Sujet de thèse en cours : Contrôle des transitions de phase électroniques dans des matériaux moléculaires par phononique non linéaire

supervisor Maciej Lorenc (department of materials and light)sous la direction de Maciej Lorenc dans le département Matériaux et Lumièr

Ultrafast spectroscopy applied to photoinduced phase transition

National audienc

Ultrafast spectroscopy applied to photoinduced phase transition

National audienc

Ultrafast spectroscopy applied to photoinduced phase transition

National audienc

Coherent structural dynamics in V2O3 under hydrostatic pressure

International audienceRecently, material science has moved from observing and understanding electronic and structural orders to control physical properties of materials on command by using external stimuli such as light. From this point of view, a crucial aspect is the understanding of the fundamental interactions within the system (electron-electron and electron-phonon) to control the emergent cooperative effects. Here, we combine ultrafast optical spectroscopy and high-pressure setup to monitor the out-of-equilibirum dynamics of the material under well-defined controlled thermodynamical environnement. The obtained results demonstrate the use of spectroscopy of coherent phonon as a thermodynamical phase marker of the Insulator to Metal Transition in V2O3 thin film. More intriguing, the variation of frequency of the observed phonon optical mode ( A1g ) seems to reflect the manifestation of critical coupling between lattice and electronic degrees of freedom near transition line with a drop of frequency near the critical pressure

Dynamique photo-induite ultra-rapide de matériaux moléculaire sous pression.

International audienceRecently, material science has moved from observing and understanding electronic and structuralorders to control physical properties of materials on command by using external stimuli such as light.From this point of view, a crucial aspect is the understanding of the fundamental interactions withinthe system (electron-electron and electron-phonon) to control the emergent cooperative effects.Here, we combine ultrafast optical spectroscopy and high-pressure setup to monitor the out-of-equilibirum dynamics of the material under well-defined controlled thermodynamical environnement.The obtained results demonstrate the use of spectroscopy of coherent phonon as a thermodynamicalphase marker of the Metal to Insulator Transition in (EDO-TTF)SbF6

Symmetry-Resolved Study of Lattice Vibration and Libration Modes in [Fe(phen)2(NCS)2] Crystal

International audienceIn the family of spin-crossover materials, which undergo thermal conversion between low-spin (LS) and high-spin (HS) phases, it is of great interest to study vibrational modes. On the one hand, vibration modes are characteristic of the spin state, and vibrational spectroscopies are often used for monitoring spin-state switching driven by temperature, pressure, or light. On the other hand, spin-state thermal conversion is an entropy-driven process, and the vibrational entropy change represents the main contribution to the total entropy difference between LS and HS phases at solid state. However, the discussion of vibrations in spin-crossover materials is often limited at the molecular scale. Here we study vibration modes in the [Fe(phen)(2)(NCS)(2)] crystal, and we compare symmetry-resolved vibrational spectroscopy data performed on single crystal to density functional theory calculations performed in a periodic three-dimensional crystal. We discuss the complex nature of vibrational modes in crystals, including the vibration of molecules within the crystalline lattice, with different symmetries and frequencies. We also highlight the presence of many low-frequency libration modes of different symmetries. The contribution of vibrational entropy, added to the electronic entropy, provides a total entropy difference in the solid state, which is in very good agreement with calorimetric measurements

Decoupling with X-FEL electronic and structural dynamics for understanding the emergence of functions in photomagnetic materials

International audienc

Controlling materials with lattice vibrations

National audienc

Nonlinear Optical Absorption in Nanoscale Films Revealed through Ultrafast Acoustics

International audienceHerein we describe a novel spinning pump-probe photoacoustic technique developed to study nonlinear absorption in thin films. As a test case, an organic polycrystalline thin film of quinacridone, a well-known pigment, with a thickness in the tens of nanometers range, is excited by a femtosecond laser pulse which generates a time-domain Brillouin scattering signal. This signal is directly related to the strain wave launched from the film into the substrate and can be used to quantitatively extract the nonlinear optical absorption properties of the film itself. Quinacridone exhibits both quadratic and cubic laser fluence dependence regimes which we show to correspond to two- and three-photon absorption processes. This technique can be broadly applied to materials that are difficult or impossible to characterize with conventional transmittance-based measurements including materials at the nanoscale, prone to laser damage, with very weak nonlinear properties, opaque, or highly scattering