We show that superconducting interlayer coupling, which coexists with and is depressed by stripe order in La1.885Ba0.115CuO4, can be enhanced by excitation with near-infrared laser pulses. For temperatures lower than Tc = 13 K, we observe a blue-shift of the equilibrium Josephson plasma resonance, detected by terahertzfrequency reflectivity measurements. Key to this measurement is the ability to probe the optical properties at frequencies as low as 150 GHz, detecting the weak interlayer coupling strengths. For T > Tc a similar plasma resonance, absent at equilibrium, is induced up to the spin-ordering temperature TSO ≃ 40 K. These effects are reminiscent but qualitatively different from the light-induced superconductivity observed by resonant phonon excitation in La1.675Eu0.2Sr0.125CuO6.5. Importantly, enhancement of the below-Tc interlayer coupling and its appearance above Tc are preferentially achieved when the nearinfrared pump light is polarized perpendicular to the superconducting planes, likely due to more effective melting of stripe order and the less effective excitation of quasiparticles from the Cooper pair condensate when compared to in-plane excitation.

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 ....

Many applications in photonics require all-optical manipulation of plasma waves1, which can concentrate electromagnetic energy on sub-wavelength length scales. This is difficult in metallic plasmas because of their small optical nonlinearities. Some layered superconductors support Josephson plasma waves (JPWs)2,3, involving oscillatory tunneling of the superfluid between capacitively coupled planes. Josephson plasma waves are also highly nonlinear4, and exhibit striking phenomena like cooperative emission of coherent terahertz radiation5,6, superconductor-metal oscillations7 and soliton formation8. We show here that terahertz JPWs can be parametrically amplified through the cubic tunneling nonlinearity in a cuprate superconductor. Parametric amplification is sensitive to the relative phase between pump and seed waves and may be optimized to achieve squeezing of the order parameter phase fluctuations9 or single terahertz-photon devices.