Impacting materials by light and seeing their structural dynamics

The emergence of the field of photo-induced transformation triggered by intense and ultra-short light pulse in a material has opened a new route to acting on matter. It combines many of today's issues such as far away from equilibrium phenomena, coherence and quantum control, complex correlations and non linear responsiveness, inter-conversion of energy and information. The advent of ultrafast structural dynamics motivated developing new tools to directly watch how matter works, by probing the time- and length-scales on which photo-induced processes take place. Different points will be briefly overviewed: (i) the development of laser pump and X-ray probe for watching ultrafast structural dynamics, (ii) the physical features resulting from this way of impacting materials by light, (iii) the presentation of some examples investigated by time-resolved X-ray diffraction and diffuse scattering experiments to illustrate the complex structural dynamics of a photo-induced transformation

PIPT from the Beginning to Future

The birth of the field of photoinduced phase transitions was strongly influenced by the conceptual viewpoint expressed by Professor Y. Toyozawa on the condensation of relaxed excitons. Since this first period, twenty years ago, this young field has been expanding rapidly along a diversity of directions. Nowadays, it goes hand in hand with the challenges of today's science: emergence, nonlinearity, coherence, far away from equilibrium, for example. The control of the functionality of a material via photoexcited states poses many new fundamental questions. Some of them will be overviewed: (i) the nature of the control parameters and the nature of the relevant collective variables, especially the order parameters, which characterize the evolution of the system, (ii) the difference between photoinduced transformations under continuous light irradiation and those resulting from an ultrashort laser pulse, (iii) the physical mechanisms of ultrafast photoinduced phase transitions from the formation and proliferation of phototransformed entities to the softening of a collective mode

100 ps time-resolved crystallographic investigation of the photoinduced phase transition in TFF-CA

We report the structural investigation of photoinduced phase transitions between ionic and neutral macroscopic states in an organic charge-transfer molecular material, with 100 picosecond resolution, using pulsed X-rays from a synchrotron. A femtosecond laser pulse switches the material within the nanosecond from neutral to ionic and also from ionic to neutral macroscopic state by virtue of a strong cooperativity. Data collections show that a long range ferroelectric order takes place in the ionic photoinduced phase. A two step mechanism is observed in the transformation from ionic to neutral. This associated with a disordering process of ionic strings followed by a 3D reorganisation at a macroscopic scale. Key words. Photo-induced phase transition, Time-resolved x-ray diffraction

Direct evidence of lattice-relaxed charge transfer exciton strings

We report first direct evidence, by X-ray diffuse scattering, of lattice-relaxed charge transfer exciton strings, which are one-dimensional self-trapped multiadjacent excitons. These non-linear excitations are key objects in quasi–one-dimensional organic compounds that undergo the neutral-ionic transition and are responsible for their physical properties. The one-dimensional fluctuations are very well described within the framework of the transfer integral method and physical parameters are deduced from both X-ray and dielectric measurements