Despite the great amount of attention CH₃NH₃PbI₃ has received for its solar cell application, intrinsic properties of this material are still largely unknown. Mobility of charges is a quintessential property in this aspect; however, there is still no clear understanding of electron transport, as reported values span over three orders of magnitude. Here we develop a method to measure the electron and hole deformation potentials using coherent acoustic phonons generated by femtosecond laser pulses. We apply this method to characterize a CH₃NH₃PbI₃ single crystal. We measure the acoustic phonon properties and characterize electron-acoustic phonon scattering. Then, using the deformation potential theory, we calculate the carrier intrinsic mobility and compare it to the reported experimental and theoretical values. Our results reveal high electron and hole mobilities of 2,800 and 9,400 cm² V⁻¹ s⁻¹, respectively. Comparison with literature values of mobility demonstrates the potential role played by polarons in charge transport in CH₃NH₃PbI₃.

Complete mechanical measurements are performed in submicron films using the picosecond ultrasonic technique. The Al layer deposited on the top of the sample acting as a transducer is replaced with a nanostructured Al film. Using an usual picosecond ultrasonic setup we can excite and detect high-frequency longitudinal and surface acoustic waves. From this we can deduce Young’s modulus and Poisson’s ratio of any isotropic thin film. Experimental results obtained for a thin silica layer on silicon are in very good agreement with literature.

Despite the numerous devoted studies, water at solid interfaces remains puzzling. An ongoing debate concerns the nature of interfacial water at a hydrophilic surface, whether it is more solid-like, ice-like, or liquid-like. To answer this question, a complete picture of the distribution of the water molecule structure and molecular interactions has to be obtained in a non-invasive way and on an ultrafast time scale. We developed a new experimental technique that extends the classical acoustic technique to the molecular level. Using nanoacoustic waves with a femtosecond pulsewidth and an ångström resolution to noninvasively diagnose the hydration structure distribution at ambient solid/water interface, we performed a complete mapping of the viscoelastic properties and of the density in the whole interfacial water region at hydrophilic surfaces. Our results suggest that water in the interfacial region possesses mixed properties and that the different pictures obtained up to now can be unified. Moreover, we discuss the effect of the interfacial water structure on the abnormal thermal transport properties of solid/liquid interfaces.

We study the elastic properties of two-dimensional phononic crystals made of aluminum nanocube lattices. From the experimental point of view, collective modes resulting from the coupling between cubes inside the crystal are excited and time resolved using ultrafast acoustics. We derive a general theoretical model of the collective modes. From the analytical expression we discuss the influence of numerous crystal parameters on the detected frequencies. We then present experimental results on various samples which all exhibit an excellent agreement with the model. The theoretical description reveals that collective modes propagate along the sample surface. From that we propose to use such modes for measuring the in-plane elastic properties of thin films.