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Experiments and simulations are performed to study filamentation and generation of acoustic waves in water by loosely focused multi-millijoules laser pulses. When the laser pulse duration is increased from femtosecond to nanosecond duration, a transition is observed from a filamentary propagation with extended and low energy density deposition to a localized breakdown, related to high energy density deposition. The transition suggests that Kerr self-focusing plays a major role in the beam propagation dynamics. As a result, the shape, the amplitude and the spectrum of the resulting pressure wave present a strong dependence on the laser pulse duration.

Liquid water is one of the most studied substances, yet many of its properties are difficult to rationalize. The uniqueness of water is rooted in the dynamic network of hydrogen-bonded molecules with relaxation time constants of about one picosecond. Terahertz fields oscillate on a picosecond timescale and are inherently suited to study water. Recent advances in non-linear terahertz spectroscopy have revealed large signals from water, which have been interpreted with different, sometimes competing, theoretical models. Here, we show that the non-linear transmission of liquid water at ∼1 THz is equal at 21 and 4 °C, thus suggesting that the most appropriate microscopic models should depend weakly on temperature. Among the different mechanisms proposed to date, the resonant reorientation of hydrogen-bonded water molecules might be the most appropriate to describe all of the currently available experimental results.

La fabrication additive (FA), notamment la FA de pièces métalliques, connait un essor dans les secteurs de pointe comme l’aéronautique ou le médical de par les possibilités accrues en termes de complexité géométrique, de fonctionnalités ou encore de personnalisation des pièces. Cependant, les poudres métalliques et la fusion laser mis en oeuvre dans certains procédés lors de la fabrication conduisent parfois à des défauts, comme par exemple des manques de fusion. Pour réduire les coûts de production engendrés par des pièces finies mais non conformes, la fabrication de ces pièces appelle à développer un contrôle en ligne. Les ultrasons laser (UL), non destructifs et sans contact, sont une piste prometteuse : ils combinent la sensibilité d’un contrôle par ultrasons avec la flexibilité d’un système optique.

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