F. Simoni

Nonlinear optical reorientation in hybrid aligned nematics

We give the complete theory of optically induced molecular reorientation in hybrid aligned nematic liquid crystals in which all the approximations usually adopted can be dropped. Applying a very general variational method of calculation and taking into account the anchoring energies on the boundaries of the liquid crystal cell we obtain a phase diagram (thickness versus optical field) exhibiting the possibility of ordered phase transitions from homeotropic to hybrid to planar configurations. Then we get the field dependent dielectric constant 〈δe〉 for any field strength and work out, as well, some numerical examples. In particular we show the previously assumed quadratic dependence on the field is true only at very low fields while a saturation occurs increasing the field. For larger sample thicknesses the saturation occurs at fields much lower than those necessary for the transition to a planar order. Finally we show that our analysis can be applied to the study of optical Fredericksz transitions in nema...

Mirrorless all-optical bistability in polymer dispersed liquid crystals

We report the observation of a bistable behavior from dye‐doped polymer dispersed liquid crystals. The interpretation of the phenomenon is given in the theoretical framework of an absorption which increases with the excitation of the material.

Tuning of a Dye Laser By a Liquid Crystal

Abstract We describe how a cholesteric liquid crystal device can be used in a laser cavity to induce tuning of the laser line. Tuning of the order of 10A is obtained by applying low voltages to the liquid crystals film. The problems, the advantages and the feasibility of the method is discussed, reporting several experimental data.

Electro-Optics of the Hybrid Nematic Cell

The first experimental data on the electro-optical properties of a hybrid aligned nematic cell submitted to an electric field are reported. Our theoretical treatment accounts for the observed behavior under an A. C. field, whereas the agreement in the D. C. case is not very good. This anomalous behavior can be explained by taking into account the flexoelectric characteristics of the examined cell. We show that, at small fields, the flexoelectric effect plays the role of changing the anchoring energy for the planar alignment.

Field Induced Molecular Reorientation in the Hybrid Nematic Cell

Abstract In this paper we review a general theoretical treatment of the interaction between a static or optical electric field and the molecular director of a hybrid aligned nematic liquid crystal. Preliminary experimental data are reported in the two cases for the first time.

Rotatory Power of Cholesteric Liquid Crystals

The dependence of the apparent rotatory power of cholesteric liquid crystals from applied fields is analysed. The main consequence of the theory is the prevision of a dramatic increase of the apparent rotatory power (ARP) varying the applied electric field if some conditions are fulfilled. The first direct measurements of optical rotation versus the electric field near the reflection band are reported; they show the expected large variation in the absolute value of ARP.

Nonlinear Optical Properties of Hibrid Aligned Nematics

Abstract We have studied the nonlinear optical behavior of Hibrid Nematic Liquid Crystals for light impinging on the sample at normal incidence. The results of the theory which takes into account the anchoring energy on the boundaries are reviewed. The experimental observation of the predicted effects i s also reported and the two more important properties are demonstrated: absence of a threshold for the onset of the nonlinear response and inhancement of the nonlinearity at low laser intensity in comparison with hometropically aligned samples.

Voltage Controlled Polarizer Using Cholesteric Liquid Crystals

Abstract We report the optical behavior of a cholesteric liquid crystal cell under a dc electric field. The peculiar features of the cell make it possible to use it as a voltage controlled polarizer for which an international patent is pending.

Optical Switching and Bistability in Polymer Dispersed Liquid Crystals

Abstract We report detailed measurements performed on a sample of a dye doped Polymer Dispersed Liquid Crystal (PDLC). It is shown that the laser induced phase transition of the liquid crystal droplets to the isotropic state can be exploited to get an optical switching and an optical bistable behavior. These effects can be satisfactory explained within the theoretical framework of an absorption which increases with the degree of excitation of the material.

Optical Modulation Induced by Stressed Cholesteric Liquid Crystals

Abstract Experimental Results on the rotatory power of a stresed Cholesteric Liquid Crystal are reported. The dependence on the applied strain is measured and a critical dependence on the light wavelenght is shown. Applications of this effect to optical modulation is also reported.