Matthew E. Sousa

Temporally multiplexed holographic polymer-dispersed liquid crystals

We report on a temporal multiplexing technique to fabricate multiple reflection gratings in holographic polymer-dispersed liquid crystals. By time-sequentially exposing the sample, two switchable reflection gratings are formed in a single film. The reflectance of the resulting gratings is well controlled by the individual exposure time. The observed transmission spectra are in excellent agreement with our model based on the reaction-diffusion equation to predict the kinetics of formation and the 2×2 matrix method to model the optical performance.

Patterning lyotropic liquid crystals as precursors for carbon nanotube arrays

We report on a simple patterning technique using lyotropic liquid-crystal solutions to fabricate organized arrays of high aspect ratio carbon nanostructures. An automated pen writing system is used in conjunction with nanochannel alumina templates to produce ordered arrays of nanotubes and nanofibers in a variety of shapes, widths, and carbon crystal structures set by surface anchoring phenomena. The nanostructure arrays are characterized with optical and electron microscopy and Raman spectroscopy to establish the morphology and crystal structures of the carbon nanoform arrays.

Optical retardation of in-plane switched polymer dispersed liquid crystals

We investigated the field-induced optical birefringence of a photopolymerized polymer dispersed liquid crystal (PDLC) with nearly a 50% volume ratio of phase-separated liquid crystal (LC) droplets. Measurements were performed in the in-plane switching configuration, which was attained by the use of interdigitated electrodes. The observed dependence of the sample transmittance on the external field magnitude is independent of the polarization state of the incident beam. The material transforms a linearly polarized wave into an elliptically polarized wave. Phase retardation between the two eigenpolarizations is proportional to the order parameter of the LC domains. The observed phenomena are attributed to the dominant role of the droplet proximity effects in the optical properties of the material. Dynamic response of the retardation takes place in a broad range of time scales, which are associated with relaxation of different degrees of freedom of the system. The observed dynamic features indicate an interc...

Template induced chiral ordering in nematic liquid crystalline materials : A deuterium nuclear magnetic resonance study

Glancing angle deposition is used to create porous liquid crystal alignment layers with high aspect-ratio, three-dimensional features. We use solid-state deuterium nuclear magnetic resonance to determine the alignment of alpha deuterated 4′-pentyl-4-cyanobiphenyl in a 3.2μm thick SiO2 film comprised of independent, helical structures. Comparing measured spectra with models, we determine that the liquid crystalline molecules exhibit chiral ordering dictated by the features of the template films. In the plane of deposition the liquid crystalline molecules tilt away from the substrate normal at an average angle of 51°, which is commensurate with the helical features of the chiral template.

Reflective Display Configurations Based on Total Internal Reflection and Grating-Grating Coupling of Holographic Polymer Dispersed Liquid Crystals (H-PDLC)

ABSTRACT We report on a novel electro-optical reflective configuration based on total internal reflection and grating-grating coupling fabricated in Holographic Polymer Dispersed Liquid Crystals (H-PDLC) materials. In this device, two symmetric slanted gratings are temporally multiplexed into a single film. A narrowband light is selected and diffracted from two coupled Bragg gratings creating a reflected image. These reflective configurations have switching voltages that are at least one-half that of H-PDLC reflective displays reported in the literature, and therefore have potential as low power, reflective display devices.

Novel Carbon Nanotubes Based on Disk-Rod Assemblies of Lyotropic Liquid Crystals

ABSTRACT We have fabricated carbon nanofibers and nanotubes using disk-rod assemblies of lyotropic liquid crystals in which the orientation of graphene layers can be manipulated using surface anchoring techniques common to liquid crystal displays. The bulk material properties of the carbon can be engineered by the spatial arrangement of the graphene layers due to their anisotropic nature. We have also demonstrated a method to pattern arrays of nanotubes using an automated printing technique.

P-165: Use of Lyotropic Liquid Crystals for Patterned Polarizer and Retarder Applications

We have developed a technique to pattern arrays of polarizers and retarders using an automated fluid dispensing system in conjunction with lyotropic liquid crystal polarizer technology. The resulting patterned optical elements have potential in flexible, stereoscopic and transflective displays.

Survey of switchable lasing configurations using structures of liquid crystal and polymer dispersions

We investigate the fabrication of holographic polymer dispersed liquid crystals (H-PDLCs) for use as switchable laser cavities. H-PDLCs are liquid crystal and polymer dispersions used in grating applications for displays, optical communications and optical security. By controlling the pitch of the H-PDLC and the laser dye used, we are able to fabricate a tunable laser. H-PDLCs were made in both reflection and transmission modes to vary the method by which lasing action occurs. The dye-doped H-PDLCs were pumped with nanosecond pulses from a laser with emission at 532 nm and a power of approximately 6 mJ. Lasing action was observed using a spectrometer from the H-PDLC grating; peak wavelengths occurred over a range of wavelengths, depending on the dye used, with the full width of the emission peaks approximately 6-8 nm at half maximum. The lasing action can be turned on and off by the application and removal of an electric field due to the properties of an H-PDLC. Furthermore, we investigate multidimensional architectures and quasicrystal symmetries for lasing applications. Applications for these cells include use in small-scale portable devices requiring a tunable laser source.

Optical Diffraction Properties of Polymer Dispersed Liquid Crystals Switched by Interdigitated Electrodes

ABSTRACT We investigated optical properties of a polymer dispersed liquid crystal (PDLC) film in-plane switched by a periodic pattern of interdigitated electrodes. Such a system acts as a thin optical phase grating which diffraction efficiency strongly depends on polarization state of the incident light. The intensity and dynamic response of various diffraction orders were analyzed as a function of the applied voltage amplitude. Our results demonstrate that by using a PDLC very effective phase gratings with response times below 10 ms can be achieved at the expense of slightly higher driving fields than needed for the similar bulk nematic liquid crystal gratings.

Optical diffraction gratings from polymer-dispersed liquid crystals switched by interdigitated electrodes

A spatial dependence of the refractive index perturbation induced in a 5-μm-thick polymer-dispersed liquid-crystal layer, by the use of a periodic electric field from the system of 50μm separated interdigitated electrodes, was investigated by measuring the induced in-plane birefringence of the sample. The efficiency of various diffraction orders was measured as a function of the applied voltage amplitude, and the dependence achieved for the 1st diffraction order was compared to the values calculated on the basis of the refractive index profile. The switching properties in response to the amplitude modulation of the driving voltage of various modulation depths were investigated. Our results demonstrate that a very effective phase modulation with response times considerably below 10ms can be achieved at the expense of slightly higher driving fields than needed for the bulk nematic liquid-crystal gratings.