One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional "striped" spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La(1.675)Eu(0.2)Sr(0.125)CuO(4), into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.

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 ....

In cuprate superconductors, tunnelling between planes makes three-dimensional superconductive transport possible. However, the interlayer tunnelling amplitude is reduced when an order-parameter-phase gradient between planes is established. As such, interlayer superconductivity along the c-axis can be weakened if a strong electric field is applied along the c-axis. In this Letter, we use high-field single-cycle terahertz pulses to gate interlayer coupling in La_1.84Sr_0.16CuO₄. We induce ultrafast oscillations between superconducting and resistive states and switch the plasmon response on and off, without reducing the density of Cooper pairs. In-plane superconductivity remains unperturbed, revealing a non-equilibrium state in which the dimensionality of the superconductivity is time-dependent. The gating frequency is determined by the electric field strength. Non-dissipative, bi-directional gating of superconductivity is of interest for device applications in ultrafast nanoelectronics and represents an example of how nonlinear terahertz physics can benefit nanoplasmonics and active metamaterials

Materials with strongly correlated electrons often show rich phase diagrams with dramatic differences in physical properties as doping, applied pressure, or magnetic fields are changed. Even subtle perturbations can cause colossal rearrangements in the electronic spectrum, and irradiation with light can be used to drive spectacular rearrangements in the structural, electronic, and magnetic properties. Here, we discuss the use of THz radiation to selectively excite one single degree of freedom at a time to drive a phase change. This is in contrast to what is done in most studies, which achieve photo-induced phase transitions by non-specific excitation in the visible spectral range. This chapter will combine a summary of developments in instrumentation for strong THz fields with some selected scientific applications of THz control of correlated electron systems

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