Lothar Kador and William E. Moerner demonstrated optical detection of individual dopant molecules in a solid in 1989. They measured absorption from pentacene in a para-terphenyl host crystal at low temperature. Working in the wings of an inhomogeneously broadened spectral line reduced the number of resonant molecules, while two modulation methods suppressed interfering background signals. This connected careful background control and spectroscopic selection with measurements beyond an ensemble-averaged molecular response.[ref]
Optical detection and spectroscopy of single molecules in a solid by W. E. Moerner and L. Kador. 1989 abstract: host/dopant, double modulation, background rejection and line-wing selection
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