Eric Collet

UNIVERSITY RENNES1

    •    E-mail :  eric [dot] colletuniv-rennes1 [dot] fr
    •    Phone :  +33 223236532

Prof Eric Collet is full professor at the Institute of Physics in Rennes. He is investigating photoinduced phase transitions, for which the structural and electronic changes are strongly coupled. Research topics concern photocrystallography, ultrafast time-resolved x-ray diffraction and ultrafast optical and x-ray spectroscopies. In addition to making use of experiments available in Rennes, he is using synchrotron, neutron and X-FEL large scale facilities. He is honorary member of the Institut Universitaire de France.
Pr Eric Collet received in June 2018 the Louis Ancel Price of the French Physical Society.
Lectures: advanced crystallography, phase transitions, Statistical Physics

Photoinduced phase transformations occur when a laser pulse impacts a material, thereby transforming its electronic and/or structural orders, consequently affecting the functionalities. The transient nature of photoinduced states has thus far severely limited the scope of applications. It is of paramount importance to explore whether structural feedback during the solid deformation has the capacity to amplify and stabilize photoinduced transformations. Contrary to coherent optical phonons, which have long been under scrutiny, coherently propagating cell deformations over acoustic timescales have not been explored to a similar degree, particularly with respect to cooperative elastic interactions. Herein we demonstrate, experimentally and theoretically, a self-amplified responsiveness in a spin-crossover material during its delayed volume expansion. The cooperative response at the material scale prevails above a threshold excitation, significantly extending the lifetime of photoinduced states. Such elastically driven cooperativity triggered by a light pulse offers an efficient route towards the generation and stabilization of photoinduced phases in many volume-changing materials.

We study the basic mechanisms allowing light to photoswitch at molecular scale a spin-crossover material from low- to high-spin state. Combined femtosecond x-ray absorption performed at LCLS X-FEL and optical spectroscopy reveal that the structural stabilization of the photoinduced HS 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

• R. Bertoni, M. Lorenc*, H. Cailleau, A. Tissot, J. Laisney, M.-L. Boillot, L. Stoleriu, A.Stancu, C. Enachescu, E. Collet*. Elastically driven cooperative response of a molecular material impacted by a laser pulse. Nature Materials 15, 606-610 (2016)
• E. Trzop, D. Zhang, L. Piñeiro-Lopez, F.J. Valverde-Muñoz, M. C. Muñoz, L. Palatinus, L. Guerin, H. Cailleau, J. A. Real* and E. Collet*
• First step towards Devil’s Staircase in Spin Crossover materials. Angewandte Chemie Int. Ed. (2016) selected as HOT PAPER
• M. Cammarata, R. Bertoni, M. Lorenc, H. Cailleau, S. Di Matteo, C. Mauriac, J.F. Létard, S. Matar, H. Lemke, M. Chollet, S. Ravy, C. Laulhé, E. Collet*. Sequential activation of molecular breathing and bending during spin-crossover       photoswitching revealed with femtosecond optical and x-ray absorption spectroscopy/ Physical Review Letters 113, 227402 (2014)
• E. Collet, H. Watanabe, N. Bréfuel, L. Palatinus, L. Roudaut, L. Toupet, K. Tanaka, J.-P. Tuchagues, P. Fertey, S. Ravy, B. Toudic, H. Cailleau. Aperiodic spin state ordering of bi-stable molecules and its photoinduced erasing. Phys. Rev. Lett. 109, 257206 (2012)