BGO relaxation dynamics probed with heterodyne detected optical transient gratings

We used optical laser pulses to create transient gratings (TGs) with sub-10 {\mu}m spatial periodicity in a Bismuth Germanate (310) (Bi4Ge3O12) single crystal at room temperature. The TG launches phonon modes, whose dynamics were revealed via forward diffraction of a third, time-delayed, heterodyne-detected optical pulse. Acoustic oscillations have been clearly identified in a time-frequency window not covered by previous spectroscopic studies and their characteristic dynamic parameters have been measured as a function of transferred momenta magnitude and direction.Comment: 6 pages, 4 figure

Stimulated Brillouin scattering at 1 nm-1 wavevector by extreme ultraviolet transient gratings

We crossed two femtosecond extreme ultraviolet (EUV) pulses in a beta - Ga2O3 (001) single crystal to create transient gratings (TG) of light intensity with sub-100 nm spatial periodicity. The EUV TG excitation launches phonon modes, whose dynamics were revealed via the backward diffraction of a third, time-delayed, EUV probe pulse. In addition to the modes typically observed in this kind of experiment, the phase-matching condition imposed by the TG, combined with the sharp penetration depth of the EUV excitation pulses, permitted to generate and detect phonons with a wavevector tangibly larger (approximately 1 nm-1) than the EUV TG one, via stimulated Brillouin back-scattering (SBBS) of the EUV probe. While SBBS of an optical probe was reported in previous EUV TG experiments, the extension of SBBS to short wavelength radiation can be used as a contact-less experimental tool for filling the gap between the wavevector range accessible through inelastic hard X-ray and thermal neutron scattering techniques, and the one accessible through Brillouin scattering of visible and UV light.Comment: 7 pages, 3 figure

Transient grating spectroscopy on a DyCo5_5 thin film with femtosecond extreme ultraviolet pulses

Surface acoustic waves (SAWs) are excited by femtosecond extreme ultraviolet (EUV) transient gratings (TGs) in a room-temperature ferrimagnetic DyCo$_5$ alloy. TGs are generated by crossing a pair of EUV pulses from a free electron laser (FEL) with the wavelength of 20.8\,nm matching the Co $M$-edge, resulting in a SAW wavelength of $\Lambda=44$\,nm. Using the pump-probe transient grating scheme in a reflection geometry the excited SAWs could be followed in the time range of -10 to 100\,ps in the thin film. Coherent generation of TGs by ultrafast EUV pulses allows to excite SAW in any material and to investigate their couplings to other dynamics such as spin waves and orbital dynamics

Ultrafast dynamics in (TaSe4)2I triggered by valence and core-level excitation

Dimensionality plays a key role in the emergence of ordered phases, such as charge density-waves (CDW), which can couple to, and modulate, the topological properties of matter. In this work, we study the out-of-equilibrium dynamics of the paradigmatic quasi-one-dimensional material (TaSe4)2I, which exhibits a transition into an incommensurate CDW phase when cooled to just below room temperature, namely at TCDW = 263 K. We make use of both optical laser and free-electron laser (FEL) based time-resolved spectroscopies in order to study the effect of a selective excitation on the normal-state and on the CDW phases by probing the near-infrared/visible optical properties both along and perpendicularly to the direction of the CDW, where the system is metallic and insulating, respectively. Excitation of the core-levels by ultrashort X-ray FEL pulses at 47 eV and 119 eV induces reflectivity transients resembling those recorded when only exciting the valence band of the compound - by near-infrared pulses at 1.55 eV - in the case of the insulating sub-system. Conversely, the metallic sub-system displays relaxation dynamics which depend on the energy of photo-excitation. Moreover, excitation of the CDW amplitude mode is recorded only for excitation at a low-photon-energy. This fact suggests that the coupling of light to ordered states of matter can predominantly be achieved when directly injecting delocalized carriers in the valence band, rather than localized excitations in the core levels. Complementing this, table-top experiments allow us to prove the quasi-unidirectional nature of the CDW phase in (TaSe4)2I, whose fingerprints are detected along its c-axis only. Our results provide new insights into the symmetry of the ordered phase of (TaSe4)2I perturbed by a selective excitation, and suggest a novel approach based on complementary table-top and FEL spectroscopies for the study of complex materials

Nanoscale transient polarization gratings

We present the generation of transient polarization gratings at the nanoscale, achieved using a tailored accelerator configuration of the FERMI free electron laser. We demonstrate the capabilities of such a transient polarization grating by comparing its induced dynamics with the ones triggered by a more conventional intensity grating on a thin film ferrimagnetic alloy. While the signal of the intensity grating is dominated by the thermoelastic response of the system, such a contribution is suppressed in the case of the polarization grating. This exposes helicity-dependent magnetization dynamics that have so-far remained hidden under the large thermally driven response. We anticipate nanoscale transient polarization gratings to become useful for the study of any physical, chemical and biological systems possessing chiral symmetry

Espressione delle potenzialit\ue0 dei Laser ad elettroni liberi attraverso esperimenti innovativi

The advent of X-Ray Free Electron Lasers (FELs) has opened a new era for exploring the fundamental properties of matter. These machines are the 4th generation light sources and combine the exceptional properties of conventional lasers and synchrotrons, allowing to probe the ultra-fast dynamics of atoms and molecules in simple and complex systems at a nano-scale level. This thesis work is divided in two main branches: at first, it reviews the basic theory behind FELs and points out possible improvement of these sources. Secondly, it presents some results obtained during new and peculiar experiments performed between 2017 and 2020 in different FELs facilities while showing how further improvements are needed in order to accomplish state of art results unreachable with the current state of synchrotrons and FELs. In Chapter 1, the reader is introduced to the charged particle accelerator theory culminating with its application on one of the main component of a FEL: the undulator. In order to provide its special radiation output, a FEL light source makes use of interactions between charged particles and a self-generated electromagnetic radiation inside an undulator. To let the reader easily understand the mechanism behind the radiation generated and its properties, this thesis opens with an approach to the laws of physics that rule the dynamics of the particles. Coupling the Lorentz equation with Maxwell equations we show the collective dynamics of electrons throughout the all FEL structure in Chapter 2. In Chapter 3 we present the two main places where this work of thesis has been built: the FERMI FEL in Italy and the SwissFEL machine based at the Paul Scherrer Institute in Switzerland. Here we explore the secrets of their layouts, beamlines and main experiment performed. Once the reader mastered the fundamental properties of a FEL, we show the main results achieved during this research period at the FERMI FEL and at the SwissFEL. In Chapter 4 we present the very first successfull attempt at X-Ray Transient Grating Spectroscopy on Bismuth Germanate Oxide. In order to confirm some of the results obtained through the X-ray Transient Grating technique, a few satellite measurements and simulations were also performed on the Bismuth Germanate Oxide sample, which are presented in Chapter 5. In Chapter 6 we present what we called a Real-time visualization of ibuprofen dimer vibrations with element- and enantiomeric- selectivity: here we exploit the potential of FELs through an ultrafast soft X-Ray absorption experiment that allows the visualization and disentangling of several low-frequency and near lying vibrational modes, involving specific carbon atoms in a racemic mixture of Ibuprofen. In Chapter 7 we explore the world of soft matter: here we introduce the problematics affecting the protein structure research by showing the main re- results obtained at two synchrotron facilities. Afterward, given the results we obtained, we explore the possibility to overcome such problems by introducing the Coherent and Incoherent Diffraction Imaging techniques at FELs combined to a new approach on protein bidimensional crystallization. Furthermore, we introduce a rather similar approach to a technique aiming at Catching Conical Intersection through Electronic Coherence and Noise Correlation Spectroscopy: we explore the possibility to use the TRUECARS technique to directly visualize the conical intersection dynamics, which regulates the photo-chemestry of most molecules. Chapter 8 opens with an introduction on Resonant Inelastic X-ray Spettroscopy (RIXS). Here we show the results of the application of RIXS exploiting the Ultrafast charge trapping dynamics in Cu2O in-gap states: we investigated the charge trapping dynamics in the defect-related in-gap states of Cu2O, following photoexcitation across the band gap. Finally, the thesis culminates with the Design of a soft X-Rays RIXS spectrometer for the SwissFEL's new beamline FURKA. \u200

The effect of echoes interference on phonon attenuation in a nanophononic membrane

Abstract Nanophononic materials are characterized by a periodic nanostructuration, which may lead to coherent scattering of phonons, enabling interference and resulting in modified phonon dispersions. We have used the extreme ultraviolet transient grating technique to measure phonon frequencies and lifetimes in a low-roughness nanoporous phononic membrane of SiN at wavelengths between 50 and 100 nm, comparable to the nanostructure lengthscale. Surprisingly, phonon frequencies are only slightly modified upon nanostructuration, while phonon lifetime is strongly reduced. Finite element calculations indicate that this is due to coherent phonon interference, which becomes dominant for wavelengths between ~ half and twice the inter-pores distance. Despite this, vibrational energy transport is ensured through an energy flow among the coherent modes created by reflections. This interference of phonon echos from periodic interfaces is likely another aspect of the mutual coherence effects recently highlighted in amorphous and complex crystalline materials and, in this context, could be used to tailor transport properties of nanostructured materials

Three-dimensional coherent diffraction snapshot imaging using extreme ultraviolet radiation from a free electron laser

The possibility to obtain a three-dimensional representation of a single object with sub-μm resolution is crucial in many fields, from material science to clinical diagnostics. This is typically achieved through tomography, which combines multiple two-dimensional images of the same object captured at different orientations. However, this serial imaging method prevents single-shot acquisition in imaging experiments with free electron lasers. In the present experiment, we report on a new approach to 3D imaging using extreme-ultraviolet radiation. In this method, two EUV pulses hit simultaneously an isolated 3D object from different sides, generating independent coherent diffraction patterns, resulting in two distinct bidimensional views obtained via phase retrieval. These views are then used to obtain a 3D reconstruction using a ray tracing algorithm. This EUV stereoscopic imaging approach, similar to the natural process of binocular vision, provides sub-μm spatial resolution and single-shot capability. Moreover, ultrafast time resolution and spectroscopy can be readily implemented, and a further extension to X-ray wavelengths can be envisioned as well

Hard X-ray transient grating spectroscopy on bismuth germanate

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