  We demonstrate theoretically and experimentally that the three-dimensional
orientation of a single fluorescent nano-emitter can be determined by
polarization analysis of the emitted light (while excitation polarization
analysis provides only the in-plane orientation). The determination of the
emitter orientation by polarimetry requires a theoretical description including
the objective numerical aperture, the 1D or 2D nature of the emitting dipole
and the environment close to the dipole. We develop a model covering most
experimentally relevant microscopy configurations and provide analytical
relations useful for orientation measurements. We perform polarimetric
measurements on high-quality core-shell CdSe/CdS nanocrystals and demonstrate
that they can be approximated by two orthogonal degenerated dipoles. Finally,
we show that the orientation of a dipole can be inferred by polarimetric
measurement even for a dipole in the vicinity of a gold film, while in this
case the well-established defocused microscopy is not appropriate.
