Fluorescence photon antibunching from single molecules on a surface

Fluorescence correlation spectroscopy of individual Rhodamine 6G (R6G) molecules immobilized on a silica surface is performed in air at room temperature using confocal laser scanning optical microscopy (CLSM). The high excitation irradiance in CLSM is used to saturate single R6G molecules, and to observe fluorescence photon antibunching. An experimental arrangement is described that improves the overall coincidence counting efficiency at high irradiance. Photophysical parameters obtained from the saturation data are used to model the antibunching behavior. This is the first reported example of photon antibunching from individual quantum systems on a surface.