F. Bloisi

Self-phase modulation in nematic liquid-crystal films: detailed measurements and theoretical calculations

We present a detailed study of self-phase modulation in nematic liquid crystals, performing a quantitative comparison with the theory of diffraction. A quite satisfactory fit is obtained at low optical power by taking into account only orientational effects, as it is found that experimental observations are affected by laser-induced heating of the sample at higher optical power. Finally, new features in the saturation region are reported.

Matrix assisted pulsed laser deposition of melanin thin films

Melanins constitute a very important class of organic pigments, recently emerging as a potential material for a new generation of bioinspired biocompatible electrically active devices. In this paper, we report about the deposition of synthetic melanin films starting from aqueous suspensions by matrix assisted pulsed laser evaporation (MAPLE). In particular, we demonstrate that it is possible to deposit melanin films by MAPLE even if melanin (a) is not soluble in water and (b) absorbs light from UV to IR. AFM images reveal that the film surface features are highly depending on the deposition parameters. UV-VIS and FTIR spectra show both the optical properties and the molecular structure typical of melanins are preserved.

Photoinduced long-term memory effects in n-type organic perylene transistors

In this paper, the photoexcitation response of high mobility n-type organic field-effect transistors is analyzed. White light exposure of N,N′-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8H) transistors is demonstrated to promote the occurrence of metastable conductance states with very long retention times, similar to what has been previously reported for p-type compounds. Even in the absence of a gate-source voltage VGS, the complete recovery of the initial electrical condition can take up to 20 days. However, the initial state restoring is electrically controllable by the application of a positive VGS. These effects suggest that PTCDI-C8H is an interesting n-type material for the development of light-sensitive organic circuitry.

Matrix representation of axisymmetric optical systems including spatial filters.

A matrix approach is presented that allows one to describe a complex optical system by a matrix relating the field at the output plane to the field at the input one. The elements of the optical system may be all those characterized by an ABCD ray-transfer matrix, as well as any kind of film which introduces a wavefront modulation that can be described by a complex radial transmittance function. These include, as particular cases, stops and limiting apertures. No integral has to be computed. The method holds only for circularly symmetric optical systems and laser beams.

PDLC: influence of droplet order parameter in light transmittance

A Polymer Dispersed Liquid Crystal (PDLC) is a dispersion of liquid crystal microdroplets in a polymeric binder. The material is opaque white and becomes transparent when a low frequency electric field is applied. In this paper we present experimental results about the PDLC transmittance as a function of the applied voltage. We also present a mathematical model able to describe the optical behavior of the material in the framework of Anomalous Diffraction Approach light scattering theory.

Optoelectronic polarizer by PDLC

Abstract We present a new device based on Polymer Dispersed Liquid Crystals (PDLC). Applying a transverse low frequency electric field, the scattering cross section increases or decreases depending on the polarization of the impinging light. Depending on the choice of the refractive index of the polymeric matrix, such a device can be operated as a polarizer, or as an inverse shutter. In the first case one polarization component of an unpolarized beam is transmitted, while the other is scattered. In the second case an impinging polarized beam is totally scattered when an external electric field is applied and becomes partially transmitted when the external field is switched off.

Diffraction Patterns of Self-Phase-Modulated Laser Beams

Transverse optical intensity redistribution phenomena like self-focusing, self-defocusing and ring generation happen when a Gaussian laser beam is focused onto a nonlinear medium. We introduce a method for the evaluation of the diffraction pattern that is very simple, quick and suitable for computer implementation.

Temperature dependence of the optical phase shift in a polymer dispersed liquid crystal

Abstract We report a study of the temperature dependence of the optical phase shift induced by a polymer dispersed liquid crystal subjected to a low frequency electric field. In the framework of a recently introduced model we point out the role of two parameters strictly depending on the PDLC configuration: an “effective elastic constant” K d and a droplet “voltage-constant” V d. We show that a suitable temperature dependence of such parameters is able to give a good agreement between experimental data and theoretical values.

Study of thermally induced optical bistability in a twisted nematic liquid crystal

We report the observation and study of optical bistability in the power transmitted by a twisted nematic liquid crystal bounded by crossed dichroic polarizers. A detailed study of the dependence of the phenomenon on the sample initial temperature has been performed. The effect is a good test for a simple theoretical model which describes the isotropic droplet formation in a nematic sample under laser illumination. A good agreement is found between experimental results and theoretical predictions. Applications of the effect to realize temperature sensors are envisaged.

Comparison of nondiffracting laser beams

Abstract We study the near axis power transport efficiency of truncated nondiffracting beams by presenting a mathematical method able to compute it, in the limits set by the Fresnel approximation. We apply our technique to truncated Bessel, Weber and gaussian beams showing the advantages of the nondiffracting beams over the other.