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 acceptors (5×10^-6 M, sulforhodamine 101) in a levitated microdroplet (diameter, 2a = 19 μm), 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 μm 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 ∼1000× 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.