Fluorescence Microscopy and Spectroscopy of an Isolated Micro-droplet
S. Arnold, S. Holler and N. Goddard
Mat.Sci.and Eng. B B48, 139-146(1997)
also in Quantum Optical Phenomena in Spatially Confined Materials
Proceedings of the 6^th NEC Symposium on Fundamental Approaches to New Material Phases
eds. K. Cho, K. Tajima and H. Kamimura
(Elsevier , Amsterdam, 1997)pp 130-138

Fluorescence microscopy and spectroscopy experiments are carried out on individual microdroplet (m-drop) isolated within an electrodynamic levitator-trap (Paul type) at atmospheric pressure. Imaging experiments on spatially homogeneous donor and acceptor dyes within particle of radius a ~ 10 mm reveal a 'global' transfer mechanism with a yield excedding conventional dipole-dipole transfer (Föster transfer) by ~1000X between molecules at a particular radial distance r > 0.93a [S. Arnold, S. Holler, and S.D. Druger, J. Chem. Phys. 104, 7741(1996)], but is consistent with the quantum theory of enhanced energy transfer theough morphology dependent resonances (MDRs) of the sphere [P.T. Leung, K. Young, J. Chem. Phys. 89, 2894(1988)]. To gain a better understanding for the primary step in this process, we investigate fluorescence from a particle with a dilute layer of surface active molecules. These surfactant molecules have a fixed orientation relative to the surface as revealed both qualitatively from a heuristic interpretation of polarization analyzed images [S. Arnold, L.M. Folan, Proc. SPIE 1862, 218(1993)], and quantitatively through the preferential cavity enhance selection of various polarized modes in emission spectroscopy. The spectra are adequately described through semi-classical [S.D. Druger, S. Arnold, L.M. Folan, J. Chem. Phys. 87, 2649(1987)]. This theory has also been applied to the description of reported 'anomalous' fluorescence lifetime measurements [M.D. Barnes, C-Y. Kung, W.B. Whitten, J.M Ramsey, S. Arnold, S. Holler, Phys. Rev. Lett. 76, 3931(1996)] from a dilute surface layer. Although sum rules based on density of states of high-Q whispering gallery modes fail to describe the data at particle sizes for which the homogeneous linewidth is larger than the free spectral range [M.D. Barnes, C-Y. Kung, W.B. Whitten, J.M Ramsey, S. Arnold, S. Holler, Phys. Rev. Lett. 76, 3931(1996)], semiclassical theory, including all modes, is in considerably better agreement with the data.