Periodic and aperiodic liquid crystal-polymer composite structures realized via spatial light modulator direct holography.

In this work we present the first realization and characterization of two-dimensional periodic and aperiodic POLICRYPS (Polymer Liquid Crystal Polymer Slices) structures, obtained by means of a single-beam holographic technique exploiting a high resolution spatial light modulator (SLM). A first investigation shows that the gratings, operating in the Raman Nath regime, exhibit a morphology and a electro-optical behavior that are typical of the POLICRYPS gratings realized by two-beam interference holography.

POLICRYPS structures as switchable optical phase modulators.

We report on the electrically controlled optical phase modulator behavior of light sculptured periodic structures made of polymer slices alternated to films of well aligned Liquid Crystals (POLICRYPS). Arbitrarily polarized light normally incident on the structure experiences a birefringence that depends on the anisotropy of the composite liquid crystalline material and on the geometrical cell parameters. The sample behaves as a retardation plate in good agreement with the Jones matrices formalism. Birefringence tuning is obtained by applying a suitable voltage, while a negligible birefringence variation is detected by increasing the incidence power. This makes POLICRYPS structures suitable as switchable phase retarders for high power laser beams.

Different reorientational regimes in a liquid crystalline medium undergoing multiple irradiation.

We present a numerical approach to the nemato-elasticity differential equation in a nematic liquid crystal cell when irradiated with multiple gaussian beams. Solutions have been carried out on a configuration with two coplanar beams illuminating the sample in order to compare it with particular nonlinear phenomena experimentally studied in the past. A new set of experimental measures were realized confirming the validity of the numerical model. Solutions for an instable case showing nonlocal effects are also presented as an example of the broader class of systems this approach can describe.

Self-healing generation of spatial solitons in liquid crystals.

We demonstrate that, in suitably designed cells with undoped nematic liquid crystals, extraordinary-wave spatial solitons can be excited at every applied voltage without adjustments in the input polarization. Their walk-off, hence direction of propagation, is externally controlled over angles as large as 7 degrees. The results pave the way not only to polarization-forgiving generation but also to voltage readdressing of these extraordinary-wave nematicons.

Dynamical behaviour of holographic gratings with a nematic film--Polymer slice sequence structure.

We have studied the dynamical response of a new kind of holographic grating to a pulsed electric field. The structure, named POLICRYPS, consists of a sequence of homogeneous polymer slices separated by liquid-crystal films. We have found that both the rise and fall response times depend on the pulse duration and amplitude of the applied electric field. We propose a phenomenological explanation for this behaviour, outlining the possibility of getting response times of the order of 100 micros by properly choosing the pulse shape. Results are very interesting for developments in which POLICRYPS gratings could be proposed as basic elements in photonic devices for telecom applications, and induce to a deeper analysis of the microscopic interaction between liquid crystal and polymer interfaces in these samples.

Electro-optic properties of switchable gratings made of polymer and nematic liquid-crystal slices.

We report the diffraction properties at wavelengths of 632.8 and 1550 nm for volume transmission gratings made of a sequence of continuously aligned nematic liquid-crystal layers separated by isotropic polymer slices. The gratings are generated by holographically curing a solution of liquid crystal diluted in an isotropic prepolymer by means of a laser beam at a wavelength of 352 nm with a total intensity of approximately 10 mW/cm2. A diffraction efficiency of 98% was measured, and an electric field as low as 5 V/microm switches off the phase grating. Measured angular spectra are fitted by use of the modified coupled-mode theory including the effects of grating birefringence.

Self-pulsing of the laser light transmitted by a nonlinear liquid-crystal interface near the phase transition.

We report bistability and self-pulsing effects in the switching from total internal reflection to a transmitting state at a glass-liquid-crystal interface. The role of the phase transition to the isotropic state is underlined, and the generation of 20-msec pulses from a continuous laser beam is observed.