Far-field optical nanoscopy

The resolution of a far-field optical microscope is usually limited to d=lambda/(2nsinalpha)>200nm, with nsinalpha denoting the numerical aperture of the lens and lambda the wavelength of light. Here, 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. Specifically, the marker is switched between a bright and a dark state (on or off) to detect the emission of adjacent features sequentially in time. The first viable concept of this kind was Stimulated Emission Depletion (STED) microscopy where the diameter of the region (focal spot) in which the molecule can be in the fluorescent state follows d=lambda(2n sin alpharadic1+I/I_s).I is the intensity that drives a fluorophore from the bright fluorescent state to the dark ground state by stimulated emission. Is depends (inversely) on the lifetime of the states. For I/I_srarrinfin, it follows that drarr0, meaning that the resolution can be molecular . 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.