Switching the sign of nonlinear refraction in N,N′-Bis(2,5,-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide (DBPI)

By using a closed-aperture (CA) Z-scan technique with a frequency doubled ps and ns Nd: YAG laser, opposite signs of nonlinear refraction (NLR) of N,N′-Bis(2,5,-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide (DBPI) in acidic (96% sulphuric acid) medium are observed. Using the ps pulses positive effect of NLR is observed (self-focusing). This is attributed to population transitions among singlet states. On the contrary, the ns pulses show negative effect of NLR (self-defocusing), which is attributed to the heat-induced temperature rise induced by acoustic effects and the population relaxation to triplet states.