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

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

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 Raúl Josué Hernández et al., Soft Matter, 2020, 16, 7704–7714, 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

Multi-Wavelength Optical Patterning for Multiscale Materials Design

Laser interferometry is a consolidated technique for materials structuring, enabling single step and large area patterning. Here we report the investigation of the morphological modification encoded on a thin film of a photosensitive material by the light interference pattern obtained from a laser operating in multiline mode. Four lines with equal intensity are retained, with the same p linear polarization. An azopolymer is exploited as medium for the holographic recording. Optical microscopy and profilometer measurements analyze the modification induced in the bulk and on the surface of the irradiated area. We show that the intensity profile of the interference patterns of two laser beams is the one obtained assuming each line of the laser as an independent oscillator of given intensity and wavelength, and how these light structures are faithfully replicated in the material bulk and on the topography of the free surface. Patterns at different length scales are achievable in a single step, that can be traced back to both interference fringes and wave envelopes. The proposed multi-wavelength holographic patterning provides a simple tool to generate complex light structures, able to perform multiscale modifications of photoresponsive material

Multi-Wavelength Optical Patterning for Multiscale Materials Design

Laser interferometry is a consolidated technique for materials structuring, enabling single step and large area patterning. Here we report the investigation of the morphological modification encoded on a thin film of a photosensitive material by the light interference pattern obtained from a laser operating in multiline mode. Four lines with equal intensity are retained, with the same p linear polarization. An azopolymer is exploited as medium for the holographic recording. Optical microscopy and profilometer measurements analyze the modification induced in the bulk and on the surface of the irradiated area. We show that the intensity profile of the interference patterns of two laser beams is the one obtained assuming each line of the laser as an independent oscillator of given intensity and wavelength, and how these light structures are faithfully replicated in the material bulk and on the topography of the free surface. Patterns at different length scales are achievable in a single step, that can be traced back to both interference fringes and wave envelopes. The proposed multi-wavelength holographic patterning provides a simple tool to generate complex light structures, able to perform multiscale modifications of photoresponsive material

Physical processes in single and multiple photons additive nano-manufacturing of three-dimensional polymeric and metallic structures for advanced optics

Dottorato di Ricerca in Scienze e Tecnologie Fisiche, Chimiche e dei Materiali. Ciclo XXXIn the field of nanotechnologies the Two-Photons Direct Laser Writing (TP-DLW) is the most advanced optical technique for creating arbitrarily complex 3D structures in organic resists, featuring details down to 50 nm, well below the diffraction limit. More recently, this technique has been used in “resists” containing a photosensitive metallic precursor, activated by the two-photon absorption (TPA) process, allowing for the creation of metallic nanoparticles clusters inside to the focus figure of a highly focused laser beam, where the TPA threshold intensity is reached. The aim of my PhD work was the elucidation of the physical processes involved in the realization of 3D nanostructures made in different materials for applications in micro-fluidics and advanced optics. In particular, I carried out studies on both isotropic and anisotropic photoresists, and on metallic precursors. Concerning the isotropic photoresists, I have investigated the capabilities and the limits of the TP-DLW technique, on the fabrication of microfluidic systems and elements of millimetric size, with micro- and nano-features printed inside the channels. The best results in printing such millimetric structures in terms of geometrical compliance and fabrication time are achieved, by combining the single (SPA) and the two-photon absorption (TPA) processes. The latter one allowed for the creation of a shell, an internal structural scaffold and eventual microscopic details, whereas the former one to polymerize the bulk of the object. However, the development step of microfluidic systems (i.e. the removal of the un-polymerized resist) is quite challenging in general, due to possible swellings and consequent distortions in the structure geometry. In my PhD, I developed an effective protocol to face this issue. The application of the TP-DLW technique to anisotropic reactive mesogens (RMs) resulted in very interesting achievements, as it allowed for the fabrication of 3D solid structures, maintaining the optical properties of liquid crystals, in combination with the mechanical properties of polymers. Effects of the direct laser writing on the internal molecular order of the reactive mesogens have been thoroughly investigated, to ensure a fine control on the optical properties of 3D objects made in liquid crystalline elastomers. Analyses of the physical processes, which occur during TP-DLW and allow for tuning of the optical response of the printed 3D solid structures are shown. Appropriate doping of the reactive mesogens with dyes and chiral dopant agents were performed to investigate different fields of applications. In particular, a chiral agent confers helical order to the RMs, which show selective Bragg reflection of the impinging light in both wavelength and polarization. Micro-fabrication of 3D chiral structures is a brand new field that is paving the way to the creation of photonic devices, such as micro-laser of defined shape, white light reflective object, anti-counterfeiting and data storage systems. I performed a series of experiments aimed at demonstrating the possibility to manipulate the helical structural order of the liquid crystals during TP-DLW. As a consequence, multi-colour three-dimensional structure can be created. Finally, the possibility to include metallic details in polymeric objects or even to create metallic structures would pave the way for the DLW of metallic/polymeric nano-composites. I performed experiments with polymeric or hydrogel matrices doped with a suitable metallic precursor, in a free surface drop cast, or in cell segregated thin film, onto a glass substrate. In such system, I was able to create 1D gratings made of GNPs stripes with single or multiple laser sweep. I demonstrated that the stripe width increases with the laser power and the exposure time, showing a behaviour similar to the photo-polymerization, as expected. I also analysed the influence of the exposure time over the nano-particles size distribution and density and showed that by suitably adjusting the exposure time it is possible to maximize the occurrence of a given diameter. The experiments were aimed at elucidating the involved physical phenomena, beyond the bare optical absorption. In particular, the key-role of thermal and diffusive processes have been analysed. TPA leads to the photo-reduction of ions of AuCl4 – and the creation of GNPs, but to a local heating of the sample as well. Due to the very fast heating, a thermal shock-wave is generated and is responsible of the local dehydration in the spot area. Due the concentration gradients of the ions of gold precursor and of water, different diffusive processes take place, occurring on different timescales. Therefore, different characteristic times are observed for the ion and the water diffusion, in the polymeric matrix. My experiments demonstrate that the diffusive effects can be exploited for controlling the NPs density and size when a given energy dose is delivered in multiple shots, by tuning the time interval between each shots. Preliminary experiments on the possibility to control the growth of GNPs through the application of specific electric field during TP-DLW were performed as well. Last but not least, the possibility to use TP-DLW of metal precursor to realize smart platform rich in GNPs suitable to different application is shown. In particular, I demonstrate that, controlling the pitch and the size of GNPs stripes, it is possible to create both thermo-platform whose thermal response to external light is tuneable, and detecting substrates for Surface-Enhanced Raman Spectroscopy (SERS). The Raman spectra were recorded from samples immersed in a solution of rhodamine-6G (R6G), as well as, after exposure of the samples in xylene. SERS enhancement factors of up to ~104 were obtained for both rhodamine-6G and xylene.Università della Calabri