Z-polarized confocal microscopy.

In light microscopy the transverse nature of the electromagnetic field precludes a strongly focused longitudinal field component, thus confining polarization spectroscopy and imaging to two dimensions (x,y). Here we describe a simple confocal microscopy arrangement that optimizes for signal from molecules with transition dipoles oriented parallel to the optic axis. In the proposed arrangement, we not only generate a predominant longitudinally (z) polarized focal field, but also engineer the detection scheme in such a way that in a bulk of randomly oriented molecules, the microscope's effective point-spread function is dominated by the contribution of those molecules that are oriented along the optic axis. Our arrangement not only implicitly allows for the determination of the orientation of transition dipoles of single molecules in three dimensions, but also highlights the contribution of z-oriented molecules in three-dimensional imaging.