Coherent optical dipole coupling of two individual molecules at nanometre separation

A novel approach that combines high resolution laser spectroscopy with scanning probe microscopy to determine the locations of individual fluorescent molecules in all three dimensions with nanometer accuracy and to study their optical interaction is reported. The schematic arrangement of the setup is described. Applying the technique, two individual molecules are located that are separated by 12 /spl plusmn/ 2 nm. In addition, it is shown that these two molecules are coupled via the coherent dipole-dipole coupling, leading to two sub- and superradiant states. By increasing the intensity of the incident laser beam, a two-photon transition is put into evidence that takes place at the frequency midway between the original transition frequencies of the system. Finally, photon statistics of the fluorescence light is measured and photon bunching is observed when exciting the system at the new frequency whereas excitation of the system at any of the original resonances yields photon antibunching.