Giancarlo Abbate

Non linearly induced self waveguiding structure in dye doped nematic liquid crystals confined in capillaries

We report on experiments dealing with the propagation of a collimated laser beam in a dye doped nematic confined in a capillary of optical fiber size. The nematic is aligned in a such a way that the source beam is self focused. The behavior of the beam - focusing, multifocus regime, filamentation and undulation - already observed in larger cylindrical geometry and pure nematic is shown here to be reproduced in the dye doped medium, at much lower powers. Another feature is reported here: a stable regime looking like a waveguide appears in certain conditions, in which the beam to propagate in a narrow tube. This regime is simply modeled in terms of saturated reorientation of the nematic.

Photonic band gaps analysis of Thue-Morse multilayers made of porous silicon

Dielectric aperiodic Thue-Morse structures up to 128 layers have been fabricated by using porous silicon technology. The photonic band gap properties of Thue-Morse multilayers have been theoretically investigated by means of the transfer matrix method and the integrated density of states. The theoretical approach has been compared and discussed with the reflectivity measurements at variable angles for both the transverse electric and transverse magnetic polarizations of light. The photonic band gap regions, wide 70 nm and 90 nm, included between 0 and 30 degrees , have been observed for the sixth and seventh orders, respectively.

Electro-optical switch and continuously tunable filter based on a Bragg grating in a planar waveguide with a liquid crystal overlayer

The possibility of using smectic liquid crystals in active wave- guide devices is explored through the analysis of both an integrated electro-optic switch and a continuously tunable filter. The two devices are based on a Bragg grating in planar waveguide with a liquid crystal over- layer, which enables changing the spectral behavior of the device. The fast and bistable switching of smectic C* in the surface-stabilized liquid crystal structure is used to investigate the possibility of realizing an inte- grated electro-optical switch. The principal advantage of this device is its spectral signature, which enables us to overcome the problems of inten- sity dependent devices. The soft-mode of smectic A* liquid crystals, enabling a continuous modulation of extraordinary refractive index, is used to design an integrated wavelength filter in the wavelength range of interest for optical communications. The principal advantages of such device include fast tuning speed, wide tuning range, low power dissipa- tion, and low cost.

A thermal (2D+1) spatial optical soliton in a dye doped liquid crystal

It has been possible to excite a spatial soliton in a dye doped liquid crystal confined in a capillary tube. The phenomenon is explained by a pure thermal nonlinear effect due to dye absorption.

Lasing Thresholds of Cholesteric Liquid Crystals Lasers

ABSTRACT Cholesteric liquid crystals (CLCs) are one dimensional photonic band-gap materials. Due to distributed feedback, low threshold mirrorless lasing can occur in dye-doped and pure CLCs. In order to optimize the lasing conditions, we have studied the dependence of the lasing threshold on dye concentration and sample thickness. In particular, we have studied dye concentrations in the range of 0.1–3.0 wt%, and cell thicknesses are in the range of 5–50 μm. We have found that the system has a shallow lasing threshold minimum and can operate efficiently in the range of dye concentrations of 0.3–2.4 wt% and sample thicknesses of 10–50 μm. We discuss the physical processes responsible for the observed behaviour.

Two-dimensional photonic quasicrystals by single beam computer-generated holography

Recently important efforts have been dedicated to the realization of a new kind of photonic crystals, known as photonic quasicrystals, in which the lack of the translational symmetry is compensated by rotational symmetries not achievable by the conventional periodic crystals. Here we show a novel approach to their fabrication based on the use of a programmable Spatial Light Modulator encoding Computer-Generated Holograms. Using this single beam technique we fabricated Penrose-tiled structures possessing rotational symmetry up to 23-fold, and a two-dimensional Thue-Morse structure, which is an aperiodic structure not achievable by multiple beam holography.

New Generation of Holographic Gratings Based on Polymer-LC Composites: POLICRYPS and POLIPHEM

Soft materials exhibit very attractive mechanical, optical, and electro-optical properties and easy processing techniques. Combined with holographic techniques, they can be patterned quite easily creating many unique structures. We present here few examples, of electrically-switchable passive opto-electronic components based on composite materials obtained from initial mixtures of photocurable monomers and liquid crystalline materials. Holographic exposure results in volume gratings consisting in alternate areas of photo-crosslinked polymer and phase separated LCs, respectively. We discuss the main features of this new generation of gratings and show how most of their performances can be enhanced with respect to other kinds of composite gratings, like H-PDLCs.

Wavelength dependence of optical reorientation in dye-doped nematics

Abstract The wavelength dependence of the optical reorientation of nematic liquid crystals is investigated in the presence of two dyes, using the z-scan technique. A strong correlation is found between the magnitude of the dyeinduced amplification of the optical torque and the dichroism of the system.

Optical sensing using dark mode excitation in an asymmetric dimer metamaterial.

We study the presence of dark and bright modes in a planar metamaterial with a double rod unit cell introducing geometric asymmetry in rod lengths. The dark mode displays a Fano-type resonance with a sharp asymmetric profile, rendering it far more sensitive than the bright mode to slight variations of the dielectric environment. This peculiar feature may envisage the possible application of the asymmetric dimer metamaterial as an optical sensor for chemical or biological analysis, provided that the effect of material losses on the dark mode quality factor is properly taken into account.

Optical characterization of liquid crystals by combined ellipsometry and half-leaky-guided-mode spectroscopy in the visible-near infrared range

In this paper, we present our results for the anisotropic refractive index measurements of commonly used liquid crystal (LC) materials (E7 and 5CB) in the whole visible–near infrared range. In order to achieve a high accuracy in the obtained values, we have employed a combination of two techniques, namely, Mueller matrix spectroscopic ellipsometry in transmission and half-leaky-guided-mode (HLGM). Measurements with the HLGM technique are usually performed at a single wavelength. In order to obtain a spectroscopic measurement based on the HLGM technique in a wide wavelength range, we modified our commercial VASE® ellipsometer. In particular, we designed a sample holder by means of which measurements in guiding structures are also possible. Thus, we take advantage of the wide-spectrum light, emitted from a standard Xe lamp of the ellipsometer, also in the HLGM setup. The complementary and in-one-setup utilization of both techniques has allowed us to overcome most of the problems previously encountered in ap...