Nanometer scale optical studies of twin domains and defects in lanthanum aluminate crystals

A near-field scanning optical microscope (NSOM) in transmission geometry was used to study optical characteristics of twin domains, domain boundaries, and other defects in lanthanum aluminate (LaAlO3) single crystals. Large optical contrast between domains was observed using linearly polarized NSOM light. This domain contrast can be changed and even reversed by varying the direction of the polarization axis with respect to the local crystallographic axis. We further demonstrate that the contrast depends critically on the numerical aperture of the collection objective lens, enabling us to differentiate between possible contrast mechanisms. In addition, microscopic stress birefringence associated with the domain boundaries was observed under crossed-polar conditions. Submicron circular optical features on LaAlO3 are attributed to surface terminations of dislocations.