We report on photo-thermal eﬀects observed in gold nanoparticles (GNPs) dispersed in Nematic Liquid Crystals (NLCs). Under a suitable optical radiation, GNPs exhibit a strong light absorption/scattering; the eﬀect depends on the refractive index of the medium surrounding the nanoparticles, which can be electrically or optically tuned. In this way, the system represents an ideal nano-source of heat, remotely controllable by light to adjust the temperature at the nanoscale. Photo-induced temperature variations in GNPs dispersed in NLCs have been investigated by implementing a theoretical model based on the thermal heating equation applied to an anisotropic medium; theoretical predictions have been compared with results of experiments carried out in a NLC medium hosting GNPs. Both theory and experiments represent a step forward to understand the physics of heat production at the nanoscale, with applications that range from photonics to nanomedicine.
By exploiting Metamaterials (MTMs) and Photonic Quasi-Crystals (PQCs), it is possible to realize man-made structures characterized by a selective EM response, which can be also controlled by combining the distinctive properties of reconfigurable soft-matter. By finely controlling lattice parameters of a given photonic structure, it is possible to optimize its extraction characteristics at a precise wavelength, or minimize the extraction of undesired modes. In general, however, once a structure is realized, its extraction properties cannot be varied. To cross this problem, it is possible to combine capabilities offered by both MTMs and PQCs with the reconfigurable properties of smart materials, such as Liquid Crystals (LCs); in this way, a completely new class of “reconfigurable metamaterials” (R-MTM) can be realized. We report here on the realization and characterization of a switchable photonic device, working in the visible range, based on nanostructured photonic quasi-crystals, layered with an azodye-doped nematic LC (NLC). The experimental characterization shows that its filtering effect is remarkable with its extraction spectra which can be controlled by applying an external voltage or by means of a laser light. The vertical extraction of the light, by the coupling of the modes guided by the PQC slab to the free radiation via Bragg scattering, consists of an extremely narrow orange emission band at 621 nm with a full width at half-maximum (FWHM) of 8 nm. In our opinion, these results represent a breakthrough in the realization of innovative MTMs based active photonic devices such as tunable MTMs or reconfigurable lasers and active filters.
We report on the realization and characterization of a polymeric template sculptured in
photosensitive material, on a chemical inert surface. The structure is devoted to micro/nanoconfinement
and stabilization of a wide range of organic and nano-particle components with selfarrangement
properties at the nanoscale . High quality morphology of a polymeric, micropatterned,
array is obtained by combining a, nano-precision level, optical holographic setup and a
multi-step chemico-physical process. The "universal" template represents the basic platform to be
filled with different organic materials, which can also include metallic nano-particles. The long
range self-organization is induced without making use of any kind of surface chemistry. Due to their
capability of exhibiting self organization, light responsive Liquid Crystals (LC)  and short pitch
Cholesterics LC  have been exploited, and experimental studies have been carried out in order to
investigate the photo-optical and elecro-optical response of obtained composite structures for the
realization of photonic devices. Finally, the possibility of including metallic nano-particles has been
also investigated, with the aim of inducing a "metamaterial" behavior of the realized structure.
We report on the fabrication and characterization of a micro periodic structure realized in soft-composite materials
containing metallic nanoparticles. The particles are used to infiltrate a passive polymer template realized by combining a
holographic curing setup and a microfluidic etching process. In other experiments, small amounts of nanoparticles are
dissolved in the original mixture utilized for the realization of polymer-liquid-crystal-polymer-slices gratings
(POLICRYPS); this enables to fabricate POLICRYPS-like structures showing novel electromagnetic properties.
Obtained structures are characterized in term of impinging probe polarization in the UV/visible range. Correlation
between the optical response and external perturbations (electric field, temperature) is also reported. These first attempts
are oriented to the fabrication of devices with tunable metamaterial properties.
The study of optical solitons and light filaments steering in liquid crystals requires utilization of particular
cells designed for top view investigation and realized with an input interface which enables both to control the
molecular director configuration and to prevent light scattering. Up to now, the director orientation imposed by
this additional interface has been only estimated by experimental observations. In this paper, we report on the
design and characterization of liquid crystal cells for investigation of optical spatial solitons as well as on a simple
model describing the configuration of the molecular director orientation under the anchoring action of multiple
interfaces. The model is based on the elastic continuum theory and only strong anchoring is considered for
boundary conditions. Controlling of the director orientation at the input interface, as well as in the bulk, allows
to obtain configurations that can produce distinct optical phenomena in a light beam propagating inside the cell.
For a particular director configuration, it is possible to produce two waves: the extraordinary and the ordinary
one. With a different director configuration, the extraordinary wave only is obtained, which propagates inside
the cell at an angle of more than 7° with respect to the impinging wave vector direction. Under this peculiar
configuration and by applying an external voltage, it is possible to have a good control of the propagation
direction of the optical spatial soliton.
We have observed, for the first to the best of our knowledge, time 2+1 dimensional spatial solitons with an Argon-ion beam in planar cells containing an undoped nematic liquid crystal. The cells provided planar anchoring for the liquid crystal molecules, with transparent electrodes for applying an external voltage across the 75µm-thick crystal. The voltage allowed to pre-orientate the director field m the prmcipal plane contammg the lmearly polarized optical field vector thereby elimmatmg the Fréedericks threshold In this regime, powers low enough to prevent thermal effects could be employed to obtam transverse localization through a local light induced mdex mcrease, trappmg the beam into a spatial soliton Self trapping as-well-as guidance of a copolarized probe at 633nm, were demonstrated at biases of 1v with 3.9 mW powers m the green at 514nm. A beam orthogonally polarized with respect to the director tilt-plane failed to produce self localization thereby confirming the reorientational nature of the phenomenon.
We show that a nematic liquid crystal acted on by a laser field is an apparently simple physical system that, however, undergoes variety of complex dynamic phenomena. We have studied the dynamics of the NLC-reorientation in a homeotropic NLC-cell in a laser beam. The nature and features of observed phenomena are essentially determined by the geometry of interaction. For normal incidence of the light beam, we have found that the knowledge of the speed of reorientation is needed to fully characterize the transition of the system form the unstable to the stable equilibrium. In the case of oblique incidence of light, the director can start persistent oscillations that become irregular and evolve towards a chaotic dynamics in a particular range of the light intensity.
Investigations of interaction of laser beams with liquid crystals reveal new fundamental features of nonlinear dynamic systems. Those features are connected with the giant values of optical nonlinearity, multitude of control parameters, specific properties of light propagation in highly anisotropic and inhomogeneous media, and multidimensional (vector) character of light-induced modulations of liquid crystals. Exceptional opportunities to visualize complex spatio-temporal phenomena in real time, in simple experimental situations, and with utilization of low-power lasers and thin layers of materials, provide a great potential for the design of low-cost and portable set-ups for modeling, demonstrations, and general educational purposes.