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Josephson Plasma Waves are linear electromagnetic modes that propagate along the planes of cuprate superconductors, sustained by oscillatory tunnelling supercurrents. These waves exhibit a plasmonic resonance in the GHz , to THz frequency range, depending on the interlayer coupling strength. For strong electromagnetic fields, the supercurrents approach their critical value and the electrodynamics become highly nonlinear , . Josephson Plasma Solitons (JPS) are breather excitations , that emerge in this regime, bound vortex/antivortex pairs that self-localize , , and propagate without dispersion. We experimentally excite such solitons using intense narrowband radiation from an infrared Free Electron Laser, tuned to the 2-THz Josephson Plasma Resonance of La1.84Sr0.16CuO4. The JPS is revealed by the appearance of long-lived transparency window in the spectral region immediately below the plasma resonance, which is opaque to small-amplitude waves. This reshaping of the optical properties originates from Fano-type coupling between probe waves and the JPS. Optical control of solitonic excitations in superconductors opens up possibilities in THz opto-plasmonics or in the manipulation of high-Tc superconductivity.

Here, we show that disruption of charge-density-wave (stripe) order by charge transfer excitation, enhances the superconducting phase rigidity in La1.885Ba0.115CuO4. Time-resolved resonant soft x-ray diffraction demonstrates that charge order melting is prompt following near-infrared photoexcitation whereas the crystal structure remains intact for moderate fluences. THz time-domain spectroscopy reveals that, for the first 2 ps following photoexcitation, a new Josephson plasma resonance edge, at higher frequency with respect to the equilibrium edge, is induced indicating enhanced superconducting interlayer coupling. Furthermore, the fluence dependence of the charge-order melting and the enhanced superconducting interlayer coupling are correlated with a saturation limit of ~0.5mJ/cm2. When using a combination of x-ray and optical spectroscopies we establish a hierarchy of timescales between enhanced superconductivity, melting of charge order, and rearrangement of the crystal structure.

Motivated by the appearance of optically induced superconductivity in the stripe-ordered La${}_{1.885}$Ba${}_{0.115}$CuO${}_{4}$ cuprate, this group uses time-resolved THz spectroscopy to map out the enhancement of interlayer superconducting coupling as a function of both temperature and time delay, for different pump wavelengths and fluences.

Static strain in complex oxide heterostructures has been extensively used to engineer electronic and magnetic properties at equilibrium. In the same spirit, deformations of the crystal lattice with light may be used to achieve functional control across hetero-interfaces dynamically. Here, by exciting large amplitude infrared-active vibrations in a LaAlO3 substrate we induce magnetic order melting in a NdNiO3 film across a hetero-interface. Femtosecond Resonant Soft X-ray Diffraction is used to determine the spatial and temporal evolution of the magnetic disordering. We observe a magnetic melt front that grows from the substrate interface into the film, at a speed that suggests electronically driven propagation. Light control and ultrafast phase front propagation at hetero-interfaces may lead to new opportunities in optomagnetism, for example by driving domain wall motion to transport information across suitably designed devices., Comment: 35 pages, 8 Figures (both incl. Supplement)

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 terahertz-frequency 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 near-infrared 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.

We report femtosecond resonant soft x-ray diffraction measurements of the dynamics of the charge order and of the crystal lattice in nonsuperconducting, stripe-ordered La1.875Ba0.125CuO4. Excitation of the in-plane Cu-O stretching phonon with a midinfrared pulse has been previously shown to induce a transient superconducting state in the closely related compound La1.675Eu0.2Sr0.125CuO4. In La1.875Ba0.125CuO4, we find that the charge stripe order melts promptly on a subpicosecond time scale. Surprisingly, the low temperature tetragonal (LTT) distortion is only weakly reduced, reacting on significantly longer time scales that do not correlate with light-induced superconductivity. This experiment suggests that charge modulations alone, and not the LTT distortion, prevent superconductivity in equilibrium.

Using ultrafast resonant soft x ray diffraction we demonstrate photoinduced melting of antiferromagnetic order in the correlated electron insulator NdNiO3. Time dependent analysis of the resonant diffraction spectra allows us to follow the temporal evolution of the charge imbalance between adjacent Ni sites. A direct correlation between the melting of magnetic order and charge rebalancing is found. Furthermore we demonstrate that the magnetic ordering on the Ni and Nd sites which are locked together in equilibrium become decoupled during this nonthermal process. © 2013 American Physical Society.

Time-resolved x-ray diffraction measures the dynamics of antiferromagnetic correlations by reconstructing the reciprocal-space scattering volume for the magnetic Bragg peak. Modifications in the scattering line shape along the three principal reciprocal lattice directions are measured.

Using time-resolved resonant soft x-ray diffraction, we measure the evolution of the full three-dimensional scattering volume of the antiferromagnetic superlattice reflection in the single-layer manganite La${}_{0.5}$Sr${}_{1.5}$MnO${}_{4}$on femtosecond time scales following photoexcitation. We find that the in-plane correlations are unchanged as a metastable state is entered, however there are subtle changes in the $c$-axis correlations. We observe a transient shift of the scattering ellipsoid along ($00L$) at very short times, and at longer time scales the short-range $c$-axis correlations are more robust than they are in equilibrium. Such results are not obtainable with any other techniques and hint at previously unresolved processes in the dynamics of photomelting in strongly correlated systems.

We used ultrafast resonant soft x-ray diffraction to probe the picosecond dynamics of spin and orbital order in La0:5Sr1:5MnO4 after photoexcitation with a femtosecond pulse of 1.5 eV radiation. Complete melting of antiferromagnetic spin order is evidenced by the disappearance of a ð 1 ; 1 ; 1 Þ diffraction peak. On the other hand, the ð 1 ; 1 ; 0Þ diffraction peak, reflecting orbital order, is only partially reduced. We interpret the results as evidence of destabilization in the short-range exchange pattern with no significant relaxation of the long-range Jahn-Teller distortions. Cluster calculations are used to analyze different possible magnetically ordered states in the long-lived metastable phase. Nonthermal coupling between light and magnetism emerges as a primary aspect of photoinduced phase transitions in manganites.

Saturated absorption and intervalley scattering in n-type semiconductors were observed using noncollinear THz-pump--THz probe spectroscopy with ultrabroadband probe pulses.