Reverse saturable absorption dynamics in indocyanine green

In this work, we studied the nonlinear absorption dynamics of indocyanine green (ICG) in dimethyl sulfoxide (DMSO) at 532 nm. Single pulse and pulse train Z-scan techniques were used to obtain singlet and triplet states dynamics, which were analyzed with a three- and five-energy level diagram, respectively. The excited singlet state absorption cross-section was determined to be 75 times higher than the ground state one, giving rise to reverse saturated absorption. Reverse saturated absorption also occurs from the triplet state, after its population through an intersystem crossing process, whose characteristic time is 4 ns. The triplet state absorption cross-section determined is 31 times higher than the ground state one. The high excited singlet and triplet states absorption cross-sections of ICG indicates its use in application requiring reverse saturated absorption, such as sensitizer for photodynamic therapy and optical limiting.