Weak Gravitational Lensing of Finite Beams.

The standard theory of weak gravitational lensing relies on the infinitesimal light beam approximation. In this context, images are distorted by convergence and shear, the respective sources of which unphysically depend on the resolution of the distribution of matter-the so-called Ricci-Weyl problem. In this Letter, we propose a strong-lensing-inspired formalism to describe the lensing of finite beams. We address the Ricci-Weyl problem by showing explicitly that convergence is caused by the matter enclosed by the beam, regardless of its distribution. Furthermore, shear turns out to be systematically enhanced by the finiteness of the beam. This implies, in particular, that the Kaiser-Squires relation between shear and convergence is violated, which could have profound consequences on the interpretation of weak-lensing surveys.

Isolation of a new disaccharide nucleoside from Helleborus caucasicus: structure elucidation and total synthesis of hellecaucaside A and its ß-anomer.

Hellecaucaside A, a new disaccharide nucleoside featuring a 2'-O-α-D-ribofuranosyluridine skeleton and a 4-hydroxybenzoyl group at the 5' position, was isolated from the underground part of Helleborus caucasicus. The structure of the compound was elucidated by means of chemical degradation and spectroscopic analyses, such as 1D/2D NMR, chiral-GC, and HRMS. The total synthesis of hellecaucaside A and its ß-anomer was accomplished, unequivocally confirming the structure of the natural product.

Can all cosmological observations be accurately interpreted with a unique geometry?

The recent analysis of the Planck results reveals a tension between the best fits for (Ω(m0), H(0)) derived from the cosmic microwave background or baryonic acoustic oscillations on the one hand, and the Hubble diagram on the other hand. These observations probe the Universe on very different scales since they involve light beams of very different angular sizes; hence, the tension between them may indicate that they should not be interpreted the same way. More precisely, this Letter questions the accuracy of using only the (perturbed) Friedmann-Lemaître geometry to interpret all the cosmological observations, regardless of their angular or spatial resolution. We show that using an inhomogeneous "Swiss-cheese" model to interpret the Hubble diagram allows us to reconcile the inferred value of Ω(m0) with the Planck results. Such an approach does not require us to invoke new physics nor to violate the Copernican principle.

Local refractive index probed via the fluorescence decay of semiconductor quantum dots.

We present a novel approach for convenient tuning of the local refractive index around nanostructures. We apply this technique to study the influence of the local refractive index on the radiative decay time of CdSe/ZnS quantum dots with three distinct emission wavelengths. The dependence of the luminescence decay time on the environment is well described by an effective medium approach. A critical distance of about 80 nm is found for the determination of the effective local index of refraction. An estimation for the emitting-state quantum efficiency can be extracted.