C. Versace

Random lasing and weak localization of light in dye-doped nematic liquid crystals

The first observation of random laser action in a partially ordered, optically anisotropic nematic liquid crystal with long-range dielectric tensor fluctuations is reported. Above a given pump power the fluorescence curve collapses and the typical narrowing and explosion effect leads to discrete sharp peaks. The unexpected surviving of interference effects in recurrent multiple scattering provide the required optical feedback for lasing in nematics. Coherent backscattering of light waves in orientationally ordered nematic liquid crystals manifests a weak localization of light which strongly supports diffusive laser action in presence of gain medium. Intensity fluctuations of the speckle-like emission pattern indicate the typical spatio-temporal randomness of diffusive laser emission. A comparison of the laser action is reported for systems with different order degree: fully disordered semiconductor powders, self-ordered cholesterics and partially ordered nematic liquid crystals.

PEG-templated mesoporous silica nanoparticles exclusively target cancer cells

Mesoporous silica nanoparticles (MSNs) have been proposed as DNA and drug delivery carriers, as well as efficient tools for fluorescent cell tracking. The major limitation is that MSNs enter cells regardless of a target-specific functionalization. Here we show that non functionalized MSNs, synthesized using a PEG surfactant-based interfacial synthesis procedure, do not enter cells, while a highly specific, receptor mediated, cellular internalization of folic acid (FOL) grafted MSNs (MSN-FOL), occurs exclusively in folate receptor (FR) expressing cells. Neither the classical clathrin pathway nor macropinocytosis is involved in the MSN endocytic process, while fluorescent MSNs (MSN-FITC) enter cells through aspecific, caveolae-mediated, endocytosis. Moreover, internalized particles seem to be mostly exocytosed from cells within 96 h. Finally, cisplatin (Cp) loaded MSN-FOL were tested on cancerous FR-positive (HeLa) or normal FR-negative (HEK293) cells. A strong growth arrest was observed only in HeLa cells treated with MSN-FOL-Cp. The results presented here show that our mesoporous nanoparticles do not enter cells unless opportunely functionalized, suggesting that they could represent a promising vehicle for drug targeting applications.

Investigation of flexoelectric properties of a palladium-containing nematic liquid crystal, Azpac, and its mixtures with MBBA

Abstract Flexoelectricity of pure Azpac (an HOAB-palladium complex) was studied using planar nematic layers under an in-plane electric field. Longitudinal domains were observed with a period inversely proportional to the applied d.c. electric field. These domains were considered as a first experiment manifestation of the theoretical prediction of R. B. Meyer (1969) and their study has permitted the evaluation of the difference in flexo-coefficientse 1z - e 3x. Their appearance can also be followed for the non-complexed HOAB, using instant video-microscopy frames. Mixtures of Azpac, up to 10wt%, and MBBA were oriented homeotropically and band flexoelectric deformations were observed, both in d.c. and in a.c. (1 to 1000 Hz) electric fields normal to the director. The dynamics of director reorientation were studied by the method of flexoelectric light modulation. A system comprising a He-Ne laser and a lock-in amplifier interfaced by a PC was developed. By operating this in a frequency sweep regime, viscoela...

Random lasing in freely suspended dye-doped nematic liquid crystals.

Random lasing in fully disordered systems having organic and inorganic nature has been the subject of extensive studies since the beginning of the past decade. The interest mainly emerges from the unexpected role played by disorder in the laser action. The disorder was considered detrimental for the optical feedback in cavity laser, until it was demonstrated that multiple-scattering materials including a gain medium act as random laser. Here, a completely new approach is reported, where freely suspended complex fluid films doped with fluorescent molecules under optical excitation generate narrowband lasing peaks. The constellation of localized modes is selected by properly choosing the gain profile. The idea to have laser action in absence of mirrors and boundaries realizes an unparalleled tunable and moldable laser source.

Thermal behavior of random lasing in dye doped nematic liquid crystals

The role of the thermally modulated order parameter in the diffusive laser action observed in dye doped nematic liquid crystals was investigated. Above a given pump energy a randomly distributed series of bright tiny spots appear, giving rise to a strongly fluctuating emission pattern. The spectral analysis reveals discrete sharp peaks (about 0.5nm) slightly blueshifted with respect to the highest efficiency region of the gain medium. A comparative study was performed in systems having different sizes and confining geometries, corroborating the idea that the random lasing observed in dye doped nematic phase is fluctuation driven through a recurrent multiple scattering process.

Asymmetric electro-optical response in a liquid crystal cell containing a layer of amorphous tungsten trioxide

In this work, we present the experimental evidence of a polarity sensitive electro-optic response in a nematic liquid crystal. The liquid crystal cell was made by using a standard sandwich configuration, with one of the indium tin oxide electrodes covered by a thin layer of tungsten trioxide (WO3), deposited by sputtering. The optical response was inhibited when the electrode covered by WO3 film was anodically charged, while the usual optical response occurred under a reverse field. An ionic diffusion process was associated with the establishment of an internal electric field, which inhibited unipolarly the optical switching.

Thermo-recurrent nematic random laser

This experimental work is aimed to investigate the thermal behavior of random laser action in dye doped nematic liquid crystals. The study evidenced an important temperature dependence of the random lasing characteristics in the nematic phase and in close proximity of the nematicisotropic (N-I) phase transition. A lowering of the laser emission intensity as the temperature increases is strictly related to the shift of the lasing threshold as function of the temperature even though the pump energy is kept fixed. The optical losses increasing owing to the thermal fluctuation enhanced scattering drive the input-output smoother behavior until the system stops to lase, because below threshold. The unexpected reoccurrence of random lasing at higher temperature, in proximity of N-I transition is found to be related to a different scattering mechanism, the micro-droplets nucleation and critical opalescence.

Changes of the electro-optic response of nematic liquid crystal cells due to inserted titania-vanadia films

We report a polarity-sensitive electro-optical response observed in nematic liquid crystal cells characterized by asymmetric insertion of thin films of titania-vanadia oxide with a Ti∕V atomic ratio of 1∕1, prepared by sol-gel synthesis on transparent indium tin oxide. The electro-optical effect is opposite to previously reported ones but is obtained using the same materials with a slightly different sol-gel route. The structural properties of titania-vanadia films are extensively studied with particular attention paid to the thermal history of the films. The measurements made of the films and on the nematic liquid crystal cell demonstrate that the crystalline structure of the film is responsible of the observed effect. Thermal annealing of the films results in formation of structures that dramatically change the electric properties of the films. A qualitative model is proposed to explain the surface charge distribution inside the nematic liquid crystal cell that results in the observed electro-optical ef...

Insertion of thin films of WO3 in liquid crystal cells

A new kind of a liquid crystal cell has been made by using a standard sandwich configuration with one of the ITO electrodes, covered by a thin layer of tungsten trioxide deposited by r.f. sputtering in a reactive atmosphere of Ar and O2 gas. In this kind of cell the optical polarisation switching of the cell (observed under a crossed polarizer microscope) occurs only for one of the two directions of the a.c. applied field, while in the usual liquid crystal cells the electro-optic response does not depend on the sign of field. The inhibiting switching configuration corresponds to the anodic polarisation of tungsten trioxide film in which the de-intercalation of cations occurs. Preliminary impedance measurements reveal an ionic diAusion process in such devices (Warburg impedance behavior). These preliminary results suggest speculations about a reverse internal electric field, which is responsible for the increased threshold of optical switching in one direction only of the applied field. # 1999 Elsevier Science Ltd. All rights reserved.

Observations of Nonlinear Optical Effects in Metallorganic Liquid Crystals

Abstract We report the observation of light induced optical nonlinearities of thermal origin in metallorganic liquid crystals. Experimental data about self focusing and self phase modulation are presented and discussed. An estimation of the value of the nonlinear refractive index of the sample is performed.