<title>On the nature of increase in multiphoton-excited fluorescence from a spherical particle in the backward direction</title>

Abstract

The angular distribution of the spontaneous fluorescence excited by the laser radiation inside a spherical particle has been considered. The position and the effective volume of radiating in the particle the fluorescence sources has been calculated numerically as functions of the particle radius in the cases of excitation of single-, two-, and three-photon fluorescence. Within the framework of the geometric-optics approach, an equation determining the ratio of the mean brightness of fluorescence from the front and rear hemispheres of the particle is obtained. It is shown, that in the case of localization of fluorescence source of near to a shadow hemisphere of a particle there is asymmetry in the angular distribution of the spontaneous fluorescence in the forward and backward directions, which becomes more pronounced when the order of multiphoton-excitation process increases.