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 h as an asymmetrical angular distribution and is enhanced in the backward dir ection. The enhancement ratios (of intensities at 180 degrees and 90 degree s) are 9, 5, and 1.8 for three-, two-, and one-photon excitation, respectiv ely. Even larger ratios an expected for microspheres with an index of refra ction larger than that used in the experiments. Because of the reciprocity principle and concentration of the incident wave inside particles, the back ward enhancement is expected to occur even with nonspherical particles.