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ItemOptics in the Natural World: Iridescent Colors of Butterflies and the Twisted Beetle!(Georgia Institute of Technology, 2013-01-08) Srinivasarao, MohanThe study of photonics in nature contains beautiful and diverse examples of sub-wavelength structural features that create observed colors through thin layered or multilayered interference, diffraction, zero order diffraction and light scattering. In this talk I will discuss two such examples: Butterflies and Beetles, both of which attracted the attention of great scientists including Newton, Rayleigh, Michelson and Raman, among others. Structural coloration based on diffraction, multilayer reflection, cholesteric analogues or photonic crystal-like structures is pervasive especially in the world of insects. The color of Papilio palinurus results from microbowl lined with a multilayer of air and chitin. The green color is the result of color mixing of the yellow light reflecting from the bottom of the bowl and the blue light reflecting from the sides of the bowl. We have used breath-figure templated assembly as the starting point to mimic the structure of Papilio palinurus. We were able to produce microbowls which were then coated with a multilayer of alternating titanium oxide and aluminum oxide. The resulting structure exhibits the same color mixing as the original butterfly structure does. In the second part of the talk, we take a closer look at the colors produced by iridescent, metallic green beetle, Chrysina Gloriosa that selectively reflects left circularly polarized light when illuminated with unpolarized light which is observed to possess a nearly hexagonal cellular pattern on its exoskeleton. Using crystallographic concepts and Voronoi analysis of the structure present on the exoskeleton, we determine that these cells (~10 microns each) are organized with pentagons and heptagons, interdispersed typically as clusters, between hexagons. In an optical microscope, each cell appears to contain a bright yellow core, placed in greenish cell that has yellowish borders. Using confocal microscope and the auto-fluorescence of the exoskeleton matrix, we visualize that these cells consist of nearly concentric, nested arcs that lie on surface of a shallow cone. The observed textures are reminiscent of the texture of a cholesteric liquid crystalline phase with a free surface, and present an interesting analogy that provides the basis for structure and color present on beetles.
ItemHomeotropically aligning phase separated columnar structures for fabrication of flexible electrooptical devices(Georgia Institute of Technology, 2011-11) Acharya, Bharat R. ; Choi, Hyunchul ; Srinivasarao, Mohan ; Kumar, SatyendraA method of achieving homeotropic alignment of liquid crystals (LCs) by ultraviolet light induced phase separation of LC from its mixture with photo-curable pre-polymer is reported. Vertical polymer columns and micro-fibrils developed during the phase separation promote homeotropic alignment of the LC director (i.e., alignment perpendicular to the LC-substrate interface), suitable for devices based on LCs possessing negative dielectric anisotropy. These vertical structures extend between two substrates and permit the fabrication of highly flexible electro-optical devices with high contrast coupled with fast response times. This simple single-step technique eliminates the need for the traditional polymer alignment layer pre-deposited on substrates. (C) 2011 American Institute of Physics. [doi:10.1063/1.3663966]
ItemRaman Scattering Study of Phase Biaxiality in a Thermotropic Bent-Core Nematic Liquid Crystal(Georgia Institute of Technology, 2010-07) Park, Min Sang ; Yoon, Beom-Jin ; Park, Jung Ok ; Prasad, Veena ; Kumar, Satyendra ; Srinivasarao, MohanPolarized Raman spectroscopy was used to investigate the development of orientational order and the degree of phase biaxiality in a bent-core mesogenic system. The values of the uniaxial order parameters < P-200 > and < P-400 >, and biaxial order parameters < P-220 >, < P-420 >, and < P-400 >, and their evolution with temperature were determined. The temperature dependence of almost all order parameters reveals a second order transition from the uniaxial to biaxial nematic phase with < P-220 > increasing to similar to 0.22 before a first order transition to the smectic-C phase, upon cooling.
ItemNematic biaxiality in a bent-core material(Georgia Institute of Technology, 2010-05) Yoon, Hyung Guen ; Kang, Shin-Woong ; Dong, Ronald Y. ; Marini, Alberto ; Suresh, Kattera A. ; Srinivasarao, Mohan ; Kumar, SatyendraThe results of a recent investigation of the nematic biaxiality in a bent-core mesogen _A131_ are in apparent disagreement with earlier claims. Samples of mesogen A131 used in the two studies were investigated with polarized optical microscopy, conoscopy, carbon-13 NMR, and crossover frequency measurements. The results demonstrate that textural changes associated with the growth of biaxial nematic order appear at _149 °C. The Maltese cross observed in the conoscopic figure gradually splits into two isogyres at lower temperatures indicating phase biaxiality. Presence of the uniaxial to biaxial nematic phase transition is further confirmed by temperature trends of local order parameters based on 13C chemical shifts in NMR experiments. Frequency switching measurements also clearly reveal a transition at 149 °C. Differences between the two reports appear to be related to the presence of solvent, impurities, and/or adsorbed gases in samples of A131 used in the study of Van Le et al. _Phys. Rev. E 79, 030701 _2009__.
ItemThe Evolution Study Of Thin Film Structure During The Film Formation(Georgia Institute of Technology, 2010-01) Park, Min Sang ; Aiyar, Avisheck ; Park, Jung Ok ; Reichmanis, Elsa ; Srinivasarao, Mohan
ItemMicroscopic observations and simulations of Bloch walls in nematic thin films(Georgia Institute of Technology, 2006-10) Zhou, Jian ; Park, Jung Ok ; De Luca, Gino ; Rey, Alejandro D. ; Srinivasarao, MohanWe study Bloch wall defects formed by quenching nematic thin films from planar anchoring to homeotropic anchoring through a temperature-driven anchoring transition. The director profiles of the walls are directly visualized using fluorescence confocal polarizing microscopy, and shown to agree well with the simulation based on the Frank elasticity theory. A pure twist wall exists if the ratio of sample thickness to surface extrapolation length p is smaller than or close to 1; while a diffuse Bloch wall is obtained if p is much greater than 1.
ItemConfocal fluorescence microscopy of hydrogel contact lens(Georgia Institute of Technology, 2006-05-01) Bottomley, Lawrence A. ; Park, Jung 0. ; Srinivasarao, Mohan