Characterizing the photoluminescence of fluorescein-labeled cellulose in aqueous and alcohol solutions: influence of the cellulose backbone.

Although many dyes have been introduced into cellulose, whether bound to its backbone or within a cellulose matrix, few studies have determined whether the backbone statically or dynamically quenches the photoluminescence of the dye. To advance cellulosic fluorescent films, the influence of the cellulose backbone on photoluminescence must be understood. We determined the fluorescence properties of fluorescein isothiocyanate (FITC) and fluorescein-labeled cellulose (FLC) in water and alcohol, including their quantum yields [Formula: see text], lifetimes [Formula: see text], and rates of radiative [Formula: see text] and nonradiative [Formula: see text] decay.

Reversibly switching liquid crystals between three orthogonal orientation states for use in rapid-response THz phase shifters.

Liquid crystal (LC) devices for terahertz phase shifters inevitably use a thick cell gap for the required retardation, severely delaying the LC response. To improve the response, we virtually demonstrate novel LC switching between in-plane and out-of-plane for reversible switching between three orthogonal orientation states, broadening the range of continuous phase shifts. This LC switching is realized using a pair of substrates, each with two pairs of orthogonal finger-type electrodes and one grating-type electrode for in- and out-of-plane switching.

Kinetic arrest during the drying of cellulose nanocrystal films from aqueous suspensions analogous to the freezing of thermal motions.

A comprehensive understanding of controlling the iridescence of cellulose films by manipulating the alignment and helical pitch of cellulose nanocrystals (CNCs) is required to advance cellulose photonics and its optoelectronic applications. Aqueous suspensions of CNCs exhibit a cholesteric liquid crystal (LC) phase with structural color; however, attaining a uniformly colored film is extremely difficult. Presumably, because multiple interrelated factors influence the CNC molecular alignment and helical pitch, existing models are not necessarily conclusive and remain a subject of debate.

Newly discovered dimensional effects of electrodes on liquid crystal THz phase shifters enable novel switching between in-plane and out-of-plane.

To unveil a novel switching mechanism in liquid crystal (LC)-based phase shifters for the THz range, we analyse how the dimensions of the electrode structures enable a new type of switching, namely, THz in-plane and THz out-of-plane (TIP-TOP) switching. Specifically, we determine how varying these electrode dimensions influences the LC in-plane states with the corresponding phase shifts by calculating these effects in virtual devices. Interestingly, we found that significant dimensional effects of the in-plane electrode structures statically and dynamically influence the phase shift and response time of LC switching.

Interactions Between Hydrated Cerium(III) Cations and Carboxylates in an Aqueous Solution: Anomalously Strong Complex Formation with Diglycolate, Suggesting a Chelate Effect.

Interactions between hydrated Ce and various carboxylates are of fundamental interest. Anomalously strong interactions with Ce occur when diglycolic acid (DGA) is added into a Ce aqueous solution, unlike various other carboxylic acids. Herein, the complex-formation constants of Ce with these acids are evaluated via absorption and emission spectra.

Randomization and Constraint of Molecular Alignment and Orientation: Temperature-Dependent Anisotropy and Phase Transition in Vapor-Deposited Thin Films of an Organic Cross-Shaped Molecule.

We observe potential randomization and constraint of molecular alignment and orientation in an organic semiconductor molecule with increasing temperature up to the phase-transition temperature. Variable-angle spectroscopic ellipsometry and second-harmonic generation are used to study the changes in the molecular alignment in vapor-deposited organic thin films as samples are heated and cooled in a cycle from room temperature to the phase-transition temperature. The films consist of sterically bulky and cross-shaped molecules, 2-cyano-9,10-di(2-naphthyl)anthracene, and the anisotropy of its two moieties is probed.

A functionally separated nanoimprinting material tailored for homeotropic liquid crystal alignment.

In order to homeotropically align liquid crystals (LCs) at the nanosized surface grooves processed by nanoimprint lithography technology (NIL), we propose to design a hybrid-type homeotropic polymer material consisting of two distinct moieties with largely different thermo-mechanical properties and surface activity. Surface contact angle measurements and sum-frequency vibrational spectroscopy allow us to conclude that the polymer film is a functionally separated composite suitable for the homeotropic LC alignment processed by NIL. As one of the potential applications using the hybrid-type homeotropic polymer, we demonstrate that the nanoimprinted grooves at the polymer surface can achieve a zenithal nematic LC bistability.

Imaging colloidal particle induced topological defects in a nematic liquid crystal using third harmonic generation microscopy.

The nature of the third-harmonic generation (THG) process in a nematic liquid crystal is investigated for the case of tightly focused, low intensity, laser beams. Colloidal particle induced topological defects in a liquid crystal are visualized in three-dimensions using the dependence of the THG signal on both changes in non-linear susceptibility and the orientation of the liquid crystal director relative to the incident laser polarization state. We have found that the interpretation of THG images in a liquid crystal is complicated not only by the change in polarisation of the electric field as it propagates through the medium but also by anisotropic refractive index mismatch induced aberrations.

High-frequency dielectric relaxation of liquid crystals: THz time-domain spectroscopy of liquid crystal colloids.

Terahertz time-domain spectroscopy has been used to study the dielectric relaxation of pure 4'-n-pentyl-4-cyanobiphenyl (5CB) liquid crystal (LC) and its mixtures with 10 mum SiO2 particles in the frequency range 0.2-2 THz. For the pure sample, we find that spatial inhomogeneities consisting of oriented domains, comparable in size to our probe area (~1 mm(2)), cause a large scatter in the measured dielectric function, due to varying contributions from the ordinary and extraordinary components.

Sum-frequency vibrational spectroscopy of a helically structured conjugated polymer.

Sum-frequency vibrational spectroscopy was used as a novel technique to probe the molecular chirality of thin polymer films. Chiral vibrational spectra of poly(bithienylene-phenylene) were obtained, and the two enantiomers were distinguished by an interference method. Vibration-electronic double resonance was responsible for the observation of unusually strong chiral spectra of the phenylene vibration modes.

Mapping molecular conformation and orientation of polyimide surfaces for homeotropic liquid crystal alignment by nonlinear optical spectroscopy.

Surface-specific sum-frequency vibrational spectroscopy and second-harmonic generation were used to study the structures of polyimide (PI) surfaces for homeotropic liquid crystal (LC) alignment and the molecular orientation of LC adsobates on these surfaces. The imide ring was perpendicular to the surface with one of CO bonds protruding out of the surface and the other pointing into the bulk rather than flat on the surface. The ester CO bond in the side chain was sticking out of the surface with a tilt angle of about 45 degrees -55 degrees from the surface normal, indicating that the rigid side chain core was, more or less, along the surface normal.

Anisotropy of alkyl chains of azobenzene molecules at the air/water interface observed by sum-frequency vibrational spectroscopy.

Surface-specific sum-frequency vibrational spectroscopy has been used to study the structure of alkyl chains of azobenzene molecules at the air/water interface. The results show that the alkyl chains are well aligned before UV irradiation and protruding out of the surface with a certain distribution. Although the alkyl chains become less ordered by UV irradiation following dynamical motion due to cis-trans isomerization of the azobenzene core, the alkyl chains show anisotropy in the direction perpendicular to that of the azobenzene core by linearly polarized UV irradiation.