Optical Nanotomography of Anisotropic Fluids – C. Rosenblatt

Date: Thu. April 2nd, 2009, 4:15 pm-5:15 pm
Location: Rockefeller 301

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 fluids’s molecular orientation profile has been beyond the reach of extant optical techniques . Here I 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 the fluid — in our case a nematic 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 nearly three orders of magnitude smaller than attainable by extant methods. We demonstrate this technique in two experiments: i) We examine a nematic liquid crystal whose director orientation is controlled by a nanoscopic pattern scribed into the underlying polymer-coated substrate, and 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. ii) We measure the surface-induced orientational order parameter above the bulk nematic-isotropic phase transition temperature as a function of position above the interface, with resolution ~ 2 nm. From these measurements we conclude that the interaction potential between the surface and the liquid crystal is nonlocal, extending ~5 nm into the liquid crystal.