Western Illinois University: Macomb Campus
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Department of Physics
Olorunsola Thesis Presentation
Nov 22, 2013
Speaker: Oluwatobi Olorunsola, graduate student
Date: Monday, December 2, 2013
Time: 3:00 PM
Room: Currens Hall 206
Understanding Conoscopic Interference Patterns in Anisotropic Crystals
The interference patterns observed in conoscopy are important in studying the optical and geometrical properties of anisotropic materials. They have also been used to identify minerals and to explore the structure of biological tissues. In a conoscopic interferometer, an optically anisotropic specimen is placed between two crossed linear polarizers and illuminated by a convergent light beam. The interference patterns are produced because in an anisotropic material an incident light is split into two eigenwaves, namely the ordinary and the extraordinary waves.
We report our work on the theoretical simulation and experimental observation of the conoscopic interference patterns in anisotropic crystals. In our simulation, the interference patterns are decomposed into fringes of isogyres and isochromates. For each light propagation direction inside the crystal there exist two eigenwaves that have their own characteristic velocities and vibration directions. The isogyres are obtained by computing the angle between the polarization of the incident light and the vibration directions of the two eigenwaves. The isochromates are obtained by computing the phase retardance between the two eigenwaves inside the crystal. The interference patterns are experimentally observed in several crystals, with their optic axes either parallel or perpendicular to their surfaces. An external electric field is applied to deform the crystals from uniaxial to biaxial. The results of our experimental observation agree well with our computer simulation.
In conventional interferometers the isochromatic interference fringes are observed by using a circular polarizer and a circular analyzer, both constructed by a linear polarizer and a quarter wave plate. However, due to the dispersion of the quarter wave plates, the phase-retardance between the two light waves inside the quarter wave plates is wavelength-dependent, which results in different conoscopic interference patterns for different colors of light. In this research endeavor we investigate the details of the variation of the interference patterns in anisotropic crystals when the wavelength of the light source is changed. In addition, we designed a spinning-polarizer and spinning-analyzer method to eliminate the effect of dispersion in these interference patterns, which provides a new way to visualize the isochromatic interference fringes in conoscopy. Our method produces similar interference patterns for all colors without dispersion and without the use of additional optical elements.
About the speaker:
Oluwatobi Olorunsola is a graduate student in the Western Illinois University Department of Physics working under Dr. Pengqian Wang. Upon successful completion of his thesis, he stands to graduate in December 2013.