Imaging enhanced energy transfer in a levitated aerosol particle

Energy transfer experiments are carried out at dilute concentrations of donors ~10 M , coumarine 334! and acceptors ~5310 M , sulforhodamine 101! in a levitated microdroplet ~diameter, 2a519 mm!, using an aerosol particle fluorescence microscope. Microphotographs in donor and acceptor luminescence show that the transfer mechanism is not of a Förster type, but is mediated by morphology dependent resonances ~MDRs! of the microdroplet. The transfer is vanishingly small in the central region of the droplet (r,0.9a), and grows to a pronounced maximum beneath the surface ~active region!, consistent with the theory of MDR-enhanced energy transfer. The angular intensity profile of the acceptor image, along with current theory, suggests that the energy transfer is a maximum with the donor and acceptor at equal distances on opposite sides of the droplet center, ;18 mm apart. From photometry we measure an overall ratio of acceptor to total luminescence of 7%. Within the active region the transfer efficiency is above 50%. This yield is ;10003 that expected from Förster transfer. This effect may be understood from a modification in the photon density of states in this region, which leads to efficient photon emission into MDRs. © 1996 American Institute of Physics. @S0021-9606~96!02819-9#