Finding a single molecule in a haystack: laser spectroscopy of solids from (square root of)N to N=1

Single-absorber optical spectroscopy in solids may be regarded as the problem of finding a single dopant impurity molecule in a 'haystack' composed of 1012 - 1018 background host molecules and up to approximately 106 additional impurity molecules. Detailed studies of the low-temperature inhomogeneously broadened 0-0 S1 ← S0 electronic transition of pentacene dopant molecules in p-terphenyl crystals have yielded both (1) observations of spectral structure scaling as N, where N is the number of impurity molecules in resonance, and (2) the optical absorption spectrum of a single impurity molecule in a solid (N = 1). Recent advances in fluorescence excitation of very small volumes have greatly improved the signal-to-noise ratio for a single molecule.