Non-linear fluorescence in dye solutions induced by a low power laser

Measurements of the fluorescence intensity I_F, excited by two-photon pumping, have been carried out in different dye solutions by utilizing a low power cw laser (I_L ≈ 12 mW) and a particular experimental technique. A considerable departure from the quadratic law I_F ∝ I²_L with the varying of 2hω - ΔE_i quantity has been detected. This behaviour has bee n accounted for by considering the dependence of the non-linear and linear cross sections relative to the S₀ → S₁ and S₀ → S₂ transitions on the laser frequency.