Small Atomic displacements Recorded in Bismuth by the Optical Reflectivity of Femtosecond Laser-Pulse Excitations

Subtle atomic motion in a Bi crystal excited by a 35 fs-laser pulse has been recovered from the transient reflectivity of an optical probe measured with an accuracy of 10-5. Analysis shows that a novel effect reported here-an initial negative drop in reflectivity-relates to a delicate coherent displacement of atoms by the polarization force during the pulse. We also show that reflectivity oscillations with a frequency coinciding with that of cold Bi are related to optical phonons excited by the electron temperature gradient through electron-phonon coupling

Photoinduced phenomena and structural analysis associated with the spin-state switching in the [FeII(DPEA)(NCS)2] complex

International audienceOut-of-equilibrium photoinduced switching from the low-spin to the high-spin state has been investigated on the iron(II) complex [Fe(II)(DPEA)(NCS)2] by both optical reflectivity and magnetic measurements under continuous light irradiation at low temperature. The photoinduced HS state can be observed up to 47 K and the relaxation process has been followed. Structural changes of both the temperature- and the photoinduced spin-state switching have been analyzed in detail by x-ray diffraction indicating no change of symmetry. Short intermolecular contacts and intramolecular deformations associated with the change of molecular spin state have been quantified. Actually a crossover behavior is observed at thermal equilibrium with however a quasiabrupt shape indicating significant cooperative effects. These aspects are compared between the temperature- and photoinduced spin crossovers

Effets photo-induits coopératifs: du photomagnétisme sous irradiation continue aux phénomènes ultrarapides- étude par spectroscopie et diffraction X.

The control with ultra-short laser pulses of the collective and concerted transformation of molecules driving a macroscopic state switching on an ultra-fast time scale in solid state opens new prospects in materials science. The goal is to realize at the material level what happens at the molecular level in femtochemistry. These processes are highly cooperative and highly non-linear, leading to self-amplification and self-organization within the material, a so-called photo-induced phase transition with a new long range order (structural, magnetic, ferroelectric...). Two families of molecular compounds have been studied here: first of all, spin transition materials changing from a diamagnetic state over to a paramagnetic state under the effect of temperature or under continuous laser excitation. It concerns photo-active molecular bistability prototype materials in solid state, whose switching has been studied during X-ray diffraction, optical reflectivity and magnetism experiments. Then we have studied charge-transfer molecular systems, prototype compounds for ultrafast photo-induced phase transitions: insulator-metal, neutral-ionic... As well as ultrafast optical experiments, time-resolved X-ray crystallography is a key technique in order to follow at the atomic level the different steps of the photo-induced transformation and thus to observe the involved mechanisms. We have underlined a process of photo-formation of one-dimensional nano-domains of lattice-relaxed charge-transfer excitations, governing the photo-induced phase transition of the molecular charge-transfer complex TTF-CA by the first time-resolved diffuse scattering measurements. Moreover, a new femtosecond laser-plasma source and a optical pump-probe spectroscopy set-up with a highly sensitive detecting system have been developed in this work. The results presented here will be an illustration of the present scientific challenges existing on the one hand with the development of projects of major scale (new ultrafast sources) and on the other hand with ultrafast photo-switching.Le contrôle au moyen d'impulsions laser ultracourtes de la transformation collective et concertée de molécules à l'état solide pouvant induire une commutation ultrarapide de l'état macroscopique d'un matériau offre des perspectives nouvelles. L'objectif est de réaliser à l'échelle du matériau ce qui est réalisé à l'échelle moléculaire en femtochimie. Ces processus sont hautement non linéaires et coopératifs, pouvant conduire à une auto-amplification et une auto-organisation au sein du matériau, et donc à une transition de phase photo-induite vers un nouvel ordre à longue distance (structural, magnétique, ferroélectrique...). Deux familles de matériaux ont été ici étudiées: tout d'abord, des matériaux à transition de spin, passant d'un état diamagnétique à paramagnétique, sous l'effet de la température, ou sous irradiation laser continue. Il s'agit de matériaux photo-actifs prototypes de la bistabilité moléculaire à l'état solide, dont la commutation est étudiée lors d'expériences de diffraction X, de réflectivité optique et de magnétisme. Une seconde partie des études a porté sur des complexes moléculaires à transfert de charge qui sont des composés prototypes pour les transitions de phase photo-induites ultra-rapides: neutre-ionique, isolant-métal... En plus des expériences d'optique temporelle ultra-rapide, la cristallographie X résolue en temps constitue une technique clé permettant de suivre au niveau atomique les différentes étapes de la transformation photo-induite et par conséquent d'observer les mécanismes mis en jeu. Ainsi, nous avons pu mettre en évidence un processus de photo-formation de nanodomaines unidimensionnels d'excitations de transfert de charge relaxées structuralement, pilotant la transition de phase photo-induite du TTF-CA, à l'aide de premières études de diffusion diffuse résolue en temps. Une nouvelle source laser-plasma femtoseconde et un dispositif de spectroscopie pompe sonde optique à détection hautement sensible ont aussi été développés dans le cadre de ce travail. Les résultats présentés dans cette étude seront une illustration des enjeux scientifiques actuels relatifs d'une part aux développements de projets de grande ampleur (nouvelles sources ultra-brèves) et d'autre part à la photo-commutation ultra-rapide

Short-time water caging and transient electron-OH couplings in liquid phase

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Reflectivity oscillations of fs-laser excited Bismuth : Excitation of coherent phonons

International audienceWe present the experimental and theoretical studies of the optical response from the single-crystal of bismuth to the excitation by the femtosecond laser pulse. The experimental results revealed a complex, first - positive and a few picoseconds later - negative, change in time-dependent reflectivity, which could not be explained in the light of the existing theories. It is shown that reflectivity oscillations are related to the excitation of coherent phonons by the pulse with duration shorter of all relaxation times. We demonstrate that swiftly heated electrons are responsible for the phonon excitation due to the fast modification of the attractive (electronic) part of inter-atomic potential. The electronic perturbation of potential is also responsible for the red shift of phonon frequency and for the increase in the amplitude of phonons. The coherent phonon excitation process as well as the change in the reflectivity is related mainly to the modification of the electron-phonon momentum exchange frequency. The comparison between the theory and experiments shows an excellent agreement. Moreover, the reflectivity measurements allow direct recovery of the electron-phonon coupling rate in bismuth crystal, which has not been measured before

Coherent phonons imprinted into reflectivity oscillations of laser-excited Bi through electron-phonon coupling

We show that the reflectivity of laser-excited solid relates to phonons, driven by thermal forces, through the electron-phonon coupling rate. Controlled excitation of phonons is available by the optimum combination of laser and material parameters