A near-field scanning optical microscope, in transmission mode, was used to study the optical characteristics of twin domains, domain boundaries, and other defects in single crystals. A large optical contrast between domains was observed by using linearly polarized light. It was found that the domain contrast could be changed, or even reversed, by varying the direction of the polarization axis with respect to the local crystallographic axis. It was also demonstrated that the contrast depended sensitively upon the numerical aperture of the collecting objective lens. This made it possible to differentiate between possible contrast mechanisms. The microscopic stress birefringence that was associated with the domain boundaries was also observed under crossed-polar conditions. Sub-micron sized circular optical features were proposed to be the surface terminations of dislocations.

E.B.McDaniel, J.W.P.Hsu: Journal of Applied Physics, 1996, 80[2], 1085-93