25 research outputs found
Surface-induced photorefractivity in twistable nematics: toward the all-optical control of gain.
We report the first two-beam coupling investigation of the surface-induced photorefractive effect (SIPRE) in optically twistable nematic liquid crystal cell. The unique space-charge field of SIPRE is exploited to achieve optical tuning of the photorefractive gain. A reconfigurable photoaligning substrate is used to adjust the twist angle, which is proved to be a control parameter for the photorefractive gain. The amplitude of the optical modulation increases gradually with the twist. Its phase shift changes from 0 degrees to 90 degrees with the polarization state of the two interfering beams. These results pave the way to the all-optical control of the photorefractive gain
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. (c) 2007 Optical Society of Americ
Polarization gradient: exploring an original route for optical trapping and manipulation.
We report a study of the capabilities of an optical tweezer based on polarization gradient. We use a light polarization pattern that is able to simultaneously exert forces and torques in opposite directions depending on the particle's position. It allows to perform oscillatory displacements and control the sense of rotation of several particles inside a uniformly illuminated region. Unconventional trapping of spinning particles in circularly polarized fringes has been observed, which suggests the involvement of hydrodynamic forces
Polarization holograms allow highly efficient generation of complex light beams
We report a viable method to generate complex beams, such as the non-diffracting Bessel and Weber beams, which relies on the encoding of amplitude information, in addition to phase and polarization, using polarization holography. The holograms are recorded in polarization sensitive films by the interference of a reference plane wave with a tailored complex beam, having orthogonal circular polarizations. The high efficiency, the intrinsic achromaticity and the simplicity of use of the polarization holograms make them competitive with respect to existing methods and attractive for several applications. Theoretical analysis, based on the Jones formalism, and experimental results are shown
Liquid crystal as laser medium with tunable gain spectra.
Amplified spontaneous emission intensity and gain spectra in polarized light have been measured in a dye doped nematic liquid crystal for different orientation of its optical axis and pump intensity. A possibility for switching the gain of the liquid crystal by an external electric field is shown experimentally. The liquid crystal materials with field controlled gain can be used in microlasers and light micro-amplifiers in both planar and waveguiding geometry. (C) 2008 Optical Society of America
Correction: Collective motion of chiral Brownian particles controlled by a circularly-polarized laser beam
Correction for 'Collective motion of chiral Brownian particles controlled by a circularly-polarized laser beam' by RaĂșl JosuĂ© HernĂĄndez et al., Soft Matter, 2020, 16, 7704â7714, DOI: 10.1039/C9SM02404B
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
Polarization holographic recording in polymeric and liquid crystalline materials
Dottorato di Ricerca in âScienze e Tecnologie delle Mesofasi e dei Materiali Molecolari, Ciclo XIX, a.a. 2005-2006Optical studies related to polarization holographic recording in photosensitive materials, as azo
compounds, liquid crystals and polymeric mixtures, and dye doped polymers, were carried out.
The interest in this type of holographic recording is due to the improved signal to noise ratio and
the possibilities for image and signal processing, optical switch, beam steering, optical polarizers
and selective erasure. We focused our investigation on the mechanisms that can possibly induce
diffraction gratings in these different types of photosensitive materials, on the peculiarities of the
achieved diffractive devices and on their possible applications. In particular we investigated the
effects of polarization holography on azo-dye Langmuir-Blodgett films, on polymer dispersed
liquid crystal (PDLC), and on liquid crystal films confined by dye-doped polymers aligning
layers.
In the first system, conventionally used for polarization holographic recording, we investigated
the influence of the particular Langmuir-Blodgett deposition technique on the features of the
recorded structures in order to obtain pure polarization gratings. The absence of surface reliefs
gratings (SRG), the stability of the recorded devices and the high induced birefringence of the
selected material, open up the possibility of interesting applications. In particular, we report the
design and the implementation of a photopolarimeter for simultaneous measurements of Stokes
parameters of light, in which the basic element is the actual polarization grating.
PDLC is a non conventional system for polarization holographic recording, because no azocompounds
are present in the polymeric and liquid crystalline mixtures. Polarization holographic
storage produces diffraction gratings that originate mainly from the liquid crystal alignment
inside the droplets of the solid polymeric matrix, created during the polymerization and phase
separation processes. Polarization properties and electro-optical switching behaviour of the
gratings are studied. We also report the unexpected observation of SRG in a system without azo
compound, where photoisomerization and chromophore reorientation processes do not occur.
In the last systems, we exploit a new method for spatially varying liquid-crystal alignment using
patterned surfaces obtained by means of a polarization holographic exposure on a dye-doped
polyimide. This idea is based on the fact that holographic gratings on some photosensitive
material provide a periodic alignment of the nematic liquid crystals. In fact, putting in contact a
Polarization holographic recording in polymeric and liquid crystalline materials
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thin film of liquid crystal with the aligning layers, we obtain a replica-grating in the bulk with
the same properties of the gratings recorded on photosensitive layers.
We describe the high flexibility of these replica-gratings, related to the control of the diffraction
efficiency by means of an external electric field, and the very singular properties of the
polarization states of the beams diffracted from this device.
We also obtain two dimensional (2D) gratings consisting of a 2D array of differently twisted
structures of nematic liquid crystal, achieved by a crossed assembling of polarization holograms
recorded at the surfaces of the aligning substrates. These devices diffract the incident beam in
several diffracted beams with various polarization states at the same time. The energy
distribution can be controlled by means of the polarization state of the incident beam.
Additionally, the distribution of the intensity on the diffracted beams can be completely
controlled by means of a low external applied voltage. These features make the optical devices
very interesting for beam steering, beam shaping and other modifications of light intensity or
phase.UniversitĂ della Calabri