  The detection of single molecules has facilitated many advances in life- and
material-sciences. Commonly, it founds on the fluorescence detection of single
molecules, which are for example attached to the structures under study. For
fluorescence microscopy and sensing the crucial parameters are the collection
and detection efficiency, such that photons can be discriminated with low
background from a labeled sample. Here we show a scheme for filtering the
excitation light in the optical detection of single stranded labeled DNA
molecules. We use the narrow-band filtering properties of a hot atomic vapor to
filter the excitation light from the emitted fluorescence of a single emitter.
The choice of atomic sodium allows for the use of fluorescent dyes, which are
common in life-science. This scheme enables efficient photon detection, and a
statistical analysis proves an enhancement of the optical signal of more than
15% in a confocal and in a wide-field configuration.
