Imaging enhanced energy transfer in a levitated aerosol particle

Energy transfer experiments are carried out at dilute concentrations of donors (10(-4) M, coumarine 334) and accepters (5 X 10(-6) M, sulforhodamine 101) in a levitated microdroplet (diameter, 2a = 19 mu m), using an aerosol particle fluorescence microscope. Microphotographs in donor and acceptor luminescence show that the transfer mechanism is not of a Forster 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, similar to 18 mu m 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 similar to 1000X that expected from Forster 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. (C) 1996 American Institute of Physics.