44915-AC7
Nanotomography of Liquid Crystals Using Near Field Scanning Optical Microscopy
The physical properties of anisotropic fluids can be manipulated on very short length scales of 100 nm or less by appropriate treatment of the confining substrate(s). This facilitates the use of ordered fluids,such as liquid crystals, in a variety of applications ranging from displays to switchable optical elements such as gratings and lenses. Future advances will require a full understanding of the liquid crystal’s structure at the nanoscale level. But owing to diffraction limitations, high resolution three dimensional imaging of the liquid crystal's molecular orientation profile has been beyond the reach of extant optical techniques. Here we present a powerful new imaging approach based on the use of polarized light emitted from a tapered optical fiber to investigate molecular orientation in three dimensions at nanoscale levels. We immerse the fiber’s tip inside a thin layer of liquid crystal that sits atop a substrate and raster-scan the fiber at a series of heights above the surface. From the images collected in the far field we are able to obtain a three dimensional visualization of the liquid crystal's structure with a resolvable volume two to three orders of magnitude smaller than attainable by extant methods. We demonstrate this technique by examining a nematic liquid crystal whose director orientation is controlled by a nanoscopic pattern scribed into the underlying polymer-coated substrate. We are able to image the extrapolation length L ~ 200 nm over which the molecular orientation relaxes due to the liquid crystal’s elastic forces. This technique of acquisition and analysis of image slices offers the intriguing possibility of 3D nanoscale reconstruction of a variety of other soft materials.
Based on this technique, we also examined the structure of AFM rubbing anisotropy of a polyimide alignment layer. Detailed experimental measurements were performed for the topographical anisotropy that arises when the polyimide alignment layer is scribed parallel and antiparallel to the AFM cantilever orientation. By means of our optical nanotomography approach, the optical retardation of an alignable birefringent liquid that covers the scribed substrate was measured with unprecedented resolution of only a few nanometers.
