Generation of integer and fractional vortex beams based on liquid crystal electronically reconfigurable spiral phase plates.

The manufacturing and characterization of a large-size 72-electrode liquid crystal-based reconfigurable spiral phase plate (SPP) is presented. The SPP is addressed by a custom-made driver with 72 independent channels, which allows for the generation of any arbitrary integer or fractional optical vortex beam with topological charges ranging from -24 to +24. The 25 mm diameter device is fabricated using direct laser writing, leading to a fill factor over 99%. The device performance and flexibility exceed previous transparent reconfigurable SPP in terms of size, tuning range, and fill factor. The device and the light path have been simulated using the angular spectrum propagation method, showing excellent correspondence.

Identification of dyes and matrices for dye doped polymer waveguide emitters covering the visible spectrum.

Polymer based photonic devices offer the possibility cost effective roll-to-roll manufacture of photonic devices. The incorporation of luminescent dopants within a solid polymer waveguide allows for the generation of light within the device avoiding tedious mechanical light coupling. However, when a dopant is embedded in a solid matrix, depending on its concentration and the nature of materials involved, the emitted light may be quenched due to aggregation effects. In this work, thin films and ridge waveguides processed by UV-photolithography have been successfully obtained from a selection of standard photopolymerizable organic monomers, SU8, EpoCore and OrmoStamp doped with a selection of standard dyes like Rhodamine-B, Coumarin-540A and Pyrromethene-580. All structures were manufactured on glass substrates. An analysis of the solubility and optical properties including band gap energy, absorption coefficient ([Formula: see text]) and fluorescence of the doped photoresists at different concentrations has been performed. Photoresists doped with Rhodamine-B shows a higher energy of indirect allowed band gap transition (2.04-2.09 eV) compared to previously reported pure Rhodamine-B thin films (1.95-1.98 eV). Fabrication protocols of dye doped photoresists covering the entire visible spectrum is established.

Rapid detection of pathogens using lyotropic liquid crystals.

Lyotropic liquid crystals play an important role in many biological environments, such as micelles, liposomes, and phospholipid bilayers of cell membranes. In this work, we explore the performance of lyotropic liquid crystals as biosensors for macromolecules, proteins and whole microorganisms in hydrophilic media, i.e., the natural media where these specimens exist. The aim is to detect specific targets employing simple, unpowered sensors that can be used in the field, with minimum additional equipment. A number of different structures have been explored. The novelty in this work is the inclusion of a new optical effect, flow enhanced amplification, that allows for the semiquantitative detection of microscopic targets in lyotropic liquid crystal cells using the naked eye only.

Dynamic multilevel spiral phase plate generator.

The design and characterisation of a reconfigurable multi-level spiral phase plate is shown. The device is based on a pie-shape liquid-crystal structure with 24 slices driven by custom electronics that allow independent excitation control of each electrode. The electrooptical cell was manufactured using maskless laser ablation lithography and has shown an unprecedented high fill factor. The topological charge can be dynamically changed between 1, 2, 3, 4, 6, 8 and 12. The device has been calibrated and characterised at 632.8 nm but can be employed at any wavelength in the visible and near infrared spectrum, just modifying the driving parameters of the electrodes. The experimental results have been compared to predictions derived from simulations. An excellent correspondence between theoretical and experimental result has been found in all cases.

Nematic liquid crystal reorientation around multi-walled carbon nanotubes mapped via Raman microscopy.

We have studied the formation of topological defects in liquid crystal (LC) matrices induced by multiwalled carbon nanotubes (MWCNTs) and external electric fields. The defects are ascribable to a distortion of the LC molecular director in proximity of the MWCNT surface. The system is analyzed macroscopically using spectroscopic variable angle ellipsometry. Concurrently, confocal micro-Raman spectroscopy is used to study the system state at the microscale. This allows to acquire a three-dimensional, spatially-resolved map of the topological defect, determining scale length variations and orientation topography of the LC molecules around the MWCNT.

Tunable liquid crystal cylindrical micro-optical array for aberration compensation.

A tunable aberration compensation device for rectangular micro-optical systems is proposed and demonstrated. This device, which is based in nematic liquid crystal and a micro-electrode structure, forms gradients in the index of refraction as a function of voltage. We have developed a fringe skeletonizing application in order to extract the 3D wavefront from an interference pattern. This software tool obtains the optical aberrations using Chebyshev polynomials. By using phase shifted electrical signals the aberrations can be controlled independently. A complete independent control over the spherical and coma aberration has been demonstrated. Also, an independent control over the astigmatism aberration has been demonstrated in a broad range. This device has promising applications where aberration compensation is required. The independent compensation achieved for some coefficients, such as astigmatism for example, is more than 2.4 waves.

Dependence of the molecular orientational states on the surface conditions for the "V-shaped switching" in a ferroelectriclike liquid crystal sample.

The three-component mixture showing the V-shaped switching in a rubbing cell is investigated using a temperature gradient cell (TGC) where the epitaxial method of alignment, but without an alignment layer, is used. The molecular alignment and the switching characteristics in a TGC are compared with the rubbing sandwich and the open cells. The uniform or the twisted state is observed in the TGC, depending on the surface condition whether indium-tin oxide (ITO) or glass, while the uniformlike state with the average optical axis parallel to the smectic layer normal emerges only in the rubbing cell. The cancellation of the surface polarization charges by the surface free charges on the ITO surfaces determines the molecular alignment state in the TGC. From these results, we conclude that the nonpolar in-plane anchoring plays an important role in giving a uniformlike state in the rubbing cell.

A non-linear least-squares approach to the resolution of heterogeneous fluorescence from multitryptophan proteins.

Protein heterogeneous fluorescence results from the different microenvironment of each emitting chromophore. The structural and dynamic information contained in this emission can be extracted to some extent by selective quenching experiments. In this work, graphical and numerical methods are described for the analysis of protein emission in terms of three separated contributions: a fluorescence fraction which is not accessible to the quencher and two additional fractions with different solvent exposure. 'Static quenching' deviations from Stern-Volmer behaviour are also discussed. The application of these methods is exemplified on simulated quenching experiments and real data on acrylamide quenching of lysozyme fluorescence.