  Evanescent light excitation is widely used in super-resolution fluorescence
microscopy to confine light and reduce background noise. Herein we propose a
method of exploiting evanescent light in the context of emission. When a
fluorophore is located in close proximity to a medium with a higher refractive
index, its near-field component is converted into light that propagates beyond
the critical angle. This so-called Supercritical Angle Fluorescence (SAF) can
be captured using a hig-NA objective and used to determine the axial position
of the fluorophore with nanometer precision. We introduce a new technique for
3D nanoscopy that combines direct STochastic Optical Reconstruction Microscopy
(dSTORM) imaging with dedicated detection of SAF emission. We demonstrate that
our approach of a Direct Optical Nanoscopy with Axially Localized Detection
(DONALD) yields a typical isotropic 3D localization precision of 20 nm.
