We demonstrate experimentally an optical process in which the spin angular momentum carried by a circularly polarized light beam is converted into orbital angular momentum, leading to the generation of helical modes with a wavefront helicity controlled by the input polarization. This phenomenon requires the interaction of light with matter that is both optically inhomogeneous and anisotropic. The underlying physics is also associated with the so-called Pancharatnam-Berry geometrical phases involved in any inhomogeneous transformation of the optical polarization.

We study hydrated model membranes, consisting of stacked bilayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine lipids, using terahertz time-domain spectroscopy and infrared spectroscopy. Terahertz spectroscopy enables the investigation of water dynamics, owing to its sensitivity to dielectric relaxation processes associated with water reorientation. By controlling the number of water molecules per lipid molecule in the system, we elucidate how the interplay between the model membrane and water molecules results in different water dynamics. For decreasing hydration levels, we observe the appearance of new types of water dynamics: the collective bulklike dynamics become less pronounced, whereas an increased amount of both very slowly reorienting (i.e., irrotational) and very rapidly reorienting (i.e., fast) water molecules appear. Temperature-dependent measurements reveal the interconversion between the three distinct types of water present in the system.

We report the realization of a Pancharatnam-Berry phase optical element E. Hasman, Opt. Lett. 27, 1141 (2002)] for wavefront shaping working in the visible spectral domain, based on patterned liquid crystal technology. This device generates helical modes of visible light with the possibility of electro-optically switching between opposite helicities by controlling the handedness of the input circular polarization. By cascading this approach, fast switching among multiple wavefront helicities can be achieved, with potential applications to multistate optical information encoding. The approach demonstrated here can be generalized to other polarization-controlled devices for wavefront shaping, such as switchable lenses, beam-splitters, and holographic elements.

A discrete quantum walk occurs in the orbital angular momentum space of light, both for a single photon and for two simultaneous photons.

The so-called "polar catastrophe," a sudden electronic reconstruction taking place to compensate for the interfacial ionic polar discontinuity, is currently considered as a likely factor to explain the surprising conductivity of the interface between the insulators LaAlO 3 and SrTiO 3 . We applied optical second harmonic generation, a technique that a priori can detect both mobile and localized interfacial electrons, to investigating the electronic polar reconstructions taking place at the interface. As the LaAlO 3 film thickness is increased, we identify two abrupt electronic rearrangements: the first takes place at a thickness of 3 unit cells, in the insulating state; the second occurs at a thickness of 4-6 unit cells, i.e., just above the threshold for which the samples become conducting. Two possible physical scenarios behind these observations are proposed. The first is based on an electronic transfer into localized electronic states at the interface that acts as a precursor of the conductivity onset. In the second scenario, the signal variations are attributed to the strong ionic relaxations taking place in the LaAlO 3 layer.

Multiferroics and magnetoelectrics with coexisting and coupled multiple ferroic orders are materials promising new technological\ud advances. While most studies have focused on single-phase or heterostructures of inorganic materials, a new class of materials\ud called metal–organic frameworks (MOFs) has been recently proposed as candidate materials demonstrating interesting new routes\ud for multiferroism and magnetoelectric coupling. Herein, we report on the origin of multiferroicity of (CH3)2NH2Mn(HCOO)3 via direct\ud observation of ferroelectric domains using second-harmonic generation techniques. For the first time, we observe how these\ud domains are organized (sized in micrometer range), and how they are mutually affected by applied electric and magnetic fields.\ud Calculations provide an estimate of the electric polarization and give insights into its microscopic origin

Creation of patterns and structures on surfaces at the micro- and nano-scale is a field of growing interest. Direct femtosecond laser surface structuring with a Gaussian-like beam intensity profile has already distinguished itself as a versatile method to fabricate surface structures on metals and semiconductors. Here we present an approach for direct femtosecond laser surface structuring based on optical vortex beams with different spatial distributions of the state of polarization, which are easily generated by means of a q-plate. The different states of an optical vortex beam carrying an orbital angular momentum ℓ = ±1 are used to demonstrate the fabrication of various regular surface patterns on silicon. The spatial features of the regular rippled and grooved surface structures are correlated with the state of polarization of the optical vortex beam. Moreover, scattered surface wave theory approach is used to rationalize the dependence of the surface structures on the local state of the laser beam characteristics (polarization and fluence). The present approach can be further extended to fabricate even more complex and unconventional surface structures by exploiting the possibilities offered by femtosecond optical vector fields.

An experimental study of the photoinduced molecular reorientation of dyed liquid crystals for a set of guest-host combinations is reported. We find large variations in the magnitude of the effect for different dyes but also for different hosts, with polar hosts resulting often significantly more effective than nonpolar ones. The data are interpreted in terms of a kinetic mean-field model for the dye molecule rotational dynamics and interaction with the liquid crystal host. The results point to a significant variation of guest-host intermolecular forces upon photoinduced electronic excitation of dye molecules. This force variation is reflected in a variation of dye molecule physical parameters such as the rotational friction coefficient and the orientational mean-field energy.