The Electric-Birefringence Set up
The optical train for the birefringence set up is given in Fig.5. The Glan-Thompson polarizer (4) and quarter wavelength plate (5) are oriented at 45 degrees with the normal to the plane in which the schematic is drawn. The transmitted intensity is measured as a function of the angle of the polarizer (6). When properly aligned, this intensity varies sinusoidally with the polarizer angle. The difference in the angle for pure solvent and a sample where the minimum occurs gives the shift in phase angle due to the sample. The analyzer (6) is mounted on a goniometer, and the transmitted intensity is automatically scanned as a function of the angle. An example is given in Fig.6, where an aligned phase of fd-virus is compared to the pure solvent. The accuracy of the measured phase shift is about 0.3 degrees. Birefringence ca be measured at an oblique incident angle, where the maximum angle is 15 degrees. Below this angle, the apparent birefringence due to refraction at the ITO-air and -sample interface can usually be neglected. This can be seen from Fig.7, where the apparent birefringence for a pure solvent is probed as a function of the angle of the laser beam.
( K. Kang )