Complex oxides offer a wide range of functional properties, and recent advances in fabrication of freestanding membranes of these oxides are adding new mechanical degrees of freedom to this already rich functional ecosystem. Here, we demonstrate photoactuation in freestanding thin film resonators of ferroelectric Barium Titanate (BaTiO3) and paraelectric Strontium Titanate (SrTiO3). The free-standing films, transferred onto perforated supports, act as nano-drums, oscillating at their natural resonance frequency when illuminated by a frequency-modulated laser. The light-induced deflections in the ferroelectric BaTiO3 membranes are two orders of magnitude larger than in the paraelectric SrTiO3 ones. Time-resolved X-ray micro-diffraction under illumination and temperature-dependent and holographic interferometry provide combined evidence for the photostrictive strain in BaTiO3 originating from partial screening of ferroelectric polarization by photo-excited carriers, which decreases the tetragonality of the unit cell. These findings showcase the potential of photostrictive freestanding ferroelectric films as wireless actuators operated by light.

Superlattices are materials created by the alternating growth of two chemically different materials. The direct consequence of creating a superlattice is the folding of the Brillouin zone which gives rise to additional electronic bands and phonon modes. This has been successfully exploited to achieve new transport and optical properties in semiconductor superlattices, for example. Here, we show that multiferroic BiFeO$_3$/LaFeO$_3$ superlattices are more than just periodic chemical stacking. Using transmission electron microscopy, X-ray diffraction and first-principles calculations, we demonstrate the existence of a new order of FeO$_6$ octahedra, with a period along the growth direction about twice that of the chemical supercell, i.e. a superorder. The effect of this new structural order on the lattice dynamics is studied with ultrafast optical pump-probe experiments. While a mode at 1.2 THz is attributed solely to the chemical modulation of the superlattice, the existence of another 0.7 THz mode seems to be explained only by a double Brillouin zone folding in agreement with the structural description. Our work shows that multiferroic BiFeO$_3$/LaFeO$_3$ superlattices can be used to tune the spectrum of coherent THz phonons, and potentially that of magnons or electromagnons.

Ultrashort light pulses induce rapid deformations of crystalline lattices. In ferroelectrics, lattice deformations couple directly to the polarization, which opens the perspective to modulate the electric polarization on an ultrafast time scale. Here, we report on the temporal and spatial tracking of strain and polar modulation in a single-domain BiFeO 3 thin film by ultrashort light pulses. To map the light-induced deformation of the BiFeO 3 unit cell, we perform time-resolved optical reflectivity and time-resolved x-ray diffraction. We show that an optical femtosecond laser pulse generates not only longitudinal but also shear strains. The longitudinal strain peaks at a large amplitude of 0.6%. The access of both the longitudinal and shear strains enables to quantitatively reconstruct the ultrafast deformation of the unit cell and to infer the corresponding reorientation of the ferroelectric polarization direction in space and time. Our findings open new perspectives for ultrafast manipulation of strain-coupled ferroic orders.

The understanding of the lattice dynamics in ferroic compounds driven by an ultrashort light pulse is an exciting research direction due to the exceptional non-linear properties (optical, elastic, electric and magnetic) of ferroic and multiferroic materials. Photo-induced strain in ferroic materials is driven by a complex interplay between charge, phonon and spin dynamics with microscopic mechanisms that still need to be elucidated. We present recent experiments where ultrafast photoinduced strain is evaluated in BiFeO3-based multiferroic materials, with a focus on the description of the ultrafast symmetry change of the unit-cell that appears after an ultrashort laser pulse. A combination of optical and X-ray time-resolved techniques will be presented. We show that it is possible to modulate at the picosecond time scale the ferroic order by playing with the out-of-plane and in-plane light-induced strains. These new results provide new insights for the understanding of the physics of photo-induced strain, in relation with the light-induced ferroelectric modulation in nanostructured ferroic compounds and could be the first step towards their use as on-purpose ferroic architectures in devices like actuators or modulators with ultra-short light pulses.

A new photon-counting camera based on hybrid pixel technology has been developed at the SOLEIL synchrotron, making it possible to implement pump-probe-probe hard X-ray diffraction experiments for the first time. This application relies on two specific advantages of the UFXC32k readout chip, namely its high frame rate (50 kHz) and its high linear count rate (2.6 × 106 photons s-1 pixel-1). The project involved the conception and realization of the chips and detector carrier board, the data acquisition system, the server with its specific software, as well as the mechanical and cooling systems. This article reports on in-laboratory validation tests of the new detector, as well as on tests performed at the CRISTAL beamline within the targeted experimental conditions. A benchmark experiment was successfully performed, showing the advantages of the pump-probe-probe scheme in correcting for drifts of the experimental conditions.

We investigate the orthorhombic distortion and the structural dynamics of epitaxial MnAs layers on GaAs(001) using static and time-resolved x-ray diffraction. Laser-induced intensity oscillations of Bragg reflections allow us to identify the optical phonon associated with orthorhombic distortion and to follow its softening along the path towards an undistorted phase of hexagonal symmetry. The frequency of this mode falls in the THz range, in agreement with recent calculations. Incomplete softening suggests that the beta - gamma transformation deviates from a purely second-order displacive transition.

This paper presents the performance of a single-photon-counting hybrid pixel X-ray detector with synchrotron radiation. The camera was evaluated with respect to time-resolved experiments, namely pump-probe-probe experiments held at SOLEIL. The UFXC camera shows very good energy resolution of around 1.5 keV and allows the minimum threshold setting to be as low as 3 keV keeping the high-count-rate capabilities. Measurements of a synchrotron characteristic filling mode prove the proper separation of an isolated bunch of photons and the usability of the detector in time-resolved experiments.

The investigation of ultrafast dynamics, taking place on the few to sub-picosecond time scale, is today a very active research area pursued in a variety of scientific domains. With the recent advent of X-ray free-electron lasers (XFELs), providing very intense X-ray pulses of duration as short as a few femtoseconds, this research field has gained further momentum. As a consequence, the demand for access strongly exceeds the capacity of the very few XFEL facilities existing worldwide. This situation motivates the development of alternative sub-picosecond pulsed X-ray sources among which femtoslicing facilities at synchrotron radiation storage rings are standing out due to their tunability over an extended photon energy range and their high stability. Following the success of the femtoslicing installations at ALS, BESSY-II, SLS and UVSOR, SOLEIL decided to implement a femtoslicing facility. Several challenges were faced, including operation at the highest electron beam energy ever, and achievement of slice separation exclusively with the natural dispersion function of the storage ring. SOLEIL's setup also enables, for the first time, delivering sub-picosecond pulses simultaneously to several beamlines. This last feature enlarges the experimental capabilities of the facility, which covers the soft and hard X-ray photon energy range. In this paper, the commissioning of this original femtoslicing facility is reported. Furthermore, it is shown that the slicing-induced THz signal can be used to derive a quantitative estimate for the degree of energy exchange between the femtosecond infrared laser pulse and the circulating electron bunch.

Spin(SDW) and Charge-density waves (CDW) systems are ubiquitous states in solid state physics. They both correspond to a modulation of the spinor the charge-density, with twice the Fermi wave vector of the electron gas. Both states are gapped, which make them sensitive to impulsive absorption of laser infrared pulses. Interestingly enough, CDW are generally coupled to the lattice, which make them easy to observe by X-ray diffraction. In this work, we compare the dynamical behavior of CDW after an infrared laser pulse in three different DW systems: Chromium [1], K0.3MoO3 (so-called blue bronze)[2], and 1T-TaS2[3]. In all three cases, the CDW is strongly depressed after the pulse in an ultrafast way, which could lead either to a melting of the state, or to a photo-induced phase transition towards another CDW state. In the IT-TaS2 case, it has been possible to watch the birth of a CDW phase with a nucleationgrowth-coarsening process [3]. The recovery of the initial CDW state follows different mechanisms which depend on the compound. [1] Jacques, V.L.R. et al. (2016). Phys. Rev. Lett. 117, 156401. [2] Huber, T. et al. (2014). Phys. Rev. Lett. 113, 026401. [3] Laulhé, C. et al. (2015). Physica B. 460, 100 ; (2017). In preparation.

Spin-(SDW) and Charge-density waves (CDW) systems are ubiquitous states in solid state physics.They both correspond to a modulation of the spin-or the charge-density, with twice the Fermi wave vector of the electron gas.Both states are gapped, which make them sensitive to impulsive absorption of laser infrared pulses.Interestingly enough, CDW are generally coupled to the lattice, which make them easy to observe by X-ray diffraction.In this work, we compare the dynamical behavior of CDW after an infrared laser pulse in three different DW systems: Chromium [1], K0.3MoO3 (so-called blue bronze)[2], and 1T-TaS2 [3].In all three cases, the CDW is strongly depressed after the pulse in an ultrafast way, which could lead either to a melting of the state, or to a photo-induced phase transition towards another CDW state.In the IT-TaS2 case, it has been possible to watch the birth of a CDW phase with a nucleationgrowth-coarsening process [3].The recovery of the initial CDW state follows different mechanisms which depend on the compound.

We have investigated by time-resolved X-ray diffraction the manganese arsenide structure dynamics in MnAs/GaAs (001) epitaxial films, when the α → β magnetostructural phase transition is triggered by an ultra-fast optical laser pulse. Understanding the photo-induced phase transition dynamics is relevant for applications of MnAs/GaAs (001) as a magnetically active template in laser-induced magnetization switching processes. The main goal of our experiment is to determine the timescales of the structural changes that are likely to play a role in laser-driven magnetization reversal.

T-TaS2 is a 2D metallic compound which undergoes a series of electronically driven phase transitions toward charge density wave and Mott phases. Its intricate electron-phonon interactions and electron- electron correlations have been promising peculiar out-of-equilibrium dynamics. In this paper, we provide the first direct information on the atomic structure response to an ultra-fast infrared laser pulse in the commensurate phase of 1T-TaS2, by using femtosecond time-resolved X-ray diffraction. We show that ultra-fast excitation with near-infrared photons drives a displacive excitation of the amplitude mode of the commensurate charge density wave. About 3 ps after laser excitation, the system reaches a new, photo-induced state that is maintained for at least 10 ps. We give evidence that this long-lived state exhibits the same structural modulation as in the thermodynamically stable commensurate phase, with a large correlation length. Only the average amplitude of the modulation is found to decrease. We propose that the long-lived state is formed from the commensurate phase by reducing the modulation amplitude on few superlattice nodes. The underlying mechanism proposed is the annihilation of self-trapped po- larons.

The femtoslicing project at SOLEIL is currently under commissioning. It will enable to serve several beamlines with 100 fs FWHM long pulses of soft and hard X-rays with reasonable flux and with a 1 kHz repetition rate. It is based on the interaction of a femtosecond Ti:Sa laser with electrons circulating in the magnetic field of a modulator wiggler, that provides the electron beam energy modulation on the length scale of the laser pulse. The optimization of the interaction is performed using two dedicated diagnostics stations. The first one, operating in the Infra-Red (IR) is installed in the tunnel and allows the adjustment of the temporal, spectral and spatial overlap between the laser and the electron beam. The second one, located in the IR-THz AILES beamline, measures the intensity of the terahertz (THz) radiation emitted by the local dip structure produced in the core electron beam after interaction. This second setup provides refined optimization of the interaction. This paper describes the layout of these diagnostics and gives first results and characterization of the slicing experiment at SOLEIL.

The aim of the Femto-Slicing project at SOLEIL is to generate 100 - 200 fs FWHM short X-ray pulses on two beamlines, CRISTAL and TEMPO, for pump-probe experiments in the spectral range of hard and soft X-rays. We note that this capability could be extended in the future to two or three more beamlines. Femtosecond lasers are currently in operation on TEMPO and CRISTAL beamlines, for pump-probe experiments on the ps time scale, enabling time resolved photo-emission and photo-diffraction studies, respectively. The Femto-Slicing project is based on the fs laser of the CRISTAL beamline, which can be adjusted to deliver 3 to 5 mJ pulses of 30 fs duration at 2.5 to 1 kHz, respectively. This laser beam is separated in three branches: one delivering about 2 mJ to the modulator wiggler and the other ones delivering the remaining energy to the experiments on the TEMPO and CRISTAL beamlines. This layout will yield natural synchronisation between Infra-Red (IR) laser pump and X-ray probe pulses, only affected by jitter and drift associated with beam transport. In this paper, we present the progress in the implementation and commissioning of the laser beam transport system and its characterization.

We study the basic mechanisms allowing light to photoswitch at the molecular scale a spin-crossover material from a low- to a high-spin state. Combined femtosecond x-ray absorption performed at LCLS X-FEL and optical spectroscopy reveal that the structural stabilization of the photoinduced high-spin state results from a two step structural trapping. Molecular breathing vibrations are first activated and rapidly damped as part of the energy is sequentially transferred to molecular bending vibrations. During the photoswitching, the system follows a curved trajectory on the potential energy surface.

We present a room temperature high-pressure X-ray diffraction study of the layered compound 1T-TaS2 up to 20 GPa. This material is known to exhibit a variety of structural phase transitions that are ascribed to the stabilization of charge density wave states. It has been recently shown that at pressures larger than 3 GPa and up to 25 GPa, 1T-TaS2 becomes superconductor below about 5 K. It was suggested that this superconductivity coexists with different CDW states, an hypothesis that can be tested by X-ray diffraction. Our first results at room temperature show that at around 1.9 GPa, the nearly-commensurate (NCCDW) phase transforms into a phase similar to the high temperature incommensurate phase (ICCDW). Above 9 GPa, we show the existence of another IC phase, still discernable up to 20 GPa despite the pressure-induced crystal damage above 13 GPa. These results are consistent with resistivity measurements, but call for a complete exploration of the P–T phase diagram of 1T-TaS2.

We report the spin state photo-switching dynamics in two polymorphs of a spin-crossover molecular complex triggered by a femtosecond laser flash, as determined by combining femtosecond optical pump-probe spectroscopy and picosecond X-ray diffraction techniques. The light-driven transformations in the two polymorphs are compared. Combining both techniques and tracking how the X-ray data correlate with optical signals allow understanding of how electronic and structural degrees of freedom couple and play their role when the switchable molecules interact in the active crystalline medium. The study sheds light on crossing the border between femtochemistry at the molecular scale and femtoswitching at the material scale.