Far-field optical nanoscopy

The resolution of a far-field optical microscope is usually limited to d = lambda/(2 n sin alpha ) > 200 nm, with n sin alpha denoting the numerical aperture of the lens and lambda the wavelength of light. This paper will discuss lens-based fluorescence microscopy concepts that feature a resolving power on the nanoscale. All these concepts share a common basis: exploiting selected (pairs of) states and transitions of the fluorescent marker to neutralize the limiting role of diffraction. Altogether, far-field optical nanoscopy is a fascinating development in optics with high relevance to the many areas of sciences, in particular the life sciences. Since it has already been a key to answering important questions in biology, and owing to its simplicity and commercial availability, it is expected that far-fieId fluorescence ´nanoscopes´ to enter most cell biology and many nanoscience laboratories the near