  The advent of single molecule optics has had a profound impact in fields
ranging from biophysics to material science, photophysics, and quantum optics.
However, all existing room-temperature single molecule methods have been based
on fluorescence detection of highly efficient emitters. Here we demonstrate
that standard, modulation-free measurements known from conventional absorption
spectrometers can indeed detect single molecules. We report on quantitative
measurements of the absorption cross section of single molecules under ambient
condition even in their dark state, for example during photoblinking or strong
quenching. Our work extends single-molecule microscopy and spectroscopy to a
huge class of materials that absorb light but do not fluoresce efficiently.
