The crystal structure of high-Tc superconducting cuprates consists of superconducting Cu-O planes separated by thin insulating layers, between which pairs of superconducting electrons (the so-called Cooper pairs) can tunnel. The combination of the capacitive coupling of the layers and the inductive impedance due to the superconducting tunneling between the planes gives rise to collective plasma oscillations of Cooper-paired electrons, the so-called “Josephson plasma waves”. The optical response perpendicular to the Cu-O planes is characterized by a Josephson plasma resonance (JPR) that shows up typically as a sharp edge in the reflectivity at terahertz frequencies and can be directly measured by time-domain spectroscopy.
Here we set out to investigate the nonlinear response of these plasma waves to strong electromagnetic radiation at terahertz frequencies, tuned to be resonant to the JPR. The study of nonlinear excitations in superconductors and manipulation of plasma waves may provide novel methods to control the superconducting state coherently at higher temperatures.
In a first experiment  out-of-plane superconducting transport in the cuprate La1.84Sr0.16CuO4 has been gated on ultrafast timescales. Picosecond oscillations between superconducting and normal metal states could be observed, as the conduction through Cooper pairs was shut on and off by quantum interference, upon excitation with a strong THz field. Remarkably, the superconducting properties are switched on and off only along the out-of-plane direction, whilst they remain unperturbed in the copper oxygen layers. Since in-plane transport remains unperturbed, this situation amounts to an intriguing state in which the superconductivity features a time-dependent dimensionality. This study demonstrates the equivalent of transistor action at ultrafast speeds, and may be of importance as a component in optoelectronic devices.
In another experiment , a nonlinear plasma mode has excited by narrowband THz pulses from a free electron laser (link to high-field THz methods section) tuned to the Josephson resonance of the same cuprate La1.84Sr0.16CuO4. The mode is a so-called “soliton”, a bound vortex-antivortex pair that propagates through the material without dispersion and it becomes observable as it induces a transparency window in the linear plasma response. The observations demonstrate the possibility to locally and selectively deactivate superconductivity in cuprates and show the potential of such materials for quantum nonlinear optics.
The last highlight in the field of Josephson Plasmonics was recently achieved in another high-Tc cuprate, La1.905Ba0.095CuO4 . Here, amplification of Josephson Plasma Waves launched by a weak probe field was obtained by applying an intense THz pump field, which resonantly drove the Josephson phase to large amplitudes. The parametric amplification reported in this work consistently exhibited in both experiment and simulations the expected sensitivity to the relative phase of strong and weak fields mixed in the process, as well as an oscillatory dependence at twice the frequency of the drive.
The physics demonstrated here extend beyond potential applications in photonics, directly leading to coherent parametric control of the superfluid in layered superconductors, and providing a means to manipulate the properties of the material or to probe them in new ways.
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