Confocal microscopy of electro-optic materials: effect of aberrations on the axial response in ac mode

Confocal scanning optical microscopy can be used to investigate the structure of electro-optic materials. Application of an ac electric field allows one to measure sensitively small changes in the reflection of light from a sample surface, and those changes can be related to the electro-optic properties. We observe the axial dependence of the ac light intensity to be a linear combination of the dc component and its axial derivative. Our analysis shows that astigmatic aberrations and the azimuthal dependence of the optical index in anisotropic materials can explain this behavior.