OPTICAL-SPECTRA AND KINETICS OF SINGLE IMPURITY MOLECULES IN A POLYMER - SPECTRAL DIFFUSION AND PERSISTENT SPECTRAL HOLE BURNING

With high-efficiency fluorescence excitation techniques optical spectra of single impurity molecules of perylene in a polyethylene matrix can be obtained at 1.5 K. Analysis of such spectra shows a variety of spectral diffusion effects, including fast (< 2 s) resonance frequency changes on the 1-100-MHz scale, which lead to a range of apparent linewidths, as well as discontinuous jumps in the resonance frequency of 10-1000 MHz on a longer time scale. In addition, light-induced changes in the resonance frequency of a single molecule (persistent spectral hole burning) have been conclusively observed by showing that the burning time decreases with increased laser power. Surprisingly, hole-burned single molecules often spontaneously return to the original frequency in 1-100 s. Measurements of the burning time for a large number of hole-burning events for the same single molecule yield an exponential burn-time distribution, which is the first direct measurement to our knowledge of the stochastic kinetics of a single molecule. Analysis of the signal-to-noise function appropriate to these experiments gives the conditions under which other systems may permit single-molecule detection: strong absorption, high fluorescence yield, weak bottlenecks in the optical pumping process, and low hole-burning quantum efficiency.