Electrically switchable diffraction gratings having periods as small as 5 μm and incorporating nematic LC confined within channels formed in poly(vinyl alcohol) (PVA) films were prepared and characterized. Gratings were produced by first using conventional photolithographic procedures to prepare reusable surface-relief grating molds from a common photoresist (SU-8). An aqueous PVA solution was then drop coated onto the mold and dried, The PVA film was subsequently peeled from the mold and pressed (channel side down) onto an indium tin oxide (ITO) coated glass slide. The channels were then filled with LC using capillary action. Finally, a second ITO-coated slide was pressed onto the backside of the PVA film, forming a polymer/LC grating cell. The diffraction efficiency of each grating was measured as a function of applied electric field strength using 488 nm light. Beam diffraction was greatest in the absence of the field and fell to zero for applied fields of less than 10 V/μm. These studies and atomic force microscopy results showed the LC channels to be ≈ 50-100 nm deep. Multiphoton excited fluorescence microscopy (MPEFM) was used to show the LC was oriented predominantly parallel to the long axis of the channels in the zero field state. Significant nonuniformity observed in the LC orientation was attributed to channel (PVA) wall roughness. Time-resolved MPEFM was used to monitor the LC reorientation process on submicron length scales. The local LC reorientation dynamics were also strongly perturbed by channel wall roughness.
KEYWORDS: Liquid crystals, Near field scanning optical microscopy, Modulation, Polymers, Near field optics, Molecules, Data modeling, Phase shift keying, Motion models, Thin films
A new technique developed in this laboratory and based on near-field scanning optical microscopy (NSOM) is used to study the field-induced reorientation of molecules in local regions of thin film materials. A highly concentrated electric field is applied across the sample, between the metal-coated near-field probe and the sample substrate. Molecular motion induced in the sample by modulation of the electric field is observed using NSOM methods. Lock-in detection of the optical response to a sinusoidally- modulated field, recorded under cross-polarized, transmitted-light conditions allows for the recording of dynamics images. The local rate of reorientation is measured for individual locations in a sample by recording the response in either the time or frequency domains. Dynamics information is obtained with microsecond(s) ec time resolution and nanometer-scale spatial resolution. This method is applied in studies of polymer-dispersed liquid crystal films. In these materials, small droplets of nematic liquid crystal are dispersed in an otherwise uniform poly(vinyl alcohol) film. The liquid crystal droplets are birefringent, forming electrically-switchable light-scattering centers. A simple forced-oscillator model for the reorientation dynamics in the liquid crystal is presented. Variations in the time scale and extent of molecular reorientation are observed as a function of field strength, droplet size, droplet shape, and position probed. The data are interpreted based on knowledge of the important intermolecular forces active in these materials.
The technique of optical second harmonic generation (SHG) is applied to the measurement of molecular adsorption at the interface between two immiscible electrolyte solutions (ITIES). The resonant second harmonic response from 2-(N-octadecyl)aminonaphthalene-6-sulfonate (ONS) is used in conjunction with interfacial tension measurements to optically determine the relatively surface coverage of the anionic surfactant molecule at a charged water- dichloroethane interface. At a pH of 9, ONS adsorption occurs at all potentials positive of the potential of zero charge. The potential dependent adsorption of ONS can be described by a Frumkin isotherm with a free energy of adsorption that varies linearly with applied potential. The potential dependence of the SHG from the interface provides important information on the position of the adsorbed ONS molecules with respect to the ITIES. At a pH of 3, both the anionic form of ONS and the protonated zwitterionic form of ONS are present at the liquid- liquid interface.
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