Nicolas Moisan

Ultrafast spin-state photoswitching in a crystal and slower consecutive processes investigated by femtosecond optical spectroscopy and picosecond X-ray diffractionw

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.

Tuning a Schottky barrier in a photoexcited topological insulator with transient Dirac cone electron-hole asymmetry

The advent of Dirac materials has made it possible to realize two-dimensional gases of relativistic fermions with unprecedented transport properties in condensed matter. Their photoconductive control with ultrafast light pulses is opening new perspectives for the transmission of current and information. Here we show that the interplay of surface and bulk transient carrier dynamics in a photoexcited topological insulator can control an essential parameter for photoconductivity-the balance between excess electrons and holes in the Dirac cone. This can result in a strongly out of equilibrium gas of hot relativistic fermions, characterized by a surprisingly long lifetime of more than 50 ps, and a simultaneous transient shift of chemical potential by as much as 100 meV. The unique properties of this transient Dirac cone make it possible to tune with ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that is impossible with conventional optoelectronic materials.

Multi-phonon dynamics of the ultra-fast photoinduced transition of (EDO-TTF)2SbF6

We report here the first observation of the photoinduced insulating-to-metal phase transition in the (EDO-TTF)2SbF6 salt, which occurs on the picosecond time-scale. The time-resolved optical experiments performed with 80 fs time-resolution demonstrate that the dynamical process involves several low-frequency phonons, as the crystalline structure is destabilized upon laser excitation.

Phase transition in (EDO-TTF)2PF6: domain growth in the thermal hysteresis and ultra-fast photoinduced effects

The first order phase transition between the metal (M) and insulating (I) phases of the molecular compound (EDO-TTF)2PF6 is investigated by single crystal x-ray diffraction. The coexistence of the insulating and metallic phases and the growth of the domains in the thermal hysteresis are clearly observed during the phase transition. We also present ultra-fast optical experiments using a nitrogen gas flow cryostat. A change of the reflectivity in the photoinduced phase was observed just after excitation. We will also discuss the influence of the excitation light polarisation on the efficiency of the photo-induced I–M phase transition.