Signatures of photo-induced superconductivity have been reported in cuprate materials subjected to a coherent phonon drive. A 'cold' superfluid was extracted from the transient Terahertz conductivity and was seen to coexist with 'hot' uncondensed quasi-particles, a hallmark of a driven-dissipative system of which the interplay between coherent and incoherent responses are not well understood. Here, time resolved spontaneous Raman scattering was used to probe the lattice temperature in the photo-induced superconducting state of YBa2Cu3O6.48. An increase in lattice temperature of approximately 80 K was observed by measuring the time dependent Raman scattering intensity of an undriven 'spectator' phonon mode. This is to be compared with an estimated increase in quasi-particle temperatures of nearly 200 K. These temperature changes provide quantitative information on the nature of the driven state and its decay, and may provide a strategy to optimize this effect.

In cuprate high-T_{c} superconductors, resonant excitation of certain lattice vibrations has been shown to induce transient terahertz reflectivity features suggestive of nonequilibrium superconductivity above the critical temperature T_{c}. A microscopic mechanism for these observations is still lacking. Here, time-resolved measurements of scattering-angle- and polarization-dependent second-harmonic generation in driven YBa_{2}Cu_{3}O_{6+x} reveal a three-order-of-magnitude amplification of a 2.5-THz electronic mode, which is unique because of its symmetry, momentum, and temperature dependence. A theory for amplification of finite-momentum Josephson plasma polaritons, which are assumed to be well formed below T_{c} but incoherent throughout the pseudogap phase, explains all these observations. A theoretical solution for the Fresnel-Floquet reflection that starts from the coherently oscillating Josephson plasma polaritons provides a possible mechanism for the nonequilibrium superconductorlike terahertz reflectivity reported earlier. Beyond the immediate case of cuprates, this work underscores the role of nonlinear mode mixing to amplify fluctuating modes above the transition temperature in a wide range of materials.

We use resonant soft x-ray diffraction to track the photoinduced dynamics of the antiferromagnetic structure in a NdNiO3 thin film. Femtosecond laser pulses with a photon energy of 0.61 eV, resonant with electron transfer between long-bond and short-bond nickel sites, are used to excite the material and drive an ultrafast insulatormetal transition. Polarization-sensitive soft x-ray diffraction, resonant to the nickel L-3 edge, then probes the evolution of the underlying magnetic spiral as a function of time delay with 80 ps time resolution. By modeling the azimuthal dependence of the scattered intensity for different linear x-ray polarizations, we benchmark the changes of the local magnetic moments and the spin alignment. The measured changes are consistent with a reduction of the long-bond site magnetic moments and an alignment of the spins towards a more collinear structure at early time delays.

Optical excitation in the cuprates has been shown to induce transient superconducting correlations above the thermodynamic transition temperature T-C, as evidenced by the terahertz-frequency optical properties in the nonequilibrium state. In YBa2Cu3O6+x, this phenomenon has so far been associated with the nonlinear excitation of certain lattice modes and the creation of new crystal structures. In other compounds, like La2-xBaxCuO4, similar effects were reported also for excitation at near-infrared frequencies, and were interpreted as a signature of the melting of competing orders. However, to date, it has not been possible to systematically tune the pump frequency widely in any one compound, to comprehensively compare the frequency-dependent photosusceptibility for this phenomenon. Here, we make use of a newly developed nonlinear optical device, which generates widely tunable high-intensity femtosecond pulses, to excite YBa2Cu3O6.5 throughout the entire optical spectrum (3-750 THz). In the far-infrared region (3-24 THz), signatures of nonequilibrium superconductivity are induced only for excitation of the 16.4- and 19.2-THz vibrational modes that drive c-axis apical oxygen atomic positions. For higher driving frequencies (25-750 THz), a second resonance is observed around the charge transfer band edge at approximately 350 THz. These findings highlight the importance of coupling to the electronic structure of the CuO2 planes, mediated either by a phonon or by charge transfer.

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. The fluence dependence of the charge-order melting and the enhanced superconducting interlayer coupling are correlated with a saturation limit of similar to 0.5 mJ/cm(2). 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.

We discuss the mechanism of coherent phonon generation by Stimulated Ionic Raman Scattering, a process different from conventional excitation with near visible optical pulses. Ionic Raman scattering is driven by anharmonic coupling between a directly excited infrared-active phonon mode and other Raman modes. We experimentally study the response of YBa2Cu3O6+x to the resonant excitation of apical oxygen motions at 20 THz by mid-infrared pulses, which has been shown in the past to enhance the interlayer superconducting coupling. We find coherent oscillations of four totally symmetric (Ag) Raman modes and make a critical assessment of the role of these oscillatory motions in the enhancement of superconductivity.

We use femtosecond resonant soft x-ray diffraction to measure the optically stimulated ultrafast changes of charge density wave correlations in underdoped YBa2Cu3O6.6. We find that when coherent interlayer transport is enhanced by optical excitation of the apical oxygen distortions, at least 50% of the in-plane charge density wave order is melted. These results indicate that charge ordering and superconductivity may be competing up to the charge ordering transition temperature, with the latter becoming a hidden phase that is accessible only by nonlinear phonon excitation.

We present a pulse-shaping scheme operating in the mid-infrared (MIR) wavelength range up to 20 μm. The spectral phase is controlled by a specially designed large stroke 32-actuator deformable mirror in a grating-based 4f configuration. We demonstrate the shaper capability of compressing the MIR pulses, imparting parabolic and third-order spectral phases and splitting the spectral content to create two independent pulses.

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.

We investigate the order parameter dynamics of the stripe-ordered nickelate, La(1.75)Sr(0.25)NiO(4), using time-resolved resonant x-ray diffraction. In spite of distinct spin and charge energy scales, the two order parameters' amplitude dynamics are found to be linked together due to strong coupling. Additionally, the vector nature of the spin sector introduces a longer reorientation time scale which is absent in the charge sector. These findings demonstrate that the correlation linking the symmetry-broken states does not unbind during the nonequilibrium process, and the time scales are not necessarily associated with the characteristic energy scales of individual degrees of freedom.

By measuring time-dependent optical birefringence with 4-femtosecond (fs) laser pulses, we determine the time scale for photoinduced melting of orbital order in the single-layer manganite La0.5Sr1.5MnO4. Such high time resolution is required to distinguish atomic motions that control the Jahn-Teller distortion from even faster electronic rearrangements. The experiment reveals an 18 fs bottleneck for the loss of orbital order, corresponding to about one-quarter period of the in-plane Jahn-Teller mode. Furthermore, we observe coherent oscillations of this Jahn-Teller mode. Both their amplitude and the birefringence drop exhibit a threshold in their fluence dependence, indicating cooperativity in the lattice response.

A charge modulation has recently been reported in (Y,Nd)Ba2Cu3O6+x [G. Ghiringhelli et al., Science 337, 821 (2012)]. Here we report Cu L-3 edge soft x-ray scattering studies comparing the lattice modulation associated with the charge modulation in YBa2Cu3O6.6 with that associated with the well-known charge and spin stripe order in La1.875Ba0.125CuO4. We find that the correlation length in the CuO2 plane is isotropic in both cases, and is 259 +/- 9 angstrom for La1.875Ba0.125CuO4 and 55 +/- 15 angstrom for YBa2Cu3O6.6. Assuming weak interplanar correlations of the charge ordering in both compounds, we conclude that the order parameters of the lattice modulations in La1.875Ba0.125CuO4 and YBa2Cu3O6.6 are of the same order of magnitude.

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.

The nonlinear lattice dynamics of La0.7Sr0.3MnO3, as initiated by strong mid-infrared femtosecond pulses made resonant with a specific lattice vibration, are measured with ultrafast X-ray diffraction at the LCLS free electron laser. Our experiments show that large amplitude excitation of an infrared-active stretching mode leads also to a displacive motion along the coordinate of a second, anharmonically coupled, Raman mode. This rectification of the vibrational field is described within the framework of the Ionic Raman Scattering theory and explains how direct lattice excitation in the nonlinear regime can induce a structural phase transition.

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.

Femtosecond midinfrared pulses are used to directly excite the lattice of the single-layer manganite La${}_{0.5}$Sr${}_{1.5}$MnO${}_{4}$. Magnetic and orbital orders, as measured by femtosecond resonant soft x-ray diffraction with an x-ray free-electron laser, are reduced within a few picoseconds. This effect is interpreted as a displacive exchange quench, a prompt shift in the equilibrium value of the magnetic- and orbital-order parameters after the lattice has been distorted. Control of magnetism through ultrafast lattice excitation may be of use for high-speed optomagnetism.

Radiation pulse from a Self-Amplified Spontaneous Emission Free Electron Laser (SASE FEL) consists out of spikes (wavepackets). Energy loss in the electron beam (averaged over radiation wavelength) also exhibit spiky behavior on a typical scale of coherence length, and follows the radiation pulse envelope. These modulations of the electron beam energy are converted into large density (current) modulations on the same temporal scale with the help of a dispersion section, installed behind the x-ray undulator. Powerful optical radiation is then generated with the help of a dedicated radiator (afterburner). We have recently demonstrated this principle at the Free Electron Laser in Hamburg (FLASH), and this paper present the first experimental results.