First-order optical Freedericksz transition in a dye-doped nematic liquid crystal.

A first-order optical Freedericksz transition in a nematic liquid crystal doped by a small quantity of a dichroic dye with negative dichroism is observed. The initial homeotropic state corresponds to maximum absorption for the linearly polarized incident beam, while reorientation reduces absorption increasing the optical torque: this process produces the positive feedback leading to the observed bistable behavior.

Dark nematicons.

We experimentally demonstrate and model dark spatial solitons in azo-doped liquid crystals, in the presence of saturation and nonlocality of the effective nonlinearity due to changes in molecular order. The guiding properties of dark solitons are probed with a weak input of different wavelength.

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.

Periodic structures generated by light in chiral liquid crystals.

We discuss materials that reveal fundamental intercoupling of light and chirality in creation of complex structures. These materials are based on cholesteric liquid crystals (CLCs) photosensitized by azobenzene nematics. Transformation of the one-dimensional periodic structure of such CLCs into complex spatial patterns takes place on macroscopic scales, over the whole area of the CLC layer, under the influence of low power radiation including LED, ambient illumination, and sunlight. The obtained structures, with their origin in the strain of the CLC layers caused by trans-cis photoisomerization precede a shift in the bandgap position of the CLCs. The effect is observed both in red-shifting as well as blue-shifting CLCs.

Nonlinear refraction in photoinduced isotropic state of liquid crystalline azobenzenes.

The photoinduced isotropic state of liquid crystalline azobenzenes exhibits one of the strongest nonlinear optical refraction known for isotropic materials. The optical nonlinearity of these materials is large in a wide band of the visible spectrum and is accompanied with high, over 90%, transmission for red wavelengths.

Photosensitive liquid crystals with nanoparticulate internal structure.

Stabilization of thermodynamic relaxation of photoinduced cis isomers of azobenzene liquid-crystal molecules is observed in nanoparticulate networks. The phenomenon permits bistability of the phase state (anisotropic and isotropic) of the material and reversible all-optical switching between those states, resulting in strong changes in the light-scattering properties of the material. Recording of complex optical structures with high spatial resolution with the aid of laser beams of different wavelengths is demonstrated.

BRIEFING: beam reconstruction by iteration of an electromagnetic field with an induced nonlinearity gauge.

Measurement of the local intensity and phase distributions in the small focal waist of a laser beam with the use of a thin nonlinear-optical layer is discussed. We suggested and tested in a computer experiment the beam reconstruction by iteration of an electromagnetic field with an induced nonlinear gauge (BRIEFING) algorithm, which allows one to extract the absolute intensity (in watts per square centimeter) and phase distributions from two intensity patterns in the far-field zone: with and without nonlinearity.