Saturable and reverse saturable absorption of a linear polymer in dimethylformamide

Nonlinear absorption properties of a linear polymer (P1) in dimethylformamide (DMF) have been investigated using an open aperture Z-scan technique with 8 ns laser pulses at 532 nm. It was found that P1 shows saturable absorption (SA) at low intensities and a transition from SA to reverse saturable absorption (RSA) with an increase in intensities. The analysis is based on rate equations and indicates that the absorption transition in P1 arises from the fact that the absorption cross-section of the first excited-state σ₁₂ is smaller than that of the ground-state σ₀₁ and that of the second excited-state σ₂₃, which is also the condition for the transition.