Photophysics of Surfactant Molecules in Microdroplets

Abstract

Over the last several years, the study of fluorescence properties in both time and frequency domain of atoms in micron-sized optical cavities has held considerable interest in the physics and quantum optics communities. In addition to fascinating purely scientific aspects of the phenomenon of cavity quantum electrodynamics (QED), the ability to modify molecular fluorescence properties in a microcavity offers potentially significant sensitivity advantages for ultrasensitive - or, single molecule - fluorescence detection. For example, two important quantities which limit sensitivity in single molecule fluorescence detection - the saturated absorption rate and the integrated fluorescence yield - can be significantly increased by enhancing the fluorescence decay rate. We have shown previously that fluorescence decay rates1 as well as the integrated fluorescence yield2 of rhodamine 6G can be significantly enhanced in glycerol microdroplets. However, exploitation of these effects in order to gain sensitivity in single molecule fluorescence detection is nontrivial for at least two important reasons. First, the magnitude of decay rate enhancement depends on the position of the molecule within the droplet; molecules near the center of the droplet are not coupled to high Q resonances while molecules near the surface may strongly interact with cavity resonances associated with droplet. Thus, diffusion limits the amount of time a given molecule may interact with the resonances thereby limiting the fluorescence decay rate. Second, the fluorescence decay rate depends on the orientation of the transition moment with respect to the cavity "axis"; e.g., the surface normal for spherical cavities.