Fluorescence resonance energy transfer scanning near-field optical microscopy.

The method of fluorescence resonance energy transfer scanning near-field optical microscopy (FRET SNOM) consists in the separation of a FRET pair between an SNOM tip and a sample. The donor (or acceptor) centre is located at the tip apex and scanned in the vicinity of a sample while acceptor fluorescence (or donor-fluorescence quenching) is detected. It is shown that the spatial resolution for such an approach is governed not by the aperture size but by the FRET characteristic radius (Förster radius), and thus can attain the value of 2-7 nm with the same (or higher) sensitivity as characteristic for the aperture SNOM. The theoretical fundamentals of the method, its experimental realization and connections with other types of near-field optical microscopy are discussed. Coherent FRET SNOM, which can be realized at liquid helium temperatures, and its possible applications for quantum informatics, are briefly outlined.