January 2000

A Millennium of Liquid Crystals

Eileen M. Korenic

The structure of a soap film is now understood to be a triple layer of soap-water-soap, in which the water molecules are hydrogen-bonded to each other, giving strength and flexibility to the film. The soap molecules are typically long hydrocarbon chains with a polar hydrophilic end dissolved in the water layer, and nonpolar hydrophobic ends pointing towards the outside of the overall trilayer. It is this lamellar or layered structure that can properly be called a liquid crystal.

Optical Coherence Tomography: High Resolution Imaging Using Echoes of Light

Optical coherence tomography (OCT) is a fundamentally new type of optical imaging modality. OCT performs high resolution, cross-sectional tomographic imaging of the internal microstructure in materials and biological systems by measuring the echo time delay and magnitude of backscattered light. I Image resolutions of 1-15 um can be achieved, one to two orders of magnitude higher than conventional ultrasound. Imaging can be performed in situ and in real time. The unique features of this technology enable a broad range of research and clinical applications. OCR imaging has its roots in ultrafast optics and draws upon many optical technologies including white light interferometry, fiber optics, and Fourier transform spectrometry.

Cirrus, Ice Crystals, and Halos

There have, however, been considerable advances in our understanding of cirrus and halos. Modern works on halos can be credited in part to Walter Tape. Tape recently co-authored a paper with Gunther P. Konnen entitled "A general setting for halo theory." This paper puts forth a framework for the systematic treatment of halos due to refraction in preferentially oriented ice wedges, and an atlas is constructed of such halos. Initially, Tape is not constrained either by the interfacial angles, or the orientations of real ice crystals. Instead, he considers "all possible" refraction halos. As a result, no assumptions arc made regarding the wedge angle, and only a weak assumption is made regarding the allowable wedge orientations. The atlas is thus a very general collection of refraction halos that includes known halos as a small fraction.

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