Abstract

We have developed a technique that allows individual dye molecules in solution to be detected as they pass through a tightly focused laser beam.¹ A mode-locked laser is used for excitation, and time-gated single photon counting is used to discriminate the weak bursts of fluorescence from the promptly scattered Raman noise arising from the many solvent molecules. By developing a dual-channel instrument that incorporates a dichroic beam splitter in the fluorescence collection optics and two synchronized mode locked lasers for excitation, we have demonstrated the ability to detect and spectrally identify single molecules in an ultradilute solution containing both rhodamine 6G and sulforhodamine 101.² Also, by digitizing and storing the arrival times of the fluorescence photons with respect to the excitation pulses we are able to measure the fluorescence lifetimes of individual molecules to a reasonable precision by using maximum entropy techniques. We also discuss recent progress towards making other spectroscopic measurements on single molecules, such as the quantum yield, absorption cross-section, and photodegradation rate, and future applications.

© 1992 Optical Society of America