  Fluorescence detection, either involving propagating or near-field emission,
is widely being used in spectroscopy, sensing and microscopy. Total internal
reflection fluorescence (TIRF) confines fluorescence excitation by an
evanescent (near-) field and it is a popular contrast generator for
surface-selective fluorescence assays. Its emission equivalent, supercritical
angle fluorescence (SAF) is comparably less established although it achieves a
similar optical sectioning as does TIRF. SAF emerges when a fluorescing
molecule is located very close to an interface and its near-field emission
couples to the higher-refractive index medium (n2 > n1) and becomes
propagative. Then, most fluorescence is detectable on the side of the
higher-index substrate and a large fraction of this fluorescence is emitted
into angles forbidden by Snell's law. SAF as well as the undercritical angle
fluorescence (UAF) (far-field emission) components can be collected with
microscope objectives having a high-enough detection aperture NA > n2 and be
separated in the back-focal plane (BFP) by Fourier filtering. The BFP image
encodes information about the fluorophore radiation pattern, and it can be
analysed to yield precise information about the refractive index in which the
emitters are embedded, their nanometric distance from the interface and their
orientation. A SAF microscope can retrieve this near-field information through
wide-field optics in a spatially resolved manner, and this functionality can be
added to any existing inverted microscope.
