Time-domain Brillouin Scattering as a Local Temperature Probe in Liquids

We present results of time-domain Brillouin scattering (TDBS) to determine the local temperature of liquids in contact to an optical transducer. TDBS is based on an ultrafast pump-probe technique to determine the light scattering frequency shift caused by the propagation of coherent acoustic waves in a sample. Since the temperature influences the Brillouin scattering frequency shift, the TDBS signal probes the local temperature of the liquid. Results for the extracted Brillouin scattering frequencies recorded at different liquid temperatures and at different laser powers - i.e. different steady state background temperatures- are shown to demonstrate the usefulness of TDBS as a temperature probe. This TDBS experimental scheme is a first step towards the investigation of ultrathin liquids measured by GHz ultrasonic probing.Comment: arXiv admin note: substantial text overlap with arXiv:1702.0107

Anisotropy in the dielectric function of Bi2_2Te3_3 from first principles: From the UV-visible to the infrared range

The dielectric properties of Bi$_2$Te$_3$, a layered compound crystallizing in a rhombohedral structure, are investigated by means of first-principles calculations at the random phase approximation level. A special attention is devoted to the anisotropy in the dielectric function and to the local field effects that strongly renormalize the optical properties in the UV-visible range when the electric field is polarized along the stacking axis. Furthermore, both the Born effective charges for each atom and the zone center phonon frequencies and eigenvectors needed to describe the dielectric response in the infrared range are computed. Our theoretical near-normal incidence reflectivity spectras in both the UV-visible and infrared range are in fairly good agreement with the experimental spectras, provided that the free carriers Drude contribution arising from defects is included in the infrared response. The anisotropic plasmon frequencies entering the Drude model are computed within the rigid band approximation, suggesting that a measurement of the reflectivity in the infrared range for both polarizations might allow to infer not only the type of doping but also the level of doping.Comment: 29 pages, 10 figure

Molecular dynamics of glycerol and glycerol-trehalose bioprotectant solutions nanoconfined in porous silicon

Glycerol and trehalose-glycerol binary solutions are glass-forming liquids with remarkable bioprotectant properties. Incoherent quasielastic neutron scattering (QENS) is used to reveal the different effects of nanoconfinement and addition of trehalose on the molecular dynamics in the normal liquid and supercooled liquid phases, on a nanosecond timescale. Confinement has been realized in straight channels of diameter D=8 nm formed by porous silicon. It leads to a faster and more inhomogeneous relaxation dynamics deep in the liquid phase. This confinement effect remains at lower temperature where it affects the glassy dynamics. The glass transitions of the confined systems are shifted to low temperature with respect to the bulk ones. Adding trehalose tends to slow down the overall glassy dynamics and increases the non-exponential character of the structural relaxation. Unprecedented results are obtained for the binary bioprotectant solution, which exhibits an extremely non-Debye relaxation dynamics as a result of the combination of the effects of confinement and mixing of two constituents

Solvent contribution to the stability of a physical gel characterized by quasi-elastic neutron scattering

The dynamics of a physical gel, namely the Low Molecular Mass Organic Gelator {\textit Methyl-4,6-O-benzylidene-$\alpha$ -D-mannopyranoside ($\alpha$-manno)} in water and toluene are probed by neutron scattering. Using high gelator concentrations, we were able to determine, on a timescale from a few ps to 1 ns, the number of solvent molecules that are immobilised by the rigid network formed by the gelators. We found that only few toluene molecules per gelator participate to the network which is formed by hydrogen bonding between the gelators' sugar moieties. In water, however, the interactions leading to the gel formations are weaker, involving dipolar, hydrophobic or $\pi-\pi$ interactions and hydrogen bonds are formed between the gelators and the surrounding water. Therefore, around 10 to 14 water molecules per gelator are immobilised by the presence of the network. This study shows that neutron scattering can give valuable information about the behaviour of solvent confined in a molecular gel.Comment: Langmuir (2015