Enhanced backward multiphoton-excited fluorescence from microcavities

Summary form only given. Incoherent emission (fluorescence and Raman scattering) of an ensemble of randomly oriented molecules is isotropic. However, when these molecules are homogeneously embedded in a sample shape (such as spheres, spheroids, cylinders, ...) which concentrates the internal intensity distribution, the resulting fluorescence emission can become anisotropic. Based on reciprocity principle, it is expected that multiphoton excited fluorescence is enhanced in the direction of the excitation source. The experiments, which demonstrate this new effect were all performed on 35-/spl mu/m radius ethanol or methanol droplets containing 1 g/l Coumarin 510 dye. Droplets were generated by a piezo-driven nozzle source, synchronized to the excitation laser. Ultrashort laser pulses were used as they have the major advantage of depositing low energies in the particle and preventing competitive stimulated Raman and Brillouin effects. The central wavelengths used for the one-, two-, and three-photon excitation of the microdroplets were 425 nm, 850 nm and 1.2 /spl mu/m respectively. The angular dependences of the 2-photon and 3-photon excited fluorescence from the microdroplets are presented. Enhancements as high as a factor 5 (2-photon) and 10 (3-photon) in the backward direction (180/spl deg/) are observed, compared to the fluorescence intensity at 90/spl deg/. This remarkable effect is very well matched to theoretical calculations. The incoherent emission was calculated by integrating the product of the internal pump distributions and the probabilities of detecting emission from molecules at different positions. The molecules rotate before emitting. The integration was also over the excitation and emission frequencies. The calculated internal pump intensities show which regions inside the droplets contributed most strongly to the multiphoton excited fluorescence. Consistent with the reciprocity principle, the fluorescence from these molecules in a spherical particle- travel preferentially back toward the oncoming plane wave.