The simplest tool to study three-dimensional arrangements is polarizing microscopy (PM). PM tests the orientation of optical axes of the liquid crystal specimen; these optical axes are closely related to the molecular arrangements in the medium. Unfortunately, PM yields only two-dimensional (2D) textures in the so-called plane of observation which is perpendicular to the optical axis of the microscope. This 2D image integrates the true 3D configuration of optical birefringence over the path of light. As the result of such an integration, the director profile along the direction of observation (="vertical cross section" of the specimen) is hard to decipher. Regrettably, it is precisely the director configuration in the vertical cross-section that is often the most valuable and desirable.
The fluorescence confocal polarizing microscopy (FCPM) allows one to recover the missing information and to obtain a truly 3D image of the liquid crystal director, both in the plane of observation and along the direction of observation. The principle of imaging is different from the traditional PM. The FCPM maps the intensity of polarized fluorescent light emitted by the liquid crystal sample, rather than the pattern of integrated birefringence as the PM texture does. This feature allows one to avoid the ambiguity of the in-plane PM textures that do not distinguish between two mutually perpendicular director configurations. More importantly, the confocal scheme allows one to collect the fluorescent light from a very small region of the sample and thus to optically slice the specimen by scanning the focused laser beam. The obtained map of fluorescence intensity is the 3D image of orientation of the fluorescent probe.
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