as the result of disruptive quantum interference between the linear plasma modes of the cuprate and an optically injected Josephson vortex lattice, which features the periodicity of the driving field, giving rise to three-level quantum interference optical transparency.</p><p>Both observations demonstrate the potential of layered superconductors for quantum nonlinear optics and are of relevance for applications in ultrafast nanoelectronics.</p>
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