P. Pagliusi

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.

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.

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.

Polarization-dependent optomechanics mediated by chiral microresonators

Chirality is one of the most prominent and intriguing aspects of nature, from spiral galaxies down to aminoacids. Despite the wide range of living and non-living, natural and artificial chiral systems at different scales, the origin of chirality-induced phenomena is often puzzling. Here we assess the onset of chiral optomechanics, exploiting the control of the interaction between chiral entities. We perform an experimental and theoretical investigation of the simultaneous optical trapping and rotation of spherulite-like chiral microparticles. Due to their shell structure (Bragg dielectric resonator), the microparticles function as omnidirectional chiral mirrors yielding highly polarization-dependent optomechanical effects. The coupling of linear and angular momentum, mediated by the optical polarization and the microparticles chiral reflectance, allows for fine tuning of chirality-induced optical forces and torques. This offers tools for optomechanics, optical sorting and sensing and optofluidics.

Mirrorless lasing from nematic liquid crystals in the plane waveguide geometry without refractive index or gain modulation

Lasing from dye doped liquid crystals was observed in the nematic and isotropic phases in the plane waveguide geometry without mirrors and special modulation of the gain or refraction index by a holographic or other technique. The light was generated in a liquid crystal layer confined by two glasses with semitransparent electrodes, which formed an optical waveguide. A strong light amplification occurred along the length of a narrow stripe produced by the pump beam with the necessary feedback provided by the walls of the waveguide in the direction perpendicular to the stripe. The conditions for lasing were fulfilled for the lowest group velocity waveguide modes bouncing at the angles very close to the total reflection angle at the waveguide-glass interface. These modes leaked into the glasses were guided therein at very grazing angles and left the cell from its edges.

Surface relief gratings on polymer dispersed liquid crystalsby polarization holography

We report the observation of surface relief gratings (SRGs) on polymer dispersed liquid crystal films after polarization holographic recording, demonstrating the formation of SRGs in systems without azo compounds, where photoisomerization and chromophore reorientation processes do not occur. Permanent SRGs, several hundred nanometers deep, are recorded on the surface of a polymeric material containing oriented liquid crystal droplets. The results suggest that SRG growth under uniform intensity irradiation is not exclusively related to the photoisomerization, but is a more general phenomenon which can involve different photoinduced chemical and physical mechanisms sensitive to the light polarization state. These effects contribute to the formation of anisotropic structures during the recording process.

Simple voltage tunable liquid crystal laser

Lasing has been observed in a dye-doped liquid crystal placed between one substrate with a large transparent electrode and the other with a system of parallel nontransparent electrode stripes of 15μm period. The stripes operate as a shadow mask providing the spatial modulation of the pump beam and material gain and, with the voltage applied, it additionally creates a Bragg resonator structure due to the spatial modulation of refraction. With that simple cell, a voltage induced tuning of the spectral positions of laser lines over 25nm has been demonstrated.

Lasing in cholesteric liquid crystal cells: Competition of Bragg and leaky modes

Measurements of the angular dependency of luminescence and lasing spectra were carried out for a planar layer of a dye-doped cholesteric liquid crystal (CLC). In addition to the known Bragg emission mode propagating along the cell normal, we observed that leaky modes also depend on the pump intensity and on the dimension and shape of the transverse cross section of the excitation beam. The leaky modes are valuable as a specific lasing regime but impose serious limitation on the energy of the Bragg lasing modes. This has to be taken into consideration when designing high energy lasing devices based on CLC.

Dynamic grating features for the surface-induced photorefractive effect in undoped nematics

We investigate the highly sensitive surface-induced photorefractive gratings, observed in undoped nematic planar cells for low dc voltage (few volts) and laser intensity (few milliwatts per square centimeter). Forced-light-scattering and two-beam coupling measurements on E7 nematic samples, aligned with rubbed polyvinyl alcohol layers, verify the orientational effect of a space-charge field and the crucial role of the polymer–liquid-crystal interface in the photoinduced processes, through a wavelength-dependent photoelectric activation. The experimental behavior supports the surface-induced photorefractive effect model, according to which light with the proper wavelength locally reduces the interfacial charge density, thus producing a space-charge electric field in the cell.

Polarization Holographic Recording in Amorphous Polymer with Photoinduced Linear and Circular Birefringence

Polarization grating recording in an amorphous and nonchiral azo copolymer has been investigated. The reported study shows that the amorphous polymeric film undergoes a light-guided inhomogeneous supramolecular modification as a consequence of the illumination with proper polarized light patterns, acquiring new functionalities. Both linear and circular, spatially modulated, photoinduced birefringences occur, attaining their peak values in the linearly and circularly polarized regions of the light pattern, respectively. The photoinduced anisotropic structures strongly affect the polarization state of the light propagating through them, and the characterization of their optical diffraction enables measurement of the amplitude of the linear and circular birefringences. The recorded gratings show long-time stability and full reconfigurability functional to the multiple holographic recording.