Optical excitation of Josephson plasma solitons in a cuprate superconductor

Abstract

Josephson plasma waves are linear electromagnetic modes that propagate along the planes of cuprate superconductors, sustained by interlayer tunnelling supercurrents. For strong electromagnetic fields, as the supercurrents approach the critical value, the electrodynamics become highly nonlinear. Josephson plasma solitons (JPSs) are breather excitations predicted in this regime, bound vortex-antivortex pairs that propagate coherently without dispersion. We experimentally demonstrate the excitation of a JPS in La 1.84 Sr 0.16 CuO 4 , using intense narrowband radiation from an infrared free-electron laser tuned to the 2-THz Josephson plasma resonance. The JPS becomes observable as it causes a transparency window in the opaque spectral region immediately below the plasma resonance. Optical control of magnetic-flux-carrying solitons may lead to new applications in terahertz-frequency plasmonics, in information storage and transport and in the manipulation of high-T c superconductivity.T erahertz-frequency nonlinear optics holds great potential for device applications in data storage and manipulation at high bit rates, as well as for applications in the coherent control of matter. New tabletop and accelerator-based sources, which generate electric fields at megavolt per centimetre strengths, are opening up new opportunities in this area. Recent advances have relied on direct control of selected vibrational resonances 1-6 or on the use of field enhancement in metamaterial structures 7 . In cuprate superconductors, direct excitation of the order-parameter phase has been shown to modulate the superfluid density on the ultrafast timescale 8 , effectively demonstrating non-dissipative routes to control the macroscopic state of the solid.Here, the terahertz nonlinear optics of cuprate superconductors is studied experimentally and theoretically in the general case in which nonlinear propagation effects are combined with the local response of ref. 8. The intrinsic nonlinearity of interlayer tunnelling is shown to generate solitonic modes that concentrate the electromagnetic energy in space and time, propagating without distortion inside the material.The terahertz-frequency electrodynamics of cuprate superconductors are, for fields polarized perpendicular to the planes, dominated by superconducting tunnelling between layers 9 . Cuprates are in fact stacks of extended Josephson junctions 10,11 , with distributed tunnelling inductance L J (x, y, t ) between capacitively coupled planes (x and y are the spatial coordinates in the planes and t is time).For low fields, L J is independent of space and time and a single Josephson plasma resonance (JPR) is found at ω JPR = 2π/ √ L J C (C is the equivalent capacitance of the planes, which is assumed to be constant in space and time). In most cuprates, ω JPR ranges between gigahertz (refs 12,13) and terahertz (ref. 14) frequencies. As characteristic for a plasmonic response, the superconductor is transparent and frequency dispersive for ω > ω JPR and has unity reflectivity for ω < ω JPR ....

ELBE Center for High-Power Radiation Sources

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Energy thresholds of discrete breathers in thermal equilibrium and relaxation processes

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Improving the Critical Temperature of MgB2 Superconducting Metamaterials Induced by Electroluminescence

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