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Abstract:
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.
This thesis takes a novel direction as it reports on the non-dissipative control of
the Josephson plasma in the layered cuprate superconductor La
1
.
84
Sr
0
.
16
CuO
4
using
terahertz electromagnetic waves. To achieve sufficiently intense sub-millimetre radi-
ation, 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.
Using the tilted pulse front technique, out-of-plane superconducting transport in
La
1
.
84
Sr
0
.
16
CuO
4
is gated bi-directionally on ultrafast timescales. The applied electric
field modulates the interlayer coupling, leading to picosecond oscillations between su-
perconducting and resistive states. Thereby, the modulation frequency is determined
by the electric field strength in spirit of the a. c. Josephson effect. Throughout the os-
cillations, in-plane properties remain unperturbed, revealing an exotic state in which
the dimensionality of the superconductivity is time-dependent.
Using a free electron laser, it is shown that resonant terahertz excitation of non-
linear Josephson plasma waves in La
1
.
84
Sr
0
.
16
CuO
4
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.
Both observations demonstrate the potential of layered superconductors for quan-
tum nonlinear optics and are of relevance for applications in ultrafast nanoelectronics.
