Non-linear optical response of single molecules

We review several recent non-linear optical experiments done with single molecules in molecular crystals at low temperatures. We summarize pump–probe experiments showing light-shift, hyper-Raman and subharmonic resonances, electro-optical effects in which laser and RF frequencies are mixed by a single molecule, and the proposed use of a molecule as a triggered single-photon emitter. The comparison of experimental results to a simple two-level model with optical Bloch equations shows that single molecules are accurately described as electronic two-level systems and that they are very convenient testbenches for many investigations in non-linear and quantum optics