G. Cipparrone

Phototunable lasing in dye-doped cholesteric liquid crystals

In this letter, we report the results of phototunable lasing in dye-doped cholesteric liquid crystals (DD–CLC). Photoexcitation of DD–CLC films gives rise to laser emission in the violet-UV range. Control of the structure of the chiral dopant driven by UV phototransformation is exploited in order to obtain a permanent variation of the cholesteric pitch. Laser emission wavelength tuning, by means of photoinduced shifting of the selective reflection band of the cholesteric liquid crystals is established. A tuning interval of about 35 nm, in the wavelength range of 385–415 nm, is observed.

Widely tunable ultraviolet-visible liquid crystal laser

Quasicontinuous tuning of a dye doped cholesteric liquid crystal (CLC) mirrorless laser in the ultraviolet-visible wavelength range is demonstrated using a single original device based on CLC as resonator and several resonant dyes. The thought is to combine the CLC pitch gradient and the distribution of different dyes. In the same cell, six dyes are combined in order to nearly cover the whole wavelength range from ultraviolet (370 nm) to red (680 nm). Some of the used dyes work as emitter, while others work in the Forster regime to decouple the excitation and emission processes. The relevant aspect of the device is that a simple translation of the cell respect to the same pump beam enables fine tuning of the laser wavelength in almost all the visible range, up to the ultraviolet.

Highly efficient liquid crystal based diffraction grating induced by polarization holograms at the aligning surfaces

Low-scattering and highly efficient orientational gratings are obtained, exploiting polarization holography to modulate the in-plane anchoring axis at both surfaces of a planar nematic cell. Polarization-sensitive azo-dye-doped polyimide films are used as aligning layers. For proper values of cell thickness and spatial periodicity of the gratings, the director configuration in the nematic bulk is a perfect replica of the polarization gratings recorded on the aligning layers. High first-order diffraction efficiency, up to 98%, is observed in thin grating regime. External ac voltage allows to adjust the efficiency over the whole range for arbitrary cell thickness.

Chiral self-assembled solid microspheres: a novel multifunctional microphotonic device.

Solid chiral microspheres with unique and multifunctional optical properties are produced from cholesteric liquid crystal-water emulsions using photopolymerization processes. These self-organizing microspheres exhibit different internal configurations of helicoidal structures with radial, conical or cylindrical geometries, depending on the physicochemical characteristics of the precursor liquid crystal emulsion.

Permanent polarization gratings in photosensitive Langmuir–Blodgett films

An investigation was carried out on thin permanent phase polarization gratings recorded in Langmuir–Blodgett films using two orthogonal circularly polarized Ar-ion laser beams. The films are composed of amphiphilic azo-dye molecules and manifest an extremely large value of photoinduced optical anisotropy, Δn≈0.36. The experimental results are in excellent accord with the theoretical model based on the Jones matrix representation of the polarization pattern. The gratings are stable at least, for half a year.

Polymer dispersed liquid crystals: effects of photorefractivity and local heating on holographic recording

Abstract We report a detailed investigation of the photorefractive origin of permanent orientational gratings recorded by holographic technique in dye-doped polymer-dispersed liquid crystal (PDLC) films. This investigation was performed using wave-mixing characterization by two beam coupling (TBC) experiments. We determined the TBC gain by means of asymmetric energy transfer measurements and the phase shift by TBC translation technique measurements. The photorefractive origin of the effect were proved by the experimental results. Nevertheless, some peculiarities showed the presence of other mechanisms that combined with the photorefractivity to give the observed storage effect. The long time stability and some characteristic of the recorded structure have been explained as a thermal fixing of the grating. During the writing process, due to the strong light absorption, the sample was locally heated; under these conditions, the space charge field effect can modify the droplets interfaces and consequently the orientation of liquid crystal inside the droplets. After removing the two writing beams, the new configuration was frozen.

Orientational gratings in dye-doped polymer-dispersed liquid crystals induced by the photorefractive effect

We report the observation of orientational gratings induced by the photorefractive effect in dye-doped polymer-dispersed liquid crystals. The photorefractive origin of the grating-induction effect is supported by the results of two-beam coupling experiments and by the possibility of erasing the grating by uniform illumination. For all the samples a stable memory effect was observed.

Electrically tunable two-dimensional liquid crystals gratings induced by polarization holography

Two-dimensional (2D) gratings made up of an array of differently twisted nematic structures are obtained by crossed assembling of 1D polarization holograms recorded at the photoaligning substrates. The rotating linear polarization pattern, produced by the interference of two opposite circularly polarized beams, is recorded on the azo-dye doped polyimide aligning layers. The 2D gratings diffract light in different directions with different polarization states, that can be optically controlled. Orthogonal circularly and linearly polarized diffraction orders are simultaneously obtained irradiating the grating with a linearly polarized beam. An external ac voltage allows to completely control the diffracted energy distribution.

Diffraction gratings in polymer-dispersed liquid crystals recorded by means of polarization holographic technique

We report an observation of holographic gratings recorded exposing a homogeneous mixture of pre-polymer and liquid crystal to a polarization light pattern. This polarization pattern is obtained by a superposition of two waves with orthogonal linear polarization. The idea to use this technique to realize holographic gratings come from the possibility of controlling the liquid crystal alignment inside the droplets, through the photoalignment, during the photoinduced polymerization process. On the contrary, the usual methods of writing diffraction gratings in polymer-dispersed liquid crystals are based on intensity holographic technique that controls the polymerization process. Diffraction gratings, composed of liquid crystal droplets having a particular internal alignment, are observed.

Permanent polarization gratings in photosensitive Langmuir–Blodgett films for polarimetric applications

The study of a diffractive device for polarimetric applications is reported. The device consists of a diffraction grating recorded by two interfering opposite circularly polarized beams and is based on a Langmuir–Blodgett film of an azo-compound material. This material provides long-time stability and high photoinduced birefringence that makes the grating highly diffraction efficient. We show how polarization gratings permanently stored in these films are suitable for applications as photopolarimeter components.