Liquid crystals in focus

Liquid Crystal lenses may be used for imaging or projection systems, in portable devices and vision correction in head-mounted devices. There are many types of LC lenses with tunable focal length, but only few have achieved practical importance, due to their small size or due to their limited focusing capability. The three most important classes of LC lenses with variable focus are lenses with curved surfaces, flat gradient index lenses and composite lenses. Fresnel lenses, included in flat gradient lenses, achieve better aperture size (1-2 cm) in thin cells and fast response, but they are on-off lenses or they have a complicated multilevel electrode structures to achieve different focal power. In this work we present a novel approach to make tunable LC Fresnel lenses, with a very simple electrode structure

Electrooptic response in tunable liquid crystal microlenses

VII Reunión española de optoelectrónica: OPTOEL2011: Santander, 29 junio - 1 julio de 2011Se ha fabricado y caracterizado un array de microlentes cilíndricas sintonizables de cristal líquido (CL). Se ha diseñado un electrodo de control en forma de peine que, junto a una capa metálica de alta resistividad, crea un gradiente de tensión en el dispositivo. Este gradiente de tensión genera un índice de refracción gradual en el ancho de la lente cuando las lentes se controlan mediante un campo eléctrico de control externo de amplitud optimizada. Se han estudiado los retardos de fase generados por la distribución de campo en cada lente utilizando métodos interferométricos. Así mismo, se han realizado medidas de la distribución angular de la luminancia a través de la lentícula con instrumentación específicaOne-dimensional array of cylindrical tunable liquid crystal (LC) lenses has been manufactured and characterized. A comb electrode has been designed as a control electrode on the top glass substrates that, in addition to a high resistance metallic layer, causes a voltage gradient inside the device. This voltage gradient generates a radial graded refractive index along each lens, as lenses are switched by applying amplitude optimized waveforms. Phase retardation profiles, generated by the radial electric field distribution on each lens, were measured by a convectional interferometric technique. Also, luminance angular distribution through the lenses has been measured with special equipments

An All-Organic Flexible Visible Light Communication System

Visible light communication systems can be used in a wide variety of applications, from driving to home automation. The use of wearables can increase the potential applications in indoor systems to send and receive specific and customized information. We have designed and developed a fully organic and flexible Visible Light Communication system using a flexible OLED, a flexible P3HT:PCBM-based organic photodiode (OPD) and flexible PCBs for the emitter and receiver conditioning circuits. We have fabricated and characterized the I-V curve, modulation response and impedance of the flexible OPD. As emitter we have used a commercial flexible organic luminaire with dimensions 99 × 99 × 0.88 mm, and we have characterized its modulation response. All the devices show frequency responses that allow operation over 40 kHz, thus enabling the transmission of high quality audio. Finally, we integrated the emitter and receiver components and its electronic drivers, to build an all-organic flexible VLC system capable of transmitting an audio file in real-time, as a proof of concept of the indoor capabilities of such a system.This Project was funded by Comunidad de Madrid through the SINFOTON-CM Research Program (S2013/MIT-2790), and the Spanish Ministry of Economy, the Agencia Estatal de Investigación and European Union's FEDER through the TEC2016-77242-C3-(1-R, 2-R and 3-R) AEI/FEDER, UE Projects

Evaluation of replicas manufactured in a 3D-printed nanoimprint unit

Nanoimprint lithography has become a useful tool to prepare elements containing nanoscale features at quite reasonable cost, especially if the fabrication elements are created in the own laboratory. We have designed and fabricated a whole nanoimprint manufacturing system and analyzed the resulting surfaces using ad hoc packages developed on an open-software AFM image analysis suite. To complete the work, a number of polymers have been thoroughly studied in order to select the best material for this implementation. It turned out that the best alternative was not always the same, but depended on the application. A comparative study of the polymers, which takes into account the values and dispersion of numerous sample parameters, has been carried out. As a large number of samples was prepared, an automatized procedure for characterization of nanoimprint surfaces had to be set up. The procedure includes figures of merit for comparative purposes. Materials without the requirement of a solvent were found to be superior for most nanoimprint applications. A large dispersion of the samples was found

mm-Wave Metasurface Unit Cells Achieving Millisecond Response Through Polymer Network Liquid Crystals

The slow response time of planar Liquid Crystal (LC)-based phase-shift metasurface and Reconfigurable Intelligent Surfaces (RIS) cells is addressed in this paper by introducing a polymer network LC (PNLC) mixture suitable at mm-wave bands. Since the conventional effective isotropic model used in optical cells for describing the PNLC is not suitable in RF, an effective anisotropic and uniaxial model for such mixture is provided and experimentally validated at 100 GHz for the first time. In order to compare the temporal performance and tunability of the PNLC, transmissive and reflective cells, containing conventional LC and PNLC, have been manufactured and measured at optical and mm-wave frequencies. The temporal responses of PNLC are also compared for both RF and optical cells, obtaining relevant differences between their improvement factors, which are also discussed. Specifically, a 50 fold response time improvement is attained in cells designed to work at 100 GHz, although at the expense of a 3X tunability reduction. The model, which is robust to varying angle of incidence and cell dimensions, has been experimentally validated by designing and manufacturing a PNLC reflectarray cell of a different geometry. The cell shows reconfigurability times of 210ms, representing a significant improvement with respect to state-of-the-art response time in mm-wave cells, which are in the order of several seconds

A straightforward method of measuring MPRT using LC test cells

A simple method based on integration over time to measure motion-picture response time (MPRT) has been developed. Liquid-crystal response time (LCRT) alone cannot express the motion blur perceived by the observer; one must also take into account the transition between intermediate gray levels and the sample-and-hold effect. The method shows similar results to other previously reported methods based on temporal integration, while being simpler and more straightforward. Indeed, just monopixel test cells are required for measuring MPRT. This method can be used for comparison between different materials, alignments surfaces, or other manufacturing details with no need of fabricating the whole LCD structure. Nevertheless, the method could also be used for characterization of commercial displays with major changes. A comparison of MPRT values for three different liquid-crystal materials is presented in this work. The behavior of the MPRT parameter in the case of an ideal liquid-crystal material with an LCRT equal to zero has also been studied. The results obtained for this material have been used as a reference to establish comparisons with real materials

Reorientation of single-wall carbon nanotubes in negative anisotropy liquid crystals by an electric field

Single-wall carbon nanotubes (SWCNT) are anisotropic nanoparticles that can cause modifications in the electrical and electro-optical properties of liquid crystals. The control of the SWCNT concentration, distribution and reorientation in such self-organized fluids allows for the possibility of tuning the liquid crystal properties. The alignment and reorientation of CNTs are studied in a system where the liquid crystal orientation effect has been isolated. Complementary studies including Raman spectroscopy, microscopic inspection and impedance studies were carried out. The results reveal an ordered reorientation of the CNTs induced by an electric field, which does not alter the orientation of the liquid crystal molecules. Moreover, impedance spectroscopy suggests a nonnegligible anchoring force between the CNTs and the liquid crystal molecules