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International audience; During the last decade, time-domain THz spectroscopy and imaging became widely used techniques in various fields of physics, chemistry, and biology. Advancements in THz generation techniques[1] have opened new perspectives in solid-state physics and material science: THz pulsed radiation can now be used to probe to drive at the picosecond time scale different degrees of freedom of a solid[2]. Non-destructive THz remote sensing and control of a solid are now one of the widely discussed topics in the scientific community. For example, the excitation of antiferromagnon and optical phonons with pulsed THz radiation were recently reported. In the meantime, no clear explanation[3] of a coherent acoustic phonon (CAP) generation process with THz radiation exists so far, while CAP driven by visible or near-infrared (NIR) light are well studied[4]. In that regard, a back-to-back study of NIR and THz ultrafast dynamics in a set of ?simple? materials should help to understand the physics behind the CAP excitation mechanism with intense THz pulsed radiation. We performed the pump-probe measurements with a sub-picosecond time resolution on the PVD deposited nm-thin Cr and Al films and on a set of Bi2Te3 MBE grown topological insulators. Studied materials were irradiated with femtosecond pulses with a central wavelength of 800 nm (375 THz/1.54 eV) or 600 �m (0.5 THz/2 meV) and probed at 400 nm (749.5 THz/3.1 eV). A direct comparison between THz and NIR excitation complemented by THz-TDS and FDTD simulations shows that the mechanism of CAP excitation with THz is similar to NIR, and linked to the well-known thermoelastic process. Refferences [1] H.Hirori et al., Appl. Phys. Lett. 98, 091106 (2011). [2] T. Kampfrath et al., Nat. Photonics 7, 680 (2013). [3] J.M. Manceau, et al., Appl. Phys. Lett. 97, (2010). [4] P. Ruello and V.E. Gusev, Ultrasonics 56, 21 (2015).

International audience; The superlattices(SLs), such as GaAs/AlAs, have been extensively studied and developed as a source of THz acoustic phonons. Due to the periodic modulation of mechanical properties, the dispersion of acoustic branches in center Brillouin-Zone is indeed folded which offers the opportunity to directly excite coherent THz acoustic phonons with femtosecond light pulses. Meanwhile, perovskite oxides caught attention because of their coupled ferroelectric and magnetic orders with the phonon subsystem, which reveal peculiar mechanisms of GHz phonon generation. Coupling the superlattice geometry with ferroelectric properties is then a natural exciting direction to explore new phonon photogeneration processes. THz coherent acoustic phonon generation in PZT/SRO superlattices has been already demonstrated. In this communication, we will show and discuss, with femtoseceond pump-probe setup, how we can generate THz coherent acoustic phonons in ferroelectric \mathrmBiFeO₃/LaFeO₃ superlattices. In this case, the light can directly interact with ferroelectric \mathrmBiFeO₃ instead of interacting with metallic oxide \mathrmSrRuO₃ only. Furthermore, we also investigate the GHz acoustic phonon generation in \mathrmBiFeO₃ thin films harboring pristine periodic domains and well defined "in plane" polarization. We explore in the latter case the generation of shear acoustic phonons. All these results bring new insights on the crucial role of ferroelectric domain nano-structuration for controlling the coherent acoustic phonon spectrum.

The size reduction of metals, from bulk to nanoparticles, induces significant modifications of their properties. For instance, the optical properties evolve and a new resonance, the localized surface plasmon resonance, appears in the optical spectrum and is responsible for the change of colors of metallic nanoparticles. This work is focused on studies of metals’ properties at the nanometric scale. In the first part, the vibrational and thermal properties are studied with a femtosecond spectroscopy technique. It is shown that it is possible to excite and detect optically vibrational frequencies in the terahertz domain by studying platinum nanoparticles formed by less than 100 atoms. The study of the thermal properties of the metallic nanoparticles (gold and silver) has shown that the boundary effect increases. This thermal boundary resistance, known as the Kapitza resistance, plays a dominant role in the heat transfer at the nanometric scale. A correlation between the experimental values of the thermal boundary resistance and the acoustic impedances of the boundary’s materials has been found. We have also shown that the Kapitza resistance is a decreasing function of the temperature in the 70-300K range. In the second part, the effect of the size reduction on the optical properties of non-spherical nanoparticles is observed. The Spatial Modulation Spectroscopy technique is used in order to locate and study individual gold nanorods. It is shown that the two geometrical parameters (the length and the diameter) of the nanorods influence the spectral linewidth of the localized surface plasmon resonance. This effect is not predicted by existing classical or quantum models, En passant de l’état massif à la nanoparticule les matériaux métalliques voient certaines de leurs caractéristiques modifiées de manière notable comme par exemple les propriétés optiques avec l’apparition d’une résonance dans le spectre optique, la Résonance Plasmon de Surface Localisée (RPSL) responsable du changement de couleur des nanoparticules métalliques. Les propriétés vibrationnelles et thermiques de nanoparticules métalliques ont été étudiées à l’aide d’une technique de Spectroscopie Femtoseconde. Nous avons montré qu’il était possible d’exciter et de détecter optiquement des fréquences de vibrations mécaniques dans le domaine térahertz pour des nanoparticules de platine composées de moins de cent atomes. D’autre part l’augmentation des effets dus aux interfaces a été mis en évidence sur les propriétés thermiques de nanoparticules d’or et d’argent. La résistance thermique à l’interface, résistance de Kapitza, voit son rôle augmenter lors du transfert thermique à l’échelle nanométrique. Une corrélation entre les valeurs mesurées et les impédances acoustiques des matériaux composants les interfaces a été mise en évidence. Nous avons aussi montré qu’elle augmente quand la température diminue de 300K à 70K. Les propriétés optiques de nanoparticules non sphériques ont été étudiées à l’aide de la Spectroscopie à Modulation Spatiale. Cette technique a permis de repérer puis de caractériser des nano-bâtonnets d’or individuels. Nous avons montré que la largeur spectrale de la RPSL est fortement dépendante de la géométrie des nanoparticules (diamètre et longueur). Cette double dépendance n’est pas prédite par les modèles classiques ou quantique existants

The size reduction of metals, from bulk to nanoparticles, induces significant modifications of their properties. For instance, the optical properties evolve and a new resonance, the localized surface plasmon resonance, appears in the optical spectrum and is responsible for the change of colors of metallic nanoparticles. This work is focused on studies of metals’ properties at the nanometric scale. In the first part, the vibrational and thermal properties are studied with a femtosecond spectroscopy technique. It is shown that it is possible to excite and detect optically vibrational frequencies in the terahertz domain by studying platinum nanoparticles formed by less than 100 atoms. The study of the thermal properties of the metallic nanoparticles (gold and silver) has shown that the boundary effect increases. This thermal boundary resistance, known as the Kapitza resistance, plays a dominant role in the heat transfer at the nanometric scale. A correlation between the experimental values of the thermal boundary resistance and the acoustic impedances of the boundary’s materials has been found. We have also shown that the Kapitza resistance is a decreasing function of the temperature in the 70-300K range. In the second part, the effect of the size reduction on the optical properties of non-spherical nanoparticles is observed. The Spatial Modulation Spectroscopy technique is used in order to locate and study individual gold nanorods. It is shown that the two geometrical parameters (the length and the diameter) of the nanorods influence the spectral linewidth of the localized surface plasmon resonance. This effect is not predicted by existing classical or quantum models; En passant de l’état massif à la nanoparticule les matériaux métalliques voient certaines de leurs caractéristiques modifiées de manière notable comme par exemple les propriétés optiques avec l’apparition d’une résonance dans le spectre optique, la Résonance Plasmon de Surface Localisée (RPSL) responsable du changement de couleur des nanoparticules métalliques. Les propriétés vibrationnelles et thermiques de nanoparticules métalliques ont été étudiées à l’aide d’une technique de Spectroscopie Femtoseconde. Nous avons montré qu’il était possible d’exciter et de détecter optiquement des fréquences de vibrations mécaniques dans le domaine térahertz pour des nanoparticules de platine composées de moins de cent atomes. D’autre part l’augmentation des effets dus aux interfaces a été mis en évidence sur les propriétés thermiques de nanoparticules d’or et d’argent. La résistance thermique à l’interface, résistance de Kapitza, voit son rôle augmenter lors du transfert thermique à l’échelle nanométrique. Une corrélation entre les valeurs mesurées et les impédances acoustiques des matériaux composants les interfaces a été mise en évidence. Nous avons aussi montré qu’elle augmente quand la température diminue de 300K à 70K. Les propriétés optiques de nanoparticules non sphériques ont été étudiées à l’aide de la Spectroscopie à Modulation Spatiale. Cette technique a permis de repérer puis de caractériser des nano-bâtonnets d’or individuels. Nous avons montré que la largeur spectrale de la RPSL est fortement dépendante de la géométrie des nanoparticules (diamètre et longueur). Cette double dépendance n’est pas prédite par les modèles classiques ou quantique existants

Controlling light polarization is one of the most essential routines in modern optical technology. Since the demonstration of optical pulse shaping by spatial light modulators and its potential in controlling the quantum reaction pathways, it paved the way for many applications as a coherent control of the photoionization process or as polarization shaping of terahertz (THz) pulses. Here, we evidenced efficient nonresonant and noncollinear $\chi^{(2)}$-type light-matter interaction in femtosecond polarization-sensitive time-resolved optical measurements. Such nonlinear optical interaction of visible light and ultrashort THz pulses leads to THz modulation of visible light polarization in bulk LiNbO$_3$ crystal. Theoretical simulations based on the wave propagation equation capture the physical processes underlying this nonlinear effect. Apart from the observed tunable polarization modulation of visible pulses at ultrahigh frequencies, this physical phenomenon can be envisaged in THz depth-profiling of materials.