Fig. 3.

Generation of harmonics by nonlinear fluorescence. (a) The nonlinear dependence of the fluorescent emission rate on the illumination intensity in the saturation regime. (b) The emission pattern resulting from sinusoidally patterned illumination with peak pulse energy densities of (from the bottom to top curve) 0.25, 1, 4, 16, and 64 times the saturation threshold. (c) Amplitude of each Fourier component of such emission patterns as a function of the illumination pulse energy (log scale). Equivalently, the curves indicate the fraction of the image signal that stems from each harmonic of the illumination pattern. As the illumination energy increases, more and more harmonics come into play (lower curves). The calculations used the scalar model of light and were for steady-state illumination (a) and for the pulse length of 0.64 ns used in the experiments and a realistic fluorescent lifetime of 3.5 ns (b and c).