Finding a needle in a haystack: Optical detection and spectroscopy of single absorbers in solids

Laser FM spectroscopy with double modulation has been used for the detection and spectroscopy of a single absorber in a solid (single-molecule detection). The expected single-molecule line shapes and FM/Stark data and background signals are shown and discussed. The observed shape and approximate size of the features, the position relative to the inhomogeneous line center, the dependence on vm, and insensitivity to any features wider than ≅ 100 MHz from nonrigid impurities indicate that the spectra reported are due to single molecules of pentacene in p-terphenyl.