Enhanced Backward-Directed Multiphoton-Excited Fluorescence from Dielectric Microcavities

We demonstrate theoretically and experimentally that one-, two-, and three-photon excited fluorescence from dye molecules in spherical microcavities has an asymmetrical angular distribution and is enhanced in the backward direction. The enhancement ratios (of intensities at 180 degrees and 90 degrees ) are 9, 5, and 1.8 for three-, two-, and one-photon excitation, respectively. Even larger ratios are expected for microspheres with an index of refraction larger than that used in the experiments. Because of the reciprocity principle and concentration of the incident wave inside particles, the backward enhancement is expected to occur even with nonspherical particles.