The crystal structure of high-Tc cuprates consists of superconducting Cu-O planes separated by insulating layers, between which pairs of superconducting electrons (the so-called Cooper pairs) can tunnel. A combination of capacitive coupling of the layers and inductive impedance (due to interlayer superconducting tunnelling) gives rise to collective plasma oscillations of Cooper pairs, known as “Josephson plasma waves”. The optical response along the direction perpendicular to the Cu-O planes is characterized by a Josephson plasma resonance that shows up typically as a sharp edge in the reflectivity and can be directly measured by time-domain terahertz spectroscopy.
In a series of experiments, we have investigated the nonlinear response of these plasma waves to strong electromagnetic radiation at terahertz frequencies. Firstly, we have been able to gate of the out-of-plane superconducting current in La1.84Sr0.16CuO4 on sub-picosecond timescales . Ultrafast oscillations between superconducting and normal metallic optical properties were induced upon strong driving with single-cycle THz pulses with a frequency content entirely below the Josephson plasma resonance. Remarkably, the superconducting response was switched on and off only along the out-of-plane direction, whilst it remained unperturbed within the Cu-O layers. This new, intriguing state, in which superconductivity displays a time-dependent dimensionality, is of interest for device applications in ultrafast electronics and represents an example of how nonlinear THz physics can benefit nanoplasmonics and active metamaterial
In another experiment, performed on the same high-Tc cuprate, we have demonstrated the possibility to directly excite nonlinear modes of the superconductor, the so-called Josephson plasma solitons, using intense narrowband radiation from a terahertz free electron laser tuned to the Josephson plasma resonance . These modes consist of bound vortex–antivortex pairs that propagate coherently without dispersion, and become observable as they cause a transparency window in the opaque spectral region immediately below the plasma resonance. Optical control of solitons may lead to new applications in terahertz plasmonics, in information storage and transport and in the manipulation of high-Tc superconductivity.
The most recent investigations in the field of Josephson plasmonics have been performed on another family of compounds, La2-xBaxCuO4, for which we have first shown that terahertz Josephson plasma waves can be parametrically amplified through the cubic tunneling nonlinearity . Parametric amplification is sensitive to the relative phase between terahertz pump and seed waves, and may be optimized to achieve squeezing of the order-parameter phase fluctuations or terahertz single-photon devices.
Another manifestation of the nonlinear character of Josephson tunneling consists in the generation of high harmonics. In La1.885Ba0.115CuO4 we have discovered a giant terahertz third harmonic response, persisting even above the superconducting Tc, all the way up to the stripe ordering temperature . This observation could be explained by hypothesizing the existence of “superfluid stripes” in the normal state, whose peculiar alignment causes interlayer superconducting tunneling to vanish on average. Our results reveal that this frustration is removed in the nonlinear optical response, thanks to the mixing of optically silent tunneling modes which drive large dipole-carrying supercurrents.
The nonlinear Josephson physics discussed here clearly extends beyond potential applications in photonics, directly leading to coherent parametric control of the superfluid in layered superconductors, and providing a means to manipulate the material properties or to probe them in new ways.
Related Methods & Experiments
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|Parametric Amplification of a Terahertz Quantum Plasma Wave|
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