Individual molecular detection and photon statistics

M. Kreiter

Some molecular properties are inaccessible in ensemble experiments due to Inhomogeneous broadening or lack of synchronization, while they are easily observable on individuals. Single molecule fluorescence microscopy and spectroscopy was used to study organic fluorophores near metal films, by utilizing surface-plasmon polaritons both for excitation and for emission. It was shown that in spite of the nearby metal, single molecule detection is feasible and may be even enhanced compared to the situation without gold[1]. In this configuration, the influence of the nearby metal on the photophysics of the organic dyes was studied. This methodology opened the door for the study of dynamical processes near metal surfaces with the toolbox of single-molecule fluorescence.

Luminescent semiconductor quantum dots were studied in a single-dot experiment in order to get some insight in the luminescence blinking in this system. These chromophores are known to enter a non-emissive „dark state“ for very long times. A still unsolved fundamental question arises due to the observation that the length of on- and off- periods follows a statistics that cannot be understood in the framework of defined states that are connected by rates. Based on single dot luminescence experiments, two mechanisms for the blinking could be identified and separated [2]. A memory in the blinking could be experimentally proven[3].

[1] F. D. Stefani, K. Vasilev, N. Bocchio, N. Stoyanova, and M. Kreiter. Surface-Plasmon-Mediated Single-Molecule Fluorescence Through a Thin Metallic Film. Phys. Rev. Lett, 94, 023005 (2005)
[2] F. D. Stefani, W. Knoll, M. Kreiter, X. Zhong and M. Y. Han. Quantification of photo-induced and spontaneous quantum-dot luminescence blinking.Phys. Rev. B 72, 125305 (2005)
[3] F. D. Stefani, W. Knoll, M. Kreiter, X. Zhong, M. Y. Han. Memory in quantum-dot photoluminescence blinking. New J. Phys. 7, 197 (2005)